ALBERT

Description: Amorphous silicates are common in extraterrestrial materials, especially in carbonaceous chondrites of petrologic types 1 and 2. In addition, high percentage of amorphous components and poorly crystalline phyllosilicates were found in the mudstones at Gale Crater by the CheMin instruments on-board of Mar Curiosity rover, which illustrates the importance of characterizing amorphous silicates in future planetary surface explorations. The structure of an amorphous silicate can vary in two aspects: the degree of polymerization and the degree of crystallinity. Here, we present the first phase study on characterizing synthetic alkali and alkali earth silicate glasses with different degrees of polymerization using vibration spectroscopy. Compared with crystalline silicates, their Raman and mid-IR spectra show broad spectral peaks, but have the similar peak positions. We find that a change in the degree of polymerization of these silicate glasses affects their Raman band positions, and especially the ratio of Raman band intensities, as well as the positions of the Christiansen feature and reststrahlen bands in their mid-IR absorbance spectra. Based on these observations, we establish a calibration curve that could enable semi-quantification of the polymerization degree of silicate glasses in planetary surface/subsurface materials during future robotics planetary surface exploration missions.

Description: Orbital magnetic field data show that portions of the Moon's crust are strongly magnetized, and paleomagnetic data of lunar samples suggest that Earth-strength magnetic fields could have existed during the first several hundred million years of lunar history. The origin of the fields that magnetized the crust are not understood and could be the result of either a long-lived core generated dynamo or transient fields associated with large impact events. Core-dynamo models usually predict that the field would be predominantly dipolar, with the dipole axis aligned with the rotation axis. We test this hypothesis by modeling the direction of crustal magnetization using a global magnetic field model of the Moon derived from Lunar Prospector and Kaguya magnetometer data. We make use of a model that assumes that the crust is unidirectionally magnetized. The intensity of magnetization can vary with the crust, and the best-fitting direction of magnetization is obtained from a non-negative least squares inversion. From the best fitting magnetization direction we obtain the corresponding North magnetic pole predicted by an internal dipolar field. Some of the obtained paleopoles are associated with the current geographic poles, while other well-constrained anomalies have paleopoles at equatorial latitudes, preferentially at 90° East and West longitudes. One plausible hypothesis for this distribution of paleopoles is that the Moon possessed a long-lived dipolar field, but that the dipole was not aligned with the rotation axis as a result of large scale heat flow heterogeneities at the core-mantle boundary.

Description: Planets and satellites can undergo physical librations, which consist of forced periodic variations in their rotation rate induced by gravitational interactions with nearby bodies. This mechanical forcing may drive turbulence in interior fluid layers such as subsurface oceans and metallic liquid cores through a libration-driven elliptical instability (LDEI) that refers to the resonance of two inertial modes with the libration-induced base flow. LDEI has been studied in the case of a full ellipsoid. Here, we address for the first time the question of the persistence of LDEI in the more geophysically-relevant ellipsoidal shell geometries. In the experimental setup, an ellipsoidal container with spherical inner cores of different sizes is filled with water. Direct side-view flow visualizations are made in the librating frame using Kalliroscope particles. A Fourier analysis of the light intensity fluctuations extracted from recorded movies shows that the presence of an inner core leads to spatial heterogeneities but does not prevent LDEI. Particle image velocimetry (PIV) and direct numerical simulations (DNS) are performed on selected cases to confirm our results. Additionally, our survey at a fixed forcing frequency and variable rotation period ( i.e. variable Ekman number, E ) shows that the libration amplitude at the instability threshold varies as ∼ E 0.65 . This scaling is explained by a competition between surface and bulk dissipation. When extrapolating to planetary interiors conditions, this leads to the E 1/2 scaling commonly considered. We argue that Enceladus' subsurface ocean and the core of the exoplanet 55 CnC e should both be unstable to LDEI.

Description: Solar wind interactions with the surfaces of asteroids and small moons eject atoms and molecules from the uppermost several n m of regolith grains through a process called sputtering. A small fraction of the sputtered species, called secondary ions, leave the surface in an ionized state, and these are diagnostic of the surface composition. Detection of secondary ions using ion mass spectrometry (IMS) provides a useful method of analysis due to low backgrounds and high instrument sensitivities. However, the sputtered secondary ion yield and the atomic composition of the surface are not 1-to-1 correlated. Thus, relative yield fractions based on experimental measurements are needed to convert measured spectra to surface composition. Here, available experimental results are combined with computationally derived solar wind sputtering yields to estimate secondary ion fluxes from asteroid-sized bodies in the Solar System. The Monte Carlo simulation code SDTrimSP is used to estimate the total sputtering yield due to solar wind ion bombardment for a diverse suite of meteorite and lunar soil compositions. Experimentally measured relative secondary ion yields are then used to determine the abundance of refractory species ( M g + , A l + , C a + , F e + ) relative to S i + , and it is shown that relative abundances can be used to distinguish whether a body is primitive or has undergone significant geologic reprocessing. Estimates of the sputtered secondary ion fluxes are used to determine the IMS sensitivity required to adequately resolve major element ratios for nominal orbital geometries.

Description: Without the protection of the atmosphere, the soils on lunar surfaces undergo a series of optical, physical, and chemical changes during micrometeorite bombardment. To simulate the micrometeorite-bombardment process and analyze the impact characteristics, four types of rocks, including terrestrial basalt and anorthosite supposed to represent lunar rock, an H-type chondrite (the Huaxi ordinary chondrite) and an iron meteorite (the Gebel Kamil iron meteorite) supposed to represent micrometeorite impactors, are irradiated by a nanosecond-pulse laser in a high vacuum chamber. Based on laser irradiation experiments, the laser pits are found to be of different shapes and sizes which vary with the rock type. Many melt and vapor deposits are found on the mineral surfaces of all the samples, and nanophase iron (npFe) or Fe-Ni alloy particles are typically distributed on the surfaces of ilmenite, kamacite or other minerals near kamacite. By analyzing the focused ion beam (FIB) ultrathin slices of laser pits with a transmission electron microscope (TEM), the results show that the subsurface structures can be divided into three classes and that npFe can be easily found in Fe-bearing minerals. These differences in impact characteristics will help determine the source material of npFe and infer the type of micrometeorite impactors. During micrometeorite bombardment, in the mare regions, the npFe are probably produced simultaneously from lunar basalt and micrometeorites with iron-rich minerals, while the npFe in the highlands regions mainly come from micrometeorites.

Description: The presence of oxidants such as hydrogen peroxide (H 2 O 2 ) and perchlorate (ClO 4 - ), which have been detected on Mars, has significant implications for chemistry and astrobiology. These oxidants can increase the reactivity of the martian soil, accelerate the decomposition of organic molecules, and depress the freezing point of water. The study by Crandall et al “Can Perchlorates be Transformed to Hydrogen Peroxide Products by Cosmic Rays on the Martian Surface” reveals a new formation mechanism by which hydrogen peroxide and other potential oxidants can be generated via irradiation of perchlorate by cosmic rays. This study represents an important next step in developing a full understanding of martian surface and subsurface chemistry, particularly with respect to degradation of organic molecules and potential biosignatures.

Description: Observations of a water vapor exosphere around Ceres suggest the dwarf planet may be episodically outgassing at a rate of ~6 kg s -1 from unknown sources. With data from the Dawn mission as constraints, we use a coupled thermal and vapor diffusion model to explore three different configurations of water ice (global buried pore-filling ice, global buried excess ice, and local exposed surface ice) that could be present on Ceres. We conclude that a buried ice table cannot alone explain the vapor production rates previously measured, but newly exposed surface ice, given the right conditions, can exceed that vapor production rate. Sublimation lag deposits form that bury and darken this surface ice over a large range of timescales (from 〈1 yr to ~100s of kyrs) that depend on latitude and ice regolith content. Sublimating water vapor can loft regolith particles from the surface of exposed ice, possibly prolonging the visible lifespan of those areas. We find this process is only effective for regolith grains smaller than ~1s of μm.

Description: O 2 , H 2 and H 2 O 2 radiolysis from water ice is pervasive on icy astrophysical bodies, but the lack of a self-consistent, quantitative model of the yields of these water products versus irradiation projectile species and energy has been an obstacle to estimating the radiolytic oxidant sources to the surfaces and exospheres of these objects. A major challenge is the wide variation of O 2 radiolysis yields between laboratory experiments, ranging over four orders of magnitude from 5×10 -7 to 5×10 -3 molecules per eV for different particles and energies. We revisit decades of laboratory data to solve this long standing puzzle, finding an inverse projectile range dependence in the O 2 yields, due to preferential O 2 formation from an ~30 Å thick oxygenated surface layer. Highly penetrating projectile ions and electrons with ranges ≳30 Å are therefore less efficient at producing O 2 than slow/heavy ions and low-energy electrons (≲ 400 eV) which deposit most energy near the surface. Unlike O 2 the H 2 O 2 yields from penetrating projectiles fall within a comparatively narrow range of (0.1-6)×10 -3 molecules per eV, and do not depend on range, suggesting H 2 O 2 forms deep in the ice uniformly along the projectile track, e.g. by reactions of OH radicals. We develop an analytical model for O 2 , H 2 and H 2 O 2 yields from pure water ice for electrons and singly-charged ions of any mass and energy, and apply the model to estimate possible O 2 source rates on several icy satellites. The yields are upper limits for icy bodies on which surface impurities may be present.

Description: Lithology distribution across the Moon is pivotal for understanding lunar evolution. However, so far, the distribution of lunar rock suites is still uncertain; as a result, many related core issues on lunar evolution have long been in dispute. This work reports on a new lithology distribution map across the Moon and discusses some critical issues of the lunar evolutionary process. The oxide abundances derived from Chang’E-1 Interference Imaging Spectrometer imagery and the Th contents inferred by Lunar Prospector Gamma Ray and Neutron Spectrometer data are employed to generate the lithological map by using the decision tree C5.0 algorithm. The following conclusions are inferred from this new map. (1) Magnesian suite is widely distributed across the Feldspathic Highlands Terrane and in the periphery of the South Pole-Aitken Terrane. Thus, the viewpoints have been validated that the early magnesian magmatism may be a global phenomenon and that KREEP basalt is not necessary for the petrogenesis of Mg-rich rocks. Moreover, the regions of Dryden, Chaffee S, Theophilus and Moscoviense are confirmed as magnesian suite exposures. (2) The observation that the Feldspathic Highlands Terrane has a higher Mg/(Mg+Fe) value than the maria is related to the flood of the Mg-suite across the Feldspathic Highlands Terrane. (3) The specific exposed locations of the alkali suite across the Moon has long been unsolved, and this work discloses that the alkali suite prevails in the outskirts of the Procellarum KREEP Terrane, the center of the South Pole-Aitken Terrane and some isolated locales. (4) Focusing on distinguishing mare basalts from other mafic rocks, 7 geochemical indices are proposed to determine the potential exposures of mare basalts across the Moon. (5) In Mare Insularum, a series of KREEP volcanisms may have occurred and lasted longer than the mare volcanism.

Description: The Bagnold dunes in Gale Crater, Mars are the first active aeolian dune field explored in situ on another planet. The Curiosity rover visited the Bagnold dune field to understand modern winds and modern aeolian processes, rates, and structures; to constrain dune material composition, provenance, and the extent and type of compositional sorting of materials; and to collect knowledge that informs the interpretation of past aeolian processes that are preserved in the Martian sedimentary rock record. The Curiosity rover conducted a coordinated campaign of activities lasting four months, interspersed with other rover activities, and employing all of the rover's science instruments and several engineering capabilities. Described in thirteen manuscripts and summarized here, the major findings of the Bagnold Dunes Campaign, phase I include: the characterization of and explanation for a distinctive, meter-scale size of sinuous aeolian bedform formed in the high kinetic viscosity regime of Mars’ thin atmosphere; articulation and evaluation of a grain splash model that successfully explains the occurrence of saltation even at wind speeds below the fluid threshold; determination of the dune sands’ basaltic mineralogy and crystal chemistry in comparison with other soils and sedimentary rocks; and characterization of chemically distinctive volatile reservoirs in sand-sized versus dust-sized fractions of Mars soil, including the importance of amorphous phase(s) as volatile carriers.

Description: Sinus Aestuum is the only known location on the Moon where orbital data has detected Fe- and/or Cr-spinel. We analyzed Moon Mineralogy Mapper (M 3 ) visible to near-infrared data of the largest and strongest spinel signatures and determined these locations always correspond to impact craters. M 3 spectra show at least three types of spinels may be present, all of which exhibit a strong and broad absorption at ~2100 nm, and also one of the following: (1) a narrow 700-750 nm absorption; (2) a broad 600-900 nm absorption; or (3) both a weaker 700 nm and stronger 1000 nm absorption. All the spinel detections occur on either larger highland massifs that make up Sinus Aestuum east and west, or smaller highland kīpukas and buried highlands within the mare. Almost all of the spinel signatures occur within the mapped pyroclastic dark mantle deposit (DMD). The strong correlation between spinel and DMD distribution on the highlands at Sinus Aestuum is best explained if the spinels were emplaced during the same explosive eruption(s) that deposited the pyroclastics in the Sinus Aestuum DMD. Our observations are most consistent with models of melt-rock reactions in the anorthositic lunar crust that produce contaminated (high-Al) regions within a volcanic dike or magmatic reservoir that was capable of erupting pyroclastic glass beads containing pleonaste spinel [Mg,Fe]Al 2 O 4 . Over billions of years, this surface layer of spinels and pyroclastics became heterogeneously mixed into and partially buried within the highland regolith where younger impact craters may sometimes expose it.

