Mineralogical composition of sands in Meridiani Planum determined from Mars Exploration Rover data and comparison to orbital measurements

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), Mossbauer, 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

Powering Mercury's dynamo

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 to alleviate ambiguities in the original Mariner 10 observations

Thermal stability of ice on Ceres with rough topography

The dwarf planet Ceres may have an ice-rich 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 (CH_3OH, NH_3, SO_2, and CO_2) may be cold trapped in smaller abundances. A model for the transport, gravitational escape, and photoionization of H_2O 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 (∼10–100 years), unless it has high albedo and thermal inertia, in which case lifetimes of > 10^4 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

Global surface slopes and roughness of the Moon from the Lunar Orbiter Laser Altimeter

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

Mars Orbiter Laser Altimeter pulse width measurements and footprint-scale roughness

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 footprint-scale 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 slope-corrected roughness over ∼75-m-diameter footprints that may be used for quantitative geomorphic surface characterization, required, for example, for landing site selection. The recalibration uses a total least-squares 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

A Reappraisal of Near-Tropical Ice Stability on Mars

Two arguments have suggested the presence of subsurface water ice at latitudes lower than 30\textdegree~on Mars. First, the absence of CO2 frost on pole-facing slopes was explained by the presence of subsurface ice. Second, models suggested that subsurface ice could be stable underneath these slopes. We revisit these arguments with a new slope microclimate model. Our model shows that below 30{\deg} latitude, slopes are warmer than previously estimated as the air above is heated by warm surrounding plains. This additional heat prevents the formation of CO2 and subsurface water ice for most slopes. Higher than 30{\deg}S, our model suggests the presence of subsurface water ice. In sparse cases (steep dusty slopes), subsurface ice may exist down to 25{\deg}S. While hypothetical unstable ice deposits cannot be excluded by our model, our results suggest that water ice is rarer than previously thought in the +- 30{\deg} latitude range considered for human exploration

Main-belt comets in the Palomar Transient Factory survey – I. The search for extendedness

Cometary activity in main-belt asteroids probes the ice content of these objects and provides clues to the history of volatiles in the inner Solar system. We search the Palomar Transient Factory survey to derive upper limits on the population size of active main-belt comets (MBCs). From data collected from 2009 March through 2012 July, we extracted ∼2 million observations of ∼220 thousand known main-belt objects (40 per cent of the known population, down to ∼1-km diameter) and discovered 626 new objects in multinight linked detections. We formally quantify the ‘extendedness’ of a small-body observation, account for systematic variation in this metric (e.g. due to on-sky motion) and evaluate this method's robustness in identifying cometary activity using observations of 115 comets, including two known candidate MBCs and six newly discovered non-MBCs (two of which were originally designated as asteroids by other surveys). We demonstrate a 66 per cent detection efficiency with respect to the extendedness distribution of the 115 sampled comets, and a 100 per cent detection efficiency with respect to extendedness levels greater than or equal to those we observed in the known candidate MBCs P/2010 R2 (La Sagra) and P/2006 VW_(139). Using a log-constant prior, we infer 95 per cent confidence upper limits of 33 and 22 active MBCs (per million main-belt asteroids down to ∼1-km diameter), for detection efficiencies of 66 and 100 per cent, respectively. In a follow-up to this morphological search, we will perform a photometric (disc-integrated brightening) search for MBCs