ALBERT

Description: No abstract is available for this article.

Description: [1]  The Sample Analysis at Mars (SAM) instrument suite detected SO 2 , H 2 S, OCS and CS 2 from ~450-800 ºC during evolved gas analysis (EGA) of materials from the Rocknest aeolian deposit in Gale Crater, Mars. This was the first detection of evolved sulfur species from a martian surface sample during in situ EGA. SO 2 (~3-23  μ mol) is consistent with the thermal decomposition of Fe-sulfates or Ca-sulfites, or evolution/desorption from sulfur-bearing amorphous phases. Reactions between reduced sulfur phases such as sulfides and evolved O 2 or H 2 O in the SAM oven are another candidate SO 2 source. H 2 S (~41-109 nmol) is consistent with interactions of H 2 O, H 2 and/or HCl with reduced sulfur phases and/or SO 2 in the SAM oven. OCS (~1-5 nmol) and CS 2 (~0.2-1 nmol) are likely derived from reactions between carbon-bearing compounds and reduced sulfur. Sulfates and sulfites indicate some aqueous interactions, although not necessarily at the Rocknest site; Fe-sulfates imply interaction with acid solutions whereas Ca-sulfites can form from acidic to near-neutral solutions. Sulfides in the Rocknest materials suggest input from materials originally deposited in a reducing environment, or from detrital sulfides from an igneous source. The presence of sulfides also suggests that the materials have not been extensively altered by oxidative aqueous weathering. The possibility of both reduced and oxidized sulfur compounds in the deposit indicates a non-equilibrium assemblage. Understanding the sulfur mineralogy in Rocknest materials, which exhibit chemical similarities to basaltic fines analyzed elsewhere on Mars, can provide insight in to the origin and alteration history of martian surface materials.

Description: [1]  Moon Mineralogy Mapper (M 3 ) spectroscopic observations are used to assess the mineralogy of five sites that have recently been proposed to include lunar Dark Mantle Deposits (DMDs) (Gustafson et al., 2012). Volcanic glasses have, for the first time, clearly been identified at the location of three of the proposed pyroclastic deposits. This is the first time that volcanic glasses have been identified at such a small scale on the lunar surface from remote sensing observations. Deposits at Birt-E, Schluter, and Walther A appear to be glassy DMDs. Deposits at Birt-E and Schluter show (1) morphological evidence suggesting a likely vent, and (2) mineralogical evidence indicative of the presence of volcanic glasses. The Walther A deposits, although they show no morphological evidence of vents, have the spectroscopic characteristics diagnostic of volcanic glasses. The deposits of the Freundlich-Sharonov basin are separated in two areas: (1) the Buys-Ballot deposits lack mineralogical and morphological evidence, thus are found to be associated with mare volcanism not with DMDs, and (2) the Anderson crater deposits, which do not exhibit glassy DMD signatures but they appear to be associated with possible vent structures and so may be classifiable as DMDs. Finally, dark deposits near the crater Kopff are found to be associated with likely mare volcanism and not associated with DMDs. The spectral identification of volcanic glass seen in many of the potential DMDs is a strong indicator of their pyroclastic origin.

Description: While recent analyses of lunar samples indicate the Moon had a core dynamo from at least 4.2-3.56 Ga, mantle convection models of the Moon yield inadequate heat flux at the core-mantle boundary to sustain thermal core convection for such a long time. Past investigations of lunar dynamos have focused on a generally homogeneous, relatively dry Moon, while an initial compositionally stratified mantle is the expected consequence of a post-accretionary lunar magma ocean. Furthermore, recent re-examination of Apollo samples and geophysical data suggests that the Moon contains at least some regions with high water content. Using a finite-element model, we investigate the possible consequences of a heterogeneously wet, compositionally stratified interior for the evolution of the Moon. We find that a post-overturn model of mantle cumulates could result in a core heat flux sufficiently high to sustain a dynamo through 2.5 Ga and a maximum surface, dipolar magnetic field strength of less than 1 μ T for a 350-km core and near ~2 μ T for a 450 km core. We find that if water was transported or retained preferentially in the deep interior, it would have played a significant role in transporting heat out of the deep interior and reducing the lower mantle temperature. Thus, water, if enriched in the lower mantle, could have influenced core dynamo timing by over 1.0 Gyr and enhanced the vigor of a lunar core dynamo. Our results demonstrate the plausibility of a convective lunar core dynamo even beyond the period currently indicated by the Apollo samples.

Description: We spectrally characterize ( λ  = 0.35-10  μ m) the low humidity polymorph of kieserite (MgSO 4 •H 2 O), which is abundant on Mars and likely present on Europa, at various grain sizes and temperatures (100-300 K) relevant to the surfaces of Mars and Europa. Compositional analysis of these surfaces often relies on remote sensing using imaging spectrometers such as Mars Reconnaissance Orbiter CRISM, Mars Express OMEGA and Galileo NIMS. To estimate surface abundances from these observations, well-characterized laboratory spectra are required for comparison. Several variables, including temperature and grain size, affect the observed spectra and must be quantified in the laboratory to more confidently evaluate the returned data. Certain spectral features of kieserite exhibit predictable variability with changes in temperature and grain size that may be exploited to better understand the nature of kieserite on the surface of Mars. For instance, trends in our spectral analysis suggest that absorption features centered at λ  〈 3.0  μ m were primarily sensitive to temperature changes while features at λ  〉 3.0  μ m were additionally sensitive to grain size changes. We compare our laboratory spectra with selected CRISM data of suspected Martian kieserite and assess the inherent uncertainty that exists in using band center minima to determine surface composition. Incorporation of these temperature and grain size specific spectra into linear mixture models of planetary surface spectra will improve the compositional interpretation and contribute to our understanding of surface geochemistry and chemical evolution.

Description: Recent advancements in visible- to near- infrared orbital measurements of the lunar surface have allowed have allowed the character and extent of the primary anorthositic crust to be studied at unprecedented spatial and spectral resolutions. Here we assess the lunar primary anorthositic crust in global context using a spectral parameter tool for Moon Mineralogy Mapper (M 3 ) data to identify and map Fe-bearing, crystalline plagioclase based on its diagnostic 1.25  μ m absorption band. This allows plagioclase-dominated rocks, specifically anorthosites, to be unambiguously identified as well as distinguished from lithologies with minor to trace amounts of mafic minerals. Low spatial resolution global mosaics and high spatial resolution individual data strips covering more than 650 targeted craters were analyzed to identify and map the mineralogy of spectrally pure regions as small as ~400 meters in size. Spectrally pure plagioclase is identified in approximately 450 targets located across the lunar surface. Diviner thermal infrared (TIR) data is analyzed for 37 of these nearly monomineralic regions in order to understand the compositional variability of plagioclase (An#) in these areas. The average Christiansen feature (CF) value for each spectrally pure region is estimated using new laboratory measurements of a well-characterized anorthite (An 96 ) sample. Diviner TIR results suggests that the plagioclase composition across the lunar highlands is relatively uniform, highly calcium (An ≥96 ), and consistent with plagioclase compositions found in the ferroan anorthosites (An 94-98 ). Our results confirm that spectrally pure anorthosite is widely distributed across the lunar surface and most exposures of the ancient anorthositic crust are concentrated in regions of thicker crust surrounding impact basins on the lunar near- and far-sides. In addition, the scale of the impact basins and the global nature and distribution of pure plagioclase requires a coherent zone of anorthosite of similar composition in the lunar crust supporting its formation from a single differentiation event involving a magma ocean. Our identifications of pure anorthosite combined with the GRAIL crustal thickness model suggest that pure anorthosite is currently observed at a range of crustal thickness values between 9 and 63 km and that the primary anorthositic crust must have been at least 30 km thick.

Description: We present a model of the lithospheric magnetic field of Mars which is based on Mars Global Surveyor orbiting satellite data and represented by an expansion of spherical harmonic functions up to degree and order 110. Several techniques were applied in order to obtain a reliable and well-resolved model of the Martian lithospheric magnetic field. A modified Huber-Norm was used to properly treat data outliers and the MPO data was weighted based on an a priori analysis of the data. Static external fields were treated by a joint inversion of external and internal fields. Further, temporal variabilities in the data which lead to unrealistically strong anomalies were considered as noise and handled by additionally minimizing a measure of the horizontal gradient of the vertically down internal field component at surface altitude. Here, we use an iteratively reweighted least-squares algorithm to approach an absolute measure (L1-norm), allowing for a better representation of strong localized magnetic anomalies as compared to the conventional least-squares measure (L2-norm). The resulting model reproduces all known characteristics of the Martian lithospheric field and shows a rich level of detail. It is characterized by a low level of noise and robust when downward-continued to the surface. We show how these properties can help to improve the knowledge of the Martian past and present magnetic field by investigating magnetic signatures associated with impacts and volcanoes. Additionally, we present some previously undescribed isolated anomalies, which can be used to determine paleopole positions and magnetization strengths.

Description: Tharsis swell represents the largest area of volcanism on Mars and sits adjacent to the dichotomy between the northern lowlands and southern highlands. While a number of investigators have proposed that Tharsis volcanism and topographic swell was caused by a stationary mantle plume, both numerical and tank experiments find that a plume will rise beneath a region of thick lithosphere, hence somewhere beneath the southern highlands on Mars. We present models of crustal thickness which show a region of thickened crust associated with previously mapped geologic units that have been suggested to represent the path of the Tharsis plume [Hynek et al., 2011]. We find a similarity in topography and structure between the proposed Tharsis plume track on Mars and the Snake River plain/Yellowstone hotspot track in western North America.

Description: The combination of the radio tracking of the MESSENGER spacecraft and Earth-based radar measurements of the planet's spin state gives three fundamental quantities for the determination of the interior structure of Mercury: mean density ρ , moment of inertia C , and moment of inertia of the outer solid shell C m . This work focuses on the additional information that can be gained by a determination of the change in gravitational potential due to planetary tides, as parametrized by the tidal potential Love number k 2 . We investigate the tidal response for sets of interior models that are compatible with the available constraints ( ρ , C , and C m ). We show that the tidal response correlates with the size of the liquid core and the mean density of material below the outer solid shell, and that it is affected by the rheology of the outer solid shell of the planet, which depends on its temperature and mineralogy. For a mantle grain size of 1 cm, we calculate that the tidal k 2 of Mercury is in the range 0.45 to 0.52. Some of the current models for the interior structure of Mercury are compatible with the existence of a solid FeS layer at the top of the core. Such a layer, if present, would increase the tidal response of the planet.

Description: We investigate the coupled evolution of the atmosphere and mantle on Venus. Here we focus on mechanisms that deplete or replenish the atmosphere: atmospheric escape to space and volcanic degassing of the mantle. These processes are linked to obtain a coupled model of mantle convection and atmospheric evolution, including feedback of the atmosphere on the mantle via the surface temperature. During early atmospheric evolution hydrodynamic escape is dominant, while for later evolution we focus on non-thermal escape, as observed by the ASPERA instrument on the Venus Express Mission. The atmosphere is replenished by volcanic degassing from the mantle, using mantle convection simulations based on those of Armann and Tackley [2012], and include episodic lithospheric overturn. The evolving surface temperature is calculated from the amount of CO 2 and water in the atmosphere using a gray radiative-convective atmosphere model. This surface temperature in turn acts as a boundary condition for the mantle convection model. We obtain a Venus-like behavior (episodic lid) for the solid planet and an atmospheric evolution leading to the present conditions. CO 2 pressure is unlikely to vary much over the history of the planet; with only a 0.25-20% post-magma-ocean build-up. In contrast, atmospheric water vapor pressure is strongly sensitive to volcanic activity, leading to variations in surface temperatures of up to 200 K, which have an effect on volcanic activity and mantle convection. Low surface temperatures trigger a mobile lid regime that stops once surface temperatures rise again, making way to stagnant lid convection that insulates the mantle.

Description: [1]  Several giant impact basins of mid-Noachian age have been identified on Mars, and the global magnetic field appears to have vanished at about the same time. The impacts that formed these basins delivered a large amount of heat to the planetary interior, modified the pattern of mantle convection, and suppressed core cooling, potentially contributing to the cessation of dynamo activity. Here we investigate the thermal evolution of Mars in response to the largest basin-forming impacts, using a new method of coupling models of mantle convection with parameterized core cooling. [2]  We find that heating by a large impact generates a strong hemispheric upwelling in the mantle, which quickly spreads into a warm layer beneath the stagnant lid. The impact heating of the core leads to spherically symmetric stratification of the core; the outermost layers are strongly heated. This acts as a thermal “blanket” that prevents cooling of the interior, and shuts down core convection. While the hottest part of the thermal blanket in the outermost core disappears relatively quickly, dynamo activity does not restart for ~100 My, and the core does not return to a fully convective state for ~1 Gy following the impact.

Description: [1]  We have completed laboratory experiments and thermochemical equilibrium models to investigate secondary mineral formation under conditions akin to volcanic, hydrothermal acid-sulfate weathering systems. Our research used the basaltic mineralogy at Cerro Negro Volcano, Nicaragua, characterized by plagioclase, pyroxene, olivine, and volcanic glass. These individual minerals and whole rock field samples were reacted in the laboratory with 1 molal sulfuric acid at varying temperatures (65, 150, 200 °C), fluid:rock weight ratios (1:1, 4:1, 10:1), and durations (1-60 days). Thermochemical equilibrium models were developed using Geochemist's Workbench. To understand the reaction products and fluids, we employed scanning electron microscopy/energy dispersive spectroscopy, x-ray diffraction, and inductively coupled plasma-atomic emission spectroscopy. The results of our experiments and models yielded major alteration minerals that include anhydrite, natroalunite, minor iron oxide, and amorphous Al-Si gel. We found that variations in experimental parameters did not drastically change the suite of minerals produced; instead, abundance, size, and crystallographic shape changed. Our results also suggest that it is essential to separate phases formed during experiments from those formed during fluid evaporation to fully understand the reaction processes. Our laboratory reacted and model predicted products are consistent with the mineralogy observed at places on Mars. However, our results indicate that determination of the formation conditions requires microscopic imagery and regional context, as well as a thorough understanding of contributions from both experiment precipitation and fluid evaporation minerals.

