ALBERT

Description: Abstract This paper presents a new, global database of lunar impact craters, estimated to be a complete census of all craters with diameters larger than 1–2 km. The database contains over 2 million craters, making it larger in number than any previously published lunar effort by more than a factor of 10. Of those craters, 1.3 million have diameters ≥1 km, 83 thousand ≥5 km, and 6972 ≥20 km. How the database was constructed along with the reliability of features are described in detail. Comparisons are made with past published databases, demonstrating good agreement for crater size and location. An ellipticity analysis is conducted, illustrating there is no dominant direction for elliptical crater orientation based on location, diameter range, or ellipticity amount, consistent with randomness for craters ≥10 km. A spatial density analysis is described, comparing the spatial density of small versus large craters, and numerous observations about the nonuniformity of the size distributions of craters across the Moon are made. The spatial density is also used in a discussion about kilometer‐scale secondary impact craters and clearly shows that they dominate the crater population in some areas of the lunar surface. While this database is complete as‐is, future work will expand it and fix errors that might be identified, so it is strongly recommended that interested individuals contact the corresponding author to be alerted to updates.

Description: Abstract The role of micrometeorite bombardment in space weathering on asteroid surfaces was studied using a 2 MV Van–De–Graaff accelerator. About 90000–100000 micron– to sub–micron sized copper particles with a mass– and velocity distribution similar to the interplanetary dust population, were fired onto the surfaces of polished Allende CV3 chondrite and eucrite NWA 6966 samples at speeds between km s−1. We find a clear relationship between micro–particle bombardment, infrared reflectance decrease, and overall spectral reddening. Differences in impact effects due to variable particle speed, size and structure are observed. Some Cu–particles form large clusters that break up upon impact and disperse. Other impactors leave imprints on the surface, implant or generate typical craters with rims and spallation features. Very small, fast particles generate small craters without spallation or significant crater rim. Mid–IR–spectra (bulk– and microscopic measurements of individual components), 3D–Laser microscopic images and XRD–spectra from the processed and unprocessed samples were collected. Mid–IR–spectra (700–6000 cm−1) over the entire sample surface, show overall darkening of features. Microscopic IR–spectra show the damage seen as reflectance decrease and spectral reddening, which is variable in the μ–range, depending on impact density and target properties (mineralogic composition). The fine–grained Allende matrix with predominantly Fe–rich olivine seems less affected than coarse–grained chondrules with Mg–rich silicates, where darkening can reach 60%. XRD–analysis also suggests chemical and crystallographical differences in the bombarded sample, due to impact shock.

Description: Abstract The processes of alteration of airless bodies exposed to the space environment are referred to be as ‘space weathering’. Multiple agents contribute generally to space weathering, to an extent that depends on the specific location of the surface within the Solar System. Typical space weathering agents encountered in the Solar System are: solar radiation, solar wind and cosmic rays, magnetospheric plasma (for example, at Jupiter or Saturn), and cosmic dust. The effect of space weathering is generally assessed by measuring the surfaces optical properties, for example by near‐infrared (IR) spectroscopy. The alteration of the surfaces is due to a cumulative effect over time of all agents. We investigate in this paper the contribution of micro‐meteoroid (dust) bombardment on different asteroids, by using the Micrometeoroid Environment Model (IMEM) for the interplanetary dust populations (IDPs), and a simplified model of Interstellar Dust (ISD) dynamics. We quantify, for different representative asteroids (Main Belt and NEOs), the particle cumulative flux, mass flux, impact velocity and the kinetic impact energy deposited. This work is primarily intended to support laboratory work investigating the effect of energy deposition onto sample surfaces, as well as astronomical observations of optical properties of asteroid surfaces.

Description: Abstract We present a comprehensive global catalog of the geomorphological features with clear or potential relevance to subsurface ice identified during the Dawn spacecraft's primary and first extended missions at Ceres. We define eight broad feature classes and describe analyses supporting their genetic links to subsurface ice. These classes include: relaxed craters; central pit craters; large domes; small mounds; lobate landslides and ejecta; pitted materials; depressions and scarps; and fractures, grooves, and channels. Features in all classes are widely distributed on the dwarf planet, consistent with multiple lines of observational evidence that ice is a key component of Ceres' crust. Independent analyses of multiple feature types suggest rheological and compositional layering may be common in the upper ~10 km of the crust. Clustering of features indicates that ice concentration is heterogeneous on nearly all length scales, from ~1 km to 100s of km. Impacts are likely the key driver of heterogeneity, causing progressive devolatilization of the low and mid‐latitude crust on billion‐year timescales, but also producing localized enhancements in near surface ice content via excavation of deep ice‐rich material and possible facilitation of cryomagmatic and cryovolcanic activity. Impacts and landslides may be the dominant mechanism for ice‐loss on modern Ceres. Our analysis suggests specific locations where future high‐resolution imaging can be used to probe (1) current volatile loss rates and (2) the history of putative cryomagmatic and cryovolcanic features. The Cerean cryosphere and its unique morphology promise to be a rich subject of ongoing research for years to come.

Description: Abstract Northwest Africa (NWA) 11042, originally classified as a primitive achondrite with no chondritic relicts, is rather a unique L‐melt rock. It is a severely shocked, igneous‐textured ultramafic rock composed of euhedral to subhedral olivine (Fa25.1±0.5) and pyroxenes (Low‐Ca pyroxene Fs20.7±0.8Wo4.2±1.0, and Ca‐rich pyroxene Fs11.5±0.5Wo37.6±1.2) with interstitial albitic plagioclase (Ab80.7±1.7Or5.0±0.7) that has been completely converted to maskelynite. Mineral compositions are similar to those of equilibrated L chondrites. Melt pockets are scattered throughout the sample, containing high‐pressure minerals including ringwoodite, wadsleyite, jadeite, and lingunite. Merrillite and apatite in NWA 11042 contain significantly higher REE abundances than those of ordinary chondrites, indicative of igneous fractional crystallization. In situ U‐Pb dating of apatite in NWA 11042 reveals an upper intercept age of 4479±43 Myr and a lower intercept age of 465±47 Myr on the normal U‐Pb concordia diagram. The upper intercept age recorded the time when NWA 11042 initially crystallized. This age is much younger than when the decay of short‐lived nuclides (e.g., 26Al) would act as a major heat source, suggesting melting and crystallization of NWA 11042 could be otherwise triggered by an impact event. The lower intercept age represents a reset age due to a later impact event, that is in coincidence with the disruption event of L chondrite parent body at ~470 Myr. NWA 11042 is an excellent example to link igneous‐textured meteorites with a chondritic parent body through shock‐induced melting.

Description: The mobility of landslides on Mars is studied based on a database of 3,118 events. To establish the volume of the landslides for the whole dataset based on the deposit area, a new volume-area relationship based on a representative dataset of 222 landslides is used. Plotting the H/L ratio between fall height H and runout L versus volume, landslide mobility is analyzed and compared with existing empirical relationships for Martian and terrestrial landslides. Analyzing the mobility in terms of normalized residuals, i.e., the relative deviation of the H/L ratio from the dataset best-fit line, mobility is found to depend on both the landslide location on Mars, and landslide typology. This allows us to identify four different types of high mobility (hypermobile) landslides. Three classes of high mobility landslides are associated respectively to meteoroid impact, the Olympus Mons aureoles, and landslides with Toreva-block failure style, and their mobility can be explained by the peculiar flow mechanics. The fourth class includes landslides associated with isolated craters, those in the regions wetted by the putative Oceanus Borealis, and the ones at high latitudes. We suggest that the common factor behind all the hypermobile landslides of this fourth kind is the presence of ice. This is confirmed by our data showing that landslides increase in mobility with latitude. The latitudinal trend mirrors the distribution of ice as detected by radar, neutron probes, and the presence of glacial and layered ejecta morphologies. Because the overall landslide distribution supports the presence of ice-lubricated conditions, two ice lubrication models are presented showing how ice melting within or underneath the landslides could enhance mobility. By proper analysis in terms of apparent friction residuals, we find that the mobility of landslides in Valles Marineris with the largest landslide concentration is lower than average. We explain this circumstance partly from the smaller role of ice in equatorial Valles Marineris, and partly because the collapses from high slope relief imply high-speed impact with the floor valley confinement, loss of momentum, and decrease in mobility. Environmental consequences imply that the present subsurface ice distribution may have been persistent throughout the Amazonian period.

Description: The lunar cratering record provides valuable information about the late accretion history of the inner Solar System. However, our understanding of the origin, rate, and timing of the impacting projectiles is far from complete. To learn more about these projectiles, we can examine crater size-frequency distributions (CSFDs) on the Moon. Here, we re-investigate the crater populations of 30 lunar basins (≥ 300 km) using the buffered non-sparseness correction (BNSC) technique, which takes crater obliteration into account, thus providing more accurate measurements for the frequencies of smaller crater sizes. Moreover, we revisit the stratigraphic relationships of basins based on N(20) crater frequencies, absolute model ages, and observation data. The BNSC-corrected CSFDs of individual basins, particularly at smaller crater diameters are shifted upwards. Contrary to previous studies, the shapes of the summed CSFDs of Pre-Nectarian (excluding South Pole-Aitken Basin), Nectarian (including Nectaris) and Imbrian (including Imbrium) basins show no statistically significant differences, and thus provide no evidence for a change of impactor population.

Description: Aeolian processes have likely been the predominant geomorphic agent for most of Mars’ history and have the potential to produce relatively young exposure ages for geologic units. Thus, identifying local evidence for aeolian erosion is highly relevant to the selection of landing sites for future missions, such as the Mars 2020 Rover mission that aims to explore astrobiologically-relevant ancient environments. Here, we investigate wind-driven activity at eight Mars 2020 candidate-landing sites to constrain erosion potential at these locations. To demonstrate our methods, we found contemporary dune-derived abrasion rates were in agreement with rover-derived exhumation rates at Gale crater and could be employed elsewhere. The Holden crater candidate site was interpreted to have low contemporary erosion rates, based on the presence of a thick sand coverage of static ripples. Active ripples at the Eberswalde and southwest Melas sites may account for local erosion and the dearth of small craters. Moderate-flux regional dunes near Mawrth Vallis were deemed unrepresentative of the candidate site, which is interpreted to currently be experiencing low levels of erosion. The Nili Fossae site displayed the most unambiguous evidence for local sand transport and erosion, likely yielding relatively young exposure ages. The down-selected Jezero crater and northeast Syrtis sites had high-flux neighboring dunes and exhibited substantial evidence for sediment pathways across their ellipses. Both sites had relatively high estimated abrasion rates, which would yield young exposure ages. The down-selected Columbia Hills site lacked evidence for sand movement and contemporary local erosion rates are estimated to be relatively low.

Description: A simplified model setup has been used in atmospheric general circulation models (AGCMs), to clarify the fluid dynamical process of terrestrial planets. In the present work, the research aim is to ascertain the dynamical effects of polar indirect circulation on superrotation and multiple equilibrium states in Venus-like planets. The model setup previously used for Venus AGCM intercomparison is applied to the Model for Interdisciplinary Research On Climate (MIROC) AGCM, and the horizontal resolution and initial conditions are altered in the long-term experiments. The structures of general circulation and planetary-scale waves in the T42 (Truncation wavenumber 42) experiment are similar to those in the T63 experiment. In the presence of the polar indirect circulation, the superrotational flow weakens in the cloud layer and its momentum is transported toward the lower atmosphere at high latitudes. In contrast, in the T21 experiment, because the polar indirect circulation is not fully resolved, the vertical momentum transport due to the indirect circulation is ineffective in the lower atmosphere, and thus the cloud-top superrotational flow becomes greater than those in the higher-resolution experiments. The multiple equilibrium states caused by different initial zonal flows appear in the T21 experiments, although they are not seen in the experiments of T42 and higher. Thus, the polar indirect circulation in the Gierasch–Rossow–Williams mechanism weakens the superrotational flow in the cloud layer and breaks the steady-state multiplicity of the general circulation.

Description: There is strong evidence that Mercury's polar deposits are water ice hosted in permanently shadowed regions. In this study, we present new Arecibo radar observations of Mercury's south pole, which reveal numerous radar-bright deposits and substantially increase the radar imaging coverage. We also use images from MESSENGER's full mission to determine the illumination conditions of Mercury's south polar region at the same spatial resolution as the north polar region, enabling comparisons between the two poles. The area of radar-bright deposits in Mercury's south is roughly double that found in the north, consistent with the larger permanently shadowed area in the older, cratered terrain at the south relative to the younger smooth plains at the north. Radar-bright features are strongly associated with regions of permanent shadow at both poles, consistent with water ice being the dominant component of the deposits. However, both of Mercury's polar regions show that roughly 50% of permanently shadowed regions lack radar-bright deposits, despite some of these locations having thermal environments that are conducive to the presence of water ice. The observed uneven distribution of water ice among Mercury's polar cold traps may suggest that the source of Mercury's water ice was not a steady, regular process but rather that the source was an episodic event, such as a recent, large impact on the innermost planet.

