Publications

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.

Estimation of planetary orbital and physical parameters from light-curve data relies heavily on the accurate interpretation of Transit Timing Variation (TTV) measurements. In this letter, we review the process of TTV measurement and compare two fitting paradigms-one that relies on making transit-by-transit timing estimates and then fitting a TTV model to the observed timings, and one that relies on fitting a global flux model to the entire light-curve data set simultaneously. The latter method is achieved either by solving for the underlying planetary motion (often referred to as "photodynamics"), or by using an approximate or empirical shape of the TTV signal. We show that, across a large range of the transit S/N regime, the probability distribution function of the mid-transit time significantly deviates from a Gaussian, even if the flux errors do distribute normally. Treating the timing uncertainties as if they are distributed normally leads, in such a case, to a wrong interpretation of the TTV measurements. We illustrate these points using numerical experiments and conclude that a fitting process that relies on a global flux fitting, rather than the derived TTVs, should be preferred.

Mars harbors ice deposits in several forms, on the surface and in the subsurface, which exchange with each other on various timescales. We seek to study the pore ice evolution over millennial time scales and how it contributes to and affects the Polar cap's evolution. We calculate the evolution of SubSurface Ice (SSI) pore filling by coupling two models, the Mars LMD Global Climate Model, which calculates the atmospheric and surface evolution on an annual timescale, and the dynamical version of the Mars Subsurface Ice Model, which calculates the evolution of the SSI on a millennial timescale. The SSI latitudinal boundary fluctuates over more than 25° in one obliquity cycle, overall extending equatorward of latitude ±35° at high obliquity, and receding to about ±60° at low obliquity. In locations where the SSI is stable continuously over orbital cycles, the simulations predict layering caused by a sublimation front at the SSI top boundary. Between 5 and 2.5 Myr ago, the subsurface lost at least ∼95 m of polar equivalent layer ice. The SSI flux routinely reaches ∼1 mm/Mars year. In addition to the direct contribution to the growth of the North Polar Layered Deposits (NPLD), the SSI causes variations in the NPLD accumulation rate due to the changes in the SSI distribution that affect the seasonal energy budget. These variations are comparable to the change in rate due to variations in orbital elements. When running paleo-climate simulations, particularly to reconstruct the NPLD profile, changes in the SSI distribution should be considered.

Exposed scarps images and ice-penetrating radar measurements in the North Polar Layered Deposits (NPLD) of Mars show alternating layers that provide an archive of past climate oscillations, that are thought to be linked to orbital variations, akin to Milankovitch cycles on Earth. We use the Laboratoire de Météorologie Dynamique Martian Global Climate Model to study paleoclimate states to enable a better interpretation of the NPLD physical and chemical stratigraphy. When a tropical ice reservoir is present, water vapor transport from the tropics to the poles at low obliquity is modulated by the intensity of summer. At times of low and relatively constant obliquity, the flux still varies due to other orbital elements, promoting polar layer formation. Ice migrates from the tropics toward the poles in two stages. First, when surface ice is present in the tropics, and second, when the equatorial deposit is exhausted, from ice that was previously deposited in mid-high latitudes. The polar accumulation rate is significantly higher when tropical ice is available, forming thicker layers per orbital cycle. However, the majority of the NPLD is sourced from ice that temporary resided in the mid-high latitudes and the layers become thinner as the source location moves poleward. The migration stages imprint different D/H ratios in different sections in the PLDs. The NPLD is isotopically depleted compared to the South Polar Layered Deposits in all simulations. Thus we predict the D/H ratio of the atmosphere in contact with NPLD upper layers is biased relative to the average global ice reservoirs.

We apply AnalyticLC, an analytic model described in an accompanying paper, to interpret Kepler data of systems that contain two or three transiting planets. We perform tests to verify that the obtained solutions agree with full N-body integrations, and that the number of model parameters is statistically justified. We probe non-co-planar interactions via impact parameter variations (TbVs), enabled by our analytic model. The subset of systems with a valid solution includes 54 systems composed of 140 planets, more than half of which are without previously reported mass constraints. Overall we provide: (i) estimates of physical and orbital properties for all systems analyzed; (ii) 102 planets with mass detections significant to better than 3σ, 43 of which have masses below 5 m ⊕; and (iii) 35 TbVs significant to better than 3σ. We focus on select systems showing strong TbVs, which can result from either interaction among the known transiting planets, or with a nontransiting object, and provide: (iv) a method to constrain the parameters of such unseen companions. These results are enabled by an accurate 3D photodynamical model, of a kind expected to become increasingly important for modeling multidecade photometric and composite (radial velocity, astrometry) data sets.

