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The gravity and shape data acquired by the Dawn spacecraft during its primary mission revealed that Ceres is partially differentiated with an interior structure consistent with a volatile-rich crust, a mantle of hydrated rock and isostatically compensated topography1–3. Detailed analyses showed that the mechanically strong crust overlays a weak, fluid-bearing upper mantle4. Previous studies, however, assumed that Ceres’s crust is a uniform layer. Here, we report findings from the new high-resolution gravity data from Dawn’s second extended mission (XM2), which reveal a complex crustal structure of Ceres. In the low-altitude regions probed by the Dawn spacecraft during the XM2 phase, we observe that gravity–topography admittance progressively shifts to a lower density solution at higher degrees, implying a radial density gradient across Ceres’s crust that is consistent with decreasing porosity with depth and/or increasing content of dense phases, such as rock and salts. That gradient brings a critical new constraint on the crustal freezing history, suggesting that the salts and silicates concentrated in the liquid phase while the crust was growing. Localized spectral analysis of the new data also shows evidence for a lower crustal density in the north polar region than in the south or near the equator, supporting impact-driven porosity variations for the observed latitudinal density differences5. On the local scale, the new data show evidence for density or rheological variations within the crust, in association with lobate landslides and ejecta deposits that were inferred to be ice-rich6,7 as well as an extensional fault system8. These inferences provide geophysical context for geological features on the surface and help us advance our understanding of the evolution of an ice-rich but heat-starved body, whose evolution was in part shaped by impacts. High-resolution gravity data from Dawn’s second extended mission could probe the global and local structure of Ceres’s crust. The results show significant spatial and vertical variations of crustal density and porosity, associated with ice features and ice-related processes driven from the interior, and impacts.

Before acquiring highest-resolution data of Ceres, questions remained about the emplacement mechanism and source of Occator crater’s bright faculae. Here we report that brine effusion emplaced the faculae in a brine-limited, impact-induced hydrothermal system. Impact-derived fracturing enabled brines to reach the surface. The central faculae, Cerealia and Pasola Facula, postdate the central pit, and were primarily sourced from an impact-induced melt chamber, with some contribution from a deeper, pre-existing brine reservoir. Vinalia Faculae, in the crater floor, were sourced from the laterally extensive deep reservoir only. Vinalia Faculae are comparatively thinner and display greater ballistic emplacement than the central faculae because the deep reservoir brines took a longer path to the surface and contained more gas than the shallower impact-induced melt chamber brines. Impact-derived fractures providing conduits, and mixing of impact-induced melt with deeper endogenic brines, could also allow oceanic material to reach the surfaces of other large icy bodies., The second extended phase of the Dawn mission provided high resolution observations of Occator crater of the dwarf planet Ceres. Here, the authors show that the central faculae were sourced in an impact-induced melt chamber, with a contribution from the deep brine reservoir, while the Vinalia Faculae were sourced by the deep brine reservoir alone.

Deciphering paleoclimate on Mars has been a driving goal of Martian science for decades. Most research has addressed this issue by studying Mars' large polar layered deposits (PLDs) as a paleoclimate proxy, but the certainty to which we know the link between climate and orbit is debated. Here, we instead consider the record of other, smaller ice deposits located within craters separated from the PLDs using images from NASA's High Resolution Imaging Science Experiment camera and signal processing techniques. We show that the climate record in Burroughs Crater (72.3°S, 116.6°E) contains robust evidence of orbital forcing, with periodicities that have wavelengths of 15.6 and 6.5 m. The ratio of these dominant wavelengths is 2.4, the same as the ratio between the periods of Mars' obliquity changes and orbital precession. This result suggests orbital control of recent Mars climate, and would imply an average ice accumulation rate of 0.13 mm/yr over 4.5 Myr in this region.

