M. A. Kreslavsky

Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars

Images from the Mars Express HRSC (High-Resolution Stereo Camera) of debris aprons at the base of massifs in eastern Hellas reveal numerous concentrically ridged lobate and pitted features and related evidence of extremely ice-rich glacier-like viscous flow and sublimation. Together with new evidence for recent ice-rich rock glaciers at the base of the Olympus Mons scarp superposed on larger Late Amazonian debris-covered piedmont glaciers, we interpret these deposits as evidence for geologically recent and recurring glacial activity in tropical and mid-latitude regions of Mars during periods of increased spin-axis obliquity when polar ice was mobilized and redeposited in microenvironments at lower latitudes. The data indicate that abundant residual ice probably remains in these deposits and that these records of geologically recent climate changes are accessible to future automated and human surface exploration.

Kilometer‐scale roughness of Mars: Results from MOLA data analysis

The Mars Orbiter Laser Altimeter (MOLA) data are used to characterize the kilometer-scale surface roughness of Mars. The median absolute value of the differential slope at a given baseline is proposed as a data-derived measure of the surface roughness at this scale. Study of the scale dependence of roughness for the smoothest terrains gives an independent estimate of 20 cm for MOLA ranging accuracy. The baseline lengths from 0.6 to ∼20 km are used, and kilometer-scale roughness is mapped for the entire surface. The maps show that different geological units have distinctive roughness characteristics. Scale dependence of roughness for a number of geological units is presented and discussed. The southern polar cap is rougher than the northern at kilometer and subkilometer scale, which suggests differences in the sublimation/condensation balance. The Vastitas Borealis Formation has a distinctive 3-km-scale background surface topography, which suggests a nonvolcanic origin for its upper layer. Young volcanic plains in Amazonis Planitia and the eastern part of Elysium Planitia are very similar to each other in their roughness characteristics and differ from other volcanic plains on Mars, which suggests a distinctive eruption style. There are systematic latitudinal variations of roughness in both the southern highlands and the northern lowlands: terrains at high latitude are smoother at short baselines; the characteristic vertical scale related to this difference is several meters. Processes that could be responsible for formation of this trend include creep of ice-rich near-surface material at high latitudes, treatment of the surface with repetitive deposition and sublimation of seasonal frost, climate-controlled deposition and/or cementation of dust at high latitudes, and repetitive sublimation and accumulation of subsurface ice at low latitudes with climate variations. All mechanisms of origin could operate more effectively under different climate conditions. Relevant morphological observations favor mechanisms involving deposition of smooth blankets at high latitudes.

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

[1] 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.

Kilometer‐scale slopes on Mars and their correlation with geologic units: Initial results from Mars Orbiter Laser Altimeter (MOLA) data

Martian surface slopes were calculated at baselines from 0.4 to 25 km using profiles obtained by the Mars Orbiter Laser Altimeter (MOLA) instrument during the aerobraking phase of the Mars Global Surveyor mission. Median slope is proposed as a characteristic measurement of the typical surface roughness at each corresponding scale. Median slope is favored over RMS slope because it is not influenced by the small number of higher slopes at the upper end of the slope-frequency distribution tail. Median slope complements interquartile scale roughness characterization in that it is more sensitive to smaller baseline slopes. A map of the median slope of the northern hemisphere is presented. Median slopes and their scale dependences are used to characterize typical kilometer-scale roughness for a set of geologic units mapped in the northern hemisphere. This analysis demonstrates that many individual units and groups of units are characterized by distinctive surface slopes and that these characteristics are sufficiently different that they hold promise for use in the definition and characterization of units. Characterization of the slope properties of geologic units provides information useful in the interpretation of their origin and evolution. For example, the generally smooth topography of the diverse Vastitas Borealis Formation subunits is dominated by about 3 km, 0.3° steep features almost indistinguishable in Viking images. The roughness characteristics of this unit differ from those of other geologic units on Mars and suggest some distinctive process(es) of formation and/or modification of kilometer-scale topography common for all subunits. The similarity of roughness characteristics of the several highland plateau units suggests that kilometer-scale topography was largely inherited from the period of heavy bombardment. The northern polar cap and layered terrains are largely very smooth at small scale. The long, steep-sloped tails of the slope-frequency distributions are compared for the dominant terrain types in the northern hemisphere of Mars and are compared with Earth continents. The Vastitas Borealis Formation found in the northern lowlands differs significantly from both cratered uplands and volcanic plains, and these latter two units can be distinguished from each other on the basis of median slope. Terrestrial continents are smoother than cratered highlands but rougher than the Vastitas Borealis Formation and most volcanic plains. An inventory of all observed slopes much steeper than the angle of repose is presented. Steep slopes occur in the upper parts of tectonic scarps, most likely representing bedrock exposures. The presence of extremely steep slopes in the presumably ductile polar cap provides evidence for geologically recent and/or ongoing formation of these slopes.

A Critical review of theoretical models of negatively polarized light scattered by atmosphereless solar system bodies

About a dozen physical mechanisms and models aspire to explain the negative polarization of light scattered by atmosphereless celestial bodies. This is too large a number for the reliable interpretation of observational data. Through a comparative analysis of the models, our main goal is to answer the question: Does any one model have an advantage over the others? Our analysis is based on new laboratory polarimetric and photometric data as well as on theoretical results. We show that the widely used models due to Hopfield and Wolff cannot realistically explain the phase-angle dependence of the degree of polarization observed at small phase angles. The so-called interference or coherent backscattering mechanism is the most promising model. Models based on that mechanism use well-defined physical parameters to explain both negative polarization and the opposition effect. They are supported by laboratory experiments, particularly those showing enhancement of negative polarization with decreasing particle size down to the wavelength of light. According to the interference mechanism, pronounced negative branches of polarization, like those of C-class asteroids, may indicate a high degree of optical inhomogeneity of light-scattering surfaces at small scales. The mechanism also seems appropriate for treating the negative polarization and opposition effects of cometary dust comae, planetary rings, and the zodiacal light.