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.

As human exploration missions to Mars are on the horizon, microbial cross-contamination remains a key issue to address. These issues can be approached today using advances in molecular metagenomics methods, which include rapid and sensitive sequencing platforms for characterizing microbial populations. Combined with analog missions, these methods provide powerful tools for assessing the challenges associated with planetary exploration. Here, we designed a protocol to monitor forward and backward contamination events and progression in an 11-days Mars analog mission in the Ramon crater in Israel. Forward contamination soil samples were collected daily from three sitestwo sites in close proximity to the habitat and one isolated site. Backward contamination was determined in samples from nitrile gloves of six analog astronauts before and after extravehicular activities Temperature, relative humidity and soil composition data were also collected for all sites. Environmental DNA samples were extracted in the main habitat and 16S (bacterial) and 18S (eukaryotic, fungal) rRNA gene amplicons were sequenced and analyzed to study microbial population diversity and composition. Shannon Diversity index analysis and Principal Coordinates analysis (PCoA) of rRNA genes indicated that differences in the diversity and population composition were significant in sites closer to the habitat when compared to a reference site. These samples also demonstrated the introduction of human-associated taxa to the environment. Backward contamination consisted of bacterial taxa found on gloves upon return from EVA and also detected in soil, altogether 44 genera, indicating backward contamination events. To our knowledge, this is the first protocol to utilize advanced molecular technologies to investigate forward and backward contamination in a Mars analog mission.

Water ice is thought to be trapped in large permanently shadowed regions in the Moons polar regions, due to their extremely low temperatures. Here, we show that many unmapped cold traps exist on small spatial scales, substantially augmenting the areas where ice may accumulate. Using theoretical models and data from the Lunar Reconnaissance Orbiter, we estimate the contribution of shadows on scales from 1 km to 1 cm, the smallest distance over which we find cold-trapping to be effective for water ice. Approximately 1020% of the permanent cold-trap area for water is found to be contained in these micro cold traps, which are the most numerous cold traps on the Moon. Consideration of all spatial scales therefore substantially increases the number of cold traps over previous estimates, for a total area of ~40,000 km², about 60% of which is in the south. A majority of cold traps for water ice is found at latitudes > 80° because permanent shadows equatorward of 80° are typically too warm to support ice accumulation. Our results suggest that water trapped at the lunar poles may be more widely distributed and accessible as a resource for future missions than previously thought.

Non-Keplerian dynamics of planetary orbits manifest in the transit light curve as variations of different types. In addition to transit timing variations, the shape of the transits contains additional information on variations in the geometry of the orbit. This study presents an analytic approach to light-curve fitting: dynamical variations in the orbital elements are transformed to a light curve using an analytic function with a restricted set of fitting parameters. Our method requires no N-body integration, resulting in a smaller number of degrees of freedom and a faster calculation. The approach described here is for the case of secular perturbations. By assuming that the orbital motion is dominated by nodal and apsidal precessions, analytic expressions for the light-curve transit parameters are derived as a function of the orbital variations. Detecting and characterizing such dynamical scenarios provides information regarding the possible existence of nontransiting companions, or the nonspherical mass distribution of the host star. The variations may imply forces out of the orbital plane, and thus probe mutual inclinations among components of the system. The derived models successfully reproduce the vanishing transit signals of KOI 120.01, and suggest a possible interesting scenario of a planet orbiting one member of a close-in binary system undergoing unusually rapid nodal regression. The model parameters are degenerate, so we provide relevant information for follow-up observations, which are suggested in order to place further constraints on this unique Kepler object.