Kuiper belt objects (KBOs) have diverse surface compositions, and the New Horizons mission to the Pluto-Charon system allows us to test hypotheses on the origin and evolution of these KBO surfaces. Previous work proposed that Charon’s organic-rich north pole formed from radiolytically processed volatiles sourced from Pluto’s escaping atmosphere. Here, we show an endogenic source of volatiles from Charon’s interior is plausible. We calculate that cryovolcanic resurfacing released 1.29 × 1015–3.47 × 1015 kg of methane to Charon’s surface from its interior. We modeled volatile transport and found the vast majority of this volcanically released methane migrates to Charon’s poles, with deposition rates sufficient to be processed into the observed organic compounds. Irradiated methane products appear on similarly sized KBOs that do not orbit a Pluto-sized object to draw an escaping atmosphere from, so interior-sourced volatiles could be a common and important process across the Kuiper belt.

The 27 satellites of Uranus are enigmatic, with dark surfaces coated by material that could be rich in organics. Voyager 2 imaged the southern hemispheres of Uranus' five largest 'classical' moons Miranda, Ariel, Umbriel, Titania, and Oberon, as well as the largest ring moon Puck, but their northern hemispheres were largely unobservable at the time of the flyby and were not imaged. Additionally, no spatially resolved datasets exist for the other 21 known moons, and their surface properties are essentially unknown. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, our knowledge of the Uranian moons' surface compositions, and the processes that modify them, is limited to disk-integrated datasets collected by ground- and space-based telescopes. Nevertheless, images collected by the Imaging Science System on Voyager 2 and reflectance spectra collected by telescope facilities indicate that the five classical moons are candidate ocean worlds that might currently have, or had, liquid subsurface layers beneath their icy surfaces. To determine whether these moons are ocean worlds, and investigate Uranus' ring moons and irregular satellites, close-up observations and measurements made by instruments onboard a Uranus orbiter are needed., Accepted in AAS Planetary Science Journal. arXiv admin note: text overlap with arXiv:2007.07284

The presence of a liquid deposit of water below the South Polar Layered Deposits (SPLD) Region has been reported based on analysis of MARSIS radar data in the Planum Australe area (Orosei et al., 2018). These radar data show bright subsurface reflections that have been interpreted to be due to liquid water buried at a depth of 1.5 km (Orosei et al., 2018). The presence of such water would have important implications for the present-day thermal state of the region. Previous work based on the flexure of the lithosphere yielded a present surface heat flow of about 20–30 mW m-2 (Ruiz et al., 2010; Parro et al., 2017) at the south polar cap. Global thermal evolution models that included variations in crustal thickness and heat production estimated a global surface heat flow of 23.2–27.3 mW m-2 (Plesa et al. 2018). These estimations are difficult to reconcile with the high heat flows, in excess of 72 mW m-2 (Sori and Bramson, 2019), that are required to explain the existence of liquid water at 1.5 km deep. In this work, we aim to re-analyze the thermal state of the region and the thermal properties that are required to stabilize liquid water under the SPLD. In order to study the thermal conditions that are compatible with the liquid water deposit, we first recalculated the depth of the bright radar reflections using a temperature-dependent relative permittivity for the water ice (Fujita et al., 2000). The depth to the putative liquid water is important, as deeper depths require lower heat fluxes to reach the melting temperature, and vice versa. We obtained a new depth to the bright reflector of 1.7 km, assuming a surface temperature of 160 K and a melting temperature of 200 K, which is appropriate if calcium perchlorate is present in the area. Then, we calculated surface heat flows and subsurface temperatures by solving the stationary heat conduction equation. We assume that the composition of the SPLD region is a mixture of 85% of water ice and 15% of dust with a density of 1220 kg m-3 (Zuber at al., 2007). We use an ice thermal conductivity that is dependent on temperature following: where T is the temperature in Kelvins (Fukusako, 1990). For the dust component, we assume a thermal conductivity of 2 W m-1 K-1, and calculate the thermal conductivity of the dust-ice mixture as a geometric mean (Beardsmore and Cull, 2001). We also include a superficial layer of CO2 with a thickness of 1 m, a density of 1600 kg m-3, and a thermal conductivity of 0.02 W m-1 K-1. We find that a surface heat flow of 61 mW m-2 is needed to obtain melting at 1.7 km depth. This result is still higher than the values previously estimated from lithosphere flexure in the region, but somewhat lower than that reported by Sori and Branson (2019). Additionally, fractures or voids near the surface of the SPLD at this site could reduce the thermal conductivity of the region and lower the required heat flow even further. In order to account for this possibility, we calculated surface heat flows for additional models which include an intermediate layer of lower thermal conductivity placed between the CO2 and the more conductive dust-ice mixture. Models with an intermediate insulating layer 15 m thick and with thermal conductivities between 0.05 and 0.1 W m K-1 yield surface heat flows of 41 and 50 mW m-2, respectively, to stabilize liquid water at 1.7 km depth. Further work will realize a more complete assessment of this location and of the regional heat flow context. References Beardsmore, G.R. et al., (2001). Crustal Heat Flow: A Guide to Measurement and Modelling. Cambridge University Press, Cambridge, 324 pp. Fujita, S. et al., (2000). A summary of the complex dielectric permittivity of ice in the megahertz range and its applications for radar sounding of polar ice sheet. Physics of Ice Core Records: 185-212 Fukusako, S., (1990). Thermophysical properties of ice, snow and sea ice. Int.J. Thermophys.11, 353–372. Martin-Torres F.J. et al., (2015), Transient liquid water and water activity at Gale Crater on Mars, NatGeosci8:357–361. Orosei R. et al., (2018). Radar evidence of subglacial liquid water on Mars, Science 1126/science.aar7268. ParroM. et al., (2017). Present-day heat flow model of Mars, Sci. Rep. 7, 45629; doi: 10.1038/srep45629. Plesa, A. C. et al., (2018). The thermal state and interior structure of Mars. Geophysical Research Letters, 45(22), 12-198. Ruiz, J. et al.,(2010). The present-day thermal state of Mars. Icarus 207, 631-637. Sori, M. M., and Bramson, A. M., (2019). Water on Mars, with a grain of salt: Local heat anomalies are required for basal melting of ice at the south pole today. Geophysical Research Letters, 46, 1222–1231. Zuber, M. T. et al., (2007). Density of Mars' south polar layered deposits. Science, 317(5845), 1718-1719.

