Elements of comparison between Martian and terrestrial mesoscale meteorological phenomena: Katabatic winds and boundary layer convection

Terrestrial and Martian atmospheres are both characterised by a large variety of mesoscale meteorological events, occurring at horizontal scales of hundreds of kilometres and below. Available measurements from space exploration and recently developed high-resolution numerical tools have given insights into Martian mesoscale phenomena, as well as similarities and differences with their terrestrial counterparts. The remarkable intensity of Martian mesoscale events compared to terrestrial phenomena mainly results from low density and strong radiative control. This is exemplified in the present paper by discussing two mesoscale phenomena encountered in the lowest atmospheric levