Publications

The Jupiter and Icy Moons Explorer (JUICE) mission of the European Space Agency (ESA) will investigate the Jovian system with multiple instruments over several years, beginning in early 2031. This paper describes the historical context and state of knowledge, as well as JUICEs scientific goals and measurement techniques of the satellites that will not be encountered in close flybys. These include the large volcanically active moon Io, the four small inner moons Metis, Adrastea, Amalthea, and Thebe, and the numerous small Irregular (outer) moons. JUICE will provide multiple opportunities to observe Io from relatively remote distances of hundreds of thousands of kilometers. These observations will enable monitoring of Ios surface for changes, and for the study of its neutral clouds and plasma torus. Io observations will be performed with the four optical remote sensing instruments and with the Particle Environment Package. For the small inner moons it is planned to obtain complete geographic longitude (scales up to 8 km/px), solar-phase and multi-color coverage, oblique polar views, and UV to near-IR spectra. Astrometric measurements will also be performed. The Irregular moons will mostly appear unresolved to the JUICE instruments. Nonetheless, long-duration disk-integrated lightcurves will be acquired to derive rotation periods, object dimensions, pole-axis orientations, and colors for most objects for the first time. From these data, convex-shape models will be generated and phase curves determined. Furthermore, the precision of the orbital elements will be improved via accurate astrometry. UV and near-IR measurements will be attempted for the largest of these objects.

Understanding Mars's water budget and distribution is crucial for evaluating its habitability and studying its evolution and past climate. Evidence for past and present glaciation includes geomorphological features: polar ice caps and subsurface ice. Among the most compelling evidence are Viscous Flow Features (VFFs), which suggest the presence of water ice due to surface patterns similar to Earth's rock glaciers and debris-covered glaciers. These features, including Lobate Debris Aprons (LDAs), are found in the mid-latitudes (30° and 50°) of both hemispheres. This study focuses on the composition of LDAs, which are large, ice-rich deposits found on slopes of massifs. Two hypotheses for their formation propose either low water ice content (30%) (\u201crock glaciers\u201d) or near-pure water ice under a debris layer (\u201cdebris-covered glaciers\u201d). Using SHARAD (SHAllow RADar) data, we calculate two key parameters dielectric constant (ϵ) and loss tangent (tanδ) to better understand the purity and composition of these deposits. Previous studies suggest LDAs consist of nearly pure water ice. Our work increases global coverage across the northern and southern hemispheres, examining five sites with enhanced data coverage and incorporating both ϵ and tanδ to improve the analysis. Our results indicate that LDAs are most likely part of a global population of features composed of >80% water ice, supporting the debris-covered glacier hypothesis. The results also point to the consistency of the debris cover across the globally distributed sites. This finding suggests that these features formed under similar climatic conditions, such as might occur under a specific orbital configuration. The high purity of the ice in LDAs has significant implications for understanding Mars's past climate and for future exploration, as LDAs represent accessible reservoirs of water ice at mid-latitudes. Additionally, our findings highlight the need for future radar-based studies to incorporate the calculation of the loss tangent, as this can alleviate the sensitivity to the topography of the LDA base inherent in ϵ.

We use PyDynamicaLC, a model using the least number ofand the least correlateddegrees of freedom needed to derive a photodynamical model, to describe some of the smallestand lowest-transit-timing-variation-amplitudeof the Kepler planets. We successfully analyze 64 systems containing 218 planets, for 88 of which we were able to determine significant masses (to better than 3σ). We demonstrate consistency with literature results over 2 orders of magnitude in mass, and for the planets that already had literature mass estimations, we were able to reduce the relative mass error by ∼22% (median value). Of the planets with determined masses, 23 are new mass determinations, with no previous significant literature values, including a planet smaller and lighter than Earth (KOI-1977.02/Kepler-345 b). These results demonstrate the power of photodynamical modeling with the appropriately chosen degrees of freedom. This will become increasingly more important as smaller planets are detected, especially as the TESS mission gathers ever longer baseline light curves and for the analysis of the future PLATO mission data.

A main source of bias in transmission spectroscopy of exoplanet atmospheres is magnetic activity of the host star in the form of stellar spots, faculae, or flares. However, the fact that main-sequence stars have a chromosphere and a corona and that these optically thin layers are dominated by line emission may alter the global interpretation of the planetary spectrum has largely been neglected. Using a JWST NIRISS/SOSS data set of hot Jupiter HAT-P-18 b, we show that even at near-IR and IR wavelengths, the presence of these layers leads to significant changes in the transmission spectrum of the planetary atmosphere. Accounting for these stellar outer layers thus improves the atmospheric fit of HAT-P-18 b and increases its best-fit atmospheric temperature from 53 6 − 101 + 189 K to 73 6 − 188 + 376 K, a value much closer to the predicted equilibrium temperature of ∼852 K. Our analysis also decreases the best-fit abundance of CO₂ by almost an order of magnitude. The approach provides a new window to the properties of chromospheres/corona in stars other than our Sun.

Planetary geologic maps are crucial tools for understanding the geological features and processes of solid bodies in the Solar System. Over the past six decades, best practices in planetary geologic mapping have emphasized clear and objective observation, geological interpretation, multi-sensor fusion, and iterative revision of maps based on new data. We summarize here four ways in which maps serve as indispensable instruments for scientific investigation, from enhancing observations to interrogating surface processes. With respect to space exploration, we underscore the role of planetary geologic maps as tools to link testable, hypothesis-driven science to exploration goals and provide actionable information for hazard identification, resource evaluation, sample collection, and potential infrastructure development. To further advance the field of planetary geologic mapping, international collaboration is essential. This includes sharing data and maps through FAIR (findable, accessible, interoperable, and reusable) platforms, establishing standardized mapping practices, promoting diverse nomenclature, and fostering continued cooperation in space exploration.

We fit a dynamical model to Kepler systems that contain four or more transiting planets using the analytic method AnalyticLC and obtain physical and orbital parameters for 101 planets in 23 systems, of which 95 are of mass significance better than 3σ, and 46 are without previously reported mass constraints or upper limits. In addition, we compile a list of 71 Kepler objects of interest that display significant transit impact parameter variations (TbVs), complementing our previously published work on two- and three-transiting-planet systems. Together, these works include the detection of significant TbV signals of 130 planets, which is, to our knowledge, the largest catalog of this type to date. The results indicate that the typical detectable TbV rate in the Kepler population is of order 10⁻² yr⁻¹ and that rapid TbV rates (≳0.05 yr⁻¹) are observed only in systems that contain a transiting planet with an orbital period less than ∼20 days. The observed TbV rates are only weakly correlated with orbital period within Keplers ≲100-day-period planets. If this extends to longer periods, it implies a limit on the utility of the transit technique for long-period planets. The TbVs we find may not be detectable in direct impact parameter measurements, but rather are inferred from the full dynamics of the system, encoded in all types of transit variations. Finally, we find evidence that the mutual inclination distribution is qualitatively consistent with the previously suggested angular momentum deficit model using an independent approach.

Massive reservoirs of subsurface water ice in equilibrium with atmospheric water vapor are found poleward of 45° latitude on Mars. The absence of CO₂ frost on steep pole-facing slopes and simulations of atmospheric-soil water exchanges suggested that water ice could be stable underneath these slopes down to 25° latitude. We revisit these arguments with a new slope microclimate model. Our model shows that below 30° latitude, slopes are warmer than previously estimated as the air above is heated by warm surrounding plains. This additional heat prevents the formation of surface CO₂ frost and subsurface water ice for most slopes. Our model suggests the presence of subsurface water ice beneath pole-facing slopes down to 30° latitude, and possibly 25° latitude on sparse steep dusty slopes. While unstable ice deposits might be present, our results suggest that water ice is rarer than previously thought in the ±30° latitude range considered for human exploration.

The Large Array Survey Telescope (LAST) is designed to survey the variable and transient sky at high temporal cadence. The array is comprised of 48 F/2.2 telescopes of 27.9 cm aperture, coupled to full-frame backside-illuminated cooled CMOS detectors with 3.76 μm pixels, resulting in a pixel scale of 1.25. A single telescope with a field of view of 7.4 deg² reaches a 5σ limiting magnitude of 19.6 in 20 s. LAST 48 telescopes are mounted on 12 independent mountsa modular design which allows us to conduct optimized parallel surveys. Here we provide a detailed overview of the LAST survey strategy and its key scientific goals. These include the search for gravitational-wave (GW) electromagnetic counterparts with a system that can cover the uncertainty regions of the next-generation GW detectors in a single exposure, the study of planetary systems around white dwarfs, and the search for near-Earth objects. LAST is currently being commissioned, with full scientific operations expected in mid 2023. This paper is accompanied by two complementary publications in this issue, giving an overview of the system and of the dedicated data reduction pipeline.