Cryovolcanism, defined here as the extrusion of icy material from depth, may be an important planetary phenomenon in shaping the surfaces of many worlds in the outer Solar System and revealing their thermal histories. However, the physics, chemistry and ubiquity of this geologic process remain poorly understood, especially in comparison to the better-studied silicate volcanism on the terrestrial planets. Ceres is the only plausibly cryovolcanic world to be orbited by a spacecraft up to now, making it the best opportunity to test the importance of cryovolcanism on bodies in the outer Solar System and compare its effects to silicate volcanism on terrestrial planets. Here, we analyse images from NASA’s Dawn mission and use the finite element method to show that Ceres has experienced cryovolcanism throughout its geologic history, with an average cryomagma extrusion rate of ~10^4 m^3 yr^(−1). This result shows that volcanic phenomena are important on Ceres, but orders of magnitude less so than on the terrestrial planets.

NASA Planetary Geology and Geophysics Program NASA Planetary Science Division [NNX13AR14G, 80NSSC17K0307]; NASA Earth and Space Sciences Fellowship (NESSF) Program [NNX16AP09H]

Crustal thickness is a crucial geophysical parameter in understanding the geology and geochemistry of terrestrial planets. Recent development of mathematical techniques suggests that previous studies based on assumptions of isostasy overestimated crustal thickness on some of the solid bodies of the solar system, leading to a need to revisit those analyses. Here, I apply these techniques to Mercury. Using MESSENGER-derived elemental abundances, I calculate a map of grain density (average 2974 ± 89 kg/m3) which shows that Pratt isostasy is unlikely to be a major compensation mechanism of Mercury's topography. Assuming Airy isostasy, I find the best fit value for Mercury's mean crustal thickness is 26 ± 11 km, 25% lower than the most recently reported and previously thinnest number. Several geological implications follow from this relatively low value for crustal thickness, including showing that the largest impacts very likely excavated mantle material onto Mercury's surface. The new results also show that Mercury and the Moon have a similar proportion of their rocky silicates composing their crusts, and thus Mercury is not uniquely efficient at crustal production amongst terrestrial bodies. Higher resolution topography and gravity data, especially for the southern hemisphere, will be necessary to refine Mercury's crustal parameters further.

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

Carbon dioxide has been detected on the trailing hemispheres of several Uranian satellites, but the exact nature and distribution of the molecules remain unknown. One such satellite, Umbriel, has a prominent high albedo annulus-shaped feature within the 131-km-diameter impact crater Wunda. We hypothesize that this feature is a solid deposit of CO 2 ice. We combine thermal and ballistic transport modeling to study the evolution of CO 2 molecules on the surface of Umbriel, a high-obliquity (∼98°) body. Considering processes such as sublimation and Jeans escape, we find that CO 2 ice migrates to low latitudes on geologically short (100s–1000 s of years) timescales. Crater morphology and location create a local cold trap inside Wunda, and the slopes of crater walls and a central peak explain the deposit's annular shape. The high albedo and thermal inertia of CO 2 ice relative to regolith allows deposits 15-m-thick or greater to be stable over the age of the solar system. We conclude that Wunda, located at low latitude (7.9° S) and near the center of the trailing hemisphere where CO 2 detections are strongest, likely contains a solid CO 2 ice deposit. We discuss prospects for similar CO 2 ice deposits on crater floors on the other major Uranian moons, and predict that they are present on Ariel, Titania, and possibly Oberon (but not Miranda or smaller satellites). Such deposits have likely not been observed due to the limited nature of Voyager 2 image coverage.

NASA Earth and Space Science Fellowship [NNX13AO55H]; NASA's Mars Reconnaissance Orbiter project

The dwarf planet Ceres's outer crust is a complex, heterogeneous mixture of ice, clathrates, salts and silicates. Numerous large domes on Ceres's surface indicate a degree of geological activity. These domes have been attributed to cryovolcanism, but that is difficult to reconcile with Ceres's small size and lack of long-lived heat sources. Here we alternatively propose that Ceres's domes form by solid-state flow within the compositionally heterogeneous crust, a mechanism directly analogous to salt tectonics on Earth. We use numerical simulations to illustrate that differential loading of a crust with compositional heterogeneity on a scale of tens of kilometres can produce dome-like features of scale similar to those observed. The mechanism requires the presence of low-viscosity and low-density, possibly ice-rich, material in the upper 1-10 km of the subsurface. Such substantial regional heterogeneity in Ceres's crustal composition is consistent with observations from the National Aeronautics and Space Administration's Dawn mission. We conclude that deformation analogous to that in terrestrial salt tectonics is a viable alternative explanation for the observed surface morphologies, and is consistent with Ceres being both cold and geologically active. National Aeronautics and Space Administration's (NASA's) Dawn Guest Investigator Program [NNH15AZ85I]; NASANational Aeronautics & Space Administration (NASA) This work was supported by the National Aeronautics and Space Administration's (NASA's) Dawn Guest Investigator Program (grant no. NNH15AZ85I). Some of the research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Special thanks to the Dawn mission operations team, who have gone above and beyond to return exceptional data from Ceres. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the US Government. Public domain – authored by a U.S. government employee

As differentiated planetesimals cool, their cores can solidify from the outside in1, as evidenced by palaeomagnetic measurements and cooling-rate estimates of iron meteorites2,3. The details of outside-in solidification and fate of residual core melt are poorly understood. For a core primarily composed of iron and nickel alloyed with lighter constituent elements such as sulfur, this inward core growth would probably be achieved by growth of solid iron–nickel dendrites4. Growth of iron–nickel dendrites results in interconnected pockets of residual melt that become progressively enriched in sulfur up to a eutectic composition of 31 wt% sulfur as iron–nickel continues to solidify4. Here, we show that regions of residual sulfur-enriched iron–nickel melt in the core attain sufficient excess pressures to propagate via dykes into the mantle. Thus, core material will intrude into the overlying rocky mantle or possibly even erupt onto the planetesimal’s surface. We refer to these processes collectively as ferrovolcanism. Our calculations show that ferrovolcanic surface eruptions are more likely on bodies with mantles less than 50 km thick. We show that intrusive ferromagmatism can produce pallasites, an enigmatic class of meteorites composed of olivine crystals entrained in a matrix of iron–nickel metal4. Ferrovolcanic eruptions may explain the observations that asteroid 16 Psyche has a bulk density inconsistent with iron meteorites5 yet shows evidence of a metallic surface composition6. Pockets of iron-rich melt within asteroids get progressively enriched in sulfur while the asteroid is cooling, generating excess pressure that can push the melt up to the surface. This ‘ferrovolcanism’ can be the origin of the stony-iron pallasite meteorites and can explain the contradicting results from asteroid 16 Psyche.