Description: While deep moonquakes are seismic events commonly observed on the Moon, their source mechanism is still unexplained. The two main issues are poorly constrained source parameters and incompatibilities between the thermal profiles suggested by many studies and the apparent need for brittle properties at these depths. In this study, we reinvestigated the deep moonquake data to re-estimate its source parameters and uncover the characteristics of deep moonquake faults that differ from those on Earth. We first improve the estimation of source parameters through spectral analysis using “new” broadband seismic records made by combining those of the Apollo long- and short-period seismometers. We use the broader frequency band of the combined spectra to estimate corner frequencies and DC values of spectra, which are important parameters to constrain the source parameters. We further use the spectral features to estimate seismic moments and stress drops for more than 100 deep moonquake events from three different source regions. This study revealed that deep moonquake faults are extremely smooth compared to terrestrial faults. Second, we re-evaluate the brittle-ductile transition temperature that is consistent with the obtained source parameters. We show that the source parameters imply the tidal stress is the main source of the stress glut causing deep moonquakes and the large strain rate from tides makes the brittle-ductile transition temperature higher. Higher transition temperatures open a new possibility to construct a thermal model that is consistent with deep moonquake occurrence and pressure condition and thereby improve our understandings of the deep moonquake source mechanism.

Description: We present new viscosity measurements of a synthetic silicate system considered an analogue for the lava erupted on the surface of Mercury. In particular, we focus on the northern volcanic plains (NVP), which correspond to the largest lava flows on Mercury and possibly in the Solar System. High-temperature viscosity measurements were performed at both superliquidus (up to 1736 K) and subliquidus conditions (1569–1502 K) to constrain the viscosity variations as a function of crystallinity (from 0 to 28%) and shear rate (from 0.1 to 5 s -1 ). Melt viscosity shows moderate variations (4 –16 Pa s) in the temperature range 1736–1600 K. Experiments performed below the liquidus temperature show an increase in viscosity as shear rate increases from 0.1 to 5 s -1 , resulting in a shear thinning behaviour, with a decrease in viscosity of ca. 1 log unit. The low viscosity of the studied composition may explain the ability of NVP lavas to cover long distances, on the order of hundreds of kilometres in a turbulent flow regime. Using our experimental data we estimate that lava flows with thickness of 1, 5 and 10 m are likely to have velocities of 4.8, 6.5 and 7.2 m/s respectively, on a 5° ground slope. Numerical modelling incorporating both the heat loss of the lavas and its possible crystallization during emplacement allows us to infer that high effusion rates (〉 10000 m 3 /s) are necessary to cover the large distances indicated by satellite data from the MESSENGER spacecraft.

Description: Chloride-bearing deposits on Mars record high-elevation lakes during the waning stages of Mars’ wet era (mid-Noachian to late Hesperian). The water source pathways, seasonality, salinity, depth, lifetime, and paleoclimatic drivers of these widespread lakes are all unknown. Here we combine reaction-transport modeling, orbital spectroscopy, and new volume estimates from high-resolution digital terrain models, in order to constrain the hydrologic boundary conditions for forming the chlorides. Considering a T = 0 °C system, we find: (1) individual lakes were 〉100 m deep and lasted decades or longer; (2) if volcanic degassing was the source of chlorine, then the water-to-rock ratio or the total water volume were probably low, consistent with brief excursions above the melting point and/or arid climate; (3) if the chlorine source was igneous chlorapatite, then Cl-leaching events would require a (cumulative) time of 〉10 yr at the melting point; (4) Cl masses, divided by catchment area, give column densities 0.1 – 50 kg Cl/m 2 , and these column densities bracket the expected chlorapatite-Cl content for a seasonally-warm active layer. Deep groundwater was not required. Taken together, our results are consistent with Mars having a usually cold, horizontally segregated hydrosphere by the time chlorides formed.

Description: No abstract is available for this article.

Description: The basin-filling materials of the northern lowlands, which cover ~1/3 of Mars’ surface, record the long-term evolution of Mars' geology and climate. The buried stratigraphy was inferred through analyses of impact crater mineralogy, detected using data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Examining 1045 impact craters across the northern lowlands, we find widespread olivine and pyroxene and diverse hydrated/hydroxylated minerals, including Fe/Mg smectite, chlorite, prehnite, and hydrated silica. The distribution of mafic minerals is consistent with infilling volcanic materials across the entire lowlands (~1–4⋅10 7 km 3 ), indicating a significant volume of volatile release by volcanic outgassing. Hydrated/hydroxylated minerals are detected more frequently in large craters, consistent with the scenario that the hydrated minerals are being excavated from deep basement rocks, beneath 1-2 km thick mafic lava flows or volcaniclastic materials. The prevalences of different types of hydrated minerals are similar to statistics from the southern highlands. No evidence of concentrated salt deposits has been found, which would indicate a long-lived global ocean. We also find significant geographical variations of local mineralogy and stratigraphy in different basins (geological provinces), independent of dust cover. For example, many hydrated and mafic minerals are newly discovered within the polar Scandia region (〉 60°N), and Chryse Planitia has more mafic mineral detections than other basins, possibly due to a previously unrecognized volcanic source.

Description: Geochemical models of secondary mineral precipitation on Mars generally assume semi-open systems (open to the atmosphere but closed at the water-sediment interface) and equilibrium conditions. However, in natural multicomponent systems, the reactive surface area of primary minerals controls the dissolution rate and affects the precipitation sequences of secondary phases; and simultaneously the transport of dissolved species may occur through the atmosphere-water and water-sediment interfaces. Here we present a suite of geochemical models designed to analyze the formation of secondary minerals in basaltic sediments on Mars, evaluating the role of (i) reactive surface areas and (ii) the transport of ions through a basalt sediment column. We consider fully open conditions, both to the atmosphere and to the sediment, and a kinetic approach for mineral dissolution and precipitation. Our models consider a geochemical scenario constituted by a basin (i.e., a shallow lake) where supersaturation is generated by evaporation/cooling, and the starting point is a solution in equilibrium with basaltic sediments. Our results show that cation removal by diffusion, along with the input of atmospheric volatiles and the influence of the reactive surface area of primary minerals, play a central role in the evolution of the secondary mineral sequences formed. We conclude that precipitation of evaporites finds more restrictions in basaltic sediments of small grain size than in basaltic sediments of greater grain size.

Description: No abstract is available for this article.

Description: The Mars Science Laboratory Curiosity rover undertook comprehensive exploration of the Kimberley waypoint within Gale crater, Mars in order to understand its context within the larger geologic picture of Gale crater and its evidence for past Martian habitability. Coordinated observations from Curiosity's rich science payload revealed important insights into new Martian crustal compositions, the prevalence and diversity of sedimentary processes within Gale crater, and surface erosion rates. Exploration at the Kimberley, in part informed by a decade of orbital observations of Gale crater, underscored the critical synergy between landed and orbital observations and furthered understanding of complex geological processes on Mars.

Description: Carbon contents in reduced Martian basalts at graphite saturation were experimentally studied at 1400-1550 °C, 1-2 GPa, and log f O 2 of IW-0.4 to IW+1.5 (IW denotes the Fe-FeO buffer). The results show that carbon solubility in Martian basalts, determined by SIMS, is 20 to 1400 ppm, increasing with increasing f O 2 . Raman and FTIR measurements on the quenched silicate glasses show that the dominant carbon species in Martian basalts is carbonate (CO 3 2- ). The experimental data generated here were combined with literature data on similar graphite-saturated carbon solubility for mafic-ultramafic compositions to develop an empirical model that can be used to predict carbon content of graphite-saturated reduced basalts at vapor-absent conditions: At IW+1.7 ≥ log f O 2 ≥ IW-1: log ( C ,  ppm ) = − 3702(±534)/ T  − 194(±49) P / T  − 0.0034(±0.043) logX H 2 O  + 0.61(±0.07) NBO / T  + 0.55(±0.02) ΔIW  + 3.5(±0.3) ( R 2  = 0.89)At IW-5.3 ≤ log f O 2 ≤ IW-1: log  ? ( C ,  ppm ) = 0.96(±0.19) logX H 2 O  − 0.25(±0.04) ?  IW  + 2.83(±0.34)( R 2  = 0.6)in which T is temperature in K, P is pressure in GPa, X H 2 O is mole fraction of water in basalts, ΔIW is the oxygen fugacity relative to the IW buffer, and NBO / T  = 2  total O / T  − 4 ( T  =  Si  +  Ti  +  Al  +  Cr  +  P ). This model was applied to predict carbon content in graphite-saturated mantle melts of the Mercury, Mars, and the Moon. The results show that graphite may be consumed during the production and extraction of some Martian basalts, and CO 2 released by volcanism on Mars cannot be an efficient greenhouse gas in the early Mars. The lunar mantle carbon may be one of the main propellant driving the fire-fountain eruption on the Moon; however, the Mercurian mantle carbon may not be an important propellant for the explosive eruption on Mercury.

Description: The Mars Science Laboratory Curiosity rover performed coordinated measurements to examine the textures and compositions of aeolian sands in the active Bagnold dune field. The Bagnold sands are rounded to subrounded, very fine- to medium- sized (~45-500 µm) with ≥6 distinct grain colors. In contrast to sands examined by Curiosity in a dust-covered, inactive bedform called Rocknest and soils at other landing sites, Bagnold sands are darker, less red, better sorted, have fewer silt-sized or smaller grains, and show no evidence for cohesion. Nevertheless, Bagnold mineralogy and Rocknest mineralogy are similar with plagioclase, olivine, and pyroxenes in similar proportions comprising 〉90% of crystalline phases, along with a substantial amorphous component (35% ± 15%). Yet, Bagnold and Rocknest bulk chemistry differ. Bagnold sands are Si-enriched relative to other soils at Gale crater, and H 2 O, S, and Cl are lower relative to all previously measured martian soils and most Gale crater rocks. Mg, Ni, Fe, and Mn are enriched in the coarse-sieved fraction of Bagnold sands, corroborated by VNIR spectra that suggest enrichment of olivine. Collectively, patterns in major element chemistry and volatile release data indicate two distinctive volatile reservoirs in martian soils: (1) amorphous components in the sand-sized fraction (represented by Bagnold) that are Si-enriched, hydroxylated alteration products and/or impact or volcanic glasses; and (2) amorphous components in the fine fraction (〈40 µm; represented by Rocknest and other bright soils) that are Fe-, S-, and Cl-enriched with low Si and adsorbed and structural H 2 O.

Description: We present APXS data for the active Bagnold dune field within the Gale impact crater (MSL mission). We derive an APXS-based Average Basaltic Soil (ABS) composition for Mars based on past and recent data from the MSL and MER missions. This represents an update to the Taylor and McLennan (2009) average martian soil, and facilitates comparison across martian datasets. The active Bagnold dune field is compositionally distinct from the ABS, with elevated Mg, Ni and Fe, suggesting mafic mineral enrichment, and uniformly low levels of S, Cl and Zn, indicating only a minimal dust component. A relationship between decreasing grain size and increasing felsic content is revealed. The Bagnold Sands possess the lowest S/Cl of all martian unconsolidated materials.. Gale soils exhibit relatively uniform major element compositions, similar to Meridiani Planum and Gusev Crater basaltic soils (MER missions). However, they show minor enrichments in K, Cr, Mn and Fe, which may signify a local contribution. The lithified eolian Stimson Formation within the Gale impact crater is compositionally similar to the ABS and Bagnold sands, which provide a modern analogue for these ancient eolian deposits. Compilation of APXS-derived soil data reveals a generally homogenous global composition for martian soils, but one that can be locally modified due to past or extant geologic processes that are limited in both space and time.

Description: As a prospective study for a future exploration of Venus, we compute the tidal response of Venus' interior assuming various mantle compositions and temperature profiles representative of different scenarios of Venus' formation and evolution. The mantle density and seismic velocities are modeled from thermodynamical equilibria of mantle minerals and used to predict the moment of inertia, Love numbers and tide-induced phase lag characterizing the signature of the internal structure in the gravity field. The viscoelasticity of the mantle is parameterized using an Andrade rheology. From the models considered here, the moment of inertia lies in the range of 0.327 to 0.342, corresponding to a core radius of 2900 to 3450 km. Viscoelasticity of the mantle strongly increases the potential Love number relative to previously published elastic models. Due to the anelasticity effects, we show that the possibility of a completely solid metal core inside Venus cannot be ruled out based on the available estimate of k 2 from the Magellan mission [ Konopliv and Yoder , 1996]. A Love number k 2 lower than 0.27 would indicate the presence of a fully solid iron core while, for larger values, solutions with an entirely or partially liquid core are possible. Precise determination of the Love numbers, k 2 and h 2 , together with an estimate of the tidal phase lag, are required to determine the state and size of the core, as well as the composition and viscosity of the mantle.

Description: The Mars Science Laboratory rover, Curiosity, is using a comprehensive scientific payload to explore rocks and soils in Gale crater, Mars. Recent investigations of the Bagnold Dune Field provided the first in situ assessment of an active dune on Mars. The CheMin X-ray diffraction instrument on Curiosity performed quantitative mineralogical analyses of the 〈150 μm size fraction of the Namib dune at a location called Gobabeb. Gobabeb is dominated by basaltic minerals. Plagioclase, Fo56 olivine, and two Ca-Mg-Fe pyroxenes account for the majority of crystalline phases along with minor magnetite, quartz, hematite, and anhydrite. In addition to the crystalline phases, a minimum ~42 wt% of the Gobabeb sample is X-ray amorphous. Mineralogical analysis of the Gobabeb dataset provides insights into the origin(s) and geologic history of the dune material and offers an important opportunity for ground truth of orbital observations. CheMin's analysis of the mineralogy and phase chemistry of modern and ancient Gale crater dune fields, together with other measurements by Curiosity's science payload, provides new insights into present and past eolian processes on Mars.

Description: How the volatile content influences the primordial surface conditions of terrestrial planets, and thus, their future geodynamic evolution is an important question to answer. We simulate the secular convective cooling of a 1D magma ocean (“ MO”) in interaction with its outgassed atmosphere. The heat transfer in the atmosphere is computed either using the grey approximation or using a k-correlated method. We vary the initial CO 2 and H 2 O contents (respectively from 0.1×10 −2 to 14×10 −2 wt% and from 0.03 to 1.4 times the Earth Ocean current mass (M EO )) and the solar distance - from 0.63 to 1.30 AU. A first rapid cooling stage, where efficient MO cooling and degassing take place, producing the atmosphere, is followed by a second quasi-steady-state where the heat flux balance is dominated by the solar flux. The end of the rapid cooling stage (“ ERCS”) is reached when the mantle heat flux becomes negligible compared to the absorbed solar flux. The resulting surface conditions at ERCS, including water ocean's formation, strongly depend both on the initial volatile content and solar distance D . For D 〉 D C , the “ critical distance”, the volatile content controls water condensation and a new scaling law is derived for the water condensation limit. Although today's Venus is located beyond D C due to its high albedo, its high CO 2 /H 2 O ratio prevents any water ocean formation. Depending on the formation time of its cloud cover and resulting albedo, only 0.3 Earth ocean mass might be sufficient to form a water ocean on early Venus.

Description: No abstract is available for this article.

Description: Surface roughness is a statistical measure of change in surface height over a given spatial horizontal scale after the effect of broad scale slope has been removed, and can be used to understand how geologic processes produce and modify a planet's topographic character at different scales. The statistical measure of surface roughness employed in this study of Mercury was the root-mean-square (RMS) deviation, and was calculated from 45–90°N at horizontal baselines of 0.5-250 km with detrended topographic data from individual Mercury Laser Altimeter tracks. As seen in previous studies, the surface roughness of Mercury has a bimodal spatial distribution, with the cratered terrain (dominated by the intercrater plains) possessing higher surface roughness than the smooth plains. The measured surface roughness for both geologic units is controlled by a trade off between impact craters generating higher surface roughness values and flood-mode volcanism decreasing surface roughness. The topography of the two terrain types has self-affine-like behavior at baselines from 0.5–1.5 km; the smooth plains collectively have a Hurst exponent of 0.88 +/- 0.01, whereas the cratered terrains have a Hurst exponent of 0.95 +/- 0.01. Subtle variations in the surface roughness of the smooth plains can be attributed to differences in regional differences in the spatial density of tectonic landforms. The northern rise, a 1,000-km-wide region of elevated topography centered at 65° N, 40° E, is not distinguishable in surface roughness measurements over baselines of 0.5–250 km.

Description: Mons Rümker is a large volcanic complex in Oceanus Procellarum on the Moon and is a candidate landing site for China's Chang'E-5 sample return mission. We conducted a comprehensive study of the topography, geomorphology, composition, and stratigraphy of the Mons Rümker region with multi-source remote sensing data in order to better understand the geology of the region and provide further support for the Chang'E-5 mission. The results show that the Rümker plateau stands 200–1300 m above the surrounding mare surface and 75% of the plateau has a slope of less than 3° at a baseline length of 30 m. Domes are the most prominent volcanic landforms in Mons Rümker and a total of 22 domes were identified and divided into two types that may represent different stages of volcanic activity. Spectral analyses indicated that Mons Rümker is covered by low-Ti basalt and the dominant mafic mineral is high-calcium pyroxene, though signs of mixing of highland materials and basalt have been found. Mons Rümker has three main basalt units and their absolute model ages are 3.71 Ga, 3.58 Ga and 3.51 Ga, respectively. Steep-sided domes could be the youngest volcanic features on the plateau with indications that they were active until the Eratosthenian. A new geologic map of the study region was produced and used to interpret and discuss the geologic evolution of the region. Finally, we propose two candidate landing sites for the Chang'E-5 mission.

Description: Estimates for volumes of mare basalts are essential to understand the thermal conditions of the lunar mantle and its lateral heterogeneity. In this study, we estimated the thicknesses and volumes of mare basalts within five farside basins, Apollo, Ingenii, Poincare, Freundlich–Sharonov, and Mendel–Rydberg, using pre-mare craters buried by mare basalts and post-mare craters that penetrated/non-penetrated mare basalts employing topographic and multiband image data obtained by SELENE (Kaguya). Furthermore, using the GRAIL crustal thickness model and the mare volumes estimated by this and previous studies, we investigated the relationship between the volumes of the mare basalts and the crustal thicknesses. The results suggest that the minimum crustal thicknesses within the basins were a dominant factor determining whether magma erupted at the surface and that the critical crustal thicknesses for magma eruption were ~10 km on the farside and 〉 20 km on the nearside. The total areas of the regions in which magmas could erupt at the surface are ~10 times larger on the nearside than on the farside. A comparison between the mare volumes within the mare basins on the nearside and the farside shows that magma production in the farside mantle might have been ~20 times smaller than that in the nearside mantle, implying a stronger dichotomy than previously estimated. These results suggest that the mare hemispherical asymmetry should be attributed to both the difference in the crustal thickness distribution and the difference in the quantity of magma production between the nearside and farside mantles.

Description: Due to their oxidizing properties, perchlorates (ClO 4 – ) are suggested by the planetary science community to play a vital role in the scarcity of organics on the Martian surface. However, alternative oxidation agents such as hydrogen peroxide (H 2 O 2 ) have received surprisingly little attention. In this study, samples of magnesium perchlorate hexahydrate (Mg(ClO 4 ) 2 ·6H 2 O) were exposed to monoenergetic electrons and D 2 + ions separately, sequentially, and simultaneously to probe the effects of galactic cosmic ray exposure of perchlorates and the potential incorporation of hydrogen (deuterium) into these minerals. The experiments were carried out under ultra-high vacuum conditions at 50 K, after which the samples were slowly heated to 300 K while the subliming products were monitored by a quadrupole mass spectrometer (QMS). In all cases, molecular oxygen (O 2 ) was detected upon the onset of irradiation and also during the warmup phase. In case of a simultaneous D 2 + - electron exposure, deuterated water (D 2 O) and deuterium peroxide (D 2 O 2 ) were also detected in the warmup phase, whereas only small amounts of D 2 O 2 were found after an exclusive D 2 + irradiation. These experiments yield the first data identifying hydrogen peroxide as a potential product in the interaction of cosmic rays with perchlorates in the Martian regolith revealing that perchlorates are capable of producing multiple oxidizing agents (O 2 , D 2 O 2 ) that may account for the destruction of organics on the Martian surface.

Description: We ran several series of two-dimensional numerical mantle convection simulations representing in idealized form the thermochemical evolution of a Mars-like planet. In order to study the importance of compositional buoyancy of melting mantle, the models were set up in pairs of one including all thermal and compositional contributions to buoyancy and one accounting only for the thermal contributions. In several of the model pairs, single large impacts were introduced as causes of additional strong local anomalies, and their evolution in the framework of the convecting mantle was tracked. The models confirm that the additional buoyancy provided by the depletion of the mantle by regular melting can establish a global stable stratification of the convecting mantle and throttle crust production. Furthermore, the compositional buoyancy is essential in the stabilization and preservation of local compositional anomalies directly beneath the lithosphere and offers a possible explanation for the existence of distinct, long-lived reservoirs in the martian mantle. The detection of such anomalies by geophysical means is probably difficult, however; they are expected to be detected by gravimetry rather than by seismic or heat flow measurements. The results further suggest that the crustal thickness can be locally overestimated by up to ∼20 km if impact-induced density anomalies in the mantle are neglected.

Description: On average, secondary impact craters are expected to deepen and become more symmetric as impact velocity ( v i ) increases with downrange distance ( L ). We have used high-resolution topography (1–2 m/pixel) to characterize the morphometry of secondary craters as a function of L for several well-preserved primary craters on Mars. The secondaries in this study ( N = 2,643) span a range of diameters (25 m ≤ D ≤400 m) and estimated impact velocities (0.4 km/s ≤ v i ≤2 km/s). The range of diameter-normalized rim-to-floor depth ( d / D ) broadens and reaches a ceiling of d / D ≈0.22 at L ≈280 km ( v i =1–1.2 km/s) whereas average rim height shows little dependence on v i for the largest craters ( h / D ≈0.02, D 〉 60 m). Populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent: planform elongations ( L 〈 110–160 km), taller downrange rims ( L 〈 280 km), and cavities that are deeper uprange ( L 〈 450–500 km). Populations of secondaries with lopsided ejecta were found to extend to at least L ∼ 700 km. Impact hydrocode simulations with iSALE-2D for strong, intact projectile and target materials predict a ceiling for d / D vs. L whose trend is consistent with our measurements. This study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations. This has applications to understanding the chronology of planetary surfaces and the long-term evolution of small crater populations.

Description: Zinc and germanium enrichments have been discovered in sedimentary rocks in Gale Crater, Mars, by the Alpha Particle X-ray Spectrometer (APXS) on the rover Curiosity . Concentrations of Zn (910 ± 840 ppm) and Ge (65 ± 58 ppm) are 10s-100s of times greater than in Martian meteorites and estimates for average silicate Mars. Enrichments occur in diverse rocks including minimally to extensively altered basaltic and alkalic sediment. The magnitude of the enrichments indicates hydrothermal fluids, but Curiosity has not discovered unambiguous hydrothermal mineral assemblages. We propose that Zn- and Ge-rich hydrothermal deposits in the source region were dispersed in siliciclastic sediments during transport into the crater. Subsequent diagenetic mobilization and fractionation of Zn and Ge is evident in a Zn-rich sandstone (Windjana; Zn ~4000 ppm, Ge ~85 ppm) and associated Cl-rich vein (Stephen; Zn ~8000 ppm, Ge ~60 ppm), in Ge-rich veins (Garden City; Zn ~1300 ppm, Ge ~650 ppm), and in silica-rich alteration haloes leached of Zn (30-200 ppm). In moderately to highly altered silica-rich rocks, Ge remained immobile relative to leached elements (Fe, Mn, Mg, Ca), consistent with fluid interaction at pH 〈〈 7. In contrast, cross-cutting Ge-rich veins at Garden City suggest aqueous mobilization as Ge-F complexes at pH 〈 2.5. Multiple jarosite detections by the CheMin XRD and variable Zn concentrations indicate diagenesis of lower Mt. Sharp bedrock under acidic conditions. The enrichment and fractionation of Zn and Ge constrains fluid events affecting Gale sediments and can aid in unraveling fluid histories as Curiosity's traverse continues.

Description: The effects of the coefficient of friction and porosity on impact cratering are not sufficiently considered in scaling laws that predict the crater size from a known impactor size, velocity, and mass. We carried out a systematic numerical study employing more than 1000 two-dimensional models of simple crater formation under lunar conditions in targets with varying properties. A simple numerical setup is used where targets are approximated as granular or brecciated materials, and any compression of porous materials results in permanent compaction. The results are found to be consistent with impact laboratory experiments for water, low strength and low porosity materials (e.g., wet sand), and sands. Using this assumption, we found that both the friction coefficient and porosity are important for estimating transient crater diameters as is the strength term in crater scaling laws, i.e. the effective strength. The effects of porosity and friction coefficient on impact cratering were parameterized and incorporated into π-group scaling laws and predict transient crater diameters within an accuracy of ±5 % for targets with friction coefficients f ≥ 0.4 and porosities Φ = 0 – 30 %. Moreover, 90 crater scaling relationships are made available and can be used to estimate transient crater diameters on various terrains and geological units with different coefficient of friction, porosity and cohesion. The derived relationships are most robust for targets with Φ 〉 10 – 15 %, applicable for a lunar environment, and could therefore yield significant insights into the influence of target properties on cratering statistics.

Description: Theoretically, spherical harmonic (SH) series expansions of the external gravitational potential are guaranteed to converge outside the Brillouin-sphere enclosing all field-generating masses. Inside that sphere, the series may be convergent or may be divergent. The series convergence behaviour is a highly unstable quantity that is little studied for high-resolution mass distributions. Here we shed light on the behaviour of SH series expansions of the gravitational potential of the Moon. We present a set of systematic numerical experiments where the gravity field generated by the topographic masses is forward-modelled in spherical harmonics and with numerical integration techniques at various heights and different levels of resolution, increasing from harmonic degree 90 to 2160 (~61 to 2.5 km scales). The numerical integration is free from any divergence issues and, therefore, suitable to reliably assess convergence vs. divergence of the SH series. Our experiments provide unprecedented detailed insights into the divergence issue. We show that the SH gravity field of degree-180 topography is convergent anywhere in free space. When the resolution of the topographic mass model is increased to degree 360, divergence starts to affect very high degree gravity signals over regions deep inside the Brillouin-sphere. For degree-2160 topography/gravity models, severe divergence (with several 1000 mGal amplitudes) prohibits accurate gravity modelling over most of the topography. As a key result, we formulate a new hypothesis to predict divergence: If the potential degree variances show a minimum, then the SH series expansions diverge somewhere inside the Brillouin-sphere, and modelling of the internal potential becomes relevant.

Description: Seven lunar crater sites of granular avalanches are studied utilizing high-resolution images (0.42-1.3 m/pixel) from the Lunar Reconnaissance Orbiter Camera; one, in Kepler crater, is examined in detail. All the sites are slopes of debris extensively aggraded by frictional freezing at their dynamic angle of repose, four in craters formed in basaltic mare and three in the anorthositic highlands. Diverse styles of mass wasting occur and three types of dry-debris-flow deposit are recognized: (1) multiple channel-and-lobe type, with coarse-grained levees and lobate terminations that impound finer debris, (2) single-surge polylobate type, with sub-parallel arrays of lobes and fingers with segregated coarse-grained margins, and (3) multiple ribbon type, with tracks reflecting reworked substrate, minor levees and no coarse terminations. The latter type results from propagation of granular erosion-deposition waves down slopes dominantly of fine regolith and it is the first recognized natural example. Dimensions, architectures and granular segregation styles of the two coarse-grained deposit types are like those formed in natural and experimental avalanches on Earth, although the timescale of motion differs due to the reduced gravity. Influences of reduced gravity and fine-grained regolith on dynamics of granular flow and deposition appear slight, but we distinguish, for the first time, extensive remobilization of coarse talus by inundation with finer debris. The (few) sites show no clear difference attributable to the contrasting mare basalt and highland megaregolith host-rocks and their fragmentation. This lunar study offers a benchmarking of deposit types that can be attributed to formation without influence of liquid or gas.

Description: The Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter (MRO), used to search for sub-surface water ice, requires corrections to range delays and image distortions caused by radiowave propagation through the martian ionosphere. These corrections yield a phase parameter that is linearly correlated with values of the total electron content (TEC), defined as the integral of the electron density profile N e (h). This new database has been validated using previous observational patterns of TEC magnitudes and variability versus latitude, local time and season. The SHARAD TEC data span the years 2007 to 2014, providing the first opportunity to study solar cycle effects upon daytime TEC magnitudes. A parameterization of TEC versus solar zenith angle and solar flux, within the context of photo-chemical-equilibrium theory, provides a flexible TEC module for the Mars Initial Reference Ionosphere (MIRI) model. A high spatial resolution study of TEC variability in the southern hemisphere confirmed a previously tentative conclusion about daytime TEC morphology controlled by the local inclination angles of crustal magnetic fields.

Description: Mars is believed to have possessed a dynamo that ceased operating approximately 4 Ga ago, although the exact time is still under debate. The scope of this study is to constrain the possible timing of its cessation by studying the magnetization signatures of craters. The study uses the latest available model of the lithospheric magnetic field of Mars, which is based on Mars Global Surveyor data. We tackle the problem of non-uniqueness that characterises the inversion of magnetic field data for the magnetization by inferring only the visible part of the magnetization, i.e., the part of the magnetization that gives rise to the observed magnetic field. Further on, we demonstrate that a zero visible magnetization is a valid proxy for the entire magnetization being zero under the assumption of a magnetization distribution of induced geometry. This assumption holds for craters whose thermoremanent magnetization has not been significantly altered since its acquisition. Our results show that the dynamo shut off after the impacts that created the Acidalia and SE Elysium basins and before the crust within the Utopia basin cooled below its magnetic blocking temperature. Accounting for the age uncertainties in the dating of these craters, we estimate that the dynamo shut off at an N(300) crater retention age of 2.5-3.2 or an absolute model age of 4.12 - 4.14 Ga. Moreover, the Martian dynamo may have been weaker in its early stage, which if true implies that the driving mechanism of the Martian dynamo was not the same throughout its history.

Description: No abstract is available for this article.

Description: Dark topographic slumps several meters wide, tens of meters in length and up to a meter in depth are observed on the slopes of Juventae Chasma (JC), Valles Marineris (VM), Mars. These slumps usually originate near the terminal points of recurring slope lineae (RSL). Near their initiation points, the slumps have topographic depressions due to the removal of materials; near their lowermost reaches, new materials are deposited in lobes. Over the course of three Mars years, ten active slumps have been observed in JC, all of which formed in or near the same season (areocentric longitude: L s 0°–120°). Mars Color Imager (MARCI) observations show low-altitude atmospheric obscurations confined within the topography of the VM and JC in the seasons when the slumps form. In one instance, data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and MARCI show evidence of H 2 O ice in the atmospheric obscuration, likely due to the formation of a low-level afternoon cloud above a dust storm, or mixing of condensate clouds with a diffuse dust cloud. The presence of atmospheric obscurations with H 2 O ice near times when the slumps form is intriguing, but no direct evidence currently exists to support that they aid in slump formation. Further monitoring of this site will help establish if RSL and/or atmospheric events play a role in the creation of contemporary slumps.

Description: Data from the Gamma-Ray Spectrometer (GRS) that flew on the MESSENGER spacecraft indicate that the O/Si weight ratio of Mercury's surface is 1.2 ± 0.1. This value is lower than any other celestial surface that has been measured by GRS and suggests that 12–20% of the surface materials on Mercury are composed of Si-rich, Si-Fe alloys. The origin of the metal is best explained by a combination of space weathering and graphite-induced smelting. The smelting process would have been facilitated by interaction of graphite with boninitic and komatiitic parental liquids. Graphite entrained at depth would have reacted with FeO components dissolved in silicate melt, resulting in the production of up to 0.4–0.9 wt.% CO from the reduction of FeO to Fe 0 —CO production that could have facilitated explosive volcanic processes on Mercury. Once the graphite-entrained magmas erupted, the tenuous atmosphere on Mercury prevented the buildup of CO over the lavas. The partial pressure of CO would have been sufficiently low to facilitate reaction between graphite and SiO 2 components in silicate melts to produce CO and metallic Si. Although exotic, Si-rich metal as a primary smelting product is hypothesized on Mercury for three primary reasons: (1) low FeO abundances of parental magmas, (2) elevated abundances of graphite in the crust and regolith, and (3) the presence of only a tenuous atmosphere at the surface of the planet within the 3.5–4.1 Ga timespan over which the planet was resurfaced through volcanic processes.

Description: We perform the first tests of various proposed explanations for observed features of the Moon's argon exosphere, including models of: spatially varying surface interactions; a source that reflects the lunar near-surface potassium distribution; and temporally varying cold trap areas. Measurements from the Lunar Atmosphere and Dust Environment Explorer (LADEE) and the Lunar Atmosphere Composition Experiment (LACE) are used to test whether these models can reproduce the data. The spatially varying surface interactions hypothesized in previous work cannot reproduce the persistent argon enhancement observed over the western maria. They also fail to match the observed local time of the near-sunrise peak in argon density, which is the same for the highland and mare regions, and is well reproduced by simple surface interactions with a ubiquitous desorption energy of 28 kJ mol −1 . A localised source can explain the observations, with a trade-off between an unexpectedly localised source or an unexpectedly brief lifetime of argon atoms in the exosphere. To match the observations, a point-like source requires source and loss rates of ∼1.9 × 10 21  atoms s −1 . A more diffuse source, weighted by the near-surface potassium, requires much higher rates of ∼1.1 × 10 22  atoms s −1 , corresponding to a mean lifetime of just 1.4 lunar days. We do not address the mechanism for producing a localised source, but demonstrate that this appears to be the only model that can reproduce the observations. Large, seasonally varying cold traps could explain the long-term fluctuation in the global argon density observed by LADEE, but not that by LACE.

Description: Juventae Chasma is an isolated chasm located north of the interconnected chasms within Valles Marineris. It contains four separate interior layered deposit (ILD) mounds. We have combined layer measurements, as well as mineralogical data of the mounds with topographic data of the remaining chasm and the surrounding plateau. Our observations indicate that the chasm underwent a complex geological history. We suggest that individual ILD mounds are not contemporaneous with each other and a series of progressive collapses enlarged an initially smaller chasm to its current shape. ILDs were deposited in a lacustrine setting and the time of ILD deposition was limited. Basin collapse and chasm enlargement continued beyond the time of ILD deposition and thus were not driven by sediment load. Glacial processes appear to have played a role in the late, post-lacustrine history of the chasm. We present a simplified model that highlights some of the complexity of the chasm evolution, but acknowledge that the true history is undoubtedly more complicated.

Description: Excess ice with a minimum age of tens of millions of years is widespread in Arcadia Planitia on Mars, and a similar deposit has been found in Utopia Planitia. The conditions that led to the formation and preservation of these mid-latitude ice sheets hold clues to past climate and subsurface structure on Mars. We simulate the thermal stability and retreat of buried excess ice sheets over 21 Myr of Martian orbital solutions and find that the ice sheets can be orders of magnitude older than the obliquity cycles that are typically thought to drive mid-latitude ice deposition and sublimation. Retreat of this ice in the last 4 Myr could have contributed ~6% of the volume of the North Polar Layered Deposits (NPLD) and more than 10% if the NPLD are older than 4 Myr. Matching the measured dielectric constants of the Arcadia and Utopia Planitia deposits requires ice porosities of ~25–35%. We model geothermally driven vapor migration through porous ice under Martian temperatures and find that Martian firn may be able to maintain porosity for timescales longer than we predict for retreat of the ice.

Description: Ceres is the largest body in the asteroid belt with a radius of approximately 470 km. In part due to its large mass, Ceres more closely approaches hydrostatic equilibrium than major asteroids. Pre-Dawn mission shape observations of Ceres revealed a shape consistent with a hydrostatic ellipsoid of revolution. The Dawn spacecraft Framing Camera has been imaging Ceres since March 2015, which has led to high-resolution shape models of the dwarf planet, while the gravity field has been globally determined to a spherical harmonic degree 14 (equivalent to a spatial wavelength of 211 km) and locally to 18 (a wavelength of 164 km). We use these shape and gravity models to constrain Ceres' internal structure. We find a negative correlation and admittance between topography and gravity at degree 2 and order 2. Low admittances between spherical harmonic degrees 3 and 16 are well explained by Airy isostatic compensation mechanism. Different models of isostasy give crustal densities between 1200 and 1400 kg/m 3 with our preferred model giving a crustal density of kg/m 3 . The mantle density is constrained to be kg/m 3 . We compute isostatic gravity anomaly and find evidence for mascon-like structures in the two biggest basins. The topographic power spectrum of Ceres and its latitude dependence suggest that viscous relaxation occurred at the long wavelengths (〉246 km). Our density constraints combined with finite element modeling of viscous relaxation suggests that the rheology and density of the shallow surface are most consistent with a rock, ice, salt and clathrate mixture.

Description: F- and M-color center formation (decay) was observed during (after) irradiation of sodium chloride crystal grains with 10 keV electrons as a function of temperature, radiation dose rate, and radiation dose. The F-centers (peak center: 460 nm) were found to form and decay at a faster rate than the M-centers (peak center: 720 nm). These effects were influenced by temperature, and possibly by irradiation dose rate. Tracking the band depth ratio of the color center features during irradiation could enable age determination of geologically very-young features on the surface of Europa and other icy ocean worlds.

Description: Surface conditions on early Mars were likely anoxic, similar to early Earth, but the timing of the evolution to oxic conditions characteristic of contemporary Mars is unresolved. Ferrous trioctahedral smectites are the thermodynamically predicted products of anoxic basalt weathering, but orbital analyses of Noachian-aged terrains find primarily Fe 3+ -bearing clay minerals. Rover-based detection of Fe 2+ -bearing trioctahedral smectites at Gale Crater suggest that ferrous smectites are the unoxidized progenitors of orbitally-detected ferric smectites. To assess this pathway, we conducted ambient-temperature oxidative alteration experiments on four synthetic ferrous smectites having molar Fe/(Mg+Fe) from 1.00 to 0.33. Smectite suspension in air-saturated solutions produced incomplete oxidation (24–38% Fe 3+ /ΣFe). Additional smectite oxidation occurred upon re-exposure to air-saturated solutions after anoxic hydrothermal recrystallization, which accelerated cation and charge redistribution in the octahedral sheet. Oxidation was accompanied by contraction of the octahedral sheet (d (060) decreased from 1.53-1.56 Å to 1.52 Å), consistent with a shift towards dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe 2+ oxidation but also led to partial Fe 3+ ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe 3+ ,Mg) 2 -OH bands at 2.28-2.30 μm, consistent with oxidative formation of dioctahedral nontronite. Accordingly, ferrous smectites are plausible precursors to observed ferric smectites on Mars, and their presence in late-Noachian sedimentary units suggests that anoxic conditions may have persisted on Mars beyond the Noachian.

Description: Seismology was developed on Earth and shaped our model of the Earth's interior over the 20th century. With the exception of the Philae lander, all in situ extraterrestrial seismological effort to date was limited to other terrestrial planets. All have in common a rigid crust above a solid mantle. The coming years may see the installation of seismometers on Europa, Titan and Enceladus, so it is necessary to adapt seismological concepts to the setting of worlds with global oceans covered in ice. Here we use waveform analyses to identify and classify wave types, developing a lexicon for icy ocean world seismology intended to be useful to both seismologists and planetary scientists. We use results from spectral-element simulations of broadband seismic wavefields to adapt seismological concepts to icy ocean worlds. We present a concise naming scheme for seismic waves and an overview of the features of the seismic wavefield on Europa, Titan, Ganymede and Enceladus. In close connection with geophysical interior models, we analyze simulated seismic measurements of Europa and Titan that might be used to constrain geochemical parameters governing the habitability of a sub-ice ocean.

Description: No abstract is available for this article.

Description: Magnetic field data acquired from orbit shows that the Moon possesses many magnetic anomalies. Though most of these are not associated with known geologic structures, some are found within large impact basins within the interior peak ring. The primary magnetic carrier in lunar rocks is metallic iron, but indigenous lunar rocks are metal poor and can not account easily for the observed field strengths. The projectiles that formed the largest impact basins must have contained a significant quantity of metallic iron, and a portion of this iron would have been retained on the Moon's surface within the impact melt sheet. Here, we use orbital magnetic field data to invert for the magnetization within large impact basins using the assumption that the crust is unidirectionally magnetized. We develop a technique based on laboratory thermoremanent magnetization acquisition to quantify the relationship between the strength of the magnetic field at the time the rock cooled and the abundance of metal in the rock. If we assume that the magnetized portion of the impact melt sheet is 1 km thick, we find average abundances of metallic iron ranging from 0.11% to 0.45 wt.%, with an uncertainty of a factor of about three. This abundance is consistent with the metallic iron abundances in sampled lunar impact melts and the abundance of projectile contamination in terrestrial impact melts. These results help constrain the composition of the projectile, the impact process, and the time evolution of the lunar dynamo.

Description: On the moon and other airless bodies, ballistically emplaced ejecta transitions from a thinning, continuous inner deposit to become discontinuous beyond approximately one crater radius from the crater rim and can further break into discrete rays and secondary craters. In contrast, on Mars, ejecta often form continuous, distinct, and sometimes thick deposits that transition to a low ridge or escarpment that may be circular or lobate. The martian ejecta type has been variously termed pancake, rampart, lobate, or layered, and in this work we refer to it as "abrupt termini" ejecta (ATE). Two main formation mechanisms have been proposed, one requiring atmospheric interaction and the other mobilization of near-surface volatiles. ATE morphologies are also unambiguously seen on Ganymede, Europa, Dione, and Tethys, but they are not as common as on Mars. We have identified up to 38 craters on Charon that show signs of ATE, including possible distal ramparts and lobate margins. These ejecta show morphologic and morphometric similarities with other moons in the Solar System which are a sub-set of the properties observed on Mars. From comparison of these ejecta on Charon and other Solar System bodies, we find the strongest support for sub-surface volatile mobilization and ejecta fluidization as the main formation mechanism for the ATE, at least on airless, icy worlds. This conclusion comes from the bodies on which they are found, an apparent preference for certain terrains, and the observation that craters with ATE can be near to similarly sized craters that only have gradational ejecta.