We developed and provide AnalyticLC, a novel analytic method and code implementation for dynamical modeling of planetary systems, including non-coplanar interactions, based on a disturbing function expansion to fourth order in eccentricities and inclinations. AnalyticLC calculates the system dynamics in 3D and the resulting model light-curve, radial-velocity, and astrometry signatures, enabling simultaneous fitting of these data. We show that for a near-resonant chain of three planets, where the two super-periods are close to each other, the TTVs of the pair-wise interactions cannot be directly summed to give the full system TTVs because the super-periods themselves resonate. We derive the simultaneous three planets correction and include it in AnalyticLC. We compare the model computed by AnalyticLC to synthetic data generated by an N-body integrator, and evaluate its accuracy. Depending on the maximal order of expansion terms kept, AnalyticLC computation time can be up to an order of magnitude faster than the state-of-the-art published N-body integrator TTVFast, with a smaller enhancement seen at higher order. The advantage increases for long-term observations as our approach's computation time does not depend on the time span of the data. Depending on the system parameters, the photometric accuracy is typically a few ppm, significantly smaller than Kepler's and other observatories' typical data uncertainty. Our highly efficient and accurate implementation allows full inversion of a large number of observed systems for planetary physical and orbital parameters, presented in a companion paper.

Impacts between planetary-sized bodies can explain the origin of satellites orbiting large (R>500 km) trans-Neptunian objects. Their water rich composition, along with the complex phase diagram of water, make it important to accurately model the wide range of thermodynamic conditions material experiences during an impact event and in the debris disk. Since differences in the thermodynamics may influence the system dynamics, we seek to evaluate how the choice of an equation of state (EOS) alters the systems evolution. Specifically, we compare two EOSs that are constructed by different approaches: either by a simplified analytic description (Tillotson), or by interpolation of tabulated data (Sesame). Approximately 50 pairs of Smoothed Particle Hydrodynamics impact simulations were performed, with similar initial conditions but different EOSs, in the parameter space in which the PlutoCharon binary is thought to form (slow impacts between Pluto-size, water rich bodies). Generally, we show that impact outcomes (e.g., circumplanetary debris disk) are consistent between EOSs. Some differences arise, importantly in the production of satellitesimals (large intact clumps) that form in the post-impact debris disk. When utilizing an analytic EOS, the emergence of satellitesimals is highly certain, while when using the tabulated EOS it is less common. This is because for the typical densities and energies experienced in these impacts, the analytic EOS predicts very low pressure values, leading to particles artificially aggregating by a tensile instability.

Water and other volatiles are fundamental tracers of habitable environments on terrestrial planets and satellites. Ice deposits in the polar regions of the Moon present an opportunity to understand the delivery and retention of water and other volatiles in the inner solar system.

Inferring planetary parameters from transit timing variations (TTVs) is challenging for small exoplanets because their transits may be so weak that determination of individual transit timing is difficult or impossible. We implement a useful combination of tools that together provide a numerically fast global photodynamical model. This is used to fit the TTV-bearing light curve, in order to constrain the masses of transiting exoplanets in loweccentricity, multiplanet systems - and small planets in particular. We present inferred dynamical masses and orbital eccentricities in four multi-planet systems from Kepler's complete long-cadence data set. We test our model against Kepler-36/KOI-277, a system with some of the most precisely determined planetary masses through TTV inversion methods, and find masses of 5.56+0.41-0.45 and 9.76+0.79-0.89 m⊕ for Kepler-36 b and c, respectively - consistent with literature in both value and error. We then improve the mass determination of the four planets in Kepler-79/KOI-152, where literature values were physically problematic to 12.5+4.5-3.6, 9.5+2.3-2.1, 11.3+2.2-2.2 and 6.3+1.0-1.0 m⊕ for Kepler-79 b, c, d, and e, respectively. We provide new mass constraints where none existed before for two systems. These are 12.5+3.2-2.6 m⊕ for Kepler-450 c, and 3.3+1.7-1.0 and 17.4+7.1-3.8 m⊕ for Kepler-595 c (previously KOI-547.03) and b, respectively. The photodynamical code used here, called PyDynamicaLC, is made publicly available.

SpaceIL's lunar lander mission Beresheet was launched on February 22, 2019 and impacted its targeted landing site in Mare Serenitatis on the Moon on April 11, 2019. The spacecraft carried a package of scientific instruments including a fluxgate magnetometer and a retroreflector array for laser ranging, as well as a suite of cameras. Orbital measurements of the magnetic field from Kaguya and Lunar Prospector guided the selection of the landing site to a location west of Posidonius crater in the Serenitatis plains, where the magnitude of the modeled magnetic field reaches 810 \u200bnT \u200bat the surface. Data was collected by the SILMAG magnetometer from Earth orbit, lunar orbit, and during the descent maneuver, although its interpretation is hindered by the presence of a spacecraft field.

Accumulations of ice and dust mixtures may acquire magnetization during deposition in a manner analogous to sedimentary rocks. Here, we consider the process of particles descending through an atmosphere and depositing in a preferential orientation that serves to record the ambient magnetic field during emplacement. We use a simple model for the settling and reorientation of ice particles with magnetic inclusions that includes magnetic torque, aerodynamic forces and gravity, to investigate the parameter space governing the process. For fields in the range of 10's 100's μT we find that ice particles of sizes up to ∼100 μm which contain smaller magnetic grains as nuclei will produce a deposit indeed magnetized in the direction aligned with the applied field, but with a moment that is independent of the field strength. For particles in the 100's μm range, the magnetic moment increases with the field strength. To demonstrate the effect experimentally, we performed a suite of laboratory deposition simulations followed by measurements of the magnetic moment of the samples. We show that in the idealized laboratory conditions dusty ice magnetizes in the direction of the applied field, with the alignment increasing with its intensity. For the chosen conditions, the magnetization increases rapidly with field intensity in the range 10 200 μT, and approaches a maximal value above that. For a mixture with dust/ice ratio of 5×10
⁻³ we obtained maximal magnetization values in the range 1.6×10
⁻⁵ 3×10
⁻³ A m
²/kg, depending on the distribution of particle sizes. We show that magnetic particle concentration in the ice determines the level of magnetic remanence, and conclude that the remanent magnetization of natural ice deposit in various settings may be measurable (if unobscured by post-depositional, wind, or other effects) and thus could provide a new paleomagnetic record on Earth and other planetary objects.

With NASA's announcement in 2019 of going back to the moon with the Artemis program, and the plan of building a Moon base in 2028 as a milestone for Mars colonization, there is an urgent need in providing methods to sustain a human crew on the Moon and Mars for substantially long periods. The Beresheet spacecraft hard landing on the Moon, April 2019, has inspired the Israeli public, children and adults alike, and the Desert Mars Analog Ramon Station (D-MARS) non profit organization was established during Beresheet developmental stages. It is dedicated for establishing an international planetary analog research center in the Negev desert, Israel. D-MARS aims to promote space-related education, science and technological innovation via space simulation or analog mission infrastructure. The Ramon Crater in the Negev desert was chosen for its Mars-like geomorphology and geology, isolation and hyper arid climate conditions allowing for a Mars simulated environment. The D-MARS habitat is a scalable deployed portable structure. It can support six analog astronauts (\u201cRamonauts\u201d) living in isolation during few weeks of an analog mission. The habitat has a science lab that includes a biological safety cabinet and additional equipment to perform geo-biological experiments internally, a single hydroponic tray, a common room, six wood encased sleeping pods, kitchenette, a small restroom. D-MARS members have performed Mars analog missions during 2018-2019. During the 2019 field campaign (February - March 2019), several analog Mars missions were performed: An 11-day main analog mission; a three-day\u201dLanding on Mars\u201d simulation mission to prepare and put into operation the infrastructure and technological equipment, simulating a crew in a landing phase on Mars; a two-day medical analog mission that included several medical emergency simulated events; a dedicated high school student group performed a three-day analog missions as part of the Young Astronaut Academy (YAA). The YAA program promotes STEM and the love of space and science. During all missions, all simulated extravehicular activities (EVAs) were done in a dedicated sim spacesuit, when operating outside of the habitat. The Ramonauts were in fairly complete isolation and communicated only with Mission Support Center (MSC), via text/image, in a dedicated webserver, with a 10-minute delay, each way. During the main 11-day mission various scientific experiments included soil analysis, environmental microorganisms metagenomics, isolation and characterization, rover utilization for external mapping and situational exploration, various psychological analyses of crew's wellness and more. This paper describes the 2019 Analog mission season, detailing the various missions and their objectives, impacts, lessons learned and future forecasts and plans.