Vinalia and Cerealia Faculae are bright and salt-rich localized areas in Occator crater on Ceres. The predominance of the near-infrared signature of sodium carbonate on these surfaces suggests their original material was a brine. Here we analyze Dawn Framing Camera's images and characterize the surfaces as composed of a central structure, either a possible depression (Vinalia) or a central dome (Cerealia), and a discontinuous mantling. We consider three materials enabling the ascent and formation of the faculae: ice ascent with sublimation and carbonate particle lofting, pure gas emission entraining carbonate particles, and brine extrusion. We find that a mechanism explaining the entire range of morphologies, topographies, as well as the common composition of the deposits is brine fountaining. This process consists of briny liquid extrusion, followed by flash freezing of carbonate and ice particles, particle fallback, and sublimation. Subsequent increase in briny liquid viscosity leads to doming. Dawn observations did not detect currently active water plumes, indicating the frequency of such extrusions is longer than years.

The South Polar Layered Deposits (SPLD) are the largest water ice reservoirs on Mars. Their accumulation is believed to result from climate oscillations that drive the movement of ice and dust on the surface. The High-Resolution Imaging Science Experiment and the Colour and Stereo Surface Imaging System have imaged exposures of its internal structure in troughs and marginal scarps. Here we use the stereo imaging products of these instruments to extract stratigraphic profiles representative of various locations throughout the SPLD. Through wavelet and series-matching analyses of these profiles, we reveal periodicities in the stratigraphy that correlate to the orbital oscillations that drive climate change on Mars and that have been observed to force the accumulation of the north polar cap. We infer that the water ice and dust of the SPLD were deposited at variable rates of 0.13-0.39 mm/year, taking a minimum of 10-30 Myr to accumulate., Plain Language Summary The single location on Mars with the most water ice is in the southern polar ice cap, in the so-called South Polar Layered Deposits (SPLD). Changes in Martian climate through time affect the locations on Mars where ice is stable, and it is believed that these changes drove the accumulation of layers upon layers of ice and dust in the SPLD. The High-Resolution Imaging Science Experiment and the Colour and Stereo Surface Imaging System are satellite cameras that have photographed troughs and scarps within the SPLD to produce 3-D views of this layered internal structure. We analyzed this structure and found patterns in the layering related to variations of Mars' orbit and spin axis. These variations drive climate change on Mars, so our analysis confirms the connection between the SPLD and Mars' astronomical parameters. From this relationship, we inferred that the ice and dust of the SPLD took at least 10-30 Myr to accumulate.

Ice sheets, such as the polar layered deposits (PLDs) of Mars, are of great interest as records of past climate. Smaller outlier ice deposits near the north and south PLDs are likely more sensitive to climate changes and thus may hold information about more recent climate history. However, the southern outlier deposits have largely remained unmapped and unanalyzed. Here, we identify 31 deposits near, but separated from, Mars's south PLDs, all of which are located within impact craters >15 km in diameter. On the basis of morphology, radar analysis, physical similarity to portions of the PLD margin, and overall similarity to previously described deposits in Mars's north polar region, we conclude that these deposits are primarily composed of water ice. An additional 66 craters contain smaller depositional features, some of which may be remnant ice deposits. The 31 outlier ice deposits represent a previously unquantified inventory of water on Mars, with a total volume between 15,000 and 38,000 km3. In addition, we identify five analogous outlier nitrogen ice deposits located within impact craters near Sputnik Planitia, the large nitrogen ice sheet on Pluto. Although important differences exist between Mars and Pluto, broad physical similarities between the two cases suggest that the topography and microclimates of impact craters cause them to be favorable locations for volatile accumulation and/or retention throughout the Solar System.