Asteroid pairs are those found to share similar heliocentric orbits but not positions. The leading theory suggests that each had a single progenitor that split due to rotational-fission of a weak, rubble-pile structured body. By constructing shape models of asteroid pairs from multiple-apparition observations and using a lightcurve inversion technique, we mapped the gravitational and rotational accelerations on the surfaces of these asteroids. This allows us to construct a map of local slopes on the asteroids' surfaces. In order to test for frictional failure, we determine the maximum rotation rate at which an area larger than half the surface area of the secondary member (assumed to be the ejected component) has a slope value >40 degrees, the angle of friction of lunar regolith, where loose material will begin sliding. We use this criterion to constrain the failure stress operating on the body, just before disruption at the commonly observed spin barrier of 2.2 h. Our current sample includes shape models of eleven primary members of asteroid pairs, observed from the Wise Observatory in the last decade. In the studied parameter space we find that the shape models only reach the spin barrier when their bulk density is larger than the ~2 g cm
^−
3 measured for the rubble pile structured 25143 Itokawa, and better matches 433 Eros' value of 2.7 g cm
^−
3, suggesting that km-sized asteroid pairs are dense compared to sub-km bodies. Assuming ejection of secondary components that are larger than those observed (up to the maximal size allowing separation), can also increase the spin barrier of the asteroids, thus supporting the previously suggested scenario of continuous disruption of the secondary. In addition, cohesion levels of hundreds of Pascals are also required to prevent these shape models from disrupting at spin rates slower than the usual spin barrier.

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.

At a mean diameter of ∼650 m, the near-Earth asteroid (455213) 2001 OE84 (OE84 for short) has a rapid rotation period of 0.486542 ± 0.000002 h, which is uncommon for asteroids larger than ∼200 m. We revisited OE84 14 years after it was first, and last, observed by Pravec et al. (2002) in order to measure again its spin rate and to search for changes. We have confirmed the rapid rotation and, by fitting the photometric data from 2001 and 2016 using the lightcurve inversion technique, we determined a retrograde sense of rotation, with the spin axis close to the ecliptic south pole; an oblate shape model of a/b=1.32±0.04 and b/c=1.8±0.2; and no change in spin rate between 2001 and 2016. Using these parameters we constrained the body's internal strength, and found that current estimations of asteroid cohesion (up to ∼80 Pa) are insufficient to maintain an intact rubble pile at the measured spin rate of OE84. Therefore, we argue that a monolithic asteroid, that can rotate at the rate of OE84 without shedding mass and without slowing down its spin rate, is the most plausible for OE84, and we give constraints on its age, since the time it was liberated from its parent body, between 2−10 million years.

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.

ULTRASAT is a scientific mini-satellite carrying a telescope with an unprecedentedly large field of view (210 squared degrees) observing in the ultraviolet (220-280 nm.UVJ, that is proposed by an Israeli/US collaboration to be constructed and launched to near geostationary orbit by 2020/21. The wide field of view and the advanced UV detectors will enable the discovery and monitoring of transient sources within a cosmic volume 300 times larger than that of the most powerful UV satellite to date. GAL-EX. thus revolutionizing our understanding of the transient UV universe.

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.

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

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.

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 images of Titan's south polar region acquired during southern summer contain lake features which disappear between observations. These features show a tenfold increases in backscatter cross-section between images acquired one year apart, which is inconsistent with common scattering models without invoking temporal variability. The morphologic boundaries are transient, further supporting changes in lake level. These observations are consistent with the exposure of diffusely scattering lakebeds that were previously hidden by an attenuating liquid medium. We use a two-layer model to explain backscatter variations and estimate a drop in liquid depth of approximately 1-m-per-year. On larger scales, we observe shoreline recession between ISS and RADAR images of Ontario Lacus, the largest lake in Titan's south polar region. The recession, occurring between June 2005 and July 2009, is inversely proportional to slopes estimated from altimetric profiles and the exponential decay of near-shore backscatter, consistent with a uniform reduction of 4 ± 1.3. m in lake depth.Of the potential explanations for observed surface changes, we favor evaporation and infiltration. The disappearance of dark features and the recession of Ontario's shoreline represents volatile transport in an active methane-based hydrologic cycle. Observed loss rates are compared and shown to be consistent with available global circulation models. To date, no unambiguous changes in lake level have been observed between repeat images in the north polar region, although further investigation is warranted. These observations constrain volatile flux rates in Titan's hydrologic system and demonstrate that the surface plays an active role in its evolution. Constraining these seasonal changes represents the first step toward our understanding of longer climate cycles that may determine liquid distribution on Titan over orbital time periods.