Description: The Mars Exploration Rover Opportunity (MER-B) has been exploring the surface of Mars since landing in 2004. Its Alpha Particle X-ray Spectrometer (APXS) is primarily used to interrogate the chemical composition of rocks and soil samples in situ. Additionally, the APXS has measured the atmosphere of Mars with a regular cadence, monitoring the change in relative atmospheric argon density. Atmospheric measurements with the MER-B APXS span over six Mars years providing an unprecedented level of statistics for careful study of the ubiquitous APXS spectral background. Several models were applied to high-frequency long-duration Spirit rover atmospheric APXS measurements. The most-stable model with the least uncertainty was applied to the MER-B data set. Seasonal variation of 10-15% in equatorial atmospheric argon density was observed-in agreement with existing literature and global climate models (GCMs). Unseen in previous work and GCMs, an abrupt deviation from the model-predicted annual mixing ratio was measured by the MER-B APXS around L s 150. The sharp change, ~10% over 10 o L s , provides strong evidence for a northward migrating front, enriched in argon, sourced from the south pole at the end of southern winter. A similar weaker front is possibly observed around L s 325, sourced from the northern polar region.

Description: Basaltic volcanism is one of the most important geologic processes of the Moon. Research on the thickness and volume of late-stage basalts of Mare Imbrium helps better understand the source of lunar volcanism and eruption styles. Based on whether apparent flow fronts exist or not, the late-stage basalts within Mare Imbrium were divided into two groups, namely, Upper Eratosthenian basalts (UEm) and Lower Eratosthenian basalts (LEm). Employing the topographic profile analysis method for UEm and the crater excavation technique for LEm, we studied the thickness and distribution of Eratosthenian basalts in Mare Imbrium. For the UEm units, their thicknesses were estimated to be ~16–34 (±2) m with several layers of individual lava (~8–13 m) inside. The estimated thickness of LEm units was ~14–45(±1) m, with a trend of reducing thickness from north to south. The measured thickness of late-stage basalts around the Chang'E-3 landing site (~37±1 m) was quite close to the results acquired by the lunar penetrating radar carried onboard the Yutu Rover (~35 m). The total volume of the late-stage basalts in Mare Imbrium was calculated to be ~8671 (±320) km 3 , which is four times lower than that of Schaber's estimation (~4 × 10 4 km 3 ). Our results indicate that the actual volume is much lower than previous estimates of the final stage of the late basaltic eruption of Mare Imbrium. Together, the area flux and transport distance of the lava flows gradually decreased with time. These results suggest that late-stage volcanic evolution of the Moon might be revised.

Description: Recent measurements by the Imaging Ultraviolet Spectrograph (IUVS) instrument on NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission show that a persistent layer of Mg + ions occurs around 90 km in the Martian atmosphere, but that neutral Mg atoms are not detectable. These observations can be satisfactorily modeled with a global ablation rate of 0.06 metric tonnes of magnesium sol -1 , out of a cosmic dust input of 2.7 ± 1.6 t sol -1 . The absence of detectable Mg at 90 km requires that at least 50% of the ablating Mg atoms ionize through hyperthermal collisions with CO 2 molecules. Dissociative recombination of MgO + .(CO 2 ) n cluster ions with electrons to produce MgCO 3 directly, rather than MgO, also avoids a buildup of Mg to detectable levels. The meteoric injection rate of Mg, Fe and other metals – constrained by the IUVS measurements - enables the production rate of metal carbonate molecules (principally MgCO 3 and FeCO 3 ) to be determined. These molecules have very large electric dipole moments (11.6 and 9.2 Debye, respectively), and thus form clusters with up to 6 H 2 O molecules at temperatures below 150 K. These clusters should then coagulate efficiently, building up metal carbonate-rich ice particles which can act as nucleating particles for the formation of CO 2 -ice clouds. Observable mesospheric clouds are predicted to occur between 65 and 80 km at temperatures below 95 K, and above 85 km at temperatures about 5 K colder.

Description: Fatigue can cause materials that undergo cyclic loading to experience brittle failure at much lower stresses than under monotonic loading. We propose that the lithospheres of icy satellites could become fatigued and thus weakened by cyclical tidal stresses. To test this hypothesis, we performed a series of laboratory experiments to measure the fatigue of water ice at temperatures of 198 K and 233 K and at a loading frequency of 1 Hz. We find that ice is not susceptible to fatigue at our experimental conditions and that the brittle failure stress does not decrease with increasing number of loading cycles. Even though fatigue was not observed at our experimental conditions, colder temperatures, lower loading frequencies, and impurities in the ice shells of icy satellites may increase the likelihood of fatigue crack growth. We also explore other mechanisms that may explain the weak behavior of the lithospheres of some icy satellites.

Description: Impact cratering is likely a primary agent of regolith generation on airless bodies. Regolith production via impact cratering has long been a key topic of study since the Apollo era. The evolution of regolith due to impact cratering, however, is not well understood. A better formulation is needed to help quantify the formation mechanism and timescale of regolith evolution. Here, we propose an analytically derived stochastic model that describes the evolution of regolith generated by small, simple craters. We account for ejecta blanketing as well as regolith infilling of the transient crater cavity. Our results show that the regolith infilling plays a key role in producing regolith. Our model demonstrates that, because of the stochastic nature of impact cratering, the regolith thickness varies laterally, which is consistent with earlier work. We apply this analytical model to the regolith evolution at the Apollo 15 site. The regolith thickness is computed considering the observed crater size-frequency distribution of small, simple lunar craters (〈 381 m in radius for ejecta blanketing and 〈 100 m in radius for the regolith infilling). Allowing for some amount of regolith coming from the outside of the area, our result is consistent with an empirical result from the Apollo 15 seismic experiment. Finally, we find that the timescale of regolith growth is longer than that of crater equilibrium, implying that even if crater equilibrium is observed on a cratered surface, it is likely the regolith thickness is still evolving due to additional impact craters.

Description: ABSTRACT The lunar highlands are isostatically compensated at large horizontal scales, but the specific compensation mechanism has been difficult to identify. With topographic data from the Lunar Orbiter Laser Altimeter and gravity data from the Gravity Recovery and Interior Laboratory, we investigate support of highland topography. Poor correlation between crustal density and elevation shows that Pratt compensation is not important in the highlands. Using spectrally weighted admittance, we compared observed values of geoid-to-topography ratio (GTR) with those predicted by isostatic models. Observed GTRs are 25.8 –5.7 +7.5 m/km for the nearside highlands and 39.3 –6.2 +5.7 m/km for the farside highlands. These values are not consistent with flexural compensation of long-wavelength topography or Airy isostasy defined under an assumption of equal mass in crustal columns. Instead, the observed GTR values are consistent with models of Airy compensation in which isostasy is defined under a requirement of equal pressures at equipotential surfaces at depth. The gravity and topography data thus reveal that long-wavelength topography on the Moon is most likely compensated by variations in crustal thickness, implying that highland topography formed early in lunar history before the development of a thick elastic lithosphere.