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

NASA Earth and Space Science Fellowship [NNX13AO55H]; NASA's Mars Reconnaissance Orbiter project (HiRISE)

We define lunar cryptovolcanism as volcanic deposits on the Moon hidden by overlying material. Notably, cryptovolcanism includes both cryptomaria (subsurface extrusive basaltic deposits that are obscured by overlying higher albedo basin and crater ejecta) and earlier candidate extrusives, such as the Mg-suite. Knowledge of the volume and extent of cryptovolcanism is necessary for a comprehensive understanding of lunar volcanic history, particularly in early (pre 3.8 Ga) epochs when abundant impact craters and basins obscured surface volcanic deposits by lateral emplacement of ejecta. We use Gravity Recovery and Interior Laboratory (GRAIL) gravity and Lunar Orbiter Laser Altimeter (LOLA) topography data to construct maps of the Moon's positive Bouguer and isostatic gravity anomalies, and explore the possibility that these features are due to mass excesses associated with cryptovolcanism by cross-referencing the regions with geologic data such as dark halo craters. We model the potential cryptovolcanic deposits as buried high-density rectangular prisms at depth in the upper crust, and find a volume of candidate buried cryptovolcanism between 0.4 × 10 6 km 3 and 4.8 × 10 6 km 3 , depending on assumptions about density and crustal compensation state. These candidate deposits correspond to a surface area of between 0.50 × 10 6 km 2 and 1.14 × 10 6 km 2 , which would increase the amount of the lunar surface containing volcanic deposits from 16.6% to between 17.9% and 19.5%. The inferred volume of cryptovolcanism is comparable to the smallest estimates of the volume of visible mare basalts and up to ∼50% of the largest estimates; the high-resolution GRAIL and LOLA observations thus would collectively indicate that early (pre 3.8 Ga) lunar volcanism is an important element of lunar thermal evolution. Alternatively, the buried material could represent the presence of intrusive Mg-suite sills or plutons.

We investigate the importance of viscous flow in shaping topography at the north polar layered deposits (NPLD) of Mars by using finite element modeling to calculate the distribution of stresses and flow velocities. Present-day impact craters on theNPLDare too small and cold for viscous relaxation tohavebeen an important mechanism in controlling their current dimensions; this effect may be ignored when analyzing crater size-frequency distributions. Scarps at the NPLDmargins, where avalanches of dust and carbon dioxide frost occur, are sufficiently steep, high, andwarm to experience significant viscous flow.We find flow velocities at the base of these steep scarps on the order of tens to hundreds of cm/yr, which are fast enough to significantly affect their slopeover kiloyear timescales. Alternatively, the scarps could be close to steady state in which observed block falls provide a competing effect to viscous flow.

Ahuna Mons is a 4-km-tall mountain on Ceres interpreted as a geologically young cryovolcanic dome. Other possible cryovolcanic features are more ambiguous, implying that cryovolcanism is only a recent phenomenon or that other cryovolcanic structures have been modified beyond easy identification. We test the hypothesis that Cerean cryovolcanic domes viscously relax, precluding ancient domes from recognition. We use numerical models to predict flow velocities of Ahuna Mons to be 10–500 m/Myr, depending upon assumptions about ice content, rheology, grain size, and thermal parameters. Slower flow rates in this range are sufficiently fast to induce extensive relaxation of cryovolcanic structures over 108–109 years, but gradual enough for Ahuna Mons to remain identifiable today. Positive topographic features, including a tholus underlying Ahuna Mons, may represent relaxed cryovolcanic structures. A composition for Ahuna Mons of >40% ice explains the observed distribution of cryovolcanic structures because viscous relaxation renders old cryovolcanoes unrecognizable.

In June 2009 the Lunar Reconnaissance Orbiter (LRO) spacecraft was launched to the Moon. The payload consists of 7 science instruments selected to characterize sites for future robotic and human missions. Among them, the Lunar Orbiter Laser Altimeter (LOLA) was designed to obtain altimetry, surface roughness, and reflectance measurements. The primary phase of lunar exploration lasted one year, following a 3-month commissioning phase. On completion of its exploration objectives, the LRO mission transitioned to a science mission. After 7 years in lunar orbit, the LOLA instrument continues to map the lunar surface. The LOLA dataset is one of the foundational datasets acquired by the various LRO instruments. LOLA provided a high-accuracy global geodetic reference frame to which past, present and future lunar observations can be referenced. It also obtained high-resolution and accurate global topography that were used to determine regions in permanent shadow at the lunar poles. LOLA further contributed to the study of polar volatiles through its unique measurement of surface brightness at zero phase, which revealed anomalies in several polar craters that may indicate the presence of water ice. In this paper, we describe the many LOLA accomplishments to date and its contribution to lunar and planetary science.