The acquisition of new global elevation data from the Lunar Orbiter Laser Altimeter, carried on the Lunar Reconnaissance Orbiter, permits quantification of the surface roughness properties of the Moon at unprecedented scales and resolution. We map lunar surface roughness using a range of parameters: median absolute slope, both directional (along-track) and bidirectional (in two dimensions); median differential slope; and Hurst exponent, over baselines ranging from ∼17 m to ∼2.7 km. We find that the lunar highlands and the mare plains show vastly different roughness properties, with subtler variations within mare and highlands. Most of the surface exhibits fractal-like behavior, with a single or two different Hurst exponents over the given baseline range; when a transition exists, it typically occurs near the 1 km baseline, indicating a significant characteristic spatial scale for competing surface processes. The Hurst exponent is high within the lunar highlands, with a median value of 0.95, and lower in the maria (with a median value of 0.76). The median differential slope is a powerful tool for discriminating between roughness units and is useful in characterizing, among other things, the ejecta surrounding large basins, particularly Orientale, as well as the ray systems surrounding young, Copernican-age craters. In addition, it allows a quantitative exploration on mare surfaces of the evolution of surface roughness with age.

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.

An investigation of the internal structure of the ice-rich Planum Boreum (PB) deposit at the north pole of Mars is presented, using 178 orbits of Mars advanced radar for subsurface and ionospheric sounding data. For each radargram, bright, laterally extensive surface and subsurface reflectors are identified and the time delay between them is converted to unit thicknesses, using a real dielectric constant of 3. Results include maps of unit thickness, for PB and its two constituent units, the stratigraphically older basal unit (BU) and the stratigraphically younger north polar layered deposits (NPLD). Maps of the individual units' surface elevation are also provided. Estimates of water ice volume in each unit are (1.3±0.2) × 10⁶ km³ in PB, (7.8±1.2) × 10⁵ km³ in the NPLD, and (4.5±1.0) × 10⁵ km³ in the BU. No lithospheric deflection is apparent under PB, in agreement with previous findings for only the Gemina Lingula lobe, which suggests that a thick elastic lithosphere has existed at the north pole of Mars since before the emplacement of the BU. The extent of BU material in the Olympia Planum lobe of PB is directly detected, providing a more accurate map of BU extent than previously available from imagery and topography. A problematic area for mapping the BU extent and thickness is in the distal portion of the 290°E-300°E region, where MARSIS data show no subsurface reflectors, even though the BU is inferred to be present from other lines of evidence.

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 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.

Constraints on the mineralogical composition of low-albedo, low-sulfur sands at Meridiani Planum are determined from Mars Exploration Rover (MER) Opportunity Miniature Thermal Emission Spectrometer (Mini-TES), Mössbauer, and Alpha Proton X-Ray Spectrometer measurements. Results of this work show that the sand is olivine basaltic in composition, with minor amounts of sulfate and a high-silica phase (glass or secondary amorphous silica). Measurements from all three instruments indicate that pyroxene is twice as abundant as olivine, and that the pyroxene composition is dominated by the low-calcium variety. The volume abundance of olivine is constrained to be 10-15%. Results from detailed analyses of MER data are used to ground truth the spectral emissivity and mineral abundances derived from orbit with Mars Global Surveyor TES data. TES-derived mineral abundances are within 5% of those derived from MER data, which is generally within the statistical errors associated with TES-derived phase abundances. The agreement lends support to global- and regional-scale variations in mineralogical composition determined from TES data in previous studies. An alternative method of least squares minimization is used for modeling the TES and Mini-TES data; the benefits of this method are demonstrated by comparison with conventional least squares techniques previously used by TES data users.

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.