Current lunar origin scenarios suggest that Earth's Moon may have resulted from the merger of two (or more) smaller moonlets. Dynamical studies of multiple moons find that these satellite systems are not stable, resulting in moonlet collision or loss of one or more of the moonlets. We perform Smoothed Particle Hydrodynamic (SPH) impact simulations of two orbiting moonlets inside the planetary gravitational potential and find that the classical outcome of two bodies impacting in free space is altered as erosive mass loss is more significant with decreasing distance to the planet. Depending on the conditions of accretion, each moonlet could have a distinct isotopic signature; therefore, we assess the initial mixing during their merger, in order to estimate whether future measurements of surface variations could distinguish between lunar origin scenarios (single vs. multiple moonlets). We find that for comparable-size impacting bodies in the accretionary regime, surface mixing is efficient, but in the hit-and-run regime, only small amount of material is transferred between the bodies. However, sequences of hit-and-run impacts are expected, which will enhance the surface mixing. Overall, our results show that large-scale heterogeneities can arise only from the merger of drastically different component masses. Surfaces of moons resulting from merger of comparable-sized components have little material heterogeneities, and such impacts are preferred, as the relatively massive impactor generates more melt, extending the lunar magma ocean phase.

The high planetary multiplicity revealed by Kepler implies that transit timing variations (TTVs) are intrinsically common. The usual procedure for detecting these TTVs is biased to long-period, deep transit planets, whereas most transiting planets have short periods and shallow transits. Here we introduce the Spectral Approach technique to TTVs that allows expanding the TTV catalog toward lower TTV amplitude, shorter orbital period, and shallower transit depth. In the spectral approach, we assume that a sinusoidal TTV exists in the data and then calculate the improvement to χ
² that this model allows over that of the linear-ephemeris model. This enables detection of TTVs even in cases where the transits are too shallow, so that individual transits cannot be timed. The spectral approach is more sensitive because it has fewer free parameters in its model. Using the spectral approach, we (a) detect 129 new periodic TTVs in Kepler data (an increase of ∼2/3 over a previous TTV catalog); (b) constrain the TTV periods of 34 long-period TTVs and reduce amplitude errors of known TTVs; and (c) identify cases of multi-periodic TTVs, for which absolute planetary mass determination may be possible. We further extend our analysis by using perturbation theory assuming a small TTV amplitude at the detection stage, which greatly speeds up our detection (to a level of few seconds per star). Our extended TTV sample shows no deficit of short-period or low-amplitude transits, in contrast to previous surveys, in which the detection schemes were significantly biased against such systems.

The heat flux incident upon the surface of an airless planetary body is dominated by solar radiation during the day, and by thermal emission from topography at night. Motivated by the close relationship between this heat flux, the surface temperatures, and the stability of volatiles, we consider the effect of the slope distribution on the temperature distribution and hence prevalence of cold-traps, where volatiles may accumulate over geologic time. We develop a thermophysical model accounting for insolation, reflected and emitted radiation, and subsurface conduction, and use it to examine several idealized representations of rough topography. We show how subsurface conduction alters the temperature distribution of bowl-shaped craters compared to predictions given by past analytic models. We model the dependence of cold-traps on crater geometry and quantify the effect that while deeper depressions cast more persistent shadows, they are often too warm to trap water ice due to the smaller sky fraction and increased reflected and reemitted radiation from the walls. In order to calculate the temperature distribution outside craters, we consider rough random surfaces with a Gaussian slope distribution. Using their derived temperatures and additional volatile stability models, we estimate the potential area fraction of stable water ice on Earth's Moon. For example, surfaces with slope RMS ∼15° (corresponding to length-scales ∼10 m on the lunar surface) located near the poles are found to have a ∼10% exposed cold-trap area fraction. In the subsurface, the diffusion barrier created by the overlaying regolith increases this area fraction to ∼40%. Additionally, some buried water ice is shown to remain stable even beneath temporarily illuminated slopes, making it more readily accessible to future lunar excavation missions. Finally, due to the exponential dependence of stability of ice on temperature, we are able to constrain the maximum thickness of the unstable layer to a few decimeters.

Seven of the nine known Mars Trojan asteroids belong to an orbital cluster 2 named after its largest member, (5261) Eureka. Eureka is probably the progenitor of the whole cluster, which formed at least 1Gyr ago(3). It has been suggested 3 that the thermal YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack) effect spun up Eureka, resulting in fragments being ejected by the rotational-fission mechanism. Eureka's spectrum exhibits a broad and deep absorption band around 1 mu m, indicating an olivine-rich composition(4). Here we show evidence that the Trojan Eureka cluster progenitor could have originated as impact debris excavated from the Martian mantle. We present new near-infrared observations of two Trojans ((311999) 2007 NS2 and (385250) 2001 DH47) and find that both exhibit an olivine-rich reflectance spectrum similar to Eureka's. These measurements confirm that the progenitor of the cluster has an achondritic composition(4). Olivine-rich reflectance spectra are rare amongst asteroids' but are seen around the largest basins on Mars(6). They are also consistent with some Martian meteorites (for example, Chassigny(7)) and with the material comprising much of the Martian mantle(8,9). Using numerical simulations, we show that the Mars Trojans are more likely to be impact ejecta from Mars than captured olivine-rich asteroids transported from the main belt. This result directly links specific asteroids to debris from the forming planets.

The hypothesis of lunar origin by a single giant impact can explain some aspects of the Earth-Moon system. However, it is difficult to reconcile giant-impact models with the compositional similarity of the Earth and Moon without violating angular momentum constraints. Furthermore, successful giant-impact scenarios require very specific conditions such that they have a low probability of occurring. Here we present numerical simulations suggesting that the Moon could instead be the product of a succession of a variety of smaller collisions. In this scenario, each collision forms a debris disk around the proto-Earth that then accretes to form a moonlet. The moonlets tidally advance outward, and may coalesce to form the Moon. We find that sub-lunar moonlets are a common result of impacts expected onto the proto-Earth in the early Solar System and find that the planetary rotation is limited by impact angular momentum drain. We conclude that, assuming efficient merger of moonlets, a multiple-impact scenario can account for the formation of the Earth-Moon system with its present properties.

Fast and accurate radiative transfer methods are essential for modeling CO₂-rich atmospheres, relevant to the climate of early Earth and Mars, present-day Venus, and some exoplanets. Although such models already exist, their accuracy may be improved as better theoretical and experimental constraints become available. Here we develop a unidimensional radiative transfer code for CO₂-rich atmospheres, using the correlated k approach and with a focus on modeling early Mars. Our model differs from existing models in that it includes the effects of CO₂ collisional line mixing in the calculation of the line-by-line absorption coefficients. Inclusion of these effects results in model atmospheres that are more transparent to infrared radiation and, therefore, in colder surface temperatures at radiative-convective equilibrium, compared with results of previous studies. Inclusion of water vapor in the model atmosphere results in negligible warming due to the low atmospheric temperatures under a weaker early Sun, which translate into climatically unimportant concentrations of water vapor. Overall, the results imply that sustained warmth on early Mars would not have been possible with an atmosphere containing only CO₂ and water vapor, suggesting that other components of the early Martian climate system are missing from current models or that warm conditions were not long lived.

The dwarf planet Ceres may have an icerich crust, and subsurface ice exposed by impacts or endogenic activity would be subject to sublimation. We model surface and subsurface temperatures on Ceres to assess lifetimes of water ice and other volatiles. Topographic shadowing allows a small but nonnegligible fraction (∼0.4%) of Ceres' surface to be perennially below the ∼110 K criterion for 1 Gyr of stability. These areas are found above 60° latitude. Other molecules (CH3OH, NH3, SO2, and CO2) may be cold trapped in smaller abundances. A model for the transport, gravitational escape, and photoionization of H2O molecules suggests net accumulation in the cold traps. Buried ice is stable within a meter for > 1 Gyr at latitudes higher than ∼50°. An illuminated polar cap of water ice would be stable within a few degrees of the poles only if it maintained a high albedo (>0.5) at present obliquity. If the obliquity exceeded 5° in the geologically recent past, then a putative polar cap would have been erased. At latitudes 0°30°, ice is stable under solar illumination only briefly (∼10100 years), unless it has high albedo and thermal inertia, in which case lifetimes of > 104 years are possible. Finally, a small hemispheric asymmetry exists due to the timing of Ceres' perihelion passage, which would lead to a detectable enhancement of ice in the northern hemisphere if the orbital elements vary slowly relative to the ice accumulation rate. Our model results are potentially testable during the Dawn science mission.