Description: The Lunar Radar Sounder (LRS) onboard Kaguya (SELENE) detected widespread horizontal reflectors under some nearside maria. Previous studies estimated that the depths of the subsurface reflectors were up to several hundreds of meters, and suggested that the reflectors were interfaces between mare basalt units. Comparison between the reflectors detected in the LRS data and surface age maps indicating formation age of each basalt unit allow us to discuss the lower limit volume of each basalt unit and its space and time variation. We estimated volumes of basalt units in the ages of 2.7 Ga to 3.8 Ga in the nearside maria including Mare Crisium, Mare Humorum, Mare Imbrium, Mare Nectaris, Mare Serenitatis, Mare Smythii, and Oceanus Procellarum. The lower limit volumes of the geologic units estimated in this study were of the order of 10 3 to 10 4  km 3 . This volume range is consistent with the total amount of erupted lava flows derived from numerical simulations of thermal erosion models of lunar sinuous rilles formation and is also comparable to the average flow volumes of continental flood basalt units formed after the Paleozoic and calculated flow volumes of Archean komatiite flows on the Earth. The lower limits of average eruption rates estimated from the unit volumes were of the order of 10 –5 to 10 –3  km 3 /yr. The estimated volumes of the geologic mare units and average eruption rate showed clear positive correlations with their ages within the same mare basin, while they vary among different maria compared within the same age range.

Description: [1]  The Radiation Assessment Detector (RAD), onboard the Mars Science Laboratory rover Curiosity, measures the energetic charged and neutral particles, and the radiation dose rate on the surface of Mars. An important factor for determining the biological impact of the Martian surface radiation is the specific contribution of neutrons, with their deeper penetration depth and ensuing high biological effectiveness. This is very difficult to measure quantitatively, resulting in considerable uncertainties in the total radiation dose. In contrast to charged particles, neutral particles (neutrons and gamma rays) are generally only measured indirectly. Measured spectra are a complex convolution of the incident particle spectrum with the detector response function, and must be unfolded. We apply an inversion method (based on a maximum-likelihood estimation) to calculate the neutron and gamma spectra from the RAD neutral particle measurements. Here we show the first measurements of neutron/gamma spectra from the surface of Mars and compare them to theoretical predictions. The measured neutron spectrum (ranging from 8 to 740 MeV) translates into a radiation dose rate of 14 ± 4 μ Gy/day and a dose equivalent rate of 61 ± 15 μ Sv/day. This corresponds to 7% of the measured total surface dose rate, and 10% of the biologically relevant surface dose equivalent rate on Mars. [2]  Measuring the Martian neutron and gamma spectra is an essential step for determining the mutagenic influences to past or present life at or beneath the Martian surface as well as the radiation hazard for future human exploration, including the shieldingdesign of a potential habitat.

Description: [1]  Mojave crater, and five other relatively young Late Hesperian to Amazonian-age martian craters exhibit channelized alluvial fans that are sourced from bedrock-eroded catchments. These catchments emerge from the crests of sloping surfaces, suggesting a formation mechanism that involved precipitation. The evidence for fluvial activity at all six craters is restricted to their interiors and the immediate surrounding regions. Detailed mapping at Mojave reveals the highest density of channels, catchments and fans interior to the crater. Similar landforms are identified outside of the crater, but not beyond ~200 km from the rim. Irregular pits on the floor of Mojave, interpreted as degassing structures from hot impact melt, directly superpose several fan surfaces, and partly destroy the fan toes. This suggests that sediment was mobilized immediately after crater formation, while the crater was still hot. Based on the patterns and timing of channel-fan development at all six craters we favor several hypotheses for the precipitation mechanism: (1) snowfall and melt on young, hot impact craters, (2) impact plume precipitation, and (3) degassing of volatiles from impact melt terrain. Scenario (1) suggests a different global or regional climate relative to modern conditions, requiring equatorial and mid-latitude snowfall accumulation. Scenarios (2) and (3) do not necessarily require unique climate conditions, as water may have been mobilized from the target or the impactor.

Description: We present and discuss here the average fields of the Venus atmosphere derived from the nighttime observations in the 1960-2350 cm -1 spectral range by the VIRTIS-M instrument on board the Venus Express satellite. These fields include: a) the air temperatures in the 1-100 mbar pressure range (~85-65 km above the surface), b) the altitude of the clouds top and c) the average CO mixing ratio. A new retrieval code based on the Bayesian formalism has been developed and validated on simulated observations, to statistically assess the retrieval capabilities of the scheme once applied to the VIRTIS data. The same code has then been used to process the entire VIRTIS-M dataset. Resulting individual retrievals have been binned on the basis of local time and latitude, to create average fields. Air temperature fields confirm the general trends previously reported in Grassi et al. , 2010, using a simplified retrieval scheme and a more limited dataset. At the lowest altitudes probed by VIRTIS (~65 km), air temperatures are strongly asymmetric around midnight, with a pronounced minima at 3LT, 70S. Moving to higher levels, the air temperatures firstly become more uniform in local time (~75 km), then display a colder region on the evening side at the upper boundary of VIRTIS sensitivity range (~80km). As already shown by Ignatiev et al. (2008) for the day side, the cloud effective altitude increases monotonically from the south pole to the equator. However, the variations observed in night data are consistent with an overall variation of just one kilometer, much smaller than the four kilometers reported for the day side. The cloud altitudes appear slightly higher on the evening side. Both observations are consistent with a less vigorous meridional circulation on the night side of the planet. Carbon monoxide is not strongly constrained by the VIRTIS-M data. However, average fields present a clear maximum of 80 ppm around 60S, well above the retrieval uncertainty. Once the intrinsic low sensitivity of VIRTIS data in the region of cold collar is kept in mind, this datum is consistent with a [CO] enrichment toward the poles driven by meridional circulation.

Description: Processes, environments, and the energy associated with the transport and deposition of sand at Gusev crater are characterized at the microscopic scale through the comparison of statistical moments for particle size and shape distributions. Bivariate and factor analyses define distinct textural groups at 51 sites along the traverse completed by the Spirit rover as it crossed the plains and went into the Columbia Hills. Fine-to-medium sand is ubiquitous in ripples and wind drifts. Most distributions show excess fine material, consistent with a predominance of wind erosion over the last 3.8-billion years. Negative skewness at West Valley is explained by the removal of fine sand during active erosion, or alternatively, by excess accumulation of coarse sand from a local source. The coarse to very coarse sand particles of ripple armors in the basaltic plains have a unique combination of size and shape. Their distribution display significant changes in their statistical moments within the ~400 m that separate the Columbia Memorial Station from Bonneville crater. Results are consistent with aeolian and/or impact deposition, while the elongated and rounded shape of the grains forming the ripples, as well as their direction of origin could point to Ma'adim Vallis as a possible source. For smaller particles on the traverse, our findings confirm that aeolian processes have dominated over impact and other processes to produce sands with the observed size and shape patterns across a spectrum of geologic (e.g., ripples, plains soils) and aerographic settings (e.g., wind shadows).

Description: Photodissociation of water deposited on an impact melt breccia collected during Apollo 16 was studied by measuring O( 3 P J=2,1,0 ) photoproducts detected with resonance-enhanced multiphoton ionization (REMPI). For each spin-orbit state, the oxygen atom time-of-flight (TOF) spectrum was measured as a function of H 2 O exposure and 157-nm irradiation time. Four Maxwell-Boltzmann distributions with translational temperatures of 10,000 K, 1800 K, 400 K, and 102 K were required to fit the data. The most likely formation mechanisms are molecular elimination following ion-electron recombination, secondary recombination of hydroxyl radicals, and photodissociation of adsorbed hydroxyls. The irradiation time required to reach maximum oxygen signal suggests that water clusters into islands when adsorbing on the lunar impact melt breccia. After enough irradiation for the oxygen atom yield to reach its maximum, the slowly decreasing signal was fit with an exponential curve to obtain a cross section that represents the rate of surface hydroxyl depletion. For 0.1, 1, and 5 Langmuir (1 L = 10 -6  Torr s) H 2 O exposure, respectively, the measured O( 3 P) depletion cross sections were 4.9 × 10 -20 , 6.6 × 10 -20 , and 4.6 × 10 -20  cm 2 . These results imply that photodissociation of water on the lunar surface cannot account for the large mass-16 (±1 amu) signal observed in the lunar atmosphere. Unless another significant source of oxygen atoms is present, this unexpectedly large signal is likely due to CH 4 or OH.

Description: The New Horizons spacecraft flew past Jupiter and its moons in February and March 2007. The flyby provided one of the most comprehensive inventories of Io's active plumes and hotspots yet taken, including the large 350 km high eruption of Tvashtar. Among the suite of instruments active during the flyby was the Linear Etalon Infrared Spectral Array (LEISA), a near-infrared imaging spectrometer covering the spectral range 1.25 to 2.5 µm. We have identified 37 distinct hotspots on Io in the nine LEISA spectral image cubes taken during the flyby. We describe the thermal emissions from these volcanoes, and fit single component blackbody curves to the hotspot spectra to derive eruption temperatures, areas and power output for the hotspots with sufficient signal-to-noise. Of these, 11 hotspots were seen by LEISA more than once, and East Girru showed short-term variability over a few days, also seen by other New Horizons instruments. This work presents a comprehensive look at the global distribution of Io's volcanism at the time of the flyby. From these measurements, we estimate the global power output of high-temperature (〉550 K) volcanism on Io to be ~8 TW. This work provides the first short-wavelength near-infrared survey with global coverage at all longitudes on the nightside of Io without sunlight contamination at these wavelengths. A major conclusion from this study is that 90% of all the volcanoes observed in the New Horizons LEISA near-infrared data in 2007 were also observed during the Galileo epoch, suggesting these are all long-lived hotspots.

Description: Lunar roughness measurements derived from the Lunar Orbiter Laser Altimeter (LOLA) are compared to 12.6 cm wavelength radar data collected by the Miniature Radio Frequency (Mini-RF) instrument and 70 cm wavelength radar data collected by the Arecibo Observatory. These data are compared to assess how surface and subsurface roughness are correlated and affected by parameters including age and composition at length scales between 0.1 and 100 meters. A range of features are analyzed including volcanic domes (Marius Hills, Rümker Hills, Gruithuisen, and Mairan Domes); mare (Imbrium, Serenitatis, and Oceanus Procellarum); pyroclastic dark mantle deposits (Sinus Aestuum, Sulpicius Gallus, and Mare Vaporum); and two young craters (Copernicus and Tycho). Statistically significant positive correlations exist between topographic roughness and both P- and S-band circular polarization ratios. The strongest correlation is observed at the longest length scales. Correlations weaken as length scales become less similar, potentially due to distinct processes controlling surface modification. Roughness is not significantly correlated with local slope. Although the Marius Hills are compositionally distinct from the Gruithuisen and Mairan domes, they are indistinguishable in roughness characteristics. Conversely, the Rümker Hills, mare, and dark mantle deposits are smoother at the length scales examined, possibly due to fine-grained mantling of regolith or pyroclastic deposits. The floor and ejecta of Tycho are the roughest surfaces measured in this study, while the floor and ejecta of Copernicus overlap the roughness distribution of the volcanic features. This study shows that many factors control the evolution of roughness over time on various length scales.

Description: The identification of widespread pyroclastic vents and deposits on Mercury has important implications for the planet's bulk volatile content and thermal evolution. However, the significance of pyroclastic volcanism for Mercury depends on the mechanisms by which the eruptions occurred. Using images acquired by the MESSENGER spacecraft, we have identified 150 sites where endogenic pits are surrounded by a relatively bright and red diffuse-edged spectral anomaly, a configuration previously used to identify sites of explosive volcanism. We find that these sites cluster at the margins of impact basins and along regional tectonic structural trends. Locally, pits and deposits are usually associated with zones of weakness within impact craters and/or with the surface expressions of individual thrust faults. Additionally, we use images and stereo-derived topographic data to show that pyroclastic deposits are dispersed up to 130 km from their source vent and commonly have either no relief or low circum-pit relief within a wider, thinner deposit. These eruptions were therefore likely driven by a relatively high concentration of volatiles, consistent with volatile concentration in a shallow magma chamber prior to eruption. The collocation of sites of explosive volcanism with near-surface faults and crater-related fractures is likely a result of such structures acting as conduits for volatile and/or magma release from shallow reservoirs, with volatile overpressure in these reservoirs a key trigger for eruption in at least some cases. Our findings suggest that widespread, long-lived explosive volcanism on Mercury has been facilitated by the interplay between impact cratering, tectonic structures and magmatic fractionation.

Description: In this work we describe a model of the perturbation of the O 2 (a 1 ∆ g ) nightglow limb profiles by the action of gravity waves (GWs) propagating in the Venus’ upper atmosphere. Data have been acquired by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) on board the ESA mission Venus Express (VEX). The high variability observed in the shape of the O 2 (a 1 ∆ g ) nightglow limb profiles between 80 and 120 km, often characterized by the presence of a double peak, suggests the occurrence of GWs at the considered altitudes. In order to model and derive the GWs properties, we apply to Venus a well known theory used to study terrestrial density fluctuations induced by the GWs propagation. The retrieved vertical wavelengths and amplitudes of the waves at the O 2 (a 1 ∆ g ) layer altitude (~100 km) are of the order of 7–16 km and 3–14 % respectively, complying with wave amplitude threshold for dynamical instability in the majority of the fitted cases. Temperature fluctuations would exceed 40 % at higher altitudes (115–120 km) thus inducing either wave breaking or dissipation. Intrinsic horizontal phase velocities are expected to vary in the range 32 m/s and 85 m/s. GWs are detected in a wide range of latitudes from the mid latitudes up to the polar regions, and we cannot exclude existence of the sources of different nature. This study also confirms the high variability induced by the action of GW propagation in the airglow profiles of the terrestrial planets and points out the need for future missions to couple simultaneous complementary GW detection techniques in order to better constrain GW properties and understand their impact on the Venus general circulation.