Description: We report electrical conductivity measurements on metal–olivine systems at 5 and 7 GPa and up to 1675°C in order to investigate the electrical properties of Core-Mantle Boundary (CMB) systems. Electrical experiments were conducted in the multi-anvil apparatus using the impedance spectroscopy technique. The samples are composed of one metal layer (Fe, FeS, FeSi 2 , or Fe-Ni-S-Si) and one polycrystalline olivine layer, with the metal:olivine ratio ranging from 1:0.7 to 1:9.2. For all samples, we observe that the bulk electrical conductivity increases with temperature from 10 -2.5 to 10 1.8 S/m, which is higher than the conductivity of polycrystalline olivine but lower than the conductivity of the pure metal phase at similar conditions. In some experiments, a conductivity jump is observed at the temperature corresponding to the melting temperature of the metallic phase. Both the metal:olivine ratio and the metal phase geometry control the electrical conductivity of the two-layer samples. By combining electrical results, textural analyses of the samples, and previous studies of the structure and composition of Mercury's interior, we propose an electrical profile of the deep interior of the planet that accounts for a layered CMB-outer core structure. The electrical model agrees with existing conductivity estimates of Mercury's lower mantle and CMB using magnetic observations and thermodynamic calculations, and thus, supports the hypothesis of a layered CMB-outermost core structure in the present-day interior of Mercury. We propose that the layered CMB-outer core structure is possibly electrically insulating, which may influence the planet's structure and cooling history.

Description: Three Mars missions have analyzed the composition of surface samples using thermal extraction techniques. The temperatures of decomposition have been used as diagnostic information for the materials present. One compound of great current interest is perchlorate, a relatively recently discovered component of Mars' surface geochemistry that leads to deleterious effects on organic matter during thermal extraction. Knowledge of the thermal decomposition behavior of perchlorate salts is essential for mineral identification and possible avoidance of confounding interactions with organic matter. We have performed a series of experiments which reveal that the hydration state of magnesium perchlorate has a significant effect on decomposition temperature, with differing temperature releases of oxygen corresponding to different perchlorate hydration states (peak of O 2 release shifts from 500 to 600°C as the proportion of the tetrahydrate form in the sample increases). Changes in crystallinity/crystal size may also have a secondary effect on the temperature of decomposition, and although these surface effects appear to be minor for our samples further investigation may be warranted. A less than full appreciation of the hydration state of perchlorate salts during thermal extraction analyses could lead to misidentification of the number and the nature of perchlorate phases present.

Description: Seismic data will be a vital geophysical constraint on internal structure of Europa if we land instruments on the surface. Quantifying expected seismic activity on Europa both in terms of large, recognizable signals and ambient background noise is important for understanding dynamics of the moon, as well as interpretation of potential future data. Seismic energy sources will likely include cracking in the ice shell and turbulent motion in the oceans. We define a range of models of seismic activity in Europa's ice shell by assuming each model follows a Gutenberg-Richter relationship with varying parameters. A range of cumulative seismic moment release between 10 16 and 10 18 Nm/yr is defined by scaling tidal dissipation energy to tectonic events on the Earth's moon. Random catalogs are generated and used to create synthetic continuous noise records through numerical wave propagation in thermodynamically self-consistent models of the interior structure of Europa. Spectral characteristics of the noise are calculated by determining probabilistic power spectral densities of the synthetic records. While the range of seismicity models predicts noise levels that vary by 80 dB, we show that most noise estimates are below the self-noise floor of high-frequency geophones, but may be recorded by more sensitive instruments. The largest expected signals exceed background noise by ∼50 dB. Noise records may allow for constraints on interior structure through autocorrelation. Models of seismic noise generated by pressure variations at the base of the ice shell due to turbulent motions in the subsurface ocean may also generate observable seismic noise.

Description: Motivated by recent evidence for subduction in Europa's ice shell, we explore the geophysical feasibility of this process. Here we construct a simple model to track the evolution of porosity and temperature within a slab that is forced to subduct. We also vary the initial salt content in Europa's ice shell and determine the buoyancy of our simulated subducting slab. We find that porosity and salt content play a dominant role in determining whether the slab is non-buoyant and subduction in Europa's ice shell is actually possible. Generally, we find that initially low porosities and high salt contents within the conductive lid are more conducive to subduction. If salt contents are laterally homogenous, and Europa has a reasonable surface porosity of 𝜙0 = 0.1, the conductive portion of Europa's shell must have salt contents exceeding ~22% for subduction to occur. However, if salt contents are laterally heterogeneous, with salt contents varying by a few percent, subduction may occur for a surface porosity of 𝜙0 = 0.1 and overall salt contents of ~5%. Thus, we argue that under plausible conditions, subduction in Europa's ice shell is possible. Moreover, assuming subduction is actively occurring or has occurred in Europa's recent past provides important constraints on the structure and composition of the ice shell.

Description: Following K-Ar dating of a mudstone and a sandstone, a third sample has been dated by the Curiosity rover exploring Gale Crater. The Mojave 2 mudstone, which contains relatively abundant jarosite, yielded a young K-Ar bulk age of 2.57 ± 0.39 Ga (1σ precision). A two-step heating experiment was implemented in an effort to resolve the K-Ar ages of primary and secondary mineralogical components within the sample. This technique involves measurement of 40 Ar released in low (500 o C) and high (930 o C) temperature steps, and a model of the potassium distribution within the mineralogical components of the sample. Using this method, the high-temperature step yields a K-Ar model age of 4.07 ± 0.63 Ga associated with detrital plagioclase, compatible with the age obtained on the Cumberland mudstone by Curiosity. The low-temperature step, associated with jarosite mixed with K-bearing evaporites and/or phyllosilicates, gave a youthful K-Ar model age of 2.12 ± 0.36 Ga. The interpretation of this result is complicated by the potential for argon loss after mineral formation. Comparison with the results on Cumberland and previously published constraints on argon retentivity of the individual phases likely to be present suggests that the formation age of the secondary materials, correcting for plausible extents of argon loss, is still less than 3 Ga, suggesting post-3 Ga aqueous processes occurred in the sediments in Gale Crater. Such a result is inconsistent with K-bearing mineral formation in Gale Lake, and instead suggests post-depositional fluid flow at a time after surface fluvial activity on Mars is thought to have largely ceased.

Description: We invert the Martian tidal response and mean mass and moment of inertia for chemical composition, thermal state, and interior structure. The inversion combines phase equilibrium computations with a laboratory-based viscoelastic dissipation model. The rheological model, which is based on measurements of anhydrous and melt-free olivine, is both temperature and grain size sensitive and imposes strong constraints on interior structure. The bottom of the lithosphere, defined as the location where the conductive geotherm meets the mantle adiabat, occurs deep within the upper mantle (∼250–500 km depth) resulting in apparent upper mantle low-velocity zones. Assuming an Fe-FeS core, our results indicate: 1) a Mantle with a Mg# (molar Mg/Mg+Fe) of ∼0.75 in agreement with earlier geochemical estimates based on analysis of Martian meteorites; 2) absence of bridgmanite- and ferropericlase-dominated basal layer; 3) core compositions (13.5–16 wt% S), core radii (1640–1740 km), and core-mantle-boundary temperatures (1560–1660 ∘ C) that, together with the eutectic-like core compositions, suggest the core is liquid; and 4) bulk Martian compositions that are overall chondritic with a Fe/Si (wt ratio) of 1.63–1.68. We show that the inversion results can be used in tandem with geodynamic simulations to identify plausible geodynamic scenarios and parameters. Specifically, we find that the inversion results are reproduced by stagnant lid convection models for a range of initial viscosities (∼10 19 –10 20 Pa·s) and radioactive element partitioning between crust and mantle around 0.001. The geodynamic models predict a mean surface heat flow between 15–25 mW/m 2 .

Description: For large-angle long-term true polar wander (TPW) there are currently two types of non-linear methods which give approximated solutions: those assuming that the rotational axis coincides with the axis of maximum moment of inertia (MoI), e.g. ?Nakada2007_iteration (), which simplifies the Liouville equation and those based on the quasi-fluid approximation (e.g. (?Ricard1993polarwander)) which approximates the Love number. Recent studies show that both can have a significant bias for certain models (?Cambiotti_assumption; ?Hu2017). Therefore, we still lack an (semi-)analytical method which can give exact solutions for large-angle TPW for a model based on Maxwell rheology. This paper provides a method which analytically solves the MoI equation and adopts an extended iterative procedure introduced in ?Hu2017 () to obtain a time-dependent solution. The new method can be used to simulate the effect of a remnant bulge or models in different hydrostatic states. We show the effect of the viscosity of the lithosphere on long-term, large-angle TPW. We also simulate models without hydrostatic equilibrium and show that the choice of the initial stress-free shape for the elastic (or highly-viscous) lithosphere of a given model is as important as its thickness for obtaining a correct TPW behaviour. The initial shape of the lithosphere can be an alternative explanation to mantle convection for the difference between the observed and model predicted flattening. Finally, it is concluded that, based on the quasi-fluid approximation, TPW speed on Earth and Mars is underestimated while the speed of the rotational axis approaching the end position on Venus is overestimated.

Description: Evidence for past basal melting of young (late Amazonian), debris-covered glaciers in Mars’ mid-latitudes is extremely rare. Thus, it is widely thought that these viscous flow features (VFFs) have been perennially frozen to their beds. We identify an instance of recent, localized wet-based mid-latitude glaciation, evidenced by a candidate esker emerging from a VFF in a tectonic rift in Tempe Terra. Eskers are sedimentary ridges deposited in ice-walled meltwater conduits and are indicative of glacial melting. We compare the candidate esker to terrestrial analogues, present a geomorphic map of landforms in the rift, and develop a landsystem model to explain their formation. We propose that the candidate esker formed during a transient phase of wet-based glaciation. We then consider the similarity between the geologic setting of the new candidate esker and that of the only other candidate esker to be identified in association with an existing mid-latitude VFF; both are within tectonic graben/rifts proximal to volcanic provinces. Finally, we calculate potential basal temperatures for a range of VFF thicknesses, driving stresses, mean annual surface temperatures, and geothermal heat fluxes, which unlike previous studies, include the possible role of internal strain heating. Strain heating can form an important additional heat source, especially in flow convergence zones, or where ice is warmer due to elevated surface temperatures or geothermal heat flux. Elevated geothermal heat flux within rifts, perhaps combined with locally-elevated strain heating, may have permitted wet-based glaciation during the late Amazonian, when cold climates precluded more extensive wet-based glaciation on Mars.

Description: We performed numerical simulations of impact crater formation on Europa to infer the thickness and structure of its ice shell. The simulations were performed using iSALE to test both the conductive ice shell over ocean and the conductive lid over warm convective ice scenarios for a variety of conditions. The modeled crater depth-diameter is strongly dependent on thermal gradient and temperature of the warm convective ice. Our results indicate that both a fully conductive (thin) shell and a conductive-convective (thick) shell can reproduce the observed crater depth-diameter and morphologies. For the conductive ice shell over ocean, the best fit is an approximately 8 km thick conductive ice shell. Depending on the temperature (255 – 265 K) and therefore strength of warm convective ice, the thickness of the conductive ice lid is estimated at 5 – 7 km. If central features within the crater, such as pits and domes, form during crater collapse, our simulations are in better agreement with the fully conductive shell (thin shell). If central features form well after the impact, however, our simulations suggest a conductive-convective shell (thick shell) is more likely. Although our study does not provide firm conclusion regarding the thickness of Europa's ice shell, our work indicates that Valhalla-class multiring basins on Europa may provide robust constraints on the thickness of Europa's ice shell.

Description: Geophysical measurements can reveal the structures and thermal states of icy ocean worlds. The interior density, temperature, sound speed, and electrical conductivity thus characterize their habitability. We explore the variability and correlation of these parameters using 1D internal structure models. We invoke thermodynamic consistency using available thermodynamics of aqueous MgSO 4 , NaCl (as seawater), and NH 3 ; pure water ice phases I, II, III, V, VI; silicates; and any metallic core that may be present. Model results suggest, for Europa, that combinations of geophysical parameters might be used to distinguish an oxidized ocean dominated by MgSO 4 from a more reduced ocean dominated by NaCl. In contrast with Jupiter's icy ocean moons, Titan and Enceladus have low-density rocky interiors, with minimal or no metallic core. The low-density rocky core of Enceladus may comprise hydrated minerals or anhydrous minerals with high porosity. Cassini gravity data indicating a high tidal potential Love number ( k 2 〉0.6), which requires a dense internal ocean ( ρ o c e a n 〉1,200 kg m −3 ) and icy lithosphere thinner than 100 km. In that case, Titan may have little or no high-pressure ice, or a surprisingly deep water-rock interface more than 500 km below the surface, covered only by ice VI. Ganymede's water-rock interface is the deepest among known ocean worlds, at around 800 km. Its ocean may contain multiple phases of high-pressure ice, which will become buoyant if the ocean is sufficiently salty. Callisto's interior structure may be intermediate to those of Titan and Europa, with a water-rock interface 250 km below the surface covered by ice V but not ice VI.

Description: We used infrared data from the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment to globally map thermophysical properties of the Moon's regolith fines layer. Thermal conductivity varies from 7.4×10 -4 W m -1 K -1 at the surface, to 3.4×10 -3 W m -1 K -1 at depths of ~1 m, given density values of 1100 kg m-3 at the surface, to 1800 kg m -3 at 1-m depth. On average, the scale height of these profiles is ~7 cm, corresponding to a thermal inertia of 55 ±2 J m -2 K -1 s -1/2 at 273 K, relevant to the diurnally active near-surface layer, ~4-7 cm. The temperature-dependence of thermal conductivity and heat capacity leads to a ~2x diurnal variation in thermal inertia at the equator. On global scales, the regolith fines are remarkably uniform, implying rapid homogenization by impact gardening of this layer on timescales 〈 1 Gyr. Regional and local scale variations show prominent impact features 〈 1 Gyr old, including higher thermal inertia (〉 100 J m -2 K -1 s -1/2 ) in the interiors and ejecta of Copernican-aged impact craters, and lower thermal inertia (〈 50 J m -2 K -1 s -1/2 ) within the lunar cold spots identified by Bandfield et al. (2014). Observed trends in ejecta thermal inertia provide a potential tool for age-dating craters of previously unknown age, complementary to the approach suggested by Ghent et al. (2014). Several anomalous regions are identified in the global 128 pixels-per-degree maps presented here, including a high-thermal inertia deposit near the antipode of Tycho crater.