Unraveling the stratigraphic record is the key to understanding ancient climate and past climate changes on Mars (Grotzinger, J. et al. [2011]. Astrobiology 11, 77-87). Stratigraphic records of river deposits hold particular promise because rain or snowmelt must exceed infiltration plus evaporation to allow sediment transport by rivers. Therefore, river deposits when placed in stratigraphic order could constrain the number, magnitudes, and durations of the wettest (and presumably most habitable) climates in Mars history. We use crosscutting relationships to establish the stratigraphic context of river and alluvial-fan deposits in the Aeolis Dorsa sedimentary basin, 10°E of Gale crater. At Aeolis Dorsa, wind erosion has exhumed a stratigraphic section of sedimentary rocks consisting of at least four unconformity-bounded rock packages, recording three or more distinct episodes of surface runoff. Early deposits (>700. m thick) are embayed by river deposits (>400. m thick), which are in turn unconformably draped by fan-shaped deposits (900. m thick) unconformably drape all previous deposits.River deposits embay a dissected landscape formed of sedimentary rock. The river deposits are eroding out of at least two distinguishable units. There is evidence for pulses of erosion during the interval of river deposition. The total interval spanned by river deposits is >(1×10⁶-2×10⁷) yr, and this is extended if we include alluvial-fan deposits. Alluvial-fan deposits unconformably postdate thrust faults which crosscut the river deposits. This relationship suggests a relatively dry interval of >4×10⁷yr after the river deposits formed and before the fan-shaped deposits formed, based on probability arguments. Yardang-forming layered deposits unconformably postdate all of the earlier deposits. They contain rhythmite and their induration suggests a damp or wet (near-) surface environment. The time gap between the end of river deposition and the onset of yardang-forming layered deposits is constrained to >1×10⁸yr by the high density of impact craters embedded at the unconformity. The time gap between the end of alluvial-fan deposition and the onset of yardang-forming layered deposits was at least long enough for wind-induced saltation abrasion to erode 20-30m into the alluvial-fan deposits. We correlate the yardang-forming layered deposits to the upper layers of Gale crater's mound (Mt. Sharp/Aeolis Mons), and the fan-shaped deposits to Peace Vallis fan in Gale crater. Alternations between periods of low mean obliquity and periods of high mean obliquity may have modulated erosion-deposition cycling in Aeolis. This is consistent with the results from an ensemble of simulations of Solar System orbital evolution and the resulting history of the obliquity of Mars. 57 of our 61 simulations produce one or more intervals of continuously low mean Mars obliquity that are long enough to match our Aeolis Dorsa unconformity data.

ULTRASAT is a proposed wide field UV orbital observatory designed to survey the sky for astrophysical transients. The telescope will stare at each field of view of -210 deg2 for up to 6 months, measuring the flux of all point sources continuously to high precision (relative precision no worse than 1%, with a goal of 0.1%). Such long, high-cadence, high-precision and continuous light curves lend themselves naturally to transiting exoplanets searches. While not part of ULTRASAT's primary mission goals, significant opportunities are presented. In particular, ULTRASAT's Near UV (220-280nm) passband and unusual pointing (for transit searches) will allow it to be sensitive to exoplanets in different populations than other longer-wavelength surveys: White dwarfs, start with extended atmosphere, and more.

The heat flux experienced by the surface of an airless planetary body is dominated by solar insolation during the day, and by topography at night. Motivated by the close relationship between this heat flux, surface temperature and volatile stability, we consider the effect of the slope distribution on the temperature distribution and hence, prevalence of cold-traps where volatiles may accumulate over geologic time. We present a thermophysical model, accounting for four effects: insolation, reflected and emitted radiation from neighbouring slopes, and subsurface conduction. We investigate specific representative geometries, as well as generic topographies defined by their slope distributions. We find that depressions, such as craters, cast shadows that are more persistent, but warmer than those transiently cast by positive relief features convex in map view, such as hills. We find that while scarce, some briefly illuminated areas may still serve as cold-traps, potentially allowing them to be imaged from orbit. Finally, we estimate the amount of surface and subsurface ice beneath topographies with different roughness and discuss its dependency on latitude.

Dune fields on Titan cover more than 17% of the moon's surface, constituting the largest known surface reservoir of organics. Their confinement to the equatorial belt, shape, and eastward direction of propagation offer crucial information regarding both the wind regime and sediment supply. Herein, we present a comprehensive analysis of Titan's dune orientations using automated detection techniques on nonlocal denoised radar images. By coupling a new dune growth mechanism with wind fields generated by climate modeling, we find that Titan's dunes grow by sediment transport on a nonmobile substratum. To be fully consistent with both the local crestline orientations and the eastward propagation of Titan's dunes, the sediment should be predominantly transported by strong eastward winds, most likely generated by equinoctial storms or occasional fast westerly gusts. Additionally, convergence of the meridional transport predicted in models can explain why Titan's dunes are confined within ±30° latitudes, where sediment fluxes converge.

It has long been suggested that water ice can exist in extremely cold regions near the lunar poles, where sublimation loss is negligible. The geographic distribution of H-bearing regolith shows only a partial or ambiguous correlation with permanently shadowed areas, thus suggesting that another mechanism may contribute to locally enhancing water concentrations. We show that under suitable conditions, water molecules can be pumped down into the regolith by day-night temperature cycles, leading to an enrichment of H₂O in excess of the surface concentration. Ideal conditions for pumping are estimated and found to occur where the mean surface temperature is below 105 K and the peak surface temperature is above 120 K. These conditions complement those of the classical cold traps that are roughly defined by peak temperatures lower than 120 K. On the present-day Moon, an estimated 0.8% of the global surface area experiences such temperature variations. Typically, pumping occurs on pole-facing slopes in small areas, but within a few degrees of each pole the equator-facing slopes are preferred. Although pumping of water molecules is expected over cumulatively large areas, the absolute yield of this pump is low; at best, a few percent of the H₂O delivered to the surface could have accumulated in the near-surface layer in this way. The amount of ice increases with vapor diffusivity and is thus higher in the regolith with large pore spaces.

We model the primary crater production of small (D20. km higher than Zunil), a factor ~2 younger than estimated using the Hartmann production function which assumes 6. mbar surface pressure. We estimate ages of 52.3. Ma and 23.9. Ma for North Ray and Cone crater respectively, consistent with cosmic ray exposure ages from Apollo samples. Our results indicate that the average cratering rate has been constant on these bodies over these time periods. Since our Monte Carlo simulations demonstrate that the existing crater chronology systems can be applied to date young surfaces using small craters on the Moon and Mars, we conclude that the signal from secondary craters in the isochrons must be relatively small at these locations, as our Monte Carlo model only generates primary craters.

The time-variable electromagnetic sky has been well-explored at a wide range of wavelengths. In contrast, the ultra-violet (UV) variable sky is relatively poorly explored, even though it offers exciting scientific prospects. Here, we review the potential scientific impact of a wide-field survey on the study of explosive and other transient events, as well as known classes of variable objects, such as active galactic nuclei and variable stars. We quantify our predictions using a fiducial set of observational parameters which are similar to those envisaged for the proposed ULTRASAT mission. We show that such a mission would be able to revolutionize our knowledge about massive star explosions by measuring the early UV emission from hundreds of events, revealing key physical parameters of the exploding progenitor stars. Such a mission would also detect the UV emission from many tens of tidal-disruption events of stars by supermassive black holes at galactic nuclei and enable a measurement of the rate of such events. The overlap of such a wide-field UV mission with existing and planned gravitational-wave and high-energy neutrino telescopes makes it especially timely.

We present maps of the topographic roughness of the Moon at hectometer and kilometer scales. The maps are derived from range profiles obtained by the Lunar Orbiter Laser Altimeter (LOLA) instrument onboard the Lunar Reconnaissance Orbiter (LRO) spacecraft. As roughness measures, we used the interquartile range of profile curvature at several baselines, from 115. m to 1.8. km, and plotted these in a global map format. The maps provide a synoptic overview of variations of typical topographic textures and utilize the exceptional ranging precision of the LOLA instrument. We found that hectometer-scale roughness poorly correlates with kilometer-scale roughness, because they reflect different sets of processes and time scales. Hectometer-scale roughness is controlled by regolith accumulation and modification processes and affected by the most recent events, primarily, geologically recent (1-2. Ga) meteoritic impacts. Kilometer-scale roughness reflects major geological (impact, volcanic and tectonic) events in earlier geological history. Young large impact craters are rough, and their roughness decreases with age. The global roughness maps revealed a few unusually dense clusters of hectometer- and decameter-size impact craters that differ in their morphology and settings from typical secondary crater clusters and chains; the origin of these features is enigmatic. The maps can assist in the geological mapping of the lunar maria by revealing contacts between volcanic plain units. The global roughness maps also clearly reveal cryptomaria, old volcanic plains superposed by younger materials, primarily crater and basin ejecta.