Formation of gullies on Mars: Link to recent climate history and insolation microenvironments implicate surface water flow origin

Features seen in portions of a typical midlatitude Martian impact crater show that gully formation follows a geologically recent period of midlatitude glaciation. Geological evidence indicates that, in the relatively recent past, sufficient snow and ice accumulated on the pole-facing crater wall to cause glacial flow and filling of the crater floor with debris-covered glaciers. As glaciation waned, debris-covered glaciers ceased flowing, accumulation zones lost ice, and newly exposed wall alcoves continued as the location for limited snow/frost deposition, entrapment, and preservation. Analysis of the insolation geometry of this pole-facing crater wall, and similar occurrences in other craters at these latitudes on Mars, shows that they are uniquely favored for accumulation of snow and ice, and a relatively more rapid exposure to warmer summer temperatures. We show that, after the last glaciation, melting of residual snow and ice in alcoves could have formed the fluvial channels and sedimentary fans of the gullies. Recent modeling shows that top-down melting can occur in these microenvironments under conditions similar to those currently observed on Mars, if small amounts of snow or frost accumulate in alcoves and channels. Accumulation and melting is even more favored in the somewhat wetter, relatively recent geological past of Mars, after the period of active glaciation.

Polarimetric and photometric properties of the Moon: Telescopic observations and laboratory simulations

Abstract This article is a sequel to Shkuratov et al. (1992, Icarus 95, 283–299). It considers the lunar positive polarization based on telescopic observations and laboratory measurements. Data obtained from point-by-point measurements as well as from images are used to study the Moon at phase angles near the polarization maximum. The first type of data shows that measurements corresponding to young craters deviate from the regression line of the log A-log P correlation (A and P are the reflectance and polarization degree, respectively). Following the approach suggested by Shkuratov et al. (1980, Astron. Circ. 1112, 3), new images of the polarimetric anomaly parameter characterized by b = log A + a · log Pmax are obtained for the western part of the lunar disc. Young mare and highland craters show up. Some anomalies presumably can be identified as volcanic ash deposits. A new empirical relationship between b and particle size was established by laboratory measurements of the lunar samples supplied by the “Luna” space probes. This relationship as well as that presented by Geake and Dollfus (1986, Mon. Not. R. Astron. Soc. 218, 75) is not universal and should be used carefully. New parameters (width and asymmetry) of the positive polarization branch, δ = (α 1 − α 2 ) α max and Δ = (α max − α 1 ) (α 2 − α max ) , where α1 and α2 are the phase angles at which P = 0.7 · Pmax are introduced. From laboratory polarimetric measurements of artificial glasses and lunar samples, some correlations between these parameters and the particle size of the surface were established.

The structure of the regolith on 67P/Churyumov-Gerasimenko from ROLIS descent imaging

The structure of the upper layer of a comet is a product of its surface activity. The Rosetta Lander Imaging System (ROLIS) on board Philae acquired close-range images of the Agilkia site during its descent onto comet 67P/Churyumov-Gerasimenko. These images reveal a photometrically uniform surface covered by regolith composed of debris and blocks ranging in size from centimeters to 5 meters. At the highest resolution of 1 centimeter per pixel, the surface appears granular, with no apparent deposits of unresolved sand-sized particles. The thickness of the regolith varies across the imaged field from 0 to 1 to 2 meters. The presence of aeolian-like features resembling wind tails hints at regolith mobilization and erosion processes. Modeling suggests that abrasion driven by airfall-induced particle “splashing” is responsible for the observed formations.

Photometric properties of the lunar surface derived from Clementine observations

Photometric properties of the lunar surface in visual and near-infrared light were studied using raw images obtained with UVVIS camera during the Clementine mission. The investigation focused on several specific regions on the lunar surface, each of which was observed by Clementine at a variety of different illumination and viewing geometries. Through these observations, the dependence of the surface brightness on the observation/illumination geometry was studied. It was shown that the disk component of this dependence, that is, the variations of brightness at constant phase angle, is different for different mare areas. The color of the lunar surface also changes with changing of the observation/illumination geometry, even if under constant phase angle. The Reiner Gamma formation displays unusual photometric properties. They are consistent with the surface being smoother than the typical mare regolith surface. The UVVIS images taken at the smallest phase angles were used to study the opposition spike, that is, the sharp increase of the surface brightness near the opposition. Steepness of the phase dependence of brightness varies over a wide range for different sites.

Near-tropical subsurface ice on Mars

Near-surface perennial water ice on Mars has been previously inferred down to latitudes of about 45° and could result from either water vapor diffusion through the regolith under current conditions or previous ice ages precipitations. In this paper we show that at latitudes as low as 25° in the southern hemisphere buried water ice in the shallow (<1 m) subsurface is required to explain the observed surface distribution of seasonal CO2 frost on pole facing slopes. This result shows that possible remnants of the last ice age, as well as water that will be needed for the future exploration of Mars, are accessible significantly closer to the equator than previously thought, where mild conditions for both robotic and human exploration lie. Copyright 2010 by the American Geophysical Union.