Description: Impact craters on Mars have been extensively modified by ancient geologic processes that may have included rainfall and surface runoff, snow and ice, denudation by lava flows, burial by aeolian material, or others. Many of these processes can leave distinct signatures on the morphometry of the modified impact crater as well as the surrounding landscape. To look for signs of potential regional differences in crater modification processes, we conducted an analysis of different morphometric parameters related to modified impact craters located in the Margaritifer Sinus, Sinus Sabaeus, Iapygia, Mare Tyrrhenum, Aeolis, and Eridania quadrangles, including depth, crater wall slope, crater floor slope, the curvature between the interior wall and the crater floor slope, and the curvature between the interior wall and surrounding landscape. A Welch's t-test analysis comparing these parameters shows that fresh impact craters (Type 4) have consistent morphologies regardless of their geographic location examined in this study, which is not unexpected. Modified impact craters both in the initial (Type 3) and terminal stages (Type 1) of modification also have statistically consistent morphologies. This would suggest that the processes that operated in the late Noachian were globally ubiquitous, and that modified craters eventually reached a stable crater morphology. However, craters preserved in advanced (but not terminal) stages of modification (Type 2) have morphopholgies that vary across the quadrangles. It is possible that these variations reflect spatial differences in the types and intensity of geologic processes that operated during the Noachian, implying that the ancient climate also varied across regions.

Description: No abstract is available for this article.

Description: We report the results of geological studies by the Opportunity Mars rover on the Endeavour Crater rim. Four major units occur in the region (oldest to youngest): the Matijevic, Shoemaker, Grasberg and Burns formations. The Matijevic formation, consisting of fine-grained clastic sediments, is the only pre-Endeavour-impact unit and might be part of the Noachian etched units of Meridiani Planum. The Shoemaker formation is a heterogeneous polymict impact breccia; its lowermost member incorporates material eroded from the underlying Matijevic formation. The Shoemaker formation is a close analog to the Bunte Breccia of the Ries Crater, although the average clast sizes are substantially larger in the latter. The Grasberg formation is a thin, fine-grained, homogeneous sediment unconformably overlying the Shoemaker formation, and likely formed as an airfall deposit of unknown areal extent. The Burns formation sandstone overlies the Grasberg, but compositions of the two units are distinct; there is no evidence that the Grasberg formation is a fine-grained subfacies of the Burns formation. The rocks along the Endeavour Crater rim were affected by at least four episodes of alteration in the Noachian and Early Hesperian: (i) vein formation and alteration of pre-impact Matijevic formation rocks; (ii) low-water/rock alteration along the disconformity between the Matijevic and Shoemaker formations; (iii) alteration of the Shoemaker formation along fracture zones; and (iv) differential mobilization of Fe and Mn, and CaSO 4 -vein formation in the Grasberg and Shoemaker formations. Episodes (ii) and (iii) possibly occurred together, but (i) and (iv) are distinct from either of these.

Description: Using Moon Mineralogy Mapper data, we characterize surface diversity across the enormous South Pole - Aitken Basin (SPA) by evaluating the abundance and composition of pyroxenes, which are overwhelmingly the most abundant mafic mineral in the region. Although SPA exhibits significant complexity due to billions of years of geologic processes subsequent to formation, the basin has retained regular patterns of compositional heterogeneity across its structure. Four distinct, roughly concentric zones are defined: (1) a central SPA compositional anomaly (SPACA), which exhibits a pervasive elevated Ca,Fe-rich pyroxene abundance, (2) a Mg-Pyroxene Annulus, which is dominated by abundant Mg-rich pyroxenes, (3) a Heterogeneous Annulus, which exhibits localized pyroxene-rich areas spatially mixed with feldspathic materials, and (4) the SPA Exterior, which is primarily feldspathic. Pyroxene compositions in the Heterogeneous Annulus are similar to those in the Mg-Pyroxene Annulus, and Mg-rich pyroxenes also underlie the more Ca,Fe-rich pyroxene surface material across SPACA. The establishment of these four distinct compositional zones across SPA constrains future basin evolution models serves to guide potential sample return (and other science) targets.

Description: Post-Noachian Martian paleochannels indicate the existence of liquid water on the surface of Mars after about 3.5 Gya [ Irwin et al. , 2015; Palucis et al. , 2016]. In order to explore the effects of variations in CO 2 partial pressure and obliquity on the possibility of surface water, we created a zero-dimensional surface energy balance model. We combine this model with physically consistent orbital histories to track conditions over the last 3.5 Gyr of Martian history. We find that melting is allowed for atmospheric pressures corresponding to exponential loss rates of or faster, but this rate is within 0.5 σ of the rate calculated from initial measurements made by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, if we assume all the escaping oxygen measured by MAVEN comes from atmospheric CO 2 [ Lillis et al. , 2017; Tu et al. , 2015]. Melting at this loss rate matches selected key geologic constraints on the formation of Hesperian river networks, assuming optimal melt conditions during the warmest part of each Mars year [ Irwin et al. , 2015; Stopar et al. , 2006; Kite et al. , 2017a,b].. The atmospheric pressure has a larger effect on the surface energy than changes in Mars's mean obliquity. These results show that initial measurements of atmosphere loss by MAVEN are consistent with atmospheric loss being the dominant process that switched Mars from a melt-permitting to a melt-absent climate [ Jakosky et al. , 2017], but non-CO 2 warming will be required if 〈2 Gya paleochannels are confirmed, or if most of the escaping oxygen measured by MAVEN comes from H 2 O.

Description: Thermal tides in the Venus atmosphere are investigated by using a GCM named as AFES-Venus. The three-dimensional structures of wind and temperature associated with the thermal tides obtained in our model are fully examined and compared with observations. The result shows that the wind and temperature distributions of the thermal tides depend complexly on latitude and altitude in the cloud layer, mainly because they consist of vertically propagating and trapped modes with zonal wavenumbers of 1–4, each of which predominates in different latitudes and altitudes under the influence of mid- and high-latitude jets. A strong circulation between the subsolar and antisolar (SS-AS) points, which is equivalent to a diurnal component of the thermal tides, is superposed on the superrotation. The vertical velocity of SS-AS circulation is about 10 times larger than that of the zonal-mean meridional circulation (ZMMC) in 60–70 km altitudes. It is suggested that the SS-AS circulation could contribute to the material transport, and its upward motion might be related to the UV dark region observed in the subsolar and early afternoon regions in low-latitudes. The terdiurnal and quaterdiurnal tides, which may be excited by the nonlinear interactions among the diurnal and semidiurnal tides in mid- and high-latitudes, are detected in the solar-fixed Y-shape structure formed in the vertical wind field in the upper cloud layer. The ZMMC is weak and has a complex structure in the cloud layer; the Hadley circulation is confined to latitudes equatorward of 30 ∘ , and the Ferrel-like one appears in mid- and high-latitudes.