Description: We investigate viscoelastic impact basin relaxation on Pluto for a variety of thermal evolution scenarios encompassing both convective and conductive ice shells. Basins smaller than 200 km in diameter do not relax appreciably, while relaxation fractions can be up to ~60% for large impact basins. The main control on basin relaxation is the amount of radiogenic heat produced in the rocky core; our results are insensitive to the formation time of the basin, the ice reference viscosity adopted, and the presence/absence of a subsurface ocean. Other volatiles, such as CO 2 or NH 3 , if present in the ice shell in sufficient quantities could increase the predicted relaxation fraction of basins. Relaxation causes extensional stresses interior to the basin; the orientation of the resulting tectonic features is controlled by the effective elastic thickness beneath the basin. Future observations of the relaxation states and tectonics of impact basins are therefore likely to provide a key constraint on Pluto's thermal history.

Description: We have analyzed three years of radio tracking data from the MESSENGER spacecraft in orbit around Mercury and determined the gravity field, planetary orientation, and ephemeris of the innermost planet. With improvements in spatial coverage, force modeling, and data weighting, we refined an earlier global gravity field both in quality and resolution, and we present here a spherical harmonic solution to degree and order 50. In this field, termed HgM005, uncertainties in low-degree coefficients are reduced by an order of magnitude relative to the earlier global field, and we obtained a preliminary value of the tidal Love number k 2 of 0.451 ± 0.014. We also estimated Mercury's pole position, and we obtained an obliquity value of 2.06 ± 0.16 arcmin, in good agreement with analysis of Earth-based radar observations. From our updated rotation period (58.646146 ± 0.000011 days) and Mercury ephemeris, we verified experimentally the planet's 32 spin-orbit resonance to greater accuracy than previously possible. We present a detailed analysis of the HgM005 covariance matrix, and we describe some near-circular frozen orbits around Mercury that could be advantageous for future exploration.

Description: A series of two-dimensional numerical models of magmatism and mantle convection in small planetary bodies are presented to discuss how the small size of the Moon exerts control over its mantle evolution. Mantle convection is modeled by a solid-state convection of internally heated materials with temperature-dependent viscosity. Magmatism is modeled as upward permeable flow of basaltic magma generated by decompression melting of upwelling mantle materials. Migration of the generated magma causes compaction/expansion of the coexisting matrix. The volume change of matrix and the buoyancy of magma induced by magmatism enhance the upwelling flow and hence the magmatism itself, when the Rayleigh number exceeds a threshold that is around the critical Rayleigh number for the onset of thermal convection. This positive feedback makes magmatism vigorous and episodic, and causes efficient cooling and stirring of the mantle in large planets like Venus and the Earth. In small planetary bodies where the Rayleigh number is lower than the threshold, however, magmatism occurs more continuously with a characteristic time of several hundred million years, cooling and convective stirring of the mantle caused by magmatism are less important, and a compositionally stratified structure develops in the mantle. These features of magmatism and mantle structure fit in with the observations on the Moon. The positive feedback is a key to understanding why the evolution of the lunar mantle is so different from that of lager planets like the Earth and Venus.

Description: With the upcoming MAVEN mission, the role of escape in the evolution of the Martian atmosphere is investigated in more detail. This work builds on our previous modeling of the atmospheric impact of the the pickup O + sputtering effects for varioussolar wind parameters, solar EUV intensities, and the surface crustal field distributions. Relationships between the incident ions properties and the ejected hot neutral components, often referred to as atmospheric sputtering, are derived for application to proposed MAVEN ion spectrometer measurements of precipitating O + . We show how our simulation results can be used to constrain the sputtering effects under present conditions and to interpolate toward estimates of sputtering efficiencies occurring in earlier epochs. Present-day sputtering under typical circumstance is estimated to be weak, but possibly detectable as an exospheric enhancement. The ultimate goal of estimating the importance of atmospheric sputtering effects on the evolution ofthe Martian atmosphere can be better deduced by the combining MAVEN measurements with models, and the sputtering response relations derived here.

Description: Transverse aeolian ridges (TARs) are small bright windblown deposits found throughout the Martian tropics that stand a few meters tall and are spaced a few tens of meters apart. The origin of these features remains mysterious more than 20 years after their discovery on Mars. This paper presents a new hypothesis, that some of the TARs could be indurated dust deposits emplaced millions of years ago during periods of higher axial obliquity. It suggests that these TARs are primary depositional bedforms that accumulated in place from dust carried by the winds in suspension, perhaps in a manner comparable to antidunes on Earth, and were subsequently indurated and eroded to their current states by eons of sandblasting. It points out examples of modern dust drifts and dune-like features that appear to have been recently formed by dust accumulating directly onto the surface from atmospheric suspension. It shows how these pristine dust deposits could evolve to explain the range of morphologies of the TARs. Finally, it explains how the known properties of many TARs are consistent with this hypothesis, including their composition, thermal behavior, and distribution.

Description: Glacial landforms on Mars, including concentric crater fill (CCF), lineated valley fill (LVF), and lobate debris aprons (LDA) were mapped between ~ ±30-50° latitude and geometric constraints were placed on the volume of these deposits based on their topography and relationship with host topography (e.g., massifs, valleys and craters). LDA deposits were found to have the largest volume (2.63 x 10 5  km 3 ), followed by CCF deposits (8.75 x 10 4  km 3 ) and LVF deposits (6.51 x 10 4  km 3 ), for a total volume of 4.16 x 10 5  km 3 . The actual ice sequestered in these deposits is calculated depending on the estimated fraction of ice currently remaining in the deposits. We adopt end member values of 30% (pore ice only) and 90% (relatively pure, debris covered glacial ice). These volumes, 1.25 x 10 5  km 3 and 3.74 x 10 5  km 3 respectively, represent an equivalent global ice layer 0.9-2.6 m thick. On the basis of extensive recent local and regional analysis of these deposits indicating a debris-covered glacial origin, we favor the larger estimate. Taken together, these glacial landforms have an average thickness of ~450 meters. The glacial deposition responsible for these features occurred or recurred during an extended period of the mid-to-late Amazonian, which implies long-term climate conditions were sufficient to produce and preserve debris-covered glacial landforms on Mars.

Description: The search for lightning is an important item on the agenda for the future exploration of Titan. Thunder, as a direct lightning signature, can be used, together with electromagnetic signals, to corroborate and quantify lightning. Using Cassini-Huygens data and model predictions, the main characteristics of thunder produced by a potential 20-km cloud-to-ground tortuous discharge are obtained and discussed. The acoustic power released right after the discharge decreaseswith increasing altitude, owing to the ambient pressure and temperature gradients. Ray tracing is used to propagate sound waves to the far field. Simulated thunder waveforms are characterized by fairly long codas—on the order of tens of seconds—arising from the small acoustic absorption (~10 −4  dB/km). In the low-loss environment, the principal thunder arrival will likely have a large signal-to-noise ratio ensuring a high detection selectivity. The spectral content depends on the amountof energy released during the discharge. For an energy density of 5 kJ/m, the dominant contribution lies between 50 and 80 Hz; for 500 kJ/m, it shifts to lower frequencies, between 10 and 30 Hz.

Description: . Analysis of lunar laser ranging and seismic data has yielded evidence that has been interpreted to indicate a molten zone in the lower-most mantle overlying a fluid core. Such a zone provides strong constraints on models of lunar thermal evolution. Here we determine thermo-chemical and physical structure of the deep Moon by inverting lunar geophysical data (mean mass and moment of inertia, tidal Love number, and electromagnetic sounding data) in combination with phase-equilibrium computations. Specifically, we assess whether a molten layer is required by the geophysical data. The main conclusion drawn from this study is that a region with high dissipation located deep within the Moon is required to explain the geophysical data. This region is located within the mantle where the solidus is crossed at a depth of 1200 km (≥1600°C). Inverted compositions for the partially molten layer (150–200 km thick) are enriched in FeO and TiO 2 relative to the surrounding mantle. The melt phase is neutrally buoyant at pressures of ~4.5–4.6 GPa, but contains less TiO 2 (〈15 wt%) than the Ti-rich (~16 wt%) melts that produced a set of high-density primitive lunar magmas (density of 3.4 g/cm 3 ). Melt densities computed here range from 3.25 to 3.45 g/cm 3 bracketing the density of lunar magmas with moderate-to-high TiO 2 contents. Our results are consistent with a model of lunar evolution in which the cumulate pile formed from crystallization of the magma ocean as it overturned, trapping heat-producing elements in the lower mantle.

Description: Positive free-air gravity anomalies associated with large lunar impact basins have been shown to represent a superisostatic mass concentration or “mascon.” High-resolution lunar gravity data from the GRAIL spacecraft reveal that these mascons are generally part of a bulls-eye pattern in which the central positive anomaly is surrounded by a negative anomaly annulus, which in turn is surrounded by an outer positive annulus. To understand the origin of this gravity pattern, we modeled numerically the entire evolution of basin formation from impact to contemporary form. With a hydrocode, we simulated impact and basin collapse. With those results as initial conditions for a finite-element model, we simulated subsequent cooling and viscoelastic relaxation of topography. We concentrated our analysis on the mare-free Freundlich-Sharonov and mare-infilled Humorum basins. By constraining these models with measured free-air and Bouguer gravity anomaly observations as well as surface topography, we can account for the evolution of lunar basins as the result of isostatic adjustment from an initially subisostatic state following basin collapse. The key to the development of a superisostatic inner basin center is its mechanical coupling to the outer basin that rises in response to sub-isostatic stresses, enabling the inner basin to rise above isostatic equilibrium. Our calculations enable us to relate basin size to the diameter and velocity of the impactor and to constrain the thermal structure of the Moon at the time of impact, the thickness of the pre-impact crust, the viscoelastic rheology, and for the Humorum basin, the thickness of its mare fill.

Description: In the present study we introduce an innovative method for the measurement of impact-induced pressure waves within geological materials. Impact experiments on dry and water-saturated sandstone targets were conducted at a velocity of 4600 m/s using 12 mm steel projectiles to investigate amplitudes, decay behavior and speed of the waves propagating through the target material. For this purpose a special kind of piezoresistive sensor capable of recording transient stress pulses within solid brittle materials was developed and calibrated using a Split-Hopkinson pressure bar. Experimental impact parameters (projectile size and speed) were kept constant and yielded reproducible signal curves in terms of rise time and peak amplitudes. Pressure amplitudes decreased by three orders of magnitude within the first 250 mm (i.e. 42 projectile radii). The attenuation for water-saturated sandstone is higher compared to dry sandstone which is attributed to dissipation effects caused by relative motion between bulk material and interstitial water. The proportion of the impact energy radiated as seismic energy (seismic efficiency) is in the order of 10 −3 . The present study shows the feasibility of real-time measurements of waves caused by hypervelocity impacts on geological materials. Experiments of this kind lead to a better understanding of the processes in the crater subsurface during a hypervelocity impact.

Description: The Tharsis region of Mars is covered in volcanic flows that can stretch for tens to hundreds of kilometers. Radar measurements of the dielectric properties of these flows can provide information regarding their composition and density. SHARAD (SHAllow RADar), a sounding radar on the Mars Reconnaissance Orbiter, detects basal interfaces beneath flows in some areas of Tharsis northwest and west of Ascraeus Mons, with additional detections south of Pavonis Mons. Comparisons with 12.6-cm ground-based radar images suggests that SHARAD detects basal interfaces primarily in dust or regolith-mantled regions. We use SHARAD data to estimate the real relative permittivity of the flows by comparing the measured time delay of returns from the subsurface with altimetry measurements of the flow heights relative to the surrounding plains. In cases where the subsurface interface is visible at different depths, spanning tens of meters, it is also possible to measure the loss tangent (tan δ) of the material. The permittivity values calculated range from 7.6 to 11.6, with an average of 9.6, while the mean loss tangent values range from 7.8 × 10 -3 to 2.9 × 10 -2 with an average of 1.0 × 10 -2 . These permittivity and loss tangent estimates for the flows northwest of Ascraeus Mons, west of Ascraeus Mons, and south of Pavonis Mons are consistent with the lab-measured values for dense, low-titanium basalt.

Description: Landscape evolution on the Moon is dominated by impact cratering in the post-maria period. In this study, we mapped 800 m to 5 km diameter craters on 〉30% of the lunar maria and extracted their topographic profiles from digital terrain models created using the Kaguya Terrain Camera. We then characterized the degradation of these craters using a topographic diffusion model. Because craters have a well-understood initial morphometry, these data provide insight into erosion on the Moon and the topographic diffusivity of the lunar surface as a function of time. The average diffusivity we calculate over the past 3 Ga is ~5.5 m 2 /My. With this diffusivity, after 3 Ga, a 1 km diameter crater is reduced to approximately ~52% of its initial depth and a 300 m diameter crater is reduced to only ~7% of its initial depth, and craters smaller than ~200-300 m are degraded beyond recognition. Our results also allow estimation of the age of individual craters on the basis of their degradation state, provide a constraint on the age of mare units, and enable modeling of how lunar terrain evolves as a function of its topography.