Description: Ozak et al. [2016] claimed that explicitly including the effect of CO 2 collisional line mixing (LM) in their radiative transfer calculations yield CO 2 atmospheres that are more transparent to infrared radiation than when spectra calculations are made using sub-Lorentzian line shapes. This would in particular imply significantly colder surface temperatures (up to 15 K) for early Mars than estimated in previous studies. Here we show that the relative cooling that Ozak et al. [2016] associated to the effect of collisional line mixing is in fact due to a wrong choice of broadening species (air instead of CO 2 ). We then calculated Line-by-Line spectra of pure CO 2 atmospheres using a line-mixing model developed for self-broadened CO 2 . Using the LMD Generic model (in 1D radiative-convective mode), we find that calculations made with the proper collisional line mixing model and with sub-Lorentzian line shapes lead to differences between early Mars surface temperatures smaller than 2 Kelvins only.

Description: Turbet and Tran [2017] have identified that we mistakenly used air-broadened CO 2 absorption spectra to generate coefficients for a rapid radiative transfer code, instead of self-broadened spectra. The resulting underestimation of absorption by CO 2 led us to suggest that when the effects of CO 2 line mixing are taken into account, surface temperatures on early Mars are up to 15 K colder than those calculated in previous studies, in which empirical corrections of the CO 2 spectrum were used ( χ -factors). Using the correct CO 2 absorption spectra, including the effects of line mixing, Turbet and Tran [2017] found that the surface temperature is colder by only 2 K than temperatures calculated with the χ -factor approach. While we acknowledge the mistake, and thank Turbet and Tran [2017] for finding and correcting it, we note that our main conclusions hold, that surface temperatures are overestimated when empirical χ -factors are used instead of a full account of CO 2 line mixing in radiative transfer calculations in CO 2 -rich planetary atmospheres. Additionally, we emphasize that the rapid radiative transfer model we developed and tested is useful for studies of planetary climate, provided the correct absorption spectra are used to generate new k -coefficients.

Description: For more than eight years the Venus Monitoring Camera (VMC) on-board the Venus Express orbiter performed continuous imaging of the Venus cloud layer in UV, visible and near-IR filters. We applied the correlation approach to sequences of the near-IR images at 965 nm to track cloud features and determine the wind field in the middle and lower cloud (49-57km). From the VMC images that spanned from December of 2006 through August of 2013 we derived zonal and meridional components of the wind field. In low-to-middle latitudes (5-65°S) the velocity of the retrograde zonal wind was found to be 68-70 m/s. The meridional wind velocity slowly decreases from peak value of +5.8±1.2 m/s at 15°S to 0 at 65-70°S. The mean meridional speed has a positive sign at 5-65°S suggesting equatorward flow. This result, together with the earlier measurements of the poleward flow at the cloud tops indicate the presence of a closed Hadley cell in the altitude range 55-65km. Long term variations of zonal and meridional velocity components were found during 1200 earth days of observation. At 20°±5°S the zonal wind speed increases from -67.18±1.81 m/s to -77.30±2.49 m/s. The meridional wind gradually increases from +1.30±1.82 m/s to +8.53±2.14 m/s. Following Bertaux et al. [2016] we attribute this long-term trend to the influence from the surface topography on the dynamical process in the atmosphere via the upward propagation of gravity waves that became apparent in the VMC observations due to slow drift of the Venus Express orbit over Aphrodite Terra.

Description: To understand the origin of the dunes on Titan, it is important to investigate the material properties of Titan's organic sand particles on Titan. The organic sand may behave distinctively compared to the quartz/basaltic sand on terrestrial planets (Earth, Venus, Mars) due to differences in interparticle forces. We measured the surface energy (through contact angle measurements) and elastic modulus (through Atomic Force Microscopy, AFM) of the Titan aerosol analog (tholin). We find the surface energy of a tholin thin film is about 70.9 mN/m and its elastic modulus is about 3.0 GPa (similar to hard polymers like PMMA and polystyrene). For two 20 μ m diameter particles, the theoretical cohesion force is therefore 3.3 μ N. We directly measured interparticle forces for relevant materials: tholin particles are 0.8±0.6 μ N, while the interparticle cohesion between walnut shell particles (a typical model materials for the Titan Wind Tunnel, TWT) is only 0.4±0.1 μ N. The interparticle cohesion forces are much larger for tholins and presumably Titan sand particles than materials used in the TWT. This suggests we should increase the interparticle force in both analog experiments (TWT) and threshold models to correctly translate the results to real Titan conditions. The strong cohesion of tholins may also inform us how the small aerosol particles (∼1 μm) in Titan's atmosphere are transformed into large sand particles (∼200 μm). It may also support the cohesive sand formation mechanism suggested by Rubin and Hesp (2009), where only unidirectional wind is needed to form linear dunes on Titan.

Description: The quasi-quadrennial oscillation (QQO) and its ∼4 year period in Jupiter's atmosphere were first discovered in 7.8 μm infrared observations spanning the 1980s and 1990s from detecting semiregular variations in equatorial brightness temperatures near 10 hPa. New observations that probe between 0.1 and 30 hPa in Jupiter's atmosphere using the Texas Echelon Cross Echelle Spectrograph (TEXES), mounted on the NASA Infrared Telescope Facility, have characterized the vertical structure of the QQO during a complete cycle between January 2012 and April 2016. These new observations show the thermal oscillation previously detected at 10 hPa and that it extends over a pressure range of 2–17 hPa. We have incorporated a spectrum of wave drag parameterizations into the Explicit Planetary Isentropic Code general circulation model to simulate the observed Jovian QQO temperature signatures inferred from the TEXES observations as a function of latitude. A new stochastic wave drag parameterization explores vertical wind structure and offers insight into the spectra of waves that likely exist in Jupiter's atmosphere to force the QQO. High-frequency gravity waves produced from convection likely contribute significantly to the QQO momentum budget. The model temperature outputs show strong correlations to equatorial and surrounding latitude temperature fields retrieved from the TEXES data sets at different epochs. Our results reproduce the QQO phenomenon as a zonal jet that descends over time in response to Jovian atmospheric forcing (e.g., gravity waves from convection).

Description: Field-aligned electrostatic potentials in the Martian ionosphere play potentially important roles in maintaining current systems, driving atmospheric escape and producing aurora. The strength and polarity of the potential difference between the observation altitude and the exobase (~180 km) determines the energy dependence of electron pitch angle distributions (PADs) measured on open magnetic field lines (i.e. those connected both to the collisional atmosphere and the IMF). Here we derive and examine a data set of ~3.6 million measurements of the potential between 185 km and 400 km altitude from PADs measured by the Mars Global surveyor MAG/ER experiment at 2 a.m./2 p.m. local time from May 1999 until November 2006. Potentials display significant variability, consistent with expected variable positive and negative divergences of the convection electric field in the highly variable and dynamic Martian plasma environment. However, superimposed on this variability are persistent patterns whereby potential magnitudes depend positively on crustal magnetic field strength, being close to zero where crustal fields are weak or non-existent. Average potentials are typically positive near the center of topologically open crustal field regions where field lines are steeper and negative near the edges of such regions where fields are shallower, near the boundaries with closed fields. This structure is less pronounced for higher solar wind pressures and (on the dayside) higher solar EUV irradiance. Its causes are uncertain at present, but may be due to differential motion of electrons and ions in Mars’ substantial but (compared to Earth) weak magnetic fields.

Description: Moon Mineralogy Mapper (M 3 ) spectroscopic data and high resolution imagery datasets were used to study the mineralogy and geology of the 207 km diameter Humboldt crater. Analyses of M 3 data, using a custom-made method for M 3 spectra continuum removal and spectral parameters calculation, reveal multiple pure crystalline plagioclase detections within the Humboldt crater central peak complex, hinting at its crustal origin. However, olivine, spinel and glass are observed in the crater walls and rims, suggesting these minerals derive from shallower levels than the plagioclase of the central peak complex. High-calcium pyroxenes are detected in association with volcanic deposits emplaced on the crater's floor. Geologic mapping was performed, and the age of Humboldt crater's units was estimated from crater counts. Results suggest that volcanic activity within this floor-fractured crater spanned over a billion years. The felsic mineralogy of the central peak complex region, which presumably excavated deeper material, and the shallow mafic minerals (olivine and spinel) detected in Humboldt crater walls and rim are not in accordance with the general view of the structure of the lunar crust. Our observations can be explained by the presence of a mafic pluton emplaced in the anorthositic crust prior to the Humboldt-forming impact event. Alternatively, the excavation of Australe basin ejecta could explain the observed mineralogical detections. This highlights the importance of detailed combined mineralogical and geological remote sensing studies to assess the heterogeneity of the lunar crust.

Description: The Opportunity rover investigated a gentle swale on the rim of Endeavour crater called Marathon Valley where a series of bright planar outcrops are cut into polygons by fractures. A wheel scuff performed on one of the soil-filled fracture zones revealed the presence of three endmembers identified on the basis of Pancam multispectral imaging observations covering ~0.4 to 1 μm: red and dark pebbles, and a bright soil clod. Multiple overlapping Alpha Particle X-ray Spectrometer (APXS) measurements were collected on three targets within the scuff zone. The field of view of each APXS measurement contained various proportions of the Pancam-based endmembers. Application of a log maximum likelihood method for retrieving the composition of the endmembers using the ten APXS measurements shows that the dark pebble endmember is compositionally similar to average Mars soil, with slightly elevated S and Fe. In contrast, the red pebble endmember exhibits enrichments in Al and Si and is depleted in Fe and Mg relative to average Mars soil. The soil clod endmember is enriched in Mg, S, and Ni. Thermodynamic modeling of the soil clod endmember composition indicates a dominance of sulfate minerals. We hypothesize that acidic fluids in fractures leached and oxidized the basaltic host rock, forming the red pebbles, and then evaporated to leave behind sulfate-cemented soil.

Description: The Mars Science Laboratory rover Curiosity engaged in a month-long campaign investigating the Bagnold dune field in Gale crater. What represents the first in situ investigation of a dune field on another planet has resulted in a number of discoveries. Collectively, the Curiosity rover team has compiled the most comprehensive survey of any extra-terrestrial aeolian system visited to date with results that yield important insights into a number of processes, including sediment transport, bedform morphology and structure, chemical and physical composition of aeolian sand, and wind regime characteristics. These findings and more are provided in detail by the JGR-Planets Special Issue Curiosity's Bagnold Dunes Campaign, Phase I.

Description: Widespread occurrence of Fe,Mg-phyllosilicates have been observed on Noachian Martian terrains. Therefore, the study of Fe,Mg-phyllosilicates formation, in order to characterize early Martian environmental conditions, is of particular interest to the Martian community. Previous studies have shown that the investigation of Fe,Mg-smectite formation alone helps to describe early Mars environmental conditions, but there are still large uncertainties in terms of pH range, oxic/anoxic conditions, etc... Interestingly, carbonates and/or zeolites have also been observed on Noachian surfaces in association with the Fe,Mg-phyllosilicates. Consequently, the present study focuses on the di/trioctahedral phyllosilicate/carbonate/zeolite formation as a function of various CO 2 contents (100% N 2 , 10% CO 2 / 90% N 2 , 100% CO 2 ), from a combined approach including closed system laboratory experiments for 3 weeks at 120°C and geochemical simulations. The experimental results show that as the CO 2 content decreases, the amount of dioctahedral clay minerals decreases in favour of trioctahedral minerals. Carbonates and dioctahedral clay minerals are formed during the experiments with CO 2 . When Ca-zeolites are formed, no carbonates and dioctahedral minerals are observed. Geochemical simulation aided in establishing pH as a key parameter in determining mineral formation patterns. Indeed, under acidic conditions dioctahedral clay minerals and carbonate minerals are formed, while trioctahedral clay minerals are formed in basic conditions with a neutral pH value of 5.98 at 120°C. Zeolites are favoured from pH 〉~7.2. The results obtained shed new light on the importance of dioctahedral clay minerals versus zeolites and carbonates versus zeolites competitions, to better define the aqueous alteration processes throughout early Mars history.

Description: Iazu is a 6.8 km-diameter crater located ~25 km south of Endeavour Crater in Meridiani Planum, Mars. The asymmetrical ejecta deposit and crater shape demonstrate that Iazu was formed by the impact of a projectile moving from west to east at 20 to 30° above the horizontal. The crater rim and walls expose a ~115 m thick section of intact banded bright and dark Burns formation polyhydrated sulfate deposits that overlie a lower section of basaltic crust of unknown thickness that has been slightly altered to Fe 3+ -Mg 2+ smectites. The lower section also exhibits several basaltic outcrops that are overlain by banded bright and dark layers similar in appearance to the bright and dark bands evident in the overlying Burns formation. We interpret the lower section as indicative of a transition from regional-scale fluvial activity to accumulation of sulfate-rich deposits, culminating in a thick overlying section of Burns formation strata. Banding in the Burns formation and underlying strata is inferred to be a consequence of periodic variations in sulfate-forming depositional environments.

Description: No abstract is available for this article.

Description: The MESSENGER observations of the seasonal variability of Mercury's Ca-exosphere are consistent with the general idea that the Ca-atoms originate from the bombardment of the surface by particles from comet 2P/Encke. The generating mechanism is believed to be a combination of different processes including the release of atomic and molecular surface particles and the photo-dissociation of exospheric molecules. Considering different generation and loss mechanisms, we perform simulations with a 3D Monte Carlo model based on the exosphere generation model by Mura et al. [2009]. We present for the first time the 3D spatial distribution of the CaO and Ca exospheres generated through the process of micrometeoroid impact vaporization and we show that the morphology of the latter is consistent with the available MESSENGER/MASCS observations. The results presented in this paper can be useful in the exosphere observations planning for BepiColombo, the upcoming ESA-JAXA mission to Mercury.