Description: One of the most intriguing, long-standing questions regarding Venus' atmosphere is the origin and distribution of the unknown UV-absorber, responsible for the absorption band detected at the near-UV and blue range of Venus' spectrum. In this work, we use data collected by MASCS spectrograph on board the MESSENGER mission during its second Venus flyby in June 2007 to address this issue. Spectra range from 0.3 μ m to 1.5 μ m including some gaseous H 2 O and CO 2 bands, as well as part of the SO 2 absorption band and the core of the UV absorption. We used the NEMESIS radiative transfer code and retrieval suite to investigate the vertical distribution of particles in the Equatorial atmosphere and to retrieve the imaginary refractive indices of the UV-absorber, assumed to be well mixed with Venus' small mode-1 particles. The results show an homogeneous Equatorial atmosphere, with cloud tops (height for unity optical depth) at 75±2 km above surface. The UV absorption is found to be centered at 0.34±0.03  μ m with a full width half maximum of 0.14±0.01 μ m. Our values are compared with previous candidates for the UV aerosol absorber, among which disulfur oxide (S 2 O) and dioxide disulfur (S 2 O 2 ) provide the best agreement with our results.

Description: The localized loss of near-surface excess ice on Mars by sublimation (and perhaps melting) can produce thermokarstic collapse features such as expanded craters and scalloped depressions, which can be indicators of the preservation of shallow ice. We demonstrate this by identifying HiRISE images containing expanded craters south of Arcadia Planitia (25–40°N), and observe a spatial correlation between regions with thermokarst and the lowest-latitude ice-exposing impact craters identified to date. In addition to widespread thermokarst north of ~35°N, we observe localized thermokarst features that we interpret as patchy ice as far south as 25°N. Few ice-exposing craters have been identified in the southern hemisphere of Mars since they are easier to find in dusty, high-albedo regions, but the relationship among expanded craters, ice-exposing impacts, and the predicted ice table boundary in Arcadia Planitia allows us to extend this thermokarst survey into the southern mid-latitudes (30–60°S) to infer the presence of ice today. Our observations suggest that the southern hemisphere excess ice boundary lies at ~45°S regionally. At lower latitudes, some isolated terrains (e.g., crater fill, pole-facing slopes) also contain thermokarst, suggesting local ice preservation. We look for spatial relationships between our results and surface properties (e.g., slope, neutron spectrometer water ice concentration) and ice table models to understand the observed ice distribution. Our results show trends with thermal inertia and dust cover, and are broadly consistent with ice deposition during a period with a higher relative humidity than today. Shallow, lower-latitude ice deposits are of interest for future exploration.

Description: Isolated landscapes largely shaped by aeolian processes can occur on Earth, while the majority of Mars’ recent history has been dominated by wind-driven activity. Resultantly, Martian landscapes often exhibit large-scale aeolian features, including yardang landforms carved from sedimentary-layered deposits. High-resolution orbital monitoring has revealed that persistent bedform activity is occurring with dune and ripple migration implying ongoing abrasion of the surface. However, little is known about the interaction between dunes and the topography surrounding them. Here we explore dune-yardang interactions in Becquerel crater in an effort to better understand local landscape evolution. Dunes there occur on the north and south sides of a 700-m-tall sedimentary deposit, which displays numerous superposed yardangs. Dune and yardang orientations are congruent suggesting they both were formed under a predominantly northerly wind regime. Migration rates and sediment fluxes decrease as dunes approach the deposit and begin to increase again downwind of the deposit where the effect of topographic sheltering decreases. Estimated sand abrasion rates (16-40 μm yr -1 ) would yield a formation time of 1.8-4.5 million years for the 70-m-deep yardangs. This evidence for local aeolian abrasion also helps explain the young exposure ages of deposit surfaces, as estimated by the crater size-frequency distribution. Comparisons to terrestrial dune activity and yardang development begin to place constraints on yardang formation times for both Earth and Mars. These results provide insight into the complexities of sediment transport on uneven terrain and are compelling examples of contemporary aeolian-driven landscape evolution on Mars.

Description: Boulder halos are circular arrangements of clasts present at martian middle to high latitudes. Boulder halos are thought to result from impacts into a boulder-poor surficial unit that is rich in ground-ice and/or sediments and that is underlain by a competent substrate. In this model, boulders are excavated by impacts and remain at the surface as the crater degrades. To determine the distribution of boulder halos and to evaluate mechanisms for their formation, we searched for boulder halos over 4,188 HiRISE images located between ~50-80° north and 50-80° south latitude. We evaluate geological and climatological parameters at halo sites. Boulder halos are about three times more common in the northern hemisphere than in the southern (19% vs. 6% of images), and have size-frequency distributions suggesting recent Amazonian formation (tens to hundreds of millions of years). In the north, boulder halo sites are characterized by abundant shallow subsurface ice and high thermal inertia. Spatial patterns of halo distribution indicate that excavation of boulders from beneath non-boulder-bearing substrates is necessary for the formation of boulder halos, but that alone is not sufficient. Rather, surface processes either promote boulder halo preservation in the north or destroy boulder halos in the south. Notably, boulder halos predate the most recent period of near-surface ice emplacement on Mars and persist at the surface atop mobile regolith. The lifetime of observed boulders at the martian surface is greater than the lifetime of the craters that excavated them. Finally, larger minimum boulder halo sizes in the north indicate thicker icy soil layers on average throughout climate variations driven by spin/orbit changes during the last 10s to 100s Ma.

Description: Thermal infrared spectra acquired by Cassini/CIRS in limb viewing geometry in 2015 are used to derive 2-D latitude-pressure temperature and thermal wind maps. These maps are used to study the vertical structure and evolution of Saturn's equatorial oscillation (SEO), a dynamical phenomenon presenting similarities with the Earth's Quasi-Biennal Oscillation (QBO) and Semi-Annual Oscillation (SAO). We report that a new local wind maximum has appeared in 2015 in the upper stratosphere, and derive the descent rates of other wind extrema through time. The phase of the oscillation observed in 2015, as compared to 2005 and 2010, remains consistent with a ∼15-year period. The SEO does not propagate downward at a regular rate, but exhibits faster descent rate in the upper stratosphere, combined with a greater vertical wind shear, compared to the lower stratosphere. Within the framework of a QBO-type oscillation, we estimate the absorbed wave momentum flux in the stratosphere to be on the order of ∼7 × 10 −6 N m −2 . On Earth, interactions between vertically propagating waves (both planetary and mesoscale) and the mean zonal flow drive the QBO and SAO. To broaden our knowledge on waves potentially driving Saturn's equatorial oscillation, we searched for thermal signatures of planetary waves in the tropical stratosphere using CIRS nadir spectra. Temperature anomalies of amplitude 1-4 K and zonal wavenumbers 1 to 9 are frequently observed and an equatorial Rossby (n=1) wave of zonal wavenumber 3 is tentatively identified in November, 2009.