Description: A morphometric and morphologic catalog of ~100 small craters imaged by the Opportunity rover over the 33.5 km traverse between Eagle and Endeavour craters on Meridiani Planum shows craters in six stages of degradation that range from fresh and blocky to eroded and shallow depressions ringed by planed-off rim blocks. The age of each morphologic class from 〈50-200 ka to ~20 Ma has been determined from the size-frequency distribution of craters in the catalog, the retention age of small craters on Meridiani Planum, and the age of the latest phase of ripple migration. The rate of degradation of the craters has been determined from crater depth, rim height and ejecta removal over the class age. These rates show a rapid decrease from ~1 m/Myr for craters 〈 1 Ma to ~ 〈0.1 m/Myr for craters 10-20 Ma, which can be explained by topographic diffusion with modeled diffusivities of ~10 -6  m 2 /yr. In contrast to these relatively fast, short-term erosion rates, previously estimated average erosion rates on Mars over ~100 Myr and 3 Gyr timescales from the Amazonian and Hesperian are of order 〈 0.01 m/Myr, which is 3-4 orders of magnitude slower than typical terrestrial rates. Erosion rates during the Middle-Late Noachian averaged over ~250 Myr and ~700 Myr intervals are around 1 m/Myr, comparable to slow terrestrial erosion rates calculated over similar timescales. This argues for a wet climate before ~3 Ga in which liquid water was the erosional agent, followed by a dry environment dominated by slow eolian erosion.

Description: This work presents the variability over seasons ( i . e ., orbital position) and solar cycle of the Martian upper atmosphere and hot carbon corona. We investigate the production and distribution of energetic carbon atoms and the impacts on the total global hot carbon loss from dominant photochemical processes at five different cases: AL (aphelion and low solar activity), EL (equinox and low solar activity), EH (equinox and high solar activity), PL (perihelion and low solar activity), and PH (perihelion and high solar activity). We compare our results with previously published results, but only on the limited cases due to the dearth of studies on solar EUV flux and seasonal variabilities. Photodissociation of CO and dissociative recombination of CO + are generally regarded as the two most important source reactions for the production of hot atomic carbon. Of these two, photodissociation of CO is found to be the dominant source in all cases considered. To describe self-consistently the exosphere and the upper thermosphere, a 3D kinetic particle simulator, the Adaptive Mesh Particle Simulator (AMPS), and the 3D Mars Thermosphere General Circulation Model (MTGCM) are one-way coupled. The basic description of this hot carbon calculation can be found in the companion paper to this one ( Lee et al ., 2014). The spatial distributions and profiles of density and temperature and atmospheric loss rates are discussed for the cases considered. Finally, our computed global escape rate of hot carbon ranges from 5.28 × 10 23  s −1 (AL) to 55.1 × 10 23  s −1 (PL).

Description: X-ray diffraction patterns of the three samples analyzed by Curiosity ’s CheMin instrument during the first year of the Mars Science Laboratory mission – the Rocknest sand; and the John Klein and Cumberland drill fines, both extracted from the Sheepbed mudstone – show evidence for a significant amorphous component of unclear origin. We developed a mass balance calculation program that determines the range of possible chemical compositions of the crystalline and amorphous components of these samples within the uncertainties of mineral abundances derived from CheMin data. In turn, the chemistry constrains the minimum abundance of amorphous component required to have realistic compositions (all oxides ≥0 wt%): 21—22 wt% for Rocknest and 15—20 wt% for Cumberland, in good agreement with estimates derived from the diffraction patterns (~27 and ~31 wt%, respectively). Despite obvious differences between the Rocknest sand and the Sheepbed mudstone, the amorphous components of the two sites are chemically very similar, having comparable concentrations of SiO 2 , TiO 2 , Al 2 O 3 , Cr 2 O 3 , FeO T , CaO, Na 2 O, K 2 O and P 2 O 5 . MgO tends to be lower in Rocknest, although it may also be comparable between the two samples depending on the exact composition of the smectite in Sheepbed. The only unambiguous difference is the SO 3 content, which is always higher in Rocknest. The observed similarity suggests that the two amorphous components share a common origin or formation process. The individual phases possibly present within the amorphous components include: volcanic (or impact) glass, hisingerite (or silica + ferrihydrite), amorphous sulfates (or adsorbed SO 4 2− ) and nanophase ferric oxides.

Description: During the 1994 Clementine lunar mapping mission, portions of 25 orbits were dedicated to a search for lunar horizon glow (LHG) using the spacecraft star tracker navigation cameras. Previous putative detections of LHG were believed to result from forward scattering of sunlight by exospheric dust grains with radii ≈ 0.1 µm, observable above the limb from within the shadow of the Moon near orbital sunrise or sunset. We have examined star tracker image sequences from five Clementine orbits in which the limb occulted the Sun, and was at least partially shadowed from earthshine, minimizing the chance of stray light contamination. No LHG appears in the image data, or in any of the net-brightness images, after subtraction of a reference zodiacal light model. However, some of the images display faint excess limb brightness that appears to be solar streamer structure. Therefore, we derive upper limits for the amount of dust in the lunar exosphere that could be hidden by these brightness fluctuations, using a dust-scattering simulation code and simple exponential dust profiles defined by surface concentration n 0 and scale height H . Simulations using grains of radius 0.1 µm show that fluctuations in the observed excess brightness can be matched by a dust exosphere with a vertical column abundance n 0 H of 5–30 cm −2 , and overlying mass 〈10 −12  g cm −2 . These dust upper limit estimates are highly dependent on assumed grain size, due to the rapid increase in per-grain brightness with grain radius.

Description: The depth/diameter ratio for new meter- to decameter-scale martian craters formed in the last ~20 years averages 0.23, only slightly deeper than that expected for simple primary craters on rocky surfaces. Large variations in d/D between impact sites indicate that differences between the sites such as target material properties, impact velocity, angle, and physical state of the bolide(s) are important in determining the depth of small craters in the strength regime. On the Moon, the d/D of random fresh small craters with similar diameters averages only 0.10, indicating that either the majority of them are unrecognized secondaries, or some proportion are degraded primaries. Older craters such as these may be shallower due to erosional infilling, which is probably not linear over time but more effective over recently-disturbed and steeper surfaces, processes that are not yet acting on the new martian craters. Brand new meter- to decameter-scale craters such as the martian ones studied here are statistically easily distinguishable as primaries, but the origins of older craters of the same size, such as the lunar ones in this study, are ambiguous.

Description: The landscape of Mars shows incised channels that often appear abruptly in the landscape, suggesting a groundwater source. However, groundwater outflow processes are unable to explain the reconstructed peak discharges of the largest outflow channels based on their morphology. Therefore, there is a disconnect between groundwater outflow processes and the resulting morphology. Using a combined approach with experiments and numerical modelling, we examine outflow processes that result from pressurized groundwater. We use a large sandbox flume, where we apply a range of groundwater pressures at the base of a layer of sediment. Our experiments show that different pressures result in distinct outflow processes and resulting morphologies. Low groundwater pressure results in seepage, forming a shallow surface lake and a channel when the lake overflows. At intermediate groundwater pressures, fissures form and groundwater flows out more rapidly. At even higher pressures, the groundwater initially collects in a subsurface reservoir that grows due to flexural deformation of the surface. When this reservoir collapses, a large volume of water is released to the surface. We numerically model the ability of these processes to produce floods on Mars and compare the results to discharge estimates based on previous morphological studies. We show that groundwater seepage and fissure outflow are insufficient to explain the formation of large outflow channels from a single event. Instead, formation of a flexure-induced subsurface reservoir and subsequent collapse generates large floods that can explain the observed morphologies of the largest outflow channels on Mars and their source areas.

Description: Studies of rocks in Gale Crater and clasts within the Martian meteorite breccia NWA 7034 (and paired stones) have expanded our knowledge of the diversity of igneous rocks that make up the Martian crust beyond those compositions exhibited in the meteorite collection or analyzed at any other landing site. Therefore, the magmas that gave rise to these rocks may have been generated at significantly different conditions in the Martian mantle than those derived from previously studied rocks. Here, we build upon our previous models of basalt formation based on rocks analyzed in Gusev Crater and Meridiani Planum to the new models of basalt formation for compositions from Gale Crater and a clast in meteorite NWA 7034. Estimates for the mantle potential temperature, T P based on Noachian age rock analyses in Gale Crater, Gusev Crater, and Bounce Rock in Meridiani Planum, and a vitrophyre clast in NWA 7034 are within error which suggests that the calculated average T P of 1450 ± 70 °C may represent an average global mantle temperature during the Noachian. The T P estimates for the Hesperian and Amazonian, based on orbital analyses of the chemistry of the crust, are lower in temperature than our estimates for the Noachian which is consistent with simple convective cooling of the interior of Mars. However, the T P estimates from the young meteorites are significantly higher than the estimates based on surface chemistry and are consistent with localized ‘hot-spot’ melting and not heating up of the interior.

Description: The Cassini Synthetic Aperture Radar has been acquiring images of Titan's surface since October 2004. To date, 59% of Titan's surface has been imaged by radar, with significant regions imaged more than once. Radar data suffer from speckle noise hindering interpretation of small-scale features and comparison of re-imaged regions for change detection. We present here a new image analysis technique that combines a denoising algorithm with mapping and quantitative measurements that greatly enhances the utility of the data and offers previously unattainable insights. After validating the technique, we demonstrate the potential improvement in understanding of surface processes on Titan and defining global mapping units, focusing on specific landforms including lakes, dunes, mountains and fluvial features. Lake shorelines are delineated with greater accuracy. Previously unrecognized dissection by fluvial channels emerges beneath shallow methane cover. Dune wavelengths and inter-dune extents are more precisely measured. A significant refinement in producing digital elevation models is shown. Interactions of fluvial and aeolian processes with topographic relief is more precisely observed and understood than previously. Benches in bathymetry are observed in northern sea Ligeia Mare. Submerged valleys show similar depth suggesting that they are equilibrated with marine benches. These new observations suggest a liquid level increase in the northern sea, which may be due to changes on seasonal or longer timescales.

Description: No abstract is available for this article.

Description: Using approximately 410 limb-viewing observations from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), we retrieve the vertical distribution of Martian dust and water ice aerosol particle sizes. We find that dust particles have an effective radius of 1.0 µm over much of the atmospheric column below 40 km throughout the Martian year. This includes the detached tropical dust layers detected in previous studies. Little to no variation with height is seen in dust particle size. Water ice clouds within the aphelion cloud belt exhibit a strong sorting of particle size with height, however, and the effective radii range from 〉3 µm below 20 km to near 1.0 µm at 40 km altitude. Conversely, water ice clouds in the seasonal polar hoods show a near-uniform particle size with an effective radius of approximately 1.5 µm throughout the atmospheric column.

Description: The Mars Science Laboratory Rover Curiosity found host rocks of basaltic composition and alteration assemblages containing clay minerals at Yellowknife Bay, Gale Crater. On the basis of the observed host rock and alteration minerals, we present results of equilibrium thermochemical modelling of the Sheepbed mudstones of Yellowknife Bay (YKB), in order to constrain formation conditions of its secondary mineral assemblage. Building on conclusions from sedimentary observations by the MSL team, we assume diagenetic, in situ alteration. The modelling shows that the mineral assemblage formed by reaction of a CO 2 -poor and oxidising, dilute aqueous solution (Gale Portage Water) in an open system with the Fe-rich basaltic-composition sedimentary rocks at 10–50 °C and Water/Rock ratio (mass of rock reacted with the starting fluid) of 100–1000, pH of ~7.5-12. Model alteration assemblages contain predominantly phyllosilicates (Fe-smectite, chlorite) the bulk composition of a mixture of which is close to that of saponite inferred from CheMin data and also to that of saponite observed in the nakhlite martian meteorites and terrestrial analogues. To match the observed clay mineral chemistry, inhomogeneous dissolution dominated by the amorphous phase and olivine is required. We therefore deduce a dissolving composition of approximately 70 % amorphous material, with 20 % olivine, and 10 % whole rock component.

Description: Observations and models of Ceres suggest that its evolution was shaped by interactions between liquid water and silicate rock. Hydrothermal processes in a heated core require both fractured rock and liquid. Using a new core cracking model coupled to a thermal evolution code, we find volumes of fractured rock always large enough for significant interaction to occur. Therefore, liquid persistence is key. It is favored by antifreezes such as ammonia, by silicate dehydration which releases liquid, and by hydrothermal circulation itself, which enhances heat transport into the hydrosphere. The effect of heating from silicate hydration seems minor. Hydrothermal circulation can profoundly affect Ceres' evolution: it prevents core dehydration via “temperature resets”, core cooling events lasting ~50 Myr during which Ceres' interior temperature profile becomes very shallow and its hydrosphere is largely liquid. Whether Ceres has experienced such extensive hydrothermalism may be determined through examination of its present-day structure. A large, fully hydrated core (radius 420 km) would suggest that extensive hydrothermal circulation prevented core dehydration. A small, dry core (radius 350 km) suggests early dehydration from short-lived radionuclides, with shallow hydrothermalism at best. Intermediate structures with a partially dehydrated core seem ambiguous, compatible both with late partial dehydration without hydrothermal circulation, and with early dehydration with extensive hydrothermal circulation. Thus, gravity measurements by the Dawn orbiter, whose arrival at Ceres is imminent, could help discriminate between scenarios for Ceres' evolution.

Description: A new Mars Global Ionosphere-Thermosphere Model (M-GITM) is presented that combines the terrestrial GITM framework [ Ridley et al. , 2006] with Mars fundamental physical parameters, ion-neutral chemistry, and key radiative processes in order to capture the basic observed features of the thermal, compositional, and dynamical structure of the Mars atmosphere from the ground to the exosphere (0-250 km). Lower, middle, and upper atmosphere processes are included, based in part upon formulations used in previous lower and upper atmosphere Mars GCMs. This enables the M-GITM code to be run for various seasonal, solar cycle, and dust conditions. M-GITM validation studies have focused upon simulations for a range of solar and seasonal conditions. Key upper atmosphere measurements are selected for comparison to corresponding M-GITM neutral temperatures and neutral-ion densities. In addition, simulated lower atmosphere temperatures are compared with observations in order to provide a first order confirmation of a realistic lower atmosphere. M-GITM captures solar cycle and seasonal trends in the upper atmosphere that are consistent with observations, yielding significant periodic changes in the temperature structure, the species density distributions, and the large scale global wind system. For instance, mid-afternoon temperatures near ~ 200 km are predicted to vary from ~210 to 350 K (equinox) and ~190 to 390 k (aphelion to perihelion) over the solar cycle. These simulations will serve as a benchmark against which to compare episodic variations (e.g. due to solar flares, dust storms) in future M-GITM studies. Additionally, M-GITM will be used to support MAVEN mission activities (2014-2016).