Description: Wind streak is a collective term for a variety of aeolian features that display distinctive albedo surface patterns. Wind streaks have been used to map near-surface winds and to estimate atmospheric circulation patterns on Mars and Venus. However, because wind streaks have been studied mostly on Mars and Venus, much of the knowledge regarding the mechanism and time-frame of their formation, and their relationship to the atmospheric circulation cannot be verified. This study aims to validate previous studies' results by a comparison of real and modeled wind data with wind streak orientations as measured from remote-sensing images. Orientations of Earth wind streaks were statistically correlated to Resultant Drift Direction (RDD) values calculated from reanalysis and wind data from 621 weather stations. The results showed good agreement between wind streak orientations and reanalysis RDD (r = 0.78). A moderate correlation was found between the wind streak orientations and the weather station data (r = 0.47); a similar trend was revealed ; on a regional scale when the analysis was performed by continent, with r ranging from 0.641 in North America to 0.922 in Antarctica. At sites where wind streak orientations did not correspond to the RDDs (i.e., a difference of 〉45°), seasonal and diurnal variations in the wind flow were found to be responsible for deviation from the global pattern. The study thus confirms that Earth wind streaks were formed by the present wind regime, and they are indeed indicative of the long-term prevailing wind direction on global and regional scales.

Description: With a Monte-Carlo model we investigate the escape of hot oxygen and carbon from the martian atmosphere for four points in time in its history corresponding to 1, 3, 10, and 20 times the present solar EUV flux. We study and discuss different sources of hot oxygen and carbon atoms in the thermosphere and their changing importance with the EUV flux. The increase of the production rates due to higher densities resulting from the higher EUV flux competes against the expansion of the thermosphere and corresponding increase in collisions. We find that the escape due to photodissociation increases with increasing EUV level. However, for the escape via some other reactions, e.g. dissociative recombination of O2+, this is only true until the EUV level reaches 10 times the present EUV flux, and then the rates start to decrease. Furthermore, our results show that Mars could not have had a dense atmosphere at the end of the Noachian epoch, since such an atmosphere would not have been able to escape until today. In the pre-Noachian era, most of a magma ocean and volcanic activity related outgassed CO 2 atmosphere could have been lost thermally until the Noachian epoch, when non-thermal loss processes such as suprathermal atom escape became dominant. Thus, early Mars could have been hot and wet during the pre-Noachian era with surface CO 2 pressures larger than 1 bar during the first 300 Myr after the planet's origin.

Description: During accretion, terrestrial bodies attain a wide range of thermal and rotational states, which are accompanied by significant changes in physical structure (size, shape, pressure and temperature profile, etc.). However, variations in structure have been neglected in most studies of rocky planet formation and evolution. Here we present a new code, the Highly Eccentric Rotating Concentric U (potential) Layers Equilibrium Structure (HERCULES) code, that solves for the equilibrium structure of planets as a series of overlapping constant-density spheroids. Using HERCULES and a smoothed particle hydrodynamics code, we show that Earth-like bodies display a dramatic range of morphologies. For any rotating planetary body, there is a thermal limit beyond which the rotational velocity at the equator intersects the Keplerian orbital velocity. Beyond this corotation limit (CoRoL), a hot planetary body forms a structure, which we name a synestia, with a corotating inner region connected to a disk-like outer region. By analyzing calculations of giant impacts and models of planet formation, we show that typical rocky planets are substantially vaporized multiple times during accretion. For the expected angular momentum of growing planets, a large fraction of post-impact bodies will exceed the CoRoL and form synestias. The common occurrence of hot, rotating states during accretion has major implications for planet formation and the properties of the final planets. In particular, the structure of post-impact bodies influences the physical processes that control accretion, core formation, and internal evolution. Synestias also lead to new mechanisms for satellite formation. Finally, the wide variety of possible structures for terrestrial bodies also expands the mass-radius range for rocky exoplanets.

Description: We use two Martian years’ temperature data from Mars Climate Sounder (MCS) cross-track observations to extract the migrating diurnal tide (MDT) and investigate its spatial and seasonal variation from the perspective of classical tidal theory by Hough mode decomposition. The results suggest that during the equinox the vertically propagating (1, 1) mode is dominant at all altitudes from the near surface to ~0.1 Pa with the magnitude growing with height. The trapped modes (1, -2), (1, -4), (1, -6) are restricted to the lower altitudes with a similar vertical structure. Both the (1, 1) and (1, -2) modes have clearly semiannual variations. The comparison between Hough modes components of MDT and dust and water ice heating rate indicates that both the dust and water ice contribute to tidal excitation. However, both the annual and semiannual variations of dust heating rate are out-of-phase with those in MDT while semiannual variation of the water ice heating rate is in-phase, indicating that the water ice may contribute to the semiannual variation of MDT. Using model wind results, we also find that the zonal mean zonal wind and its latitudinal shear at the low latitudes modify the vertical propagation condition of the MDT (1, 1) mode and further affect its seasonal variation. The semiannual variations of equatorial MDT and its corresponding mechanism on Mars are comparable to those on Earth, suggests that the two planets may have more common characteristics.

Description: The analysis of the Lunar Prospector magnetic data over South Pole Aitken basin reveals two categories of magnetic anomalies with distinctly different polarities. Modeling the anomalies in terms of vertical prism source bodies shows two separate true polar wander paths of the Moon. One is driven by the giant surface mass deficiency resulted from the formation of the South Pole Aitken basin and the other by the giant surface mass concentrations (mascons) formed inside the large impact basins, such as Serenitatis and Imbrium, and a vast volcanic deposit on Procellarum Terrane. A total of ~100 o true polar wander path suggests that the source bodies are formed and magnetized during a long period. Moreover, the distinctly different polarities of the two categories of the anomalies supports the idea that the core dynamo underwent a reversal, or shutdown and started up with a different polarity, caused by the latter impacts. The impact-induced shock pressures of 10-20 GPa traveling inside the lunar core, the appreciable angular momentum transfer to the lunar mantle by the impacting bodies, and the impact-induced stratification of the core probably reversed the polarity of the core dynamo. We also model the north Crisium magnetic anomaly, extracted from the Lunar Prospector magnetic data. The paleomagnetic pole position obtained for the source body reveals that the north Crisium anomaly belongs to the first category of the anomalies.

Description: The VIRTIS instrument on Venus Express observed thermal emission from the surface of Venus at 1 μm wavelength and thus would have detected sufficiently bright incandescent lava flows. No eruptions were detected in the observations between April 2006 and October 2008, covering an area equivalent to 7 times the planets surface on separate days. Models of the cooling of lava flows on Earth are adapted to Venus ambient conditions to predict thermal emission based on effusion rate. Taking into account the blurring of surface thermal emission by the atmosphere, the VIRTIS images would detect eruptions with effusion rates above 500 to 1000 m 3 /s. On Earth such eruptions occur but are rare. Based on an eruption rate and duration distribution fitted to historical data of three terrestrial volcanos, we estimate that only a few percent of all eruptions are detectable. With these assumptions the VIRTIS data can constrain the rate of effusive volcanism on Venus to be less than about 300 km 3 /yr, at least an order of magnitude higher than existing constraints. There remains a large uncertainty because of unknown properties of lava flows on Venus. Resolving flows in radar imaging and their thickness in altimetry might help to better constrain these properties. While VIRTIS data does not represent a significant constraint on volcanism, an optimized instrument with a 20 times better signal to noise ratio would likely be able to detect effusion rates on the order of 50 m 3 / s .

Description: The present paper describes observations of crater growth for various impact velocities up to ∼6km s −1 . Spherical polycarbonate projectiles (4.76 mm) were impacted vertically into dry sand targets at impact velocities v i ranging from 0.7 to ∼6km s −1 , and the temporal change (diameter growth) in diameter of crater cavities was measured by a high-speed profilometer with 2000frames per second. Our data show that the diameter growth follows a power-law relation at the early stages, but the data at the later stages deviates from the power-law relation. It is also shown that the power-law exponent at the early stages decreases with increasing v i , indicating a possibility that the growth rate of the transient crater cavity decreases with increasing v i . On the other hand, the power-law relation for final crater sizes in our data are consistent with the classical scaling relation for final crater sizes, which is thought to be independent of v i . Thus, our data may suggest that there is a difference in the power-law behavior between the crater growth and the final crater sizes.

Description: Impact cratering is the dominant process for transporting material on the Moon's surface. An impact transports both proximal material (continuous ejecta) locally and distal ejecta (crater rays) to much larger distances. Quantifying the relative importance of locally derived material versus distal material requires understandings of lunar regolith evolution and the mixing of materials across the lunar surface. The Moon has distinctive albedo units of darker mare basalt and brighter highland materials, and the contacts between these units are ideal settings to examine this question. Information on the amount of material transported across these contacts comes from both the sample collection and remote sensing data, though earlier interpretations of these observations are contradictory. The relatively narrow (~4-5 km wide) mixing zone at mare/highland contacts had been interpreted as consistent with most material having been locally-derived from underneath mare plains. However, even far from these contacts where the mare is thick, highland material is abundant in some soil samples (〉20%), requiring transport of highland material over great distances. Any model of impact transport on the Moon needs to be consistent with both the observed width of mare/highland contacts and the commonality of non-mare material in mare soil samples far from any contact. In this study, using a three-dimensional regolith transport model, we match these constraints and demonstrate that both local and distal material transport are important at the lunar surface. Furthermore, the nature of the distal material transport mechanism in discrete crater rays can result in substantial heterogeneity of surface materials.

Description: In December 2001, the Space Telescope Imaging Spectrograph of the Hubble Space Telescope obtained far-ultraviolet spectral images of Jupiter's moon Callisto. The leading and trailing hemispheres were observed in the spectral range 1190 Å to 1720 Å when the moon was at eastern and western elongations, respectively. We analyzed the hydrogen Lyman- α (1216 Å) signal in the two observations and found that faint atmospheric emissions extending up to several moon radii away are present in addition to the solar flux reflected off the surface. We show that the detected atmospheric Lyman- α emissions are consistent with an escaping hydrogen corona with a vertical column density in the range of (6 − 12) × 10 11  cm −2 . The derived hydrogen abundance is about two times higher when the moon is at eastern elongation, possibly related to increased water sublimation when the visibly darker leading hemisphere is illuminated by the Sun. The detected hemispheric difference is larger than the measurement uncertainties providing first evidence for asymmetries in Callisto's neutral atmosphere.

Description: Ground-penetrating radar (GPR) is a well established geophysical terrestrial exploration method largely employed and has recently become one of the most promising for planetary subsurface exploration. Several future landing vehicles like EXOMARS, 2020 NASA ROVERand Chang'e-4, to mention a few, will host GPR. A GPR survey has been conducted on volcanic deposits on Mount Etna (Italy), considered a good analogue for Martian and Lunar volcanic terrains, to test a novel methodology for subsoil dielectric properties estimation. The stratigraphy of the volcanic deposits was investigated using 500 MHz and 1 GHz antennas in two different configurations: transverse electric (TE) and transverse magnetic (TM). Sloping discontinuities have been used to estimate the loss tangents of the upper layer of such deposits by applying the amplitude-decay and frequency shift methods and approximating the GPR transmitted signal by Gaussian and Ricker wavelets. The loss tangent values, estimated using these two methodologies, were compared and validated with those retrieved from Time Domain Reflectometry (TDR) measurements acquired along the radar profiles. The results show that the proposed analysis, together with typical GPR methods for the estimation of the real part of permittivity, can be successfully used to characterize the electrical properties of planetary subsurface and to define some constraints on its lithology of the subsurface.

Description: Liquid water is likely present in the interior of Enceladus, but it is still debated whether this water forms a global ocean or a regional sea, and whether the present-day situation is stable. As the heat flux of Enceladus exceeds most heat source estimates, the liquid water is likely cooling and crystallizing, which results in expansion and pressurization of the sea or ocean. We determine, using an axisymmetric Finite Element Model, the tectonic patterns that pressurization of a regional sea or global ocean might produce at the surface of Enceladus. Tension is always predicted above where the ice is thinnest and generates cracks that might be at the origin of the Tiger Stripes. Tectonic activity is also expected in an annulus around the sea if the ice shell is in contact with, but slips freely along the rocky core of the satellite. Cracks at the North Pole are expected if the shell slips along the core or if there is a global ocean with thin ice at the pole. Water is likely injected along the base of the ice when the shell is grounded, which may lead to cycles of tectonic activity with the shell alternating between floating and grounded states and mid-latitude faulting occurring at the transition from grounded to floating states.

Description: High resolution pictures of Pluto's surface obtained by the New Horizons spacecraft revealed, among other surface features, a large nitrogen ice glacier informally named Sputnik Planitia. The surface of this glacier is separated into a network of polygonal cells with a wavelength of ∼20–40 km. This network is similar to the convective patterns obtained under certain conditions by laboratory experiments, suggesting that it is the surface expression of thermal convection. Here, we investigate the surface planform obtained for different convective systems in 3D-Cartesian geometry with different modes of heating and rheologies. We find that bottom heated systems, as assumed by previous studies, do not produce surface planforms consistent with the observed pattern. Alternatively, for a certain range of Rayleigh-Roberts number, R a H , a volumetrically heated system produces a surface planform similar to this pattern. We then combine scaling laws with values of R a H within its possible range to establish relationships between the critical parameters of Sputnik Planitia. In particular, our calculations indicate that the glacier thickness and the surface heat flux are in the ranges 2–10 km and 0.1–10 mW m −2 , respectively. However, a difficulty is to identify a proper source of internal heating. We propose that the long-term variations of surface temperature caused by variations in Pluto's orbit over millions of year produces secular cooling equivalent to internal heating. We find that this source of heating is sufficient to trigger thermal convection, but additional investigations are needed to determine under which conditions it can produce surface patterns similar to those of Sputnik Planitia.

Description: The martian magnetic field is unique among those of the terrestrial planets. It is the net result of the interaction of the solar wind and the interplanetary magnetic field (IMF) with crustal remnant magnetization and a planetary ionosphere. Internal fields of crustal origin have been the subject of extensive studies; the focus of our work is identification and characterization of contributions from external magnetic fields using the Mars Global Surveyor (MGS) vector magnetic field data. We investigate the magnitude, average spatial structure and temporal variability of the external magnetic field at MGS mapping altitude of 400 km by first subtracting the expected contributions from crustal fields using existing global crustal field models. We identify periodicities and spatial structure in the field related to the day-night cycle and Carrington rotations, as well as variations corresponding to an annual cycle and short aperiodic signals. We suggest that ionospheric currents driven by upper atmosphere winds contribute to the observed zonal structure in the daily variation in the external magnetic field. Finally, we discuss the potential for magnetic sounding studies using time-varying external fields and surface magnetometry measurements from the InSight mission to be launched in 2018.