Saturn's moon Titan has lakes and seas of liquid hydrocarbon and a dense atmosphere, an environment conducive to generating wind waves. Cassini observations thus far, however, show no indication of waves. We apply models for wind wave generation and detection to the Titan environment. Results suggest wind speed thresholds at a reference altitude of 10. m of 0.4-0.7. m/s for liquid compositions varying between pure methane and equilibrium mixtures with the atmosphere (ethane has a threshold of 0.6. m/s), varying primarily with liquid viscosity. This reduced threshold, as compared to Earth, results from Titan's increased atmosphere-to-liquid density ratio, reduced gravity and lower surface tension. General Circulation Models (GCMs) predict wind speeds below derived thresholds near equinox, when available observations of lake surfaces have been acquired. Predicted increases in winds as Titan approaches summer solstice, however, will exceed expected thresholds and may provide constraints on lake composition and/or GCM accuracy through the presence or absence of waves during the Cassini Solstice Mission. A two-scale microwave backscatter model suggests that returns from wave-modified liquid hydrocarbon surfaces may be below the pixel-scale noise floor of Cassini radar images, but can be detectable using real-aperture scatterometry, pixel binning and/or observations obtained in a specular geometry.

The observed cloud-level atmospheric circulation on the outer planets of the Solar System is dominated by strong east-west jet streams. The depth of these winds is a crucial unknown in constraining their overall dynamics, energetics and internal structures. There are two approaches to explaining the existence of these strong winds. The first suggests that the jets are driven by shallow atmospheric processes near the surface, whereas the second suggests that the atmospheric dynamics extend deeply into the planetary interiors. Here we report that on Uranus and Neptune the depth of the atmospheric dynamics can be revealed by the planets' respective gravity fields. We show that the measured fourth-order gravity harmonic, J 4, constrains the dynamics to the outermost 0.15 per cent of the total mass of Uranus and the outermost 0.2 per cent of the total mass of Neptune. This provides a stronger limit to the depth of the dynamical atmosphere than previously suggested, and shows that the dynamics are confined to a thin weather layer no more than about 1, 000 kilometres deep on both planets.

Numerical simulations and analysis show that the Moon locks into resonance with a statistical preference of facing either the current near-side or far-side toward Earth. The near-side is largely covered by dense, topographically low, dark mare basalts, the pattern of which to some, resembles the image of a man's face. Although the Moon is locked in this configuration at present, the opposite one, with the current far-side facing Earth, is of lower potential energy and hence might be naively expected. Instead, we find that the probability of selecting each configuration depends upon the ratio of the asymmetry of the potential energy maxima, dominated by the octupole moment of the Moon, to the energy dissipated per tidal cycle within the Moon. If this ratio is small, the two configurations are equally likely. Otherwise, interesting dynamical behavior ensues. In the Moon's present orbit, with the best-estimated geophysical parameters and dissipation parameter . Q=. 35, trapping into the current higher-energy configuration is preferred. With . Q=. 100 in analogy with the solid Earth, the current configuration is nearly certain. The ratio of energies and corresponding probabilities were different in the past. Relative crater counts on the leading and trailing faces indicate an impact may have unlocked the Moon before it settled into the present configuration. Our analysis constrains the geophysical parameters at the time of the last such event.

In a series of laboratory experiments, we measure thermal diffusivity, thermal conductivity, and heat capacity of icy regolith created by vapor deposition of water below its triple point and in a low pressure atmosphere. We find that an ice-regolith mixture prepared in this manner, which may be common on Mars, and potentially also present on the Moon, Mercury, comets and other bodies, has a thermal conductivity that increases approximately linearly with ice content. This trend differs substantially from thermal property models based of preferential formation of ice at grain contacts previously applied to both terrestrial and non-terrestrial subsurface ice. We describe the observed microphysical structure of ice responsible for these thermal properties, which displaces interstitial gases, traps bubbles, exhibits anisotropic growth, and bridges non-neighboring grains. We also consider the applicability of these measurements to subsurface ice on Mars and other solar system bodies.

All planetary bodies with old surfaces exhibit planetary-scale impact craters: vast scars caused by the large impacts at the end of Solar System accretion or the late heavy bombardment. Here we investigate the geophysical consequences of planetary-scale impacts into a Mars-like planet, by simulating the events using a smoothed particle hydrodynamics (SPH) model. Our simulations probe impact energies over two orders of magnitude (2×10²⁷-6×10²⁹J), impact velocities from the planet's escape velocity to twice Mars' orbital velocity (6-50km/s), and impact angles from head-on to highly oblique (0-75°). The simulation results confirm that for planetary-scale impacts, surface curvature, radial gravity, the large relative size of the impactor to the planet, and the greater penetration of the impactor, contribute to significant differences in the geophysical expression compared to small craters, which can effectively be treated as acting in a half-space. The results show that the excavated crustal cavity size and the total melt production scale similarly for both small and planetary-scale impacts as a function of impact energy. However, in planetary-scale impacts a significant fraction of the melt is sequestered at depth and thus does not contribute to resetting the planetary surface; complete surface resetting is likely only in the most energetic (6×10²⁹J), slow, and head-on impacts simulated. A crater rim is not present for planetary-scale impacts with energies >10²⁹J and angles ≤45°, but rather the ejecta is more uniformly distributed over the planetary surface. Antipodal crustal removal and melting is present for energetic (>10²⁹J), fast (>6km/s), and low angle (≤45°) impacts. The most massive impactors (with both high impact energy and low velocity) contribute sufficient angular momentum to increase the rotation period of the Mars-sized target to about a day. Impact velocities of >20km/s result in net mass erosion from the target, for all simulated energies and angles. The hypothesized impact origin of planetary structures may be tested by the presence and distribution of the geochemically-distinct impactor material.

Cassini RADAR topography data are used to evaluate Titan's hypsometric profile, and to make comparisons with other planetary bodies. Titan's hypsogram is unimodal and strikingly narrow compared with the terrestrial planets. To investigate topographic extremes, a novel variant on the classic hypsogram is introduced, with a logarithmic abscissa to highlight mountainous terrain. In such a plot, the top of the terrestrial hypsogram is quite distinct from those of Mars and Venus due to the 'glacial buzz-saw' that clips terrestrial topography above the snowline. In contrast to the positive skew seen in other hypsograms, with a long tail of positive relief due to mountains, there is an indication (weak, given the limited data for Titan so far) that the Titan hypsogram appears slightly negatively skewed, suggesting a significant population of unfilled depressions. Limited data permit only a simplistic comparison of Titan topography with other icy satellites but we find that the standard deviation of terrain height (albeit at different scales) is similar to those of Ganymede and Europa.

Mercury's coupled 3:2 spin-orbit resonance in conjunction with its relatively high eccentricity of ∼0.2 and near-zero obliquity results in both a latitudinal and longitudinal variation in annual average solar insolation and thus equatorial hot and cold regions. This results in an asymmetric temperature distribution in the lithosphere and a long wavelength lateral variation in lithosphere structure and strength that mirrors the insolation pattern. We employ a thermal evolution model for Mercury generating strength envelopes of the lithosphere to demonstrate and quantify the possible effects the insolation pattern has on Mercury's lithosphere. We find the heterogeneity in lithosphere strength is substantial and increases with time. We also find that a crust thicker than that of the Moon or Mars and dry rheologies for the crust and mantle are favorable when compared with estimates of brittle-ductile transition depths derived from lobate scarps. Regions of stronger and weaker compressive strength imply that the accommodation of radial contraction of Mercury as its interior cooled, manifest as lobate scarps, may not be isotropic, imparting a preferential orientation and distribution to the lobate scarps.

As of June 19, 2010, the Lunar Orbiter Laser Altimeter, an instrument on the Lunar Reconnaissance Orbiter, has collected over 2.0 × 10⁹ measurements of elevation that collectively represent the highest resolution global model of lunar topography yet produced. These altimetric observations have been used to improve the lunar geodetic grid to ∼10 m radial and ∼100 m spatial accuracy with respect to the Moon's center of mass. LOLA has also provided the highest resolution global maps yet produced of slopes, roughness and the 1064-nm reflectance of the lunar surface. Regional topography of the lunar polar regions allows precise characterization of present and past illumination conditions. LOLA's initial global data sets as well as the first high-resolution digital elevation models (DEMs) of polar topography are described herein.