Description: We reconstruct the abundance of thorium near the Compton-Belkovich Volcanic Complex on the Moon, using data from the Lunar Prospector Gamma Ray Spectrometer. We enhance the resolution via a pixon image reconstruction technique, and find that the thorium is distributed over a larger (40 km × 75 km) area than the (25 km × 35 km) high albedo region normally associated with Compton-Belkovich. Our reconstructions show that inside this region, the thorium concentration is 14–26 ppm. We also find additional thorium, spread up to 300 km eastward of the complex at ~2 ppm. The thorium must have been deposited during the formation of the volcanic complex, because subsequent lateral transport mechanisms, such as small impacts, are unable to move sufficient material. The morphology of the feature is consistent with pyroclastic dispersal and we conclude that the present distribution of thorium was likely created by the explosive eruption of silicic magma.

Description: The geologic history of the northern plains of Mars has been extensively debated. Specifically, the Vastitas Borealis Formation (VBF) represents an enigmatic surface unit that exhibits flow morphologies at its boundary, extensive surface fracturing, tens of thousands of small mounds, and unique crater morphologies. Here, we test the hypothesis that the VBF in the region of Chryse and Acidalia Planitiae, Mars, originated through the compaction and later expulsion of fluid-laden sediments sourced from the shallow subsurface. We find that the morphological, thermophysical, and spectral properties of the VBF marginal unit, in addition to the recent identification of fine-grained sedimentary layers in the shallow subsurface, are all consistent with such a formation mechanism. Estimates of volume loss based on “collar-like” morphologies present on high-standing buttes suggest that a minimum fluid volume of ~13,500 km 3 was expelled from the subsurface, making the VBF a significant ancient hydrologic reservoir on the martian surface. This formation mechanism lends additional insight into the sedimentary and aqueous history of the northern plains of Mars and unites morphologic, spectral, thermophysical, and structural observations under one consistent formation hypothesis.

Description: The Opportunity rover's exploration of the portion of the rim of Endeavour crater known as Cape York included examination of the sulfate-bearing Grasberg formation and the Matijevic Hill region. Multispectral visible and near infrared (VNIR) Pancam observations were used to characterize reflectance properties of rock units. Using spectral endmember detection and classification approaches including a principal components/n-dimensional visualization, automatic sequential maximum angle convex cone method, and classification through hierarchical clustering, six main spectral classes of rock surfaces were identified: light-toned veins, Grasberg fm., the smectite-bearing Matijevic formation, the hematitic “blueberry” spherules, resistant spherules within the Matijevic fm. dubbed “newberries”, and the Shoemaker formation impact breccia. Some of these could be divided into spectral sub-classes. There were three types of veins: veins in the bench unit of Cape York, thinner veins in the Matijevic fm., and boxwork pattern-forming veins. The bench unit veins had higher 535 nm band depths than the other two vein sub-classes and a steeper 934 to 1009 nm slope. The Grasberg fm. has VNIR spectral features that are interpreted to indicate higher fractions of red hematite than in the sulfate-bearing Burns Fm. The Matijevic fm. includes both light-toned, fine-grained matrix and dark-toned veneers. The latter has a weak NIR absorption band centered near 950 nm consistent with nontronite. Observations of Rock Abrasion Tool brushed and ground newberries indicated that cuttings from the RAT grind had a longer wavelength reflectance maximum and deeper 535 nm band depth, consistent with more oxidized materials. Greater oxidation of cementing materials in the newberries is consistent with a diagenetic concretion origin.

Description: No abstract is available for this article.

Description: 13C/12C and 15 N/14 N isotopic ratios are pivotal for our understanding of the Martian carbon cycle, history of the Martian atmospheric escape and origin of the organic compounds on Mars. Here we demonstrate that the carbon and nitrogen isotopic composition of the surface rocks on Mars can be significantly altered by the continuous exposure of Martian surface to cosmic rays. Cosmic rays can effectively produce 13C and 15 N isotopes via spallation nuclear reactions on oxygen atoms in various Martian rocks. We calculate that in the top meter of the Martian rocks the rates of production of both 13C and 15 N due to galactic cosmic rays (GCRs) exposure can vary within 1.5-6 atoms/cm3/s depending on rocks’ depth and chemical composition. We also find that the average solar cosmic rays (SCRs) can produce carbon and nitrogen isotopes at a rate comparable to GCRs in the top 5–10 cm of the Martian rocks. We demonstrate that if the total carbon content in a surface Martian rock is 〈10 ppm then the “light”, potentially “biological” 13C/12C ratio would be effectively erased by cosmic rays over 3.5 billion years of exposure. We found that for the rocks with relatively short exposure ages (e.g. 100 million years), cosmogenic changes in 15 N/14 N ratio are still very significant. We also show that a short exposure to CRs of ALH 84001 while on Mars can explain its high-temperature heavy nitrogen isotopic composition (15 N/14 N). Applications to Martian meteorites and the current MSL mission are discussed.

Description: Physical properties of terrains encountered by the Curiosity rover during the first 360 sols of operations have been inferred from analysis of the scour zones produced by Sky Crane Landing System engine plumes, wheel touchdown dynamics, pits produced by ChemCam laser shots, rover wheel traverses over rocks, the extent of sinkage into soils, and the magnitude and sign of rover-based slippage during drives. Results have been integrated with morphologic, mineralogic, and thermophysical properties derived from orbital data, and Curiosity-based measurements, to understand the nature and origin of physical properties of traversed terrains. The hummocky plains (HP) landing site and traverse locations consist of moderately to well consolidated bedrock of alluvial origin variably covered by slightly cohesive, hard-packed basaltic sand and dust, with both embedded and surface-strewn rock clasts. Rock clasts have been added through local bedrock weathering and impact ejecta emplacement and form a pavement-like surface in which only small clasts (〈5 to 10 cm wide) have been pressed into the soil during wheel passages. The bedded fractured (BF) unit, site of Curiosity's first drilling activity, exposes several alluvial-lacustrine bedrock units with little to no soil cover and varying degrees of lithification. Small wheel sinkage values (〈1 cm) for both HP and BF surfaces demonstrate that compaction resistance countering driven-wheel thrust has been minimal and that rover slippage while traversing across horizontal surfaces or going uphill, and skid going downhill, have been dominated by terrain tilts and wheel-surface material shear modulus values.

Description: Sedimentary rocks preserved on the surface of Mars represent a natural archive of past climate conditions. Although the details of their formation often remain poorly constrained, the recent detection of rhythmic bedding patterns in the Arabia Terra region suggests the influence of orbital variations on sedimentary deposition. Here, we detail a number of new sites which exhibit quasiperiodic stratigraphic variations, demonstrating their occurrence throughout the equatorial region of the planet. We characterize these recorded signals as well as the local geomorphic context and structural attributes. Two cyclic units are identified within Gale crater, the landing site of the Mars Science Laboratory mission, enabling estimation of possible formation timescales for the geologic units that may be studied in situ by the rover. We find a general lack of fluvial features in connection with rhythmic geologic units, contrasting these sites with the aperiodic deltaic stratigraphy found at Eberswalde crater. Possible formation scenarios and their climatic implications are discussed for the diverse set of quasiperiodic sedimentary units. We propose multiple depositional pathways for recording cyclic climate changes, including repeated evaporitic precipitation from groundwater discharge in topographic lows as well as largely anhydrous accumulation of atmospheric dust for deposits outside of confined basins. The preservation of orbital signals in sediments distributed across a wide range of geographic settings suggests a pervasive influence on Martian climate conditions through time.

Description: Data gathered with the Dynamic Albedo of Neutrons (DAN) instrument onboard Curiosity rover were analyzed for variations in subsurface neutron flux and tested for possible correlation with local geological context. A special DAN observation campaign was executed in which 18 adjacent DAN active measurements were acquired every 0.75-1.0 m to search for variations of subsurface hydrogen content along a 15 m traverse across geologic contacts between the Sheepbed and Gillespie Lake members of the Yellowknife Bay formation. It was found that several subunits in Sheepbed and Gillespie Lake could be characterized with different depth distributions of water equivalent hydrogen (WEH) and different chlorine equivalent abundance responsible for the distribution of neutron absorption elements. The variations of average WEH at the top 60 cm of the subsurface are estimated at up to 2-3%. Chlorine equivalent neutron absorption abundances ranged within 0.8-1.5%. The largest difference in WEH and Chlorine equivalent neutron absorption distribution is found between Sheepbed and Gillespie Lake.

Description: Three-dimensional (3D) inversion of gravity data has been widely used to reconstruct the density distributions of ore bodies, basins, crust, lithosphere and upper mantle. At present, gravity inversions are based on density model representation in the Cartesian coordinates, which is applicable for local to regional scales. For global model of 3D density structures of planetary interior, such as the Earth, the Moon or Mars, it is necessary to use an inversion algorithm that operates in the spherical coordinates. We develop a 3D inversion algorithm formulated with specially designed model objective function and radial weighting function in the spherical coordinates. We present a regional and a global synthetic example to illustrate the capability of the algorithm. The inverted results show density distribution features consistent with the true models. We also apply the algorithm to a set of lunar Bouguer gravity anomaly, and obtain a lunar 3D density distribution. High density anomalies are clearly identified underlying lunar basins, and a wide region of the lateral density heterogeneities are found that exist beneath the South Pole-Aitken (SPA) basin; and low density anomalies are distributed beneath the Feldspathic Highlands Terrane (FHT) on the lunar far-side. The consistency of these results with those obtained independently from other existing methods verifies the newly developed algorithm.

Description: We analyze the ROSAT PSPC soft X-ray image of the moon taken on 29 June 1990 by examining the radial profile of the surface brightness in three wedges, two 19 degree wedges (one north and one south) 13–32 degrees off the terminator towards the dark side and one wedge 38 degrees wide centered on the antisolar point. The radial profiles of both the north and the south wedges show significant limb brightening that is absent in the 38 degree wide antisolar wedge. An analysis of the soft X-ray intensity increase associated with the limb brightening shows that its magnitude is consistent with that expected due to solar wind charge exchange (SWCX) with the tenuous lunar atmosphere based on lunar exospheric models and hybrid simulation results of solar wind access beyond the terminator. Soft X-ray imaging thus can independently infer the total lunar limb column density including all species, a property that before now has not been measured, and provide a large-scale picture of the solar wind-lunar interaction. Because the SWCX signal appears to be dominated by exospheric species arising from solar wind implantation, this technique can also determine how the exosphere varies with solar wind conditions. Now along with Mars, Venus, and Earth, the moon represents another solar system body at which SWCX has been observed.

Description: The analysis of the Surface Energy Budget (SEB) yields insights into soil-atmosphere interactions and local climates, while the analysis of the thermal inertia ( I ) of shallow subsurfaces provides context for evaluating geological features. Mars orbital data have been used to determine thermal inertias at horizontal scales of ~10 4  m 2 to ~10 7  m 2 . Here we use measurements of ground temperature and atmospheric variables by Curiosity to calculate thermal inertias at Gale Crater at horizontal scales of ~10 2  m 2 . We analyze three sols representing distinct environmental conditions and soil properties, sol 82 at Rocknest (RCK), sol 112 at Point Lake (PL) and sol 139 at Yellowknife Bay (YKB). Our results indicate that the largest thermal inertia I  = 452 J m −2  K −1  s −1/2 (SI units used throughout this article) is found at YKB followed by PL with I  = 306 and RCK with I  = 295. These values are consistent with the expected thermal inertias for the types of terrain imaged by Mastcam and with previous satellite estimations at Gale Crater. In contrast with previous studies, we also calculate the SEB using data from measurements by Curiosity's Rover Environmental Monitoring Station and dust opacity values derived from measurements by Mastcam. The knowledge of the SEB and thermal inertia has the potential to enhance our understanding of the climate, the geology and the habitability of Mars.

Description: Evidence for widespread non-uniform vertical mixing of dust in Mars's tropical atmosphere (in the form of features called “detached dust layers” or DDLs) is a challenge for atmospheric modeling. We characterize the seasonal, diurnal, and geographic variability of DDL activity in retrievals from observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter (MRO). We find that dust injection above the boundary layer, which forms DDLs, is a spatially ubiquitous phenomenon in the tropics during the daytime, implying that it has a significant non-topographic component. DDL formation is more intense in northern spring and summer than in southern spring and summer, but is still common when the zonal average dust distribution appears uniformly mixed. DDLs do not appear to follow the upwelling associated with Mars's Hadley circulation or the extant climatology of local dust storm activity in the tropics. Geographic variability in the nightside vertical dust distribution does not always correlate with the dayside vertical dust distribution, implying that there is spatial and seasonal variability in the efficiency of dust deposition/removal processes. Nighttime dust removal is especially efficient over the Tharsis Montes during northern spring and summer, which suggests some association between water ice clouds and removal. Intense injection combined with efficient removal results in a high amplitude of diurnal variability in the dust distribution at 15-30 km above the surface of the tropics during much of the Martian year.

Description: [1]  We describe preliminary results from the first 100 sols of ground temperature measurements along the Mars Science Laboratory's traverse from Bradbury Landing to Rocknest in Gale. The ground temperature data show long term increases in mean temperature that are consistent with seasonal evolution. Deviations from expected temperature trends within the diurnal cycle are observed and may be attributed to rover and environmental effects. Fits to measured diurnal temperature amplitudes using a thermal model suggest that the observed surfaces have thermal inertias in the range of 265 – 375 J · m -2  · K -1  · s -1/2 , which are within the range of values determined from orbital measurements, and are consistent with the inertias predicted from the observed particle sizes on the uppermost surface near the rover. Ground temperatures at Gale crater appear to warm earlier and cool later than predicted by the model, suggesting that there are multiple unaccounted-for physical conditions or processes in our models. Where the MSL descent engines removed a mobile layer of dust and fine sediments from over rockier material, the diurnal temperature profile is closer to that expected for a homogeneous surface, suggesting that the mobile materials on the uppermost surface may be partially responsible for the mismatch between observed temperatures and those predicted for materials having a single thermal inertia. Models of local stratigraphy also implicate thermophysical heterogeneity at the uppermost surface as a potential contributor to the observed diurnal temperature cycle.