Description: Signatures of lithospheric flexure were previously identified at a dozen or more large coronae on Venus. Thin plate models fit to topographic profiles return elastic parameters, allowing derivation of mechanical thickness and surface heat flows given an assumed yield strength envelope. However, the low resolution of altimetry data from the NASA Magellan mission has hindered studying the vast majority of coronae, particularly those less than a few hundred kilometers in diameter. Here we search for flexural signatures around 99 coronae over ∼20% of the surface in Magellan altimetry data and stereo-derived topography that was recently assembled from synthetic aperture radar images. We derive elastic thicknesses of ∼2 to 30 km (mostly ∼5 to 15 km) with Cartesian and axisymmetric models at 19 coronae. We discuss the implications of low values that were also noted in earlier gravity studies. Most mechanical thicknesses are estimated as 〈19 km, corresponding to thermal gradients 〉24 K km −1 . Implied surface heat flows 〉95 mW m −2 —twice the global average in many thermal evolution models—imply that coronae are major contributors to the total heat budget or Venus is cooling faster than expected. Binomial statistics show that “Type 2" coronae with incomplete fracture annuli are significantly less likely to host flexural signatures than “Type 1" coronae with largely complete annuli. Stress calculations predict extensional faulting where nearly all profiles intersect concentric fractures. We failed to identify systematic variations in flexural parameters based on type, geologic setting, or morphologic class. Obtaining quality, high-resolution topography from a planet-wide survey is vital to verifying our conclusions.

Description: No abstract is available for this article.

Description: We investigate Titan's low- and mid-latitude surface using spectro-imaging near-infrared data from Cassini/VIMS. We use a radiative transfer code to first evaluate atmospheric contributions and then extract the haze and the surface albedo values of major geomorphological units identified in Cassini Synthetic Aperture Radar data, which exhibit quite similar spectral response to the VIMS data. We have identified three main categories of albedo values and spectral shapes, indicating significant differences in the composition among the various areas. We compare with linear mixtures of three components (water ice, tholin-like, and a dark material) at different grain sizes. Due to the limited spectral information available, we use a simplified model, with which we find that each albedo category of regions of interest can be approximately fitted with simulations composed essentially by one of the three surface candidates. Our fits of the data are overall successful, except in some cases at 0.94, 2.03, and 2.79 μm, indicative of the limitations of our simplistic compositional model and the need for additional components to reproduce Titan's complex surface. Our results show a latitudinal dependence of Titan's surface composition, with water ice being the major constituent at latitudes beyond 30°N and 30°S, while Titan's equatorial region appears to be dominated partly by a tholin-like or by a very dark unknown material. The albedo differences and similarities among the various geomorphological units give insights on the geological processes affecting Titan's surface and, by implication, its interior. We discuss our results in terms of origin and evolution theories.

Description: Ancient impact craters with wind-eroded layering on their floors provide a record of resurfacing materials and processes on early Mars. In a 54 km Noachian crater in Terra Sabaea (20.2°S, 42.6°E), aeolian deflation of a friable, dark-toned layer up to tens of meters thick has exposed more resistant, underlying light-toned material. These layers differ significantly from strata of similar tone described in other regions of Mars. The light-toned material has no apparent internal stratification, and visible/near-infrared spectral analysis suggests that it is rich in feldspar. Its origin is ambiguous, as we cannot confidently reject igneous, pyroclastic, or clastic alternatives. The overlying dark-toned layer is probably a basaltic siltstone or sandstone that was emplaced mostly by wind, although its weak cementation and inverted fluvial paleochannels indicate some modification by water. Negative-relief channels are not found on the crater floor, and fluvial erosion is otherwise weakly expressed in the study area. Small impacts onto this crater's floor have exposed deeper friable materials that appear to contain goethite. Bedrock outcrops on the crater walls are phyllosilicate-bearing. The intercrater plains contain remnants of a post-Noachian thin, widespread, likely aeolian mantle with an indurated surface. Plains near Hellas-concentric escarpments to the north are more consistent with volcanic resurfacing. A 48 km crater nearby contains similar dark-over-light outcrops but no paleochannels. Our findings indicate that dark-over-light stratigraphy has diverse origins across Mars and that some dark-toned plains with mafic mineralogy are not of igneous origin.

Description: Science operates best by sharing accurate new knowledge in clear ways. In order to check our assumptions, our methods, and our interpretations of the observations, experiments, analyses, and calculations that we do, we ask others to look at our work. We call this peer review – other experts who were not involved in a given study read and critically evaluate the descriptions of our work. They look for completeness, accuracy, whether work is new, and how clearly we have written the descriptions. We continue to be humbled by the time, effort, and careful insights that our colleagues share with each other through the process of peer review. In 2017, JGR Planets benefited from more than 610 reviews provided by 398 of our peers for papers submitted to the journal. Thank you all for your awesome efforts toward advancing planetary science now and for the future.

Description: Liquid water is one of the most important materials affecting the climate and habitability of a terrestrial planet. Liquid water vaporizes entirely when planets receive insolation above a certain critical value, which is called the runaway greenhouse threshold. This threshold forms the inner most limit of the habitable zone. Here, we investigate the effects of the distribution of surface water on the runaway greenhouse threshold for Earth-sized planets using a three-dimensional dynamic atmosphere model. We considered a 1-bar atmosphere whose composition is similar to the current Earth's atmosphere with a zonally uniform distribution of surface water. As previous studies have already showed, we also recognized two climate regimes: the land planet regime, which has dry low latitude and wet high latitude regions, and the aqua planet regime, which is globally wet. We showed that each regime is controlled by the width of the Hadley circulation, the amount of surface water, and the planetary topography. We found that the runaway greenhouse threshold varies continuously with the surface water distribution from about 130% (an aqua planet) to 180% (the extreme case of a land planet) of the present insolation at Earth's orbit. Our results indicate that the inner edge of the habitable zone is not a single sharp boundary, but a border whose location varies depending on planetary surface condition, such as the amount of surface water. Since land planets have wider habitable zones and less cloud cover, land planets would be good targets for future observations investigating planetary habitability.

Description: We present a new implementation of the hydrological cycle scheme into a general circulation model of the Martian atmosphere. The model includes a semi-Lagrangian transport scheme for water vapor and ice, and accounts for microphysics of phase transitions between them. The hydrological scheme includes processes of saturation, nucleation, particle growth, sublimation and sedimentation under the assumption of a variable size distribution. The scheme has been implemented into the Max Planck Institute Martian general circulation model (MPIŰMGCM) and tested assuming mono- and bimodal lognormal distributions of ice condensation nuclei. We present a comparison of the simulated annual variations, horizontal and vertical distributions of water vapor and ice clouds with the available observations from instruments onboard Mars orbiters. The accounting for bi-modality of aerosol particle distribution improves the simulations of the annual hydrological cycle, including predicted ice clouds mass, opacity, number density, particle radii. The increased number density and lower nucleation rates brings the simulated cloud opacities closer to observations. Simulations show a weak effect of the excess of small aerosol particles on the simulated water vapor distributions.