Ontario Lacus is the largest and best characterized lake in Titan's south polar region. In June and July 2009, the Cassini RADAR acquired its first Synthetic Aperture Radar (SAR) images of the area. Together with closest approach altimetry acquired in December 2008, these observations provide a unique opportunity to study the lake's nearshore bathymetry and complex refractive properties. Average radar backscatter is observed to decrease exponentially with distance from the local shoreline. This behavior is consistent with attenuation through a deepening layer of liquid and, if local topography is known, can be used to derive absorptive dielectric properties. Accordingly, we estimate nearshore topography from a radar altimetry profile that intersects the shoreline on the East and West sides of the lake. We then analyze SAR backscatter in these regions to determine the imaginary component of the liquid's complex index of refraction (κ). The derived value, κ = (6.1_-1.3^+1.7) × 10^-4, corresponds to a loss tangent of tan Δ = (9.2_-2.0^+2.5) × 10^-4 and is consistent with a composition dominated by liquid hydrocarbons. This value can be used to test compositional models once the microwave optical properties of candidate materials have been measured. In areas that do not intersect altimetry profiles, relative slopes can be calculated assuming the index of refraction is constant throughout the liquid. Accordingly, we construct a coarse bathymetry map for the nearshore region by measuring bathymetric slopes for eleven additional areas around the lake. These slopes vary by a factor of ∼5 and correlate well with observed shoreline morphologies.

A set of lakes filled or partially filled with liquid hydrocarbon and empty lake basins have been discovered in the high latitudes of Saturns moon Titan. These features were mapped by the radar instrument on the Cassini orbiter. Here we quantify the distribution of the lakes and basins, and show a pronounced hemispheric asymmetry in their occurrence. Whereas significant fractions of the northern high latitudes are covered by filled and empty lakes, the same latitudes in the southern hemisphere are largely devoid of such features. We propose that in addition to known seasonal changes, the observed difference in lake distribution may be caused by an asymmetry in the seasons on Titan that results from the eccentricity of Saturns orbit around the Sun. We suggest that the consequent hemispheric difference in the balance between evaporation and precipitation could lead to an accumulation of lakes in one of Titans hemispheres. This effect would be modulated by, and reverse with, dynamical variations in the orbit. We propose that much like in the Earths glacial cycles, the resulting vigorous hydrologic cycle has a period of tens of thousands of years and leads to active geologic surface modification in the polar latitudes.

Global acquisition of infrared spectra and high-resolution visible and infrared imagery has enabled the placement of compositional information within stratigraphic and geologic context. Mare Serpentis, a low albedo region located northwest of Hellas Basin, is rich in spectral and thermophysical diversity and host to numerous isolated exposures of in situ rocky material. Most martian surfaces are dominated by fine-grained particulate materials that bear an uncertain compositional and spatial relationship to their source. Thus location and characterization of in situ rock exposures is important for understanding the origin of highland materials and the processes which have modified those materials. Using spectral, thermophysical and morphologic information, we assess the local and regional stratigraphy of the Mare Serpentis surface in an effort to reconstruct the geologic history of the region. The martian highlands in Mare Serpentis are dominated by two interspersed surface units, which have distinct compositional and thermophysical properties: (1) rock-dominated surfaces relatively enriched in olivine and pyroxene, and depleted in high-silica phases, and (2) sediment or indurated material depleted in olivine and pyroxene, with relatively higher abundance of high-silica phases. This is a major, previously unrecognized trend which appears to be pervasive in the Mare Serpentis region and possibly in other highland areas. The detailed observations have led us to form two hypotheses for the relationship between these two units: either (1) they are related through a widespread mechanical and/or chemical alteration process, where less-mafic plains materials are derived from the mafic bedrock, but have been compositionally altered in the process of regolith formation, or (2) they are stratigraphically distinct units representing separate episodes of upper crust formation. Existing observations suggest that the second scenario is more likely. In this scenario, plains materials represent older, degraded, and possibly altered, "basement" rock, whereas the rocky exposures represent later additions to the crust and are probably volcanic in origin. These hypotheses should be further testable with decimeter-resolution imagery and meter-resolution short wavelength infrared spectra.

Widespread sedimentary rocks on Mars preserve evidence of surface conditions different from the modern cold and dry environment, although it is unknown how long conditions favorable to deposition persisted. We used 1-meter stereo topographic maps to demonstrate the presence of rhythmic bedding at several outcrops in the Arabia Terra region. Repeating beds are ∼10 meters thick, and one site contains hundreds of meters of strata bundled into larger units at a ∼10:1 thickness ratio. This repetition likely points to cyclicity in environmental conditions, possibly as a result of astronomical forcing. If deposition were forced by orbital variation, the rocks may have been deposited over tens of millions of years.

Current understanding of weather, climate and global atmospheric circulation on Mars is incomplete, in particular at altitudes above about 30 km. General circulation models for Mars are similar to those developed for weather and climate forecasting on Earth and require more martian observations to allow testing and model improvements. However, the available measurements of martian atmospheric temperatures, winds, water vapour and airborne dust are generally restricted to the region close to the surface and lack the vertical resolution and global coverage that is necessary to shed light on the dynamics of Mars middle atmosphere at altitudes between 30 and 80 km (ref.7). Here we report high-resolution observations from the Mars Climate Sounder instrument on the Mars Reconnaissance Orbiter. These observations show an intense warming of the middle atmosphere over the south polar region in winter that is at least 10-20 K warmer than predicted by current model simulations. To explain this finding, we suggest that the atmospheric downwelling circulation over the pole, which is part of the equator-to-pole Hadley circulation, may be as much as 50 more vigorous than expected, with consequences for the cycles of water, dust and CO"2 that regulate the present-day climate on Mars.

We explore the capability of a method of mapping the depth distribution of a hydrogen-rich layer in the top meter of Mars from the neutron currents measured by the Mars Odyssey Neutron Spectrometer. Assuming the soil can be modeled by two layers of known composition having different hydrogen contents, simulations allow an inversion of the neutron data into knowledge of depth and hydrogen content of the lower layer. The determination of these variables is sensitive to the hypothesis of chemical composition of the soil. We quantify this contribution to the uncertainty in the method first in terms of individual chemical elements and then in terms of macroscopic absorption cross sections. To minimize this source of error, an average composition was inferred from Mars Exploration Rover data. Possible compositions having a wide range of macroscopic absorption cross sections were used to evaluate the uncertainty associated with our calculations. We finally compare our results to ice table depth estimates predicted by two published theoretical models at locations where the composition is relatively well known. The fit is excellent in the southern high latitudes but questionable in the northern high latitudes. Possible explanations of these differences include the high geographical variations of the neutron currents relative to the spatial width of the response function of the instrument and the overly simple model we, of necessity, used for surface layering.

The Mars hemispheric dichotomy is expressed as a dramatic difference in elevation, crustal thickness and crater density between the southern highlands and northern lowlands (which cover ∼42% of the surface). Despite the prominence of the dichotomy, its origin has remained enigmatic and models for its formation largely untested. Endogenic degree-1 convection models with north-south asymmetry are incomplete in that they are restricted to simulating only mantle dynamics and they neglect crustal evolution, whereas exogenic multiple impact events are statistically unlikely to concentrate in one hemisphere. A single mega-impact of the requisite size has not previously been modelled. However, it has been hypothesized that such an event could obliterate the evidence of its occurrence by completely covering the surface with melt or catastrophically disrupting the planet. Here we present a set of single-impact initial conditions by which a large impactor can produce features consistent with the observed dichotomy's crustal structure and persistence. Using three-dimensional hydrodynamic simulations, large variations are predicted in post-impact states depending on impact energy, velocity and, importantly, impact angle, with trends more pronounced or unseen in commonly studied smaller impacts. For impact energies of ∼(3-6) × 10²⁹ J, at low impact velocities (6-10 km s^-1) and oblique impact angles (30-60°), the resulting crustal removal boundary is similar in size and ellipticity to the observed characteristics of the lowlands basin. Under these conditions, the melt distribution is largely contained within the area of impact and thus does not erase the evidence of the impact's occurrence. The antiquity of the dichotomy is consistent with the contemporaneous presence of impactors of diameter 1,600-2,700 km in Mars-crossing orbits, and the impact angle is consistent with the expected distribution.