Description: [1]  Cross­cutting relationships of tectonic lineaments on Europa record the history of surface deformation. We mapped the displacement and orientation of older features cross­cut by two types of lineaments: bands and double ridges. These measurements allow us to determine both the strike-perpendicular and strike­parallel displacement along investigated features. Double ridges record both ridge-perpendicular contraction and expansion, with a mean of 0.16 ± 0.06 km of contraction based on the analysis of sixteen double ridges. Bands record expansion, with a mean of 3.33 ± 0.27 km for the six bands analyzed, but with perpendicular displacement less than their apparent morphologic widths of 3­24 km. The implied global surface strain for double ridges (including those that expand) and bands is 2.22 ± 0.76% contraction and 7.60 ± 3.7% expansion, respectively. Double ridges thus may accommodate part of the surface expansion recorded by bands. Most current models for double ridges do not predict contraction. The models that satisfy the observations for bands are “slow spreading” models, cryovolcanism, and folding.

Description: [1]  With its young surface, very few impact craters and the abundance of tectonic and cryovolcanic features, Europa has likely been subjected to relatively recent endogenic activity. Morphological analyses of chaos terrains and double ridges suggest the presence of liquid water within the ice shell a few kilometers below the surface, which may result from enhanced tidal heating. A major issue concerns the thermal/gravitational stability of these water reservoirs. Here, we investigate the conditions under which water can be generated and transported through Europa's ice shell. We address particularly the downward two-phase flow by solving the equations for a two-phase mixture of water ice and liquid water in one-dimensional geometry. In the case of purely temperate ice, we show that water is transported downwards very efficiently in the form of successive porosity waves. The time needed to transport the water from the subsurface region to the underlying ocean varies between ∼ 1 and 100 kyr, depending mostly on the ice permeability. We further show that water produced in the head of tidally-heated hot plumes never accumulates at shallow depths and is rapidly extracted from the ice shell (within less than a few hundred kyr). Our calculations indicate that liquid water will be largely absent in the near subsurface, with the possible exception of cold conductive regions subjected to strong tidal friction. Recently active double ridges subjected to large tidally-driven strike-slip motions are perhaps the most likely candidates for the detection of transient water lenses at shallow depths on Europa.

Description: [1]  Pits, domes and small chaos on Europa's surface are quasi-circular features a few to a few tens of kilometers in diameter. We examine if injection of water sills into Europa's ice shell and their subsequent evolution can induce successive surface deformations similar to the morphologies of these features. We study the dynamics of water spreading within the elastic part of the ice shell and show that the mechanical properties of ice exert a strong control on the lateral extent of the sill. At shallow depths, water makes room for itself by lifting the overlying ice layer and water weight promotes lateral spreading of the sill. In contrast, a deep sill bends the underlying elastic layer and its weight does not affect its spreading. In that case, the sill lateral extent is limited by the fracture toughness of ice and the sill can thicken substantially. After emplacement, cooling of the sill warms the surrounding ice and thins the overlying elastic ice layer. As a result, preexisting stresses in the elastic part of the ice shell increase locally to the point that they may disrupt the ice above the sill (small chaos). Disruption of the surface also allows for partial isostatic compensation of water weight, leading to a topographic depression at the surface (pits), of the order of ∼ 10 2 m. Complete water solidification finally causes expansion of the initialsill volume and results in an uplifted topography (domes) of ∼ 10 2 m.

Description: We investigate the effects of space weathering at ultraviolet wavelengths using a near global seven-band (321–689 nm) mosaic from the Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC). We confirm that for moderate- to high-iron compositions (〉 ~ 5 wt% FeO), the steeply positive UV slope at wavelengths 〈415 nm shallows with increasing exposure to space weathering. We measure these differences in LROC WAC data as variations in the 321/415 nm ratio, which has low values for fresh craters in the mare and moderate-iron highlands. For low-iron highland compositions, the break in slope occurs at shorter wavelengths, and it is instead the 321/360 nm ratio that increases with exposure to the space-weathering environment, whereas the 321/415 nm ratio appears to be largely controlled by the degree of shock experienced during the impact. The effects of shock may be more important at highland craters because modest shock pressures result in the solid-state transformation of plagioclase to its glass equivalent, maskelynite, and can help distinguish between primary shocked ejecta and locally exposed fresh material in rays. While all of the “fresh” craters we examined have UV spectral properties consistent with substantial alteration due to space weathering, the UV spectra of lunar swirls (magnetically shielded from the solar wind) are consistent with exposure of immature, crystalline material. Together these results suggest that lunar space weathering is dominated by the solar wind and “saturates” in the UV at I S /FeO values of ~40 (submature).

Description: To study the transport of the ionospheric plasma on Mars, we have included a 3D multifluid dynamical core in a Martian General Circulation Model (GCM). Vertical transport modifies the ion density above ~160 km on the dayside, especially the ions produced at high altitudes like O + , N + and C + . Near the exobase, the dayside to nightside flow velocity reaches few hundreds of m/s, due to a large horizontal pressure gradient. Comparison with MEX/ASPERA-3 measurements between 290 – 500 km, suggests this flow could account for at least 20% of the flow produced by the solar wind. This flow is not sufficient to populate substantially the nightside ionosphere at high altitudes, in agreement with recent observations, because of a strong nightside downward flow produced by vertical pressure gradient. The O 2 + and NO + ion densities on the nightside at low altitudes (~130 km) are modified by this downward flow, compared to simulated densities without ion dynamics, while other ions are lost by chemical reactions. Variability at different time scales (diurnal, seasonal and solar cycle) are studied. We simulate diurnal and seasonal variations of the ionospheric composition due to the variability of the neutral atmosphere and solar flux at the top of the atmosphere. The ionospheric dynamics are not strongly affected by seasons and solar cycles and the retroaction of the ionosphere on the neutral atmosphere temperature and velocity is negligible compared to other physical processes below the exobase.

Description: We derived spatially resolved near-global Hapke photometric parameter maps of the Moon from 21 months of Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) multispectral observations using a novel " tile-by-tile" method (1° latitude by 1°longitude bins). The derived six parameters ( w , b , c , B S 0 , h S , ) for each tile were used to normalize the observed reflectance (standard angles i  =  g  = 60°, e  = 0° instead of the traditional angles i  =  g  = 30°, e = 0°) within each tile, resulting in accurate normalization optimized for the local photometric response. Each pixel in the seven-color near-global mosaic (70°S to 70°N and 0°E to 360°E) was computed by the median of normalized reflectance from large numbers of repeated observations (UV: ~50, visible: ~126 on average). The derived mosaic exhibits no significant artifacts with latitude or along the tile boundaries demonstrating the quality of the normalization procedure. The derived Hapke parameter maps reveal regional photometric response variations across the lunar surface. The b , c (Henyey-Greenstein double-lobed phase function parameters) maps demonstrate decreased backscattering in the maria relative to the highlands (except 321 nm band), probably due to the higher content of both SMFe (sub-micron iron) and ilmenite in the interiors of back scattering agglutinates in the maria. The h S (angular width of shadow hiding opposition effect) map exhibits relatively lower values in the maria than the highlands, and slightly higher values for immature highland crater ejecta, possibly related to the variation in a grain size distribution of regolith.

Description: Desorption of H 2 O ( v  = 0) following 157-nm irradiation of amorphous solid water (ASW) on a lunar impact melt breccia was measured with resonance-enhanced multiphoton ionization (REMPI). Photofragments of vibrationally excited water were detected with non-resonant ionization. The average cross section for H 2 O ( v  = 0) removal and destruction at 0.1 Langmuir (1 L = 10 −6  Torr∙s) H 2 O exposure was measured to be (7.1 ± 1.9) x 10 −19  cm 2 . Cross sections were also measured at 0.3, 1, 5, and 10 L exposures. Because these cross sections increase drastically with decreasing water coverage, water is not expected to remain intact as H 2 O on the sunlit lunar surface. Instead, photons are likely to cause H 2 O to desorb or dissociate. The OH + fragment of H 2 O ( v *) increased in intensity with increasing irradiation as hydroxyl groups built up on the surface and then recombined. The OH + signal eventually began to decrease after a dose of 5 x 10 18 photons cm −2 . Under these conditions, the cross section for H 2 O ( v *) photodesorption was measured to be 6.4 x 10 −20  cm 2 for an initial exposure of 5 L H 2 O.

Description: [1]  The landing site for Curiosity rover is located at the distal end of the Peace Vallis fan in Gale Crater. Peace Vallis fan covers 80 km 2 and is fed by a 730 km 2 catchment, which drains an uplands plains area through a 15 km wide gap in the crater rim. Valley incision into accumulated debris delivered sediment through a relatively low-density valley network to a mainstem channel to the fan. An estimated total fan volume of 0.9 km 3 matches the calculated volume of removal due to valley incision (0.8 km 3 ), and indicates a mean thickness of 9 m. The fan profile is weakly concave up with a mean slope of 1.5% for the lower portion. Numerous inverted channels outcrop on the western surface of the fan, but on the eastern portion such channels are rare suggesting a change in process from distributary channel domination on the west to sheetflow on the eastern portion of the fan. Runoff (discharge/watershed area) to produce the fan is estimated to be more than 600 m, perhaps as much as 6000 m, indicating a hydrologic cycle that likely lasted at least 1000's of years. Atmospheric precipitation (possibly snow) not seepage produced the runoff. Based on topographic data, Peace Vallis fan likely onlapped Bradbury Rise and spilled into a topographic low to the east of the rise. This argues that the light-toned fractured terrain within this topographic low corresponds to the distal deposits of Peace Vallis fan and in such a setting lacustrine deposits are expected.

Description: [1]  We use a finite-element model to solve for the response of Ganymede and Europa to tidal forcing from Jupiter, using various icy shell models with laterally variable (3-D) structure. In all cases, the shell is assumed to be underlain by a liquid water ocean. Icy shells with laterally varying thickness are derived from a thermal conduction model. 3-D shear modulus profiles for the shell are built either from a conduction model or, for Europa, by assuming a hemispherical difference in composition. Icy shell structures with a non-global ocean are built for Ganymede. Using these shell structures to calculate the tidal response of Ganymede and Europa, we conclude: (1) the presence of lateral variations in thickness or in shear modulus would not degrade future attempts to use tidal observations to decide on the existence or absence of a liquid ocean, and to determine the mean icy shell thickness. (2) Given accurate enough observations, the presence of lateral variations in thickness or in shear modulus could be determined by searching for non-degree-2 components in the tidal response. (3) In the absence of significant viscous convective flow in the shell, the effects of a laterally varying shear modulus on the tidal response would be smaller than those of a laterally varying shell thickness. (4) If the shell is partially grounded, tidal observations of either gravity or uplift would be able to roughly differentiate regions where the ice is grounded from those where it is floating.

Description: We present latitude and longitude distributions of Na + and K + fluxes from the Moon derived from Kaguya low-energy ion data. Although the latitude distribution agrees with previous ground-based telescope observations, dawn-dusk asymmetry has been determined in the longitude distribution. Our model of the lunar surface abundance and yield of Na and K demonstrates that the abundance decreases to approximately 50% at dusk compared with that at dawn due to the emission of the exospheric particles assuming the ion fluxes observed by Kaguya are proportional to the yield. It is also implied that the surface abundance of Na and K need to be supplied during the night to explain the observed lunar exosphere with dawn-dusk asymmetry. We argue that the interplanetary dust as well as grain diffusion and migration/recycling of the exospheric particles may be major suppliers.

Description: Widespread detection of phyllosilicates (clay minerals) in Noachian-age (〉3.5 Ga) terrains on Mars and their paucity in younger terrains has led to the hypothesis that the Noachian period was characterized by more clement conditions than the colder, drier conditions experienced since that time. However, recent detections of clay minerals in several Hesperian-age impact craters suggest that fluvial transport and alteration were possible after the posited early era of phyllosilicate formation. Here we present evidence that rocks within Ritchey crater (28.5˚S, 51˚W), a Hesperian impact crater, record a period of post-Noachian fluvial transport and in situ alteration. This resulted in the transport of clays from the crater wall to the crater floor and the formation of hydrated silica and Fe/Mg smectite in Ritchey's central uplift. Clay minerals associated with central uplifts of impact craters are commonly interpreted to represent pre-existing clay-bearing rocks excavated from depth, potentially providing insight into crustal and older clay-forming processes. Here we present detailed geomorphic and mineralogic maps and show that the clay minerals in Ritchey's central peak formed after or as a direct result of the impact and are thus Hesperian or younger. Clay minerals are also detected along the crater wall and floor and are consistent with a source-to-sink fluvial transport system. Clays on the crater wall were either pre-existing clays exposed by the impact or formed in situ through post-impact water-rock interaction. In either scenario, some of these clays were likely subsequently transported to the crater floor by fluvial-alluvial processes. In this context, the hydrated phases in Ritchey indicate several different formation and transport mechanisms and provide further evidence that near-surface clay mineral formation, and thus habitable conditions, existed on Mars after the Noachian.