Description: A new modeling-based study by Johnson et al. (2017) lends support to the hypothesis that portions of Europa's surface may have been removed by the process of subduction, as suggested by Kattenhorn and Prockter (2014). Using a simple 1D model that tracks the thermal and density structure of a descending ice plate, Johnson et al. show that ice plates with 10% porosity and overall salt contents of ~5%, but which differ in salt content by ~2.5% from the surrounding reference ice shell, are non-buoyant and thus likely to sink through the underlying, convecting portion of the ice shell. The feasibility of subduction in an ice shell is critical to the existence of icy plate tectonics, which is hypothesized to exist at least locally on Europa, potentially making it the only other Solar System body other than Earth with a surface modified by plate tectonics.

Description: Estimates of the Martian elastic lithosphere thickness suggest small values of ∼25 km during the Noachian for the Southern hemisphere and a large present-day difference below the two polar caps (≥300 km in the North and 〉110 km in the South). In addition, young lava flows suggest that Mars has been volcanically active up to the recent past. We run Monte Carlo simulations using a 1D parametrized thermal evolution model to investigate whether a North/South hemispheric dichotomy in crustal properties and composition can explain these constraints. Our results suggest that 55 − 65% of the bulk radioelement content are in the crust, and most of it (43 − 51%) in the Southern one. The Southern crust can be up to 480 kg/m 3 less dense than the Northern one and might contain a non-negligible proportion of felsic rocks. Our models predict a dry mantle and a wet or dry crustal rheology today. This is consistent with a mantle depleted in radioelements and volatiles. We retrieve North/South surface heat flux of 17.1 − 19.5 mW/m 2 and 24.8 − 26.5 mW/m 2 , respectively, and a large difference in lithospheric temperatures between the two hemispheres (170 − 304 K in the shallow mantle). This difference could leave a signature in the seismic signals measured by the future InSight mission.

Description: In 2005, the complex permittivity of the surface of Saturn's moon Titan was measured by the PWA-MIP/HASI (Permittivity Wave Altimetry-Mutual Impedance Probe/Huygens Atmospheric Structure Instrument) experiment on board the Huygens probe. The analysis of these measurements was recently refined but could not be interpreted in terms of composition due to the lack of knowledge on the low-frequency/low-temperature electrical properties of Titan's organic material, a likely key ingredient of the surface composition. In order to fill that gap, we developed a dedicated measurement bench and investigated the complex permittivity of analogs of Titan's organic aerosols called "tholins". These laboratory measurements, together with those performed in the microwave domain, are then used to derive constraints on the composition of Titan's first meter below the surface based on both the PWA-MIP/HASI and the Cassini Radar observations. Assuming a ternary mixture of water-ice, tholin-like dust and pores (filled or not with liquid methane), we find that at least 10% of water ice and 15% of porosity are required to explain observations. On the other hand, there should be at most 50-60% of organic dust. PWA-MIP/HASI measurements also suggest the presence of a thin conductive superficial layer at the Huygens landing site. Using accurate numerical simulations, we put constraints on the electrical conductivity of this layer as a function of its thickness (e.g., in the range 7-40 nS/m for a 7-mm thick layer). Potential candidates for the composition of this layer are discussed.

Description: Major impact events have shaped the Earth as we know it. The Late Heavy Bombardment is of particular interest because it immediately precedes the first evidence of life. The reentry of impact ejecta creates numerous chemical byproducts, including biotic precursors such as HCN. This work examines the production of HCN during the Late Heavy Bombardment in more detail. We stochastically simulate the range of impacts on the early Earth, and use models developed from existing studies to predict the corresponding ejecta properties. Using multi-phase flow methods and finite rate equilibrium chemistry, we then find the HCN production due to the resulting atmospheric heating. We use DSMC to develop a correction factor to account for increased yields due to thermochemical nonequilibrium. We then model 1D atmospheric turbulent diffusion to find the time-accurate transport of HCN to lower altitudes and ultimately surface water. Existing works estimate the necessary HCN molarity threshold to promote polymerization is 0.01 M. For a mixing depth of 100 m, we find that the Late Heavy Bombardment will produce at least 1 impact event above this threshold with probability 24.1% for an oxidized atmosphere and 56.3% for a partially reduced atmosphere. For a mixing depth of 10 m, the probability is 79.5% for an oxidized atmosphere and 96.9% for a partially reduced atmosphere. Therefore, LHB impact ejecta is likely an HCN source sufficient for polymerization in shallow bodies of water, particularly if the atmosphere were in a partially reduced state.

Description: The current version of Laboratoire de Météorologie Dynamique (LMD) Mars GCM (original-MGCM) uses annually repeating (prescribed) CO 2 snow albedo values based on the Thermal Emission Spectrometer observations. We integrate the Snow, Ice, and Aerosol Radiation (SNICAR) model with MGCM (SNICAR-MGCM) to prognostically determine H 2 O and CO 2 snow albedos interactively in the model. Using the new diagnostic capabilities of this model, we find that cryospheric surfaces (with dust) increase the global surface albedo of Mars by 0.022. Over snow-covered regions, SNICAR-MGCM simulates mean albedo that is higher by about 0.034 than prescribed values in original-MGCM. Globally, shortwave flux into the surface decreases by 1.26 W/m 2 , and net CO 2 snow deposition increases by about 4% with SNICAR-MGCM over one Martian annual cycle as compared to original-MGCM simulations. SNICAR integration reduces the mean global surface temperature, and the surface pressure of Mars by about 0.87% and 2.5% respectively. Changes in albedo also show a similar distribution to dust deposition over the globe. The SNICAR-MGCM model generates albedos with higher sensitivity to surface dust content as compared to original-MGCM. For snow-covered regions, we improve the correlation between albedo and optical depth of dust from -0.91 to -0.97 with SNICAR-MGCM as compared to the original-MGCM. Dust substantially darkens Mars' cryosphere, thereby reducing its impact on the global shortwave energy budget by more than half, relative to the impact of pure snow.

Description: No abstract is available for this article.