THE RETURN OF MARTIAN SAMPLES TO EARTH has long been recognized as an essential component of a cycle of exploration that begins with orbital reconnaissance and in situ surface investigations. Major questions about life, climate, and geology require answers from state-of-the-art laboratories on Earth. Spacecraft instrumentation cannot perform critical measurements such as precise radiometric age dating, sophisticated stable isotopic analyses, and definitive life-detection assays. Returned sample studies could respond radically to unexpected findings, and returned materials could be archived for study by future investigators with even more capable laboratories. Unlike martian meteorites, returned samples could be acquired with known context from selected sites on Mars according to the prioritized exploration goals and objectives. The ND-MSR-SAG formulated the following 11 high-level scientific objectives that indicate how a balanced program of ongoing MSR missions could help to achieve the objectives and investigations described by MEPAG (2006). (1) Determine the chemical, mineralogical, and isotopic composition of the crustal reservoirs of carbon, nitrogen, sulfur, and other elements with which they have interacted and characterize carbon-, nitrogen-, and sulfur-bearing phases down to submicron spatial scales in order to document processes that could sustain habitable environments on Mars both today and in the past. (2) Assess the evidence for prebiotic processes, past life, and extant life on Mars by characterizing the signatures of these phenomena in the form of structure/morphology, biominerals, organic molecular and isotopic compositions, and other evidence within their geologic contexts. (3) Interpret the conditions of martian water-rock interactions through the study of their mineral products. (4) Constrain the absolute ages of major martian crustal geologic processes, including sedimentation, diagenesis, volcanism/plutonism, regolith formation, hydrothermal alteration, weathering, and cratering. (5) Understand paleoenvironments and the history of nearsurface water on Mars by characterizing the clastic and chemical components, depositional processes, and postdepositional histories of sedimentary sequences. (6) Constrain the mechanism and timing of planetary accretion, differentiation, and the subsequent evolution of the martian crust, mantle, and core. (7) Determine how the martian regolith was formed and modified and how and why it differs from place to place. (8) Characterize the risks to future human explorers in the areas of biohazards, material toxicity, and dust/granular materials and contribute to the assessment of potential in situ resources to aid in establishing a human presence on Mars. (9) For the present-day martian surface and accessible shallow subsurface environments, determine the preservation potential for the chemical signatures of extant life and prebiotic chemistry by evaluating the state of oxidation as a function of depth, permeability, and other factors. (10) Interpret the initial composition of the martian atmosphere, the rates and processes of atmospheric loss/gain over geologic time, and the rates and processes of atmospheric exchange with surface condensed species. (11) For martian climate-modulated polar deposits, determine their age, geochemistry, conditions of formation, and evolution through the detailed examination of the composition of water, CO₂, and dust constituents, as well as isotopic ratios and detailed stratigraphy of the upper layers of the surface. MSR would attain its greatest value if samples are collected as sample suites that represent the diversity of the products of various planetary processes. Sedimentary materials likely contain complex mixtures of chemical precipitates, volcaniclastics, impact glass, igneous rock fragments, and phyllosilicates. Aqueous sedimentary deposits are important for performing measurements of life detection, observations of critical mineralogy and geochemical patterns, and trapped gases. On Earth, hydrothermally altered rocks can preserve a record of hydrothermal systems that provided water, nutrients, and chemical energy necessary to sustain microorganisms. They also might have preserved fossils in their mineral deposits. Hydrothermal processes alter the mineralogy of crustal rocks and inject CO₂ and reduced gases into the atmosphere. Chemical alteration that occurs at nearsurface ambient conditions (typically 2O and CO₂ between near-surface solid materials and the atmosphere, and processes that involve fluids and sublimation. Regolith studies would help facilitate future human exploration by assessing toxicity and potential resources. Polar ices would constrain present and past climatic conditions and help elucidate water cycling. Surface ice samples from the Polar Layered Deposits (PLD) or seasonal frost deposits would help to quantify surface/atmosphere interactions. Short cores could help to resolve recent climate variability. Atmospheric gas samples would constrain the composition of the atmosphere and processes that influenced its origin and evolution. Trace organic gases (e.g., methane and ethane) could be analyzed for abundances, distribution, and relationships to a potential martian biosphere. Returned atmospheric samples that contain Ne, Kr, CO₂, CH₄, and C₂H₆ would confer major scientific benefits. Chemical and mineralogical analyses of martian dust would help to elucidate the weathering and alteration history of Mars. Given the global homogeneity of martian dust, a single sample is likely to be representative of the planet. A depth-resolved suite of samples should be obtained from depths that range from cm to several m within regolith or from rock outcrop to investigate trends in the abundance of oxidants (e.g., OH, HO₂, H₂O₂, and peroxy radicals), the effects of radiation, and the preservation of organic matter. Other sample suites include impact breccias that might sample rock types that are otherwise not available locally, tephra consisting of fine-grained regolith material or layers and beds possibly delivered from beyond the landing site, and meteorites whose alteration history could provide insights into martian climatic history. The following factors would affect our ability to achieve MSR's scientific objectives: (1) Sample size. A full program of scientific investigations would likely require samples of ≥8 g for bedrock, loose rocks, and finer-grained regolith. To support required biohazard testing, each sample requires an additional 2 g, leading to an optimal size of 10 g. Textural studies of some rock types might require one or more larger samples of ∼20 g. Material should remain to be archived for future investigations. (2) Number of samples. Studies of differences between samples could provide more information than detailed studies of a single sample. The number of samples needed to address MSR scientific objectives effectively is 35 (28 rock, 4 regolith, 1 dust, 2 gas). If the MSR mission recovers the MSL cache, it should also collect 26 additional samples (20 rock, 3 regolith, 1 dust and 2 atmospheric gas). The total mass of these samples is expected to be about 345 g (or 380 g with the MSL cache). The total returned mass with sample packaging would be about 700 g. (3) Sample encapsulation. To retain scientific value, returned samples must not commingle, each sample must be linked uniquely to its documented field context, and rocks should be protected against fragmentation during transport. A smaller number or mass of carefully managed samples is far more valuable than a larger number or mass of poorly managed samples. The encapsulation of at least some samples must retain any released volatile components. (4) Diversity of the returned collection. The diversity of returned samples must be commensurate with the diversity of rocks and regolith encountered. This guideline substantially influences landing site selection and rover operation protocols. It is scientifically acceptable for MSR to visit only a single site, but visiting 2 independent landing sites would be much more valuable. (5) In situ measurements for sample selection and documentation of field context. Relatively few samples could be returned from the vast array of materials the MSR rover would encounter; thus we must be able to choose wisely. At least 3 kinds of in situ observations are needed (color imaging, microscopic imaging, and mineralogy measurement) and possibly as many as 5 (also elemental analysis and reduced-carbon analysis). No significant difference exists in the observations needed for sample selection vs. sample documentation. Revisiting a previously occupied site might result in a reduction in the number of instruments.

Cassini RADAR observations now permit an initial assessment of the inventory of two classes, presumed to be organic, of Titan surface materials: polar lake liquids and equatorial dune sands. Several hundred lakes or seas have been observed, of which dozens are each estimated to contain more hydrocarbon liquid than the entire known oil and gas reserves on Earth. Dark dunes cover some 20% of Titan's surface, and comprise a volume of material several hundred times larger than Earth's coal reserves. Overall, however, the identified surface inventories (>3 × 10⁴ km³ of liquid, and >2 × 10⁵ km³ of dune sands) are small compared with estimated photochemical production on Titan over the age of the solar system. The sand volume is too large to be accounted for simply by erosion in observed river channels or ejecta from observed impact craters. The lakes are adequate in extent to buffer atmospheric methane against photolysis in the short term, but do not contain enough methane to sustain the atmosphere over geologic time. Unless frequent resupply from the interior buffers this greenhouse gas at exactly the right rate, dramatic climate change on Titan is likely in its past, present and future.

Slope streaks are surficial mass movements that are abundant in the dust-covered regions of Mars. Targeting of slope streaks seen in Viking images with the Mars Orbiter Camera provides observations of slope streak dust activity over two to three decades. In all study areas, new and persisting dark slope streaks are observed. Slope streaks disappeared in one area, with persisting streaks nearby. New slope streaks are found to be systematically darker than persisting streaks, which indicates gradual fading. Far more slope streaks formed at the study sites than have faded from visibility. The rate of formation at the study sites was 0.03 new slope streaks per existing streak per Mars year. Bright slope streaks do not presently form in sudden events as dark slope streaks do. Instead, bright streaks might form from old dark slope streaks, perhaps transitioning through a partially faded stage.

Current geophysical knowledge of the planet Mercury is based upon observations from ground-based astronomy and flybys of the Mariner 10 spacecraft, along with theoretical and computational studies. Mercury has the highest uncompressed density of the terrestrial planets and by implication has a metallic core with a radius approximately 75% of the planetary radius. Mercurys spin rate is stably locked at 1.5 times the orbital mean motion. Capture into this state is the natural result of tidal evolution if this is the only dissipative process affecting the spin, but the capture probability is enhanced if Mercurys core were molten at the time of capture. The discovery of Mercurys magnetic field by Mariner 10 suggests the possibility that the core is partially molten to the present, a result that is surprising given the planets size and a surface crater density indicative of early cessation of significant volcanic activity. A present-day liquid outer core within Mercury would require either a core sulfur content of at least several weight percent or an unusual history of heat loss from the planets core and silicate fraction. A crustal remanent contribution to Mercurys observed magnetic field cannot be ruled out on the basis of current knowledge. Measurements from the MESSENGER orbiter, in combination with continued ground-based observations, hold the promise of setting on a firmer basis our understanding of the structure and evolution of Mercurys interior and the relationship of that evolution to the planets geological history. [Reprinted also in The MESSENGER Mission to Mercury; 978-0-387-77214-1; https://doi.org/10.1007/978-0-387-77214-1]

Home Plate is a layered plateau in Gusev crater on Mars. It is composed of clastic rocks of moderately altered alkali basalt composition, enriched in some highly volatile elements. A coarse-grained lower unit lies under a finer-grained upper unit. Textural observations indicate that the lower strata were emplaced in an explosive event, and geochemical considerations favor an explosive volcanic origin over an impact origin. The lower unit likely represents accumulation of pyroclastic materials, whereas the upper unit may represent eolian reworking of the same pyroclastic materials.