Description: A series of numerical models of magmatism and mantle convection with a stagnant lithosphere are developed to understand the mantle evolution in Venus. Magmatism is modeled as a permeable flow of basaltic magma generated by decompression melting, and the solid-state convection of mantle materials with temperature-dependent Newtonian rheology is affected by the garnet-perovskite transition and the post-spinel transition. In our preferred models, the mantle evolves in two stages: The earlier stage is characterized by a layered mantle convection punctuated by repeated bursts of hot materials from the deep mantle to the surface. Mantle bursts induce vigorous magmatism, and also cause the basaltic crust, enriched in heat-producing elements (HPEs), to recycle into the mantle. A part of the recycled basaltic crusts accumulates along the post-spinel boundary to form a barrier, and this basalt barrier causes mantle convection to become layered. At a later stage, when the HPEs have already decayed, in contrast, the basalt barrier disappears and whole-mantle convection occurs more steadily. Mild magmatism is induced by small-scale partial melting at the base of the crust and hot plumes from the deep mantle. The internal heating by the HPEs that recycled into the mantle in the earlier stage allows the magmatism of the later stage to continue throughout the calculated history of mantle evolution. The two stages arise when the barrier effect of the post-spinel transition is weak and the lithosphere is mechanically strong enough. The two-stage evolution model meshes with the observed history of magmatism and the lithosphere on Venus.

Description: No abstract is available for this article.

Description: We study the backscattering of solar wind protons from the lunar regolith using the Solar Wind Monitor (SWIM) of the Sub-keV Atom Reflecting Analyzer (SARA) on Chandrayaan-1. Our study focuses on the component of the backscattered particles that leaves the regolith with a positive charge. We find that the fraction of the incident solar wind protons that backscatter as protons, i.e., the proton-backscattering efficiency, has an exponential dependence on the solar wind speed that varies from ~0.01% to ~1% for solar wind speeds of 250 km/s to 550 km/s. We also study the speed distribution of the backscattered protons in the fast (~550 km/s) solar wind case and find both a peak speed at ~80% of the solar wind speed and a spread of ~85 km/s. The observed flux variations and speed distribution of the backscattered protons can be explained by a speed-dependent charge state of the backscattered particles.

Description: The Mars Science Laboratory (MSL) rover, Curiosity , has a titanium science observation tray (o-tray), upon which portions from drilled and scooped martian samples can be delivered for analyses by the alpha-particle X-ray spectrometer (APXS). The standard APXS calibration approach to derive elemental concentrations cannot be applied to samples on the o-tray because they 1) have a non-uniform three-dimensional distribution within the APXS field of view and 2) are thin (〈 50 µm) compared to the APXS information depth for heavy elements (〉 90 µm). To develop techniques for interpreting MSL-APXS o-tray measurements, we conducted laboratory measurements of thin particulate basalt samples on Ti metal with the Flight Equivalent APXS Unit. The experiments demonstrate that, relative to an “infinitely thick” sample, increasing areal coverage of particulates on a Ti metal substrate results in a proportional decrease in the Ti signal and increase in the sample signal. Count rates for heavier elements (Mn, Fe) drop with decreasing sample thickness because the mean thickness is smaller than the APXS information depth. Similar effects were seen in the MSL APXS o-tray measurement of Rocknest fines on sol 95, an aliquot of material delivered to SAM (Sample Analysis at Mars) and CheMin (Chemistry and Mineralogy). The thin layer effect caused a drop in Mn and Fe signals, which cannot be quantitatively compared to the in situ Rocknest target “Portage” because sample thickness was unknown. Otherwise, Rocknest fines on the o-tray had no significant compositional differences from Portage, except for slight increases in S and Cl.

Description: Titan, Saturn's largest satellite, is subject to solid body tides exerted by Saturn on the time-scale of its orbital period. The tide-induced internal redistribution of mass results in tidal stress variations, which could play a major role for Titan's geologic surface record. We construct models of Titan's interior that are consistent with the satellite's mean density, polar moment-of-inertia factor, obliquity, and tidal potential Love number k 2 as derived from Cassini observations of Titan's low-degree gravity field and rotational state. In the presence of a global liquid reservoir, the tidal gravity field is found to be consistent with a subsurface water-ammonia ocean more than 180 km thick and overlain by an outer ice shell of less than 110 km thickness. The model calculations suggest comparatively low ocean ammonia contents of less than 5 wt.-% and ocean temperatures in excess of 255 K, i.e., higher than previously thought, thereby substantially increasing Titan's potential for habitable locations. The calculated diurnal tidal stresses at Titan's surface amount to 20 kPa, almost comparable to those expected at Enceladus and Europa. Tidal shear stresses are concentrated in the polar areas, while tensile stresses predominate in the near-equatorial, mid-latitude areas of the sub- and anti-saturnian hemispheres. The characteristic pattern of maximum diurnal tidal stresses is largely compliant with the distribution of active regions such as cryovolcanic candidate areas. The latter could be important for Titan's habitability since those may provide possible pathways for liquid water-ammonia outbursts on the surface and the release of methane in the satellite's atmosphere.

Description: The expression of the Eastern Olympus Mons Basal Scarp (EOMBS) is seemingly unique along the edifice. It exhibits two slope-parallel structures: a nearly 100 km-long up-slope extensional normal fault system, and a down-slope contractional wrinkle ridge network, a combination that is found nowhere else on Olympus Mons. Through structural mapping and numerical modeling of slope stability of the EOMBS, we show that these structures are consistent with landsliding processes and volcanic spreading. The EOMBS is conditionally stable when the edifice contains pore fluid, and critically stable, or in failure, when the edifice contains a dipping overpressured confined aquifer and mechanical sublayer at depth. Failure of the fault bounded portion of the flank results in estimated volumes of material ranging from 5600–6900 km 3 , or 32 – 39% of the estimated volume of the “East” Olympus Mons aureole lobe. We suggest that the EOMBS faults may be an expression of early stage flank collapse and aureole lobe formation. Ages of deformed volcano adjacent plains associated with the wrinkle ridges indicate that this portion of the edifice may have been tectonically active at 〈 50 Ma, and may be coeval with estimated ages of adjacent outflow channels, 25–40 Ma. This observation suggests that conditions that favor flank failure, such as water at depth below the edifice, existed in the relatively recent past and potentially could drive deformation to the present day.

Description: Silicate glasses are an important constituent in the regolith of airless planetary bodies and knowledge of glass reflectance characteristics is important for remote-sensing studies of the Moon, Mercury, and asteroids. We recovered reflectance spectra for 20 vacuum-melted lunar glass simulants measured by Wells [1977], which cover a wider range of Fe and Ti contents (0-17.5 wt.% FeO and 0-15 wt.% TiO 2 ) and a wider wavelength range than those of the better-known Bell et al . [1976] study. We examine the spectra in terms of known absorptions, explore the relationship between ultraviolet spectral parameters and composition, and apply the Hapke radiative transfer model to predict the reflectance spectra of the Wells glasses. The imaginary part of the refractive index ( k ) at each wavelength was computed based on the Ti and Fe composition using the linear relationship presented by Wilcox et al . [2006] and with a new linear-exponential hybrid relationship. Comparison of the model spectra with the measured spectra reveals that the samples rich in Fe and Ti are best modeled by the linear relationship, because the linear model was developed using the Fe- and/or Ti-rich Bell et al . [1976] glasses. For Fe- and Ti-poor glasses, the hybrid model provides a better fit to the measured spectra, because this model for k is based on the wider compositional range of the Wells glasses. In the future, better linear model fits might be obtained if optical parameters were derived for a wider compositional range, from low-Fe/low-Ti to the higher-Fe/higher-Ti compositions of Apollo volcanic glasses.

Description: Small morphologically fresh impact craters (〈10 km in diameter) on Vesta's surface with a photometrically distinct ejecta blanket are expected to represent fresh surface material, and thus provide the opportunity to study the composition of the unweathered surface. Dawn-FC and VIR data reveal impact craters with bright, dark, as well as mixed, i.e. partly bright and dark, ejecta existing on Vesta's surface, which not only differ in the visible albedo from their surroundings but also in their composition. Differences in the composition are related to the visible albedo and/or the geographic location of the impact craters. Bright ejecta, only seen in the southern Vestan hemisphere, are dominated by howardite/eucrite-like material as expected for Vesta's upper crust. Dark ejecta associated with dark impact craters are dominated by a strongly absorbing, spectrally neutral compound, supporting an origin from carbon-rich impactors. Few impact craters of intermediate albedo in Vesta's southern hemisphere contain material resembling diogenites, which are expected to exist in the deeper parts of Vesta's interior. The geological settings suggest that the diogenite-like material represents a part of a layer of diogenitic material surrounding the Rheasilvia basin or local concentrations of diogenitic material as part of the ejecta excavated during the latter stage of the Rheasilvia impact event. The spectral differences between eucrite- and diogenite-dominated materials also could be verified due to spin-forbidden absorptions in the visible spectral range, which are known from laboratory spectra of pyroxenes, but, which have been identified in the VIR spectra of Vesta for the first time.

Description: No abstract is available for this article.

Description: Two important source reactions for hot atomic carbon on Mars are photodissociation of CO and dissociative recombination of CO + ; both reactions are highly sensitive to solar activity and occur mostly deep in the dayside thermosphere. The production of energetic particles results in the formation of hot coronae that are made up of neutral atoms including hot carbon. Some of these atoms are on ballistic trajectories and return to the thermosphere and others escape. Understanding the physics in this region requires modeling that captures the complicated dynamics of hot atoms in 3D. This study evaluates the carbon atom inventory by investigating the production and distribution of energetic carbon atoms using the full 3D atmospheric input. The methodology and details of the hot atomic carbon model calculation are given, and the calculated total global escape of hot carbon from the assumed dominant photochemical processes at a fixed condition, equinox (L s  = 180°) and low solar activity (F10.7 = 70 at Earth), are presented. To investigate the dynamics of these energetic neutral atoms, we have coupled a self-consistent a 3D global kinetic model, the Adaptive Mesh Particle Simulator (AMPS), with a 3D thermosphere / ionosphere model, the Mars Thermosphere General Circulation Model (MTGCM) to provide a self-consistent global description of the hot carbon corona in the upper thermosphere and exosphere. The spatial distributions of density and temperature and atmospheric loss are simulated for the case considered.

Description: Behaviors of bromide, chloride and phosphate were studied experimentally under previously proposed Martian diagenetic conditions, involving jarosite (KFe 3 (OH) 6 (SO 4 ) 2 ), goethite ( α -FeOOH) and hematite ( α -Fe 2 O 3 ). Experiments evaluated: 1) behavior of Cl - /Br - with and without aqueous phosphate during oxidation of Fe 2+ to Fe 3+ ; 2) stability of halogen-bearing jarosite; and 3) uptake of Cl - , Br - , H 2 PO 4 - and SO 4 2- by halogen-free -hematite, -goethite, and -jarosite through adsorption. Our results demonstrate that when precipitated from a solution in which Cl - is higher than Br - , jarosite preferentially incorporated at least an order of magnitude more Br - than Cl - . Such enrichment of Br - over Cl - in the solids compared to initial solutions suggests that jarosite could be a host for elevated Br on the Martian surface, and the fluids from which jarosite forms could be depleted in Br - with respect to Cl - . Moreover, the incorporation of halogens in jarosite would affect its stability during aqueous alteration, and the dissolution rates of four types of jarosite at both 25 ºC and 70 ºC were in the same order: Br,Cl-bearing 〉 Br-only 〉 halogen-free 〉 Cl-only. In addition, competitive adsorption of Cl - , Br - , SO 4 2- and H 2 PO 4 - on halogen-free -hematite, -goethite and -jarosite demonstrates that, in a sulfate-dominant aqueous system, Cl - , Br - and H 2 PO 4 - could not compete with SO 4 2- . This observation suggests that adsorption may not account for enrichment of phosphate or halogens in Fe-oxides in a sulfate-dominant aqueous system like Meridiani Planum, consistent with the absence of significant correlations of Cl and P with nano-particle Fe-oxides found in Martian soils.

Description: Over 500 quasi-circular volcano-tectonic features called coronae occur on Venus. They are believed to form via small-scale mantle upwellings, lithospheric instability, or a combination thereof. Coronae and rifts commonly occur together, including many coronae that lie outside of the fracture zone. However the genetic link between the two has remained unclear. This paper proposes a mechanism for the formation of off-rift coronae due to the rifting process. We model the interaction of a rising mantle plume associated with a rift with a hypothetical preexisting layer of dense material in the lithosphere. We show that a rift and its associated off-rift coronae could be genetically linked by the process of development of secondary ring-like dripping instabilities initiating at the plume margins. We calculate the resulting surface topographies, melt volumes, and Bouguer gravity anomalies and compare them to observations.

Description: Impact-induced fracturing creates porosity that is responsible for many aspects of the geophysical signature of an impact crater. This paper describes a simple model of dilatancy—the creation of porosity in a shearing geological material—and its implementation in the iSALE shock physics code. The model is used to investigate impact-induced dilatancy during simple and complex crater formation on Earth. Simulations of simple crater formation produce porosity distributions consistent with observations. Dilatancy model parameters appropriate for low-quality rock masses give the best agreement with observation; more strongly dilatant behaviour would require substantial post-impact porosity reduction. The tendency for rock to dilate less when shearingunder high pressure is an important property of the model. Pressure suppresses impact-induced dilatancy: in the shock wave; at depth beneath the crater floor; and in the convergent sub-crater flow that forms the central uplift. Consequently, sub-surface porosity distribution is a strong function of crater size, which is reflected in the inferred gravity anomaly. The Bouguer gravity anomaly for simulated craters smaller than 25 km is a broad low with a magnitude proportional to the crater radius; larger craters exhibit a central gravity high within a suppressed gravity low. Lower crustal pressures on the Moon relative to Earth imply that impact-induced dilatancy is more effective on the Moon than Earth for the same size impact in an initially non-porous target. This difference may be mitigated by the presence of porosity in the lunar crust.

Description: The jet activity emanating from Enceladus’ exosphere south pole region observed by Cassini is a subject of intensive study. The in situ and remote sensing observations performed since 2005 triggered an active modeling campaign. Such modeling is essential for better understanding of the measurements performed by individual instruments as well as to link them for a more complete picture of the volatile and ice grain distribution in Enceladus’ exosphere. This paper is focused on the investigation of the effect that diffuse gas sources along the Tiger Stripes have on distribution of the water vapor in Enceladus’ exosphere using the updated version of our multi-plume model. We have found that accounting for the gas production by Tiger Stripes is critical for interpretation of the Cassini data. According to our calculations sources along the Tiger Stripes (apart from those originally identified by Spitale and Porco [22]) must contribute about 23–32% to the total plume source rate, which varies in the range of (6.4 – 29) × 10 27  s –1 . The effect of the previously unidentified source suggested in the paper is found to be critical for explaining the UVIS 2007 and 2010 observations in the whole range of the elapsed times.