The presence of the global magnetic field of Mercury has implications for the interior structure of the planet and its thermal evolution. We use a thermal evolution model to explore the conditions under which excess entropy is available to drive a convective dynamo. The current state of the core is strongly affected by its sulfur concentration and the viscosity of the overlying mantle. A present-day dynamo is difficult to achieve. The minimum rate of entropy production required to drive a dynamo is attained in only the most optimistic models, and requires present-day mantle convection. An additional entropy source such as the addition of a radiogenic heat source in the core increases the probability of a present-day dynamo. Given the uncertainty, more specific characterization of the planet's interior and magnetic field is required allviate ambiguities in the original Mariner 10 observations.

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]

The diffusion coefficient of water vapor in unconsolidated porous media is measured for various soil simulants at Mars-like pressures and subzero temperatures. An experimental chamber which simultaneously reproduces a low-pressure, low-temperature, and low-humidity environment is used to monitor water flux from an ice source through a porous diffusion barrier. Experiments are performed on four types of simulants: 40-70 μm glass beads, sintered glass filter disks, 1-3 μm dust (both loose and packed), and JSC Mars-1. A theoretical framework is presented that applies to environments that are not necessarily isothermal or isobaric. For most of our samples, we find diffusion coefficients in the range of 2.8 to 5.4 cm² s^-1 at 600 Pascal and 260 K. This range becomes 1.9-4.7 cm² s^-1 when extrapolated to a Mars-like temperature of 200 K. Our preferred value for JSC Mars-1 at 600 Pa and 200 K is 3.7 ± 0.5 cm² s^-1. The tortuosities of the glass beads is about 1.8. Packed dust displays a lower mean diffusion coefficient of 0.38 ± 0.26 cm² s^-1, which can be attributed to transition to the Knudsen regime where molecular collisions with the pore walls dominate. Values for the diffusion coefficient and the variation of the diffusion coefficient with pressure are well matched by existing models. The survival of shallow subsurface ice on Mars and the providence of diffusion barriers are considered in light of these measurements.

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.

From gravity and topography data collected by the Mars Global Surveyor spacecraft we calculate gravity/topography admittances and correlations in the spectral domain and compare them to those predicted from models of lithospheric flexure. On the basis of these comparisons we estimate the thickness of the Martian elastic lithosphere (T_e) required to support the observed topographic load since the time of loading. We convert T_e to estimates of heat flux and thermal gradient in the lithosphere through a consideration of the response of an elastic/plastic shell. In regions of high topography on Mars (e.g., the Tharsis rise and associated shield volcanoes), the mass-sheet (small-amplitude) approximation for the calculation of gravity from topography is inadequate. A correction that accounts for finite-amplitude topography tends to increase the amplitude of the predicted gravity signal at spacecraft altitudes. Proper implementation of this correction requires the use of radii from the center of mass (collectively known as the planetary "shape") in lieu of "topography" referenced to a gravitational equipotential. Anomalously dense surface layers or buried excess masses are not required to explain the observed admittances for the Tharsis Montes or Olympus Mons volcanoes when this correction is applied. Derived T_e values generally decrease with increasing age of the lithospheric load, in a manner consistent with a rapid decline of mantle heat flux during the Noachian and more modest rates of decline during subsequent epochs.

Data obtained recently by the Mars Orbiter Laser Altimeter (MOLA) were used to study the statistical properties of the topography and slopes on Mars. We find that the hemispheric dichotomy, manifested as an elevation difference, can be described by long baseline tilts but in places is expressed as steeper slopes. The bimodal hypsometry of elevations on Mars becomes unimodal when referenced to the center of figure, contrary to the Earth, for which the bimodality is retained. However, ruling out a model in which the elevation difference is expressed in a narrow equatorial topographic step cannot be done by the hypsometry alone. Mars' slope distribution is longer tailed than those of Earth and Venus, indicating a lower efficiency of planation processes relative to relief-building tectonics and volcanics. We define and compute global maps of statistical estimators, including the interquartile scale, RMS and median slope, and characteristic decorrelation length of the surface. A correspondence between these parameters and geologic units on Mars is inferred. Surface smoothness is distinctive in the vast northern hemisphere plains, where slopes are typically

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