Arising from: T. M. McCollom & B. M. Hynek Nature 438, 1129-1131 (2005); McCollom & Hynek replyThe Mars Exploration Rover Opportunity discovered sulphate-rich sedimentary rocks at Meridiani Planum on Mars, which are interpreted by McCollom and Hynek as altered volcanic rocks. However, their conclusions are derived from an incorrect representation of our depositional model, which is upheld by more recent Rover data. We contend that all the available data still support an aeolian and aqueous sedimentary origin for Meridiani bedrock.

We seek a better understanding of the distribution of subsurface ice on Mars, based on the physical processes governing the exchange of vapor between the atmosphere and the subsurface. Ground ice is expected down to ∼49° latitude and lower latitudes at poleward facing slopes. The diffusivity of the regolith also leads to seasonal accumulation of atmospherically derived frost at latitudes poleward of ∼30°. The burial depths and zonally averaged boundaries of subsurface ice observed from neutron emission arc consistent with model predictions for ground ice in equilibrium with the observed abundance of atmospheric water vapor. Longitudinal variations in ice distribution are due mainly to thermal inertia and are more pronounced in the observations than in the model. These relations support the notion that the ground ice has at least partially adjusted to the atmospheric water vapor content or is atmospherically derived. Changes in albedo can rapidly alter the equilibrium depth to the ice, creating sources or sinks of atmospheric H₂O while the ground ice is continuously evolving toward a changing equilibrium. At steady state humidity and temperature oscillations, the net flux of vapor is uninhibited by adsorption. The occurrence of temporary frost is characterized by the isosteric enthalpy of adsorption.

In the paper \u201cLocalized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution\u201d by Patrick J. McGovern, Sean C. Solomon, David E. Smith, Maria T. Zuber, Mark Simons, Mark A. Wieczorek, Roger J. Phillips, Gregory A. Neumann, Oded Aharonson, and James W. Head (Journal of Geophysical Research, 107(E12), 5136, doi:10.1029/2002JE001854, 2002), the thickness of the lithosphere and lithospheric heat flow for a number of regions of Mars and as functions of time were inferred on the basis of gravity/topography admittance spectra. Observed admittances, derived from spherical harmonic expansions localized with the scheme of Simons et al. [1997], were compared with those predicted from models for the flexural response to lithospheric loading [e.g., Turcotte et al., 1981]. Gravity was calculated according to the finiteamplitude scheme of Wieczorek and Phillips [1998]. Estimates for the thickness of the elastic lithosphere Te at the time of loading for each region were converted to equivalent thermal gradient dT/dz and heat flux q by means of an elasticplastic stressenvelope formalism [McNutt, 1984]. Here we describe a correction required in the calculation of the modeled gravity anomalies; we report new estimates of Te, load density ρl, dT/dz, and q from corrected model admittances; and we discuss the implications of the new results.

We report observations of a set of surface features on Mars that form a distinct class of avalanche scars. These features have a horizontal scale of hundreds of meters, but a depth scale of meters distinguishes them from the shallower features known as slope streaks. The meters-thick avalanche scars have escaped previous attention because of weak contrast between the interiors of the scarred regions and their surroundings. Often the most visible feature is a shadow cast by the trough wall, a band 1-3 pixels wide in Mars Orbiter Camera narrow angle images, indicating maximum depths of 4-10 m. We investigate the morphology of more than 500 such features. Slopes upon which the avalanches occur average about 27°. Impact craters are seen at the heads of some avalanche scars; this subset exhibits statistically wider opening angles. The scars span an estimated several Ma in age. Those found so far occurred mainly in the Olympus Mons lower aureole. We compare shapes of slope streaks to shapes of meters-thick avalanches, and the results support the notion that the two classes are distinct. The newly-discovered avalanches resemble some terrestrial flows of loose, dry material such as dry snow and glass beads. On the basis of these analogs, we suggest a physical model.

The Mars Orbiter Laser Altimeter (MOLA) measured the pulse width and energy of altimetric laser returns during the course of two Mars years of operations. As secondary science objectives, MOLA obtains the footprintscale roughness and the bidirectional reflectivity of Mars. MOLA underwent extensive preflight calibration and pulse measurements were monitored continuously in flight, but anomalous values of roughness have been inferred. A calibration of pulse widths using inflight data yields a slopecorrected roughness over ∼75mdiameter footprints that may be used for quantitative geomorphic surface characterization, required, for example, for landing site selection. The recalibration uses a total leastsquares estimation of pulse characteristics that generalizes the method of Abshire et al. [2000]. This method, utilizing the timing at voltage threshold crossings and the area between crossings, accounts for observation errors and shows that surface roughness as small as 1 m can be resolved.

The Mars Orbiter Camera on board the Mars Global Surveyor spacecraft has returned images of numerous dark streaks that are the result of down-slope mass movement occurring under present-day martian climatic conditions. We systematically analyze over 23,000 high-resolution images and demonstrate that slope streaks form exclusively in regions of low thermal inertia (confirming earlier results), steep slopes, and, remarkably, only where peak temperatures exceed 275 K. The northernmost streaks, which form in the coldest environment, form preferentially on warmer south-facing slopes. Repeat images of sites with slope streaks show changes only if the time interval between the two images includes the warm season. Surprisingly (in light of the theoretically short residence time of H₂O close to the surface), the data support the possibility that small amounts of water are transiently present in low-latitude near-surface regions of Mars and undergo phase transitions at times of high insolation, triggering the observed mass movements.

The Mars Orbiter Laser Altimeter (MOLA), an instrument on the Mars Global Surveyor spacecraft, has measured the topography, surface roughness, and 1.064-μm reflectivity of Mars and the heights of volatile and dust clouds. This paper discusses the function of the MOLA instrument and the acquisition, processing, and correction of observations to produce global data sets. The altimeter measurements have been converted to both gridded and spherical harmonic models for the topography and shape of Mars that have vertical and radial accuracies of ∼1 m with respect to the planet's center of mass. The current global topographic grid has a resolution of 1/64° in latitude x 1/32° in longitude (1 x 2 km² at the equator). Reconstruction of the locations of incident laser pulses on the Martian surface appears to be at the 100-m spatial accuracy level and results in 2 orders of magnitude improvement in the global geodetic grid of Mars. Global maps of optical pulse width indicative of 100-m-scale surface roughness and 1.064-μm reflectivity with an accuracy of 5% have also been obtained.

Loading of the lithosphere of Mars by the Tharsis rise explains much of the global shape and long-wavelength gravity field of the planet, including a ring of negative gravity anomalies and a topographic trough around Tharsis, as well as gravity anomaly and topographic highs centered in Arabia Terra and extending northward toward Utopia. The Tharsis-induced trough and antipodal high were largely in place by the end of the Noachian Epoch and exerted control on the location and orientation of valley networks. The release of carbon dioxide and water accompanying the emplacement of ∼3 × 10⁸ cubic kilometers of Tharsis magmas may have sustained a warmer climate than at present, enabling the formation of ancient valley networks and fluvial landscape denudation in and adjacent to the large-scale trough.

Topography and gravity measured by the Mars Global Surveyor have enabled determination of the global crust and upper mantle structure of Mars. The planet displays two distinct crustal zones that do not correlate globally with the geologic dichotomy: a region of crust that thins progressively from south to north and encompasses much of the southern highlands and Tharsis province and a region of approximately uniform crustal thickness that includes the northern lowlands and Arabia Terra. The strength of the lithosphere beneath the ancient southern highlands suggests that the northern hemisphere was a locus of high heat flow early in martian history. The thickness of the elastic lithosphere increases with time of loading in the northern plains and Tharsis. The northern lowlands contain structures interpreted as large buried channels that are consistent with northward transport of water and sediment to the lowlands before the end of northern hemisphere resurfacing.

We investigate slope distributions in the northern hemisphere of Mars from topographic profiles collected by the Mars Orbiter Laser Altimeter. Analysis of the region from about 12°S to 82°N, over diverse geologic units, indicates that the range of regional-scale slopes is small, generally