New gravity measurements greatly improve the Moon’s preserved impact basin inventory., Observations from the Gravity Recovery and Interior Laboratory (GRAIL) mission indicate a marked change in the gravitational signature of lunar impact structures at the morphological transition, with increasing diameter, from complex craters to peak-ring basins. At crater diameters larger than ~200 km, a central positive Bouguer anomaly is seen within the innermost peak ring, and an annular negative Bouguer anomaly extends outward from this ring to the outer topographic rim crest. These observations demonstrate that basin-forming impacts remove crustal materials from within the peak ring and thicken the crust between the peak ring and the outer rim crest. A correlation between the diameter of the central Bouguer gravity high and the outer topographic ring diameter for well-preserved basins enables the identification and characterization of basins for which topographic signatures have been obscured by superposed cratering and volcanism. The GRAIL inventory of lunar basins improves upon earlier lists that differed in their totals by more than a factor of 2. The size-frequency distributions of basins on the nearside and farside hemispheres of the Moon differ substantially; the nearside hosts more basins larger than 350 km in diameter, whereas the farside has more smaller basins. Hemispherical differences in target properties, including temperature and porosity, are likely to have contributed to these different distributions. Better understanding of the factors that control basin size will help to constrain models of the original impactor population.

International audience; We have analyzed the Bouguer anomaly (BA) of~1200 complex craters in the lunar highlands from Gravity Recovery and Interior Laboratory observations. The BA of these craters is generally negative, though positive BA values are observed, particularly for smaller craters. Crater BA values scale inversely with crater diameter, quantifying how larger impacts produce more extensive fracturing and dilatant bulking. The Bouguer anomaly of craters larger than 93 þ 47 À19 km in diameter is independent of crater size, indicating that there is a limiting depth to impact-generated porosity, presumably from pore collapse associated with either overburden pressure or viscous flow. Impact-generated porosity of the bulk lunar crust is likely in a state of equilibrium for craters smaller than~30 km in diameter, consistent with an~8 km thick lunar megaregolith, whereas the gravity signature of larger craters is still preserved and provides new insight into the cratering record of even the oldest lunar surfaces.

Layered deposits of dusty ice in the martian polar caps have been hypothesized to record climate changes driven by orbitally induced variations in the distribution of incoming solar radiation. Attempts to identify such an orbital signal by tuning a stratigraphic sequence of polar layered deposits (PLDs) to match an assumed forcing introduce a risk of identifying spurious matches between unrelated records. We present an approach for evaluating the significance of matches obtained by orbital tuning, and investigate the utility of this approach for identifying orbital signals in the Mars PLDs. Using a set of simple models for ice and dust accumulation driven by insolation, we generate synthetic PLD stratigraphic sequences with nonlinear time–depth relationships. We then use a dynamic time warping algorithm to attempt to identify an orbital signal in the modeled sequences, and apply a Monte Carlo procedure to determine whether this match is significantly better than a match to a random sequence that contains no orbital signal. For simple deposition mechanisms in which dust deposition rate is constant and ice deposition rate varies linearly with insolation, we find that an orbital signal can be confidently identified if at least 10% of the accumulation time interval is preserved as strata. Addition of noise to our models raises this minimum preservation requirement, and we expect that more complex deposition functions would generally also make identification more difficult. In light of these results, we consider the prospects for identifying an orbital signal in the actual PLD stratigraphy, and conclude that this is feasible even with a strongly nonlinear relationship between stratigraphic depth and time, provided that a sufficient fraction of time is preserved in the record and that ice and dust deposition rates vary predictably with insolation. Independent age constraints from other techniques may be necessary, for example, if an insufficient amount of time is preserved in the stratigraphy., United States. National Aeronautics and Space Administration. Mars Data Analysis Program (award 65P-1089493)