Description: The Traverse Gravimeter Experiment (TGE) from the Apollo 17 mission was the first and only successful gravity survey on the surface of the Moon, revealing the local gravity field at Taurus-Littrow Valley (TLV). TLV is hypothesized to be a basalt-filled graben, oriented radial to Serenitatis basin. We implemented modern 3D modeling techniques using recent high-resolution Lunar Reconnaisance Orbiter topography and image datasets to reinvestigate the subsurface structure of TLV and constrain the volcanic and tectonic history of the region. Updated topography led to significant improvements in the accuracy of free-air, Bouguer and terrain corrections. To determine the underlying geometry for TLV, we tested a range of possible thicknesses, dips and wall positions for the graben fill. We found that the thickness and position previously determined by Talwani et al. [1973] represent our preferred model for the data, but with walls with dips of 30° , rather than 90° [ Talwani et al. , 1973]. We found large model misfits due to unmodelled 3D structure and density anomalies, as well as parameter trade offs. We performed a sensitivity analysis to quantify the parameter trade-offs in an ideal future survey, assuming dominantly 2D geological structure. At the TGE survey noise level (2.5 mgal), the fill thickness was constrained to ±150 m, the wall angle to and the wall positions to ±1 km of the preferred model. This information can be used to inform the design of future lunar gravimetry experiments in regions similar to TLV.

Description: The Martian atmosphere, which mainly consists of carbon dioxide (CO 2 ), is characterized by extremely low temperatures that cause CO 2 gas to freeze and dry ice to form. To date, temperatures below the CO 2 saturation temperature, which can be attributed to the effects of atmospheric waves, have been observed in the polar winter and in the mesosphere. Using data from MGS radio occultation measurements, we investigated the role of large-scale atmospheric waves including stationary and transient waves at northern high latitudes in winter on CO 2 supersaturation. A distinct longitudinal dependence of CO 2 supersaturation was observed at altitudes higher than the pressure level of 200-400 Pa, where a stationary wave with a wavenumber of 2, whose temperature amplitude had minima at 30-100 Pa, lowered the background temperature to a level close to the CO 2 saturation temperature. However, the stationary wave alone was not sufficient to cause CO 2 supersaturation. Additional temperature disturbances caused by transient waves, namely, superposition of both waves, had a significant role in CO 2 supersaturation. The longitudinal dependence of the occurrence of CO 2 supersaturation revealed by our study might affect the longitudinal distribution of CO 2 snowfall and the formation of the seasonal polar ice cap.

Description: Titan is the only moon with a substantial atmosphere, the only other thick N 2 atmosphere Earth's, the site of extraordinarily complex atmospheric chemistry that far surpasses any other solar system atmosphere, and the only other solar system body with stable liquid currently on its surface. The connection between Titan's surface and atmosphere is also unique in our Solar system; atmospheric chemistry produces materials that are deposited on the surface and subsequently altered by surface-atmosphere interactions such as aeolian and fluvial processes resulting in the formation of extensive dune fields and expansive lakes and seas. Titan's atmosphere is favorable for organic haze formation, which combined with the presence of some oxygen bearing molecules indicates that Titan's atmosphere may produce molecules of prebiotic interest. The combination of organics and liquid, in the form of water in a subsurface ocean and methane/ethane in the surface lakes and seas, means that Titan may be the ideal place in the solar system to test ideas about habitability, prebiotic chemistry, and the ubiquity and diversity of life in the Universe. The Cassini-Huygens mission to the Saturn system has provided a wealth of new information allowing for study of Titan as a complex system. Here I review our current understanding of Titan's atmosphere and climate forged from the powerful combination of Earth-based observations, remote sensing and in situ spacecraft measurements, laboratory experiments, and models. I conclude with some of our remaining unanswered questions as the incredible era of exploration with Cassini-Huygens comes to an end.

Description: The smooth plains on the floor of Mercury's Caloris basin and those almost entirely surrounding it beyond its rim are usually accepted to be younger than the rim materials, and to be lava flows rather than impact melt. High resolution imaging shows that emplacement of interior and exterior plains was concurrent, with evidence of both inward and outward flow while they were being emplaced. The Caloris rim is breached in two places by continuous smooth plains that seamlessly connect interior and exterior plains. The gross-scale spectral and compositional distinctiveness of interior and exterior plains is blurred on a scale of several tens of km, which could reflect interfingering of flow units less than a few 100 km long that tapped melt sources of different composition and/or depth inside and outside the basin followed by local mixing of regolith. Flows occurring both inside and outside the basin should be included in estimates of the total erupted volume.

Description: The energy sources involved in the early stages of the formation of terrestrial bodies can induce partial or even complete melting of the mantle, leading to the emergence of magma oceans. The fractional crystallization of a magma ocean can cause the formation of a compositional layering that can play a fundamental role for the subsequent long-term dynamics of the interior and for the evolution of geochemical reservoirs. In order to assess to what extent primordial compositional heterogeneities generated by magma ocean solidification can be preserved, we investigate the solidification of a whole-mantle Martian magma ocean, and in particular the conditions that allow solid-state convection to start mixing the mantle before solidification is completed. To this end, we performed 2-D numerical simulations in a cylindrical geometry. We treat the liquid magma ocean in a parametrized way while we self-consistently solve the conservation equations of thermochemical convection in the growing solid cumulates accounting for pressure-, temperature- and, where it applies, melt-dependent viscosity. By testing the effects of different cooling rates and convective vigor, we show that for a lifetime of the liquid magma ocean of 1 Myr or longer, the onset of solid-state convection prior to complete mantle crystallization is likely and that a significant part of the compositional heterogeneities generated by fractionation can be erased by efficient mantle mixing. We discuss the consequences of our findings in relation to the formation and evolution of compositional reservoirs on Mars and on the other terrestrial bodies of the Solar System.

Description: The identification of lobate debris deposits in Arabia Terra, along the proposed paleoshoreline of a former northern ocean, has renewed questions about the existence and stability of ocean-sized body of water in the early geologic history of Mars. The potential occurrence of impact-generated tsunamis in a northern ocean was investigated by comparing the geomorphologic characteristics of the Martian deposits with the predictions of well-validated terrestrial models (scaled to Mars) of tsunami wave height, propagation direction, runup elevation, and distance, for three potential sea levels. Our modelling suggests several potential impact craters ~30-50 km in diameter as the source of the tsunami events. Within the complex topography of flat-floored valleys and plateaus along the dichotomy boundary, the interference of the multiple reflected and refracted waves that are observed in the simulation may explain the origin of the arcuate pattern that characterizes the thumbprint terrain.

Description: As part of the Bagnold Dune campaign conducted by Mars Science Laboratory rover Curiosity, visible/near-infrared reflectance spectra of dune sands were acquired using Mast Camera (Mastcam) multispectral imaging (445-1013 nm) and Chemistry and Camera (ChemCam) passive point spectroscopy (400-840 nm). By comparing spectra from pristine and rover-disturbed ripple crests and troughs within the dune field, and through analysis of sieved grain size fractions, constraints on mineral segregation from grain sorting could be determined. In general, the dune areas exhibited low relative reflectance, a weak ~530 nm absorption band, an absorption band near 620 nm, and a spectral downturn after ~685 nm consistent with olivine-bearing sands. The finest grain size fractions occurred within ripple troughs and in the subsurface, and typically exhibited the strongest ~530 nm bands, highest relative reflectances, and weakest red/near-infrared ratios, consistent with a combination of crystalline and amorphous ferric materials. Coarser-grained samples were the darkest and bluest, and exhibited weaker ~530 nm bands, lower relative reflectances, and stronger downturns in the near-infrared, consistent with greater proportions of mafic minerals such as olivine and pyroxene. These grains were typically segregated along ripple crests and among the upper surfaces of grain flows in disturbed sands. Sieved dune sands exhibited progressive decreases in reflectance with increasing grain size, as observed in laboratory spectra of olivine size separates. The continuum of spectral features observed between the coarse- and fine-grained dune sands suggests that mafic grains, ferric materials, and airfall dust mix in variable proportions depending on aeolian activity and grain sorting.

Description: The early martian environment is interpreted as warmer and wetter, before a significant change in its global climatic conditions irreversibly led to the current hyper-arid environments. This transition is one of the most intriguing processes of Martian history. The extreme climatic change is preserved in the salt deposits, desiccated landscapes and geomorphological structures that were shaped by the evaporation of water. However, until a manned journey to Mars is feasible, many Martian materials, morphological structures, and much of its evolutionary history will continue to be poorly understood. In this regard, searching and investigating Martian analogues is still meaningful. To find an Earth environment with a whole set of Martian structures distributed at a scale comparable to Mars is even more important to test landing crafts and provide optimized working parameters for rovers. The western Qaidam Basin in North Tibetan Plateau is such a Martian analogue. The area harbors one of the most extreme hyper-arid environments on Earth, and contains a series of ancient lakes that evaporated at different evolutionary stages during the rise of the Tibetan Plateau. Large quantities of salts and geomorphological features formed during the transition of warmer-and-wet to colder-and-dry conditions provide unique references to study the modern Martian surface and interpret the orbital data. We present numerous similarities and results of investigations that suggest the Qaidam Basin as a potential analogue to study modern geomorphic processes on Mars, and suggest that this is an essential site to test future Mars sample return missions.

Description: Wrinkle ridges are among the most common tectonic structures on the terrestrial planets, and provide important records of the history of planetary strain and geodynamics. The observed broad arches and superposed narrow wrinkles are thought to be the surface manifestation of blind thrust faults, which terminate in near-surface volcanic sequences and cause folding and layer-parallel shear. However, the subsurface tectonic architecture associated with the ridges remains a matter of debate. Here we present direct observations of a wrinkle ridge thrust fault where it has been exposed by erosion in the southern wall of Melas Chasma on Mars. The thrust fault has been made resistant to erosion, likely due to volcanic intrusion, such that later erosional widening of the trough exposed the fault plane as a 70 km-long ridge extending into the chasma. A plane fit to this ridge crest reveals a thrust fault with a dip of 13° (+8°, -7°) between 1 and 3.5 km depth below the plateau surface, with no evidence for listric character in this depth range. This dip is significantly lower than the commonly assumed value of 30°, which, if representative of other wrinkle ridges, indicates that global contraction on Mars may have been previously underestimated.

Description: Outflow channels are prevalent on the surface of Mars and they are one of the strongest lines of evidence for fluvial activity. The purported presence of some of the channel networks on the Amazonian surface provides evidence for recent surficial fluvial activity. We describe morphological evidence collectively diagnostic of fluvial origin for the channels located northwest (NW) of Jovis Tholus, within the Tharsis volcanic province. The fluvial relationships are deciphered from 1) morphological analysis of catastrophic effusion associated with the graben system superposed over NW portion of an unnamed crater ejecta (19.85° N, 118.03° W), and 2) crater-size frequency distributions of unnamed crater ejecta revealing the tentative period of the fluvial activity. Channels, streamlined/curvilinear islands, terraces, divide crossing, crossover channels, networks of braided like channels and eroded ejecta are collectively suggestive of fluvial outflow that emanated from the graben. Chronological analysis revealed that the unnamed crater formed at ~3.4 Ga. Subsequently, the NW part of crater ejecta was incised by a channel network originated from the graben. Furthermore, we infer that the current channel floor has undergone volcanic and aeolian resurfacing to some extent, and the estimated modeled resurfaced channel age is ~0.2-0.5 Ga. This reveals that the fluvial activity possibly occurred after the crater formation, but before channel floor resurfacing, which is during Early-to-Middle Amazonian. Nevertheless, the potential source for the channel formation likely persisted under this region since the pre-Amazonian epoch. The suite of these fluvial features inferred within the Jovis Tholus region outflow channel system thus adds one another evidence for Amazonian fluvial activity.

Description: The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission measures both the upstream solar wind and collisional products from energetic neutral hydrogen atoms that precipitate into the upper atmosphere after their initial formation by charge exchange with exospheric hydrogen. By computing the ratio between the densities of these populations, we derive a robust measurement of the column density of exospheric hydrogen upstream of the Martian bow shock. By comparing with Chamberlain-type model exospheres, we place new constraints on the structure and escape rates of exospheric hydrogen, derived from observations sensitive to a different and potentially complementary column from most scattered sunlight observations. Our observations provide quantitative estimates of the hydrogen exosphere with nearly complete temporal coverage, revealing order of magnitude seasonal changes in column density and a peak slightly after perihelion, approximately at southern summer solstice. The timing of this peak suggests either a lag in the response of the Martian atmosphere to solar inputs, or a seasonal effect driven by lower atmosphere dynamics. The high degree of seasonal variability implied by our observations suggests that the Martian atmosphere and the thermal escape of light elements depend sensitively on solar inputs.

Description: Mid-infrared spectroscopy is a useful tool for remotely sensing the composition of Earth and other planets. Quantitative mineralogical investigations are possible using remotely sensed data, however the difficulty in modeling complex interactions of light with particles that are on the order of the wavelength limits the usefulness of the remote sensing data. As part of an effort to develop a more effective treatment of light scattering in planetary regolith, we explore the ability of T-matrix and radiative transfer (RT) hybrid models to produce emissivity spectra that are consistent with laboratory measurements. Parameters obtained from T-matrix calculations are used in three different RT models to construct emissivity spectra of enstatite particles of different sizes. Compared to the widely used Mie/RT hybrid models, the T-matrix/RT hybrid models produce more consistent emissivity spectra for the finest particle size fraction (3.3 µm). Overall, T-matrix hybrid models produce improved emissivity spectra, but larger particle sizes are still difficult to model. The improvement observed in T-matrix/RT hybrid models is a result of the inclusion of multiple scattering in closely packed media, and it demonstrates the importance of the implementation of physically realistic factors in developing a more effective light scattering model for planetary regolith. Further development and implementation of this and similar hybrid models will result in an improvement in quantitative assessments of planetary particulate surfaces from mid-infrared spectra.