Description: The giant impact hypothesis remains the leading theory for lunar origin. However, current models struggle to explain the Moon's composition and isotopic similarity with Earth. Here we present a new lunar origin model. High-energy, high-angular momentum giant impacts can create a post-impact structure that exceeds the corotation limit (CoRoL), which defines the hottest thermal state and angular momentum possible for a corotating body. In a typical super-CoRoL body, traditional definitions of mantle, atmosphere and disk are not appropriate, and the body forms a new type of planetary structure, named a synestia. Using simulations of cooling synestias combined with dynamic, thermodynamic and geochemical calculations, we show that satellite formation from a synestia can produce the main features of our Moon. We find that cooling drives mixing of the structure, and condensation generates moonlets that orbit within the synestia, surrounded by tens of bars of bulk silicate Earth (BSE) vapor. The moonlets and growing moon are heated by the vapor until the first major element (Si) begins to vaporize and buffer the temperature. Moonlets equilibrate with BSE vapor at the temperature of silicate vaporization and the pressure of the structure, establishing the lunar isotopic composition and pattern of moderately volatile elements. Eventually, the cooling synestia recedes within the lunar orbit, terminating the main stage of lunar accretion. Our model shifts the paradigm for lunar origin from specifying a certain impact scenario to achieving a Moon-forming synestia. Giant impacts that produce potential Moon-forming synestias were common at the end of terrestrial planet formation.

Description: The surface of the Moon is exposed to the solar wind and the particles can reach a few average grains deep into the regolith. This work examines the depth of penetration of solar wind particles in lunar regolith using equations derived from analytical, Monte-Carlo, and ray tracing techniques and discrete element method simulations. It is shown that the process can be approximately described using an exponential distribution function starting from some small depth with parameters depending on void ratio, solar wind incidence angle, and regolith grain size distribution. The proton implantation rate into the regolith as a function of depth is proposed with the coefficients computationally derived from Monte-Carlo simulations of proton implantation process.

Description: Mittlefehldt et al. [2018] synthesize APXS chemical measurements along more than 4.5 km of Endeavour crater's rim. Their analyses clarify details of Endeavour's geologic history, including evidence for three to four distinct episodes of aqueous alteration. Fracture driven aqueous systems and Mn mobility are particularly important both here and at Curiosity's landing site on the opposite side of the planet. The detailed documentation of APXS data products within this paper will be a key reference for researchers who want to perform future work on questions related to Mars aqueous geochemistry, impact processes, and Martian crustal and atmospheric evolution.

Description: We update the results of cloud imaging sequences from the Mars Science Laboratory (MSL) rover Curiosity to complete two Mars years of observations ( L S =160 ∘ of MY 31 to L S =160 ∘ of MY 33). Relatively good seasonal coverage is achieved within the study period, with just over 500 observations obtained, averaging one observation every 2-3 sols. Cloud opacity measurements are made using differential photometry and a simplified radiative transfer method. These opacity measurements are used to assess the inter-annual variability of the aphelion cloud belt (ACB) for MY 32 and 33. Upon accounting for a statistical bias in the data set, the variation is found to be 〈30 % within uncertainty. Diurnal variation of the ACB is also able to be examined in MY 33 owing to an increased number of early morning observations in this year. Although a gap in data around local noon prevents a complete assessment, we find that cloud opacity is moderately increased in the morning hours (07:00-09:00) compared to the late afternoon (15:00-17:00).

Description: Spacecraft observations show that weak magnetic fields of crustal origin are ubiquitous across the surface of the Moon. To investigate the origin of these magnetic anomalies, a model was developed for the magnetic power spectrum that consists of ensembles of randomly magnetized sills or prisms. Localized spectrum analyses constrained how the parameters of this model vary with position, including the size of the sources, a quantity proportional to their mean-squared dipole moment, and the depth to the top and bottom of the magnetized region. The depth to the top of the magnetized region varies from the surface to about 25 km. The magnetic carriers in the deep crust likely formed at the same time as the crust itself, implying that a core-generated dynamo field must have existed when the crust was cooling during the first 100 million years of lunar evolution. The parameter related to the strength of magnetization shows the existence of a prominent region on the nearside hemisphere that is largely unmagnetized and that correlates with a region of extremely low surface field strengths. This region lies entirely within a geological province that is highly enriched in heat-producing elements (the Procellarum KREEP Terrane), suggesting that this region escaped being magnetized because of prolonged high crustal temperatures. The nearside magnetic low may be representative of the size of that portion of the crust that is highly enriched in heat producing elements, which is almost one third the size of the Procellarum KREEP Terrane based on surface thorium abundances.

Description: Iron meteorites provide some of the most direct insights into the processes and timescales of core formation in planetesimals. Of these, group IVB irons stand out by having one of the youngest 182 Hf- 182 W model ages for metal segregation (2.9 ± 0.6 Ma after solar system formation), as well as the lowest bulk sulphur content and hence highest liquidus temperature. Here, using a new model for the internal evolution of the IVB parent body, we show that a single stage of metal-silicate separation cannot account for the complete melting of pure Fe metal at the relatively late time given by the Hf-W model age. Instead, a complex metal-silicate separation scenario is required that includes migration of partial silicate melts, formation of a shallow magma ocean and core formation in two distinct stages of metal segregation. In the first stage, a proto-core formed at ≈1.5 Ma via settling of metal particles in a mantle magma ocean, followed by metal segregation from a shallow magma ocean at ≈5.4 Ma. As these stages of metal segregation occurred at different times, the two metal fractions had different 182 W compositions. Consequently, the final 182W composition of the IVB core does not correspond to a single differentiation event, but represents the average composition of early- and late-segregated core fractions. Our best-fit model indicates a ≈100 km radius for the IVB parent body and provides an accretion age of ≈0.1 − 0.5 Ma after solar system formation. The computed solidification time is, furthermore, consistent with the Re-Os age for crystallization of the IVB core.

Description: Licus Vallis is a large valley (〉350 km long, 〉2 km wide, 〉150 m deep) that heads at the outlet breach of an ~30 km diameter impact crater. We present observations of the geomorphology and topography of this paleolake outlet valley and associated tributary valleys to constrain the history of incision of the Licus Vallis system. Licus Vallis has an abrupt increase in gradient by a factor of approximately four along its longitudinal profile, and a knickpoint that drops ~200 m over a reach of ~2 km approximately 12 km downstream from the valley head. We also describe a set of paired terraces within Licus Vallis, which are continuous for tens of kilometers and define an interior valley 〉2 km in width. We interpret the geomorphology of Licus Vallis as recording at least two discrete, major episodes of valley incision, both driven by lake overflow floods. The main portion of Licus Vallis formed by overflow flooding from a large (~10 3 –10 4 km 2 ) lake contained in an inter-crater basin. Subsequently, overflow flooding from a lake within the ~30 km diameter impact crater reactivated Licus Vallis, forming a major knickpoint at the valley head and establishing the upstream section of the valley at a lower slope. Further down the valley, this flood event incised an interior valley bounded by paired terraces. Regional tributary valleys that feed Licus Vallis also have prominent knickpoints, which have retreated farthest for downstream valleys. We conclude that these knickpoints record successive waves of incision that swept up Licus Vallis during lake overflow flooding, with erosion in the main trunk of the valley (from overflow floods) significantly outpacing erosion in the tributary valleys (from regional surface runoff). These observations of Licus Vallis illustrate how lake overflow floods may have provided an important control on the pace of landscape evolution on Mars.