Description: No abstract is available for this article.

Description: Recent observations of Ceres, made using the Heterodyne Instrument for the Far Infrared (HIFI) on board the European Space Agency's Herschel Space Observatory, have shown that water vapor is emanated from the low albedo mid-latitude regions. Based on the supposition that some dark spots on Europa may be caused by diapirism, we explored whether Ceres’ environment also can induce compositional diapirism, and whether an upwelling diapir could explain the origin and distribution of the dark spots and vaporization of Ceres. If the density of the rock that constitutes Ceres is around 2700 kg m − 3 , the undifferentiated crust is stable over geological time periods at less than 180 K. At a surface temperature of 170 K, a diapir can reach the surface if the crust is a few tens of kilometers thick. Alternatively, if the surface temperature is 150 K, a large contrast in viscosity would prevent diapirs from reaching the surface. The relatively moderate surface temperature in the mid-latitude regions of Ceres may be the reason for the formation of dark terrains occurring only in such areas. Our finite difference calculations do not show that diapirism melts the surface ice because the temperature gradient changes very little. However, if the observed vapor emission is caused by the sublimation of fresh ice, compositional diapirism could be the mechanism that transports the fresh ice to the surface. In addition to other mechanisms such as cryovolcanism, diapirism is a valid hypothesis for explaining the dark terrain and vapor emissions of Ceres.

Description: The small crater Airy-0 was selected from Mariner 9 images to be the reference for the Mars prime meridian. Initial analyses in the year 2000 tied Viking Orbiter and Mars Orbiter Camera images of Airy-0 to the evolving Mars Orbiter Laser Altimeter global digital terrain model to update the location of Airy-0. Based upon this tie and radiometric tracking of landers / rovers from earth, new expressions for the Mars spin axis direction, spin rate and prime meridian epoch value were produced to define the orientation of the Martian surface in inertial space over time. Since the Mars Global Surveyor mission and Mars Orbiter Laser Altimeter global digital terrain model were completed some time ago, a more exhaustive study has been performed to determine the accuracy of the Airy-0 location and orientation of Mars at the standard epoch. THEMIS IR image cubes of the Airy and Gale crater regions were tied to the global terrain grid using precision stereo photogrammetric image proce ssing techniques. The Airy-0 location was determined to be about 0.001 ∘ east of its predicted location using the currently defined IAU prime meridian location. Information on this new location and how it was derived will be provided to the NASA Mars Exploration Program Geodesy and Cartography Working Group for their assessment. This NASA group will make a recommendation to the IAU Working Group on Cartographic Coordinates and Rotational Elements to update the expression for the Mars spin axis direction, spin rate and prime meridian location.

Description: We propose a method to estimate seasonal changes in the composition of the Martian atmosphere, which is influenced by CO 2 condensation due to the polar nights at southern high latitudes. The method relies on measurements of the Ar concentration obtained by the Gamma Ray Spectrometer (GRS) onboard Mars Odyssey. We assume that the Martian atmosphere is composed of CO 2 , N 2 , and Ar, and is vertically well mixed. Since N 2 and Ar do not condense even during the polar nights, the ratio of N 2 and Ar remains constant, and the concentrations of N 2 and CO 2 can be estimated from Ar measurements. Estimates of the atmospheric composition were utilized for the rederivation of temperature and pressure profiles in the Mars Global Surveyor (MGS) radio occultation measurements ( ~ 70 profiles of ~ 20,000 profiles in the whole data set) at southern polar latitudes (90°S−75°S) during the autumn, winter, and spring seasons (Ls =0°−240°). The rederived profiles indicated that use of the standard global composition overestimated the temperature by at least approximately 5 K at Ls = ~120° (midwinter), when the largest CO 2 depletion occurred and the CO 2 volume mixingratio fell to 78%. The occurrence and degree of CO 2 supersaturation were several times higher and larger, respectively, in the rederived temperature profiles than in the original MGS profiles. This suggests that consideration of CO 2 depletion during southern polar nights is needed when studying CO 2 supersaturation using radio occultation profiles.

Description: We have generated regional-scale photoclinometry (PC) digital elevation models (DEMs) from Voyager and Galileo imagery of Io that resolve small-scale topographic features including paterae and layered plains. Given the difficulty of applying this technique to Io due to its anomalous surface albedo properties, we have experimented extensively with the relevant procedures in order to generate what we consider to be the most reliable DEMs. The DEMs have been used to gauge the depths of 23 paterae and the heights of 12 layered plains outcrops, and we find the very similar relief and frequent close association of the two landforms to support the existence of a mixed silicate-volatile layer covering much of the surface of Io.

Description: To explore the mechanisms of support of surface topography on Mercury, we have determined the admittances and correlations of topography and gravity in Mercury's northern hemisphere from measurements obtained by NASA's MESSENGER spacecraft. These admittances and correlations can be interpreted in the context of a number of theoretical scenarios, including flexural loading and dynamic flow. We find that long-wavelength (spherical harmonic degree l  〈 15) surface topography on Mercury is primarily supported through a combination of crustal thickness variations and deep mass anomalies. The deep mass anomalies may be interpreted either as lateral variations in mantle density or as relief on compositional interfaces. Domical topographic swells are associated with high admittances and are compensated at 300–400 km depth, in the lower reaches of Mercury's mantle. Quasi-linear topographic rises are primarily associated with shallow crustal compensation and are weakly correlated with positive mass anomalies in the mantle. The center of the Caloris basin features some of the thinnest crust on the planet, and the basin is underlain by a large negative mass anomaly. We also explore models of dynamic flow in the presence of compositional stratification above the liquid core. If there is substantial compositional stratification in Mercury's solid outer shell, relaxation of perturbed compositional interfaces may be capable of creating and sustaining long-wavelength topography.

Description: We identified ten distinct classes of mineral assemblage on Mars through statistical analyses of mineral abundances derived from Mars Global Surveyor Thermal Emission Spectrometer (TES) data at a spatial resolution of eight pixels per degree. Two classes are new regions in Sinus Meridiani and northern Hellas basin. Except for crystalline hematite abundance, Sinus Meridiani exhibits compositional characteristics similar to Meridiani Planum; these two regions may share part of a common history. The northern margin of Hellas basin lacks olivine and high-Ca pyroxene compared to terrains just outside the Hellas outer ring; this may reflect a difference in crustal compositions and/or aqueous alteration. Hesperian highland volcanic terrains are largely mapped into one class. These terrains exhibit low-to-intermediate potassium and thorium concentrations (from Gamma Ray Spectrometer (GRS) data) compared to older highland terrains, indicating differences in the complexity of processes affecting mantle melts between these different-aged terrains. A previously reported, locally-observed trend towards decreasing proportions of low-calcium pyroxene relative to total pyroxene with time is also apparent over the larger scales of our study. Spatial trends in olivine and pyroxene abundance are consistent with those observed in near-infrared data sets. Generally, regions that are distinct in TES data also exhibit distinct elemental characteristics in GRS data, suggesting that surficial coatings are not the primarily control on TES mineralogical variations, but rather reflect regional differences in igneous and large-scale sedimentary/glacial processes. Distinct compositions measured over large, low-dust regions from multiple data sets indicate that global homogenization of unconsolidated surface materials has not occurred.

Description: Hydrated chlorine salts are expected to exist on a variety of planetary bodies, including inner planets such as Mars and outer planet satellites such as Europa. However, detection by remote sensing has been limited due to a lack of comparison data in spectral libraries. In addition, at low temperatures spectral features of many H 2 O-bearing species deviate from their room temperature behavior. Thus, we acquired spectra of NaCl, NaClO 4 ∙ n H 2 O, MgCl 2 ∙ n H 2 O, Mg(ClO 4 ) 2 °6H 2 O, and Mg(ClO 3 ) 2 °6H 2 O from 0.35-2.5 µm at both 298 and 80 K to observe the effects of temperature on diagnostic spectral features. In the NIR, the strongest spectral features often arise from water molecules. Increasing hydration states increases the depth and width of water bands. Interestingly, at low temperature these bands become narrower with sharper, better-defined minima, allowing individual bands to be more easily resolved. We also measured frozen eutectic solutions of NaCl, MgCl 2, and KCl. We show that while care must be taken to acquire laboratory spectra of all hydrated phases at the relevant conditions (e.g. temperature, pressure) for the planetary body being studied, chlorine salts do possess distinct spectral features that should allow for their detection by remote sensing.

Description: Baroclinic instability in the super-rotation of Venus is investigated by a newly developed atmospheric general circulation model. First, we adopt an idealized super-rotation, i.e., solid-body rotating flow in a weakly stratified layer at cloud level, as an initial basic state in a nominal case. With the evolution of time, baroclinic instability occurs in a weakly stratified layer with large vertical shear of the basic zonal flow. Horizontal wind associated with the baroclinic instability modes is of a few m s −1 . The initial structure of the unstable modes is similar to those obtained in previous linear stability analyses. However, it is modified by nonlinear interactions in the later stage, reaching a quasi-steady state. Meridional transport of momentum and heat by these unstable modes accelerates the super-rotation by ~0.05 m s −1  day −1 at mid-latitudes. Furthermore, the dependence of baroclinic instability on the basic state, i.e., the meridional profiles of zonal flow and the vertical profiles of static stability, are subsequently investigated. For the super-rotation with mid-latitude jets at cloud level, the modes are modified from baroclinic to barotropic in the later stage. Typically, their horizontal wind is of O (10) m s −1 . Their amplitude is maintained by energy conversion from zonal-mean available potential energy associated with the baroclinic basic state. In the case where static stability is smaller than that in the nominal case, the baroclinic modes transfer angular momentum from mid-latitude to the equator near a 70 km level, and accelerate the super-rotation more than 10 m s −1 in the equatorial region.

Description: A suite of eight rocks analyzed by the Curiosity Rover while it was stopped at the Rocknest sand ripple show the greatest chemical divergence of any potentially sedimentary rocks analyzed in the early part of the mission. Relative to average martian soil and to the stratigraphically lower units encountered as part of the Yellowknife Bay formation, these rocks are significantly depleted in MgO, with a mean of 1.3 wt %, and high in Fe, averaging over 20 wt % FeO T . with values between 15 - 26 wt% FeO T . The variable iron and low magnesium, and rock texture make it unlikely that these are igneous rocks. Rock surface textures range from rough to smooth, can be pitted or grooved, and show various degrees of wind erosion. Some rocks display poorly defined layering while others seem to show possible fractures. Narrow vertical voids are present in Rocknest-3, one of the rocks showing the strongest layering. Rocks in the vicinity of Rocknest may have undergone some diagenesis similar to other rocks in the Yellowknife Bay Formation as indicated by the presence of soluble calcium phases. The most reasonable scenario is that fine-grained sediments, potentially a mixture of feldspar-rich rocks from Bradbury Rise and normal martian soil, was lithified together by an iron-rich cement.

Description: The magmatism characterizing the early history of the asteroid Vesta has long been investigated with the mafic and ultramafic meteorites howardite, eucrite and diogenite (HED). The lack of geologic context for the meteorites, however, has limited its understanding. Here we use the visible to near-IR (VIR/Dawn) orbital observations of Vesta's surface to detect relative enrichments in olivine and to study the associated geologic features. Because the near-IR signature of olivine on Vesta's surface is subtle relative to the widespread pyroxene absorption bands, a method was developed to distinguish olivine enrichments from admixture of pyroxenes with high Fe 2+ /M1, dark material, and potential Fe-bearing glass. Relative enrichment of olivine (〈~50-60 vol%) is found in 2–5 km wide, morphologically fresh areas. Our global survey reveals a dozen of these areas clustering in the eastern hemisphere of Vesta. The hemispherical coincidence with a widespread, low enrichment in diogenite-like pyroxene suggests the presence of a distinct compositional terrain. On the central mound of the Rheasilvia impact basin, no olivine enrichment was found, suggesting the absence of an olivine-dominated mantle above the basin's excavation depth or, alternatively, a low amount of olivine homogeneously mixed with diogenite-like pyroxenes. Rare olivine-enriched areas in close proximity to diogenite-like pyroxene are found as part of material ejected by the Rheasilvia impact. Such co-occurrence is reminiscent of local, ultramafic lithologies within the crust. The possible formation of such lithologies on Vesta is supported by some HED meteorites dominated by olivine and orthopyroxene.

Description: ABSTRACT Here we discuss one of the current reservoirs of water on Mars, the regolith and rocks exposed at the surface. This reservoir is characterized by the presence of H 2 O- and OH-bearing phases that produce a broad absorption at a wavelength of ~3 µm in near-infrared (NIR) reflectance spectra. This absorption is present in every ice-free spectrum of the Martian surface obtained thus far by orbital NIR spectrometers. We present a quantitative analysis of the global distribution of the 3 µm absorption using the Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) imaging spectrometer that has been mapping the surface of Mars at kilometer scale for more than ten years. Based on laboratory reflectance spectra of a wide range of hydrous minerals and phases, we estimate a model-dependent water content of 4 ± 1 wt. % in the equatorial and mid-latitudes. Surface hydration increases with latitude, with an asymmetry in water content between the northern and southern hemispheres. The surface hydration is compared to various parameters (albedo, dust, geological units, time, relative humidity, atmospheric water pressure, and in situ measurements performed by Phoenix and Curiosity) to constrain the nature of the reservoir. We conclude that the nature of the surface hydration of the Martian low latitudes is not adsorbed water but rather more tightly-bound water molecules and hydroxyl groups in the structure of the materials of the near-top surface. A frost-related process best explains the implementation of water into and onto the first microns of the high latitudes Martian regolith.

Description: No abstract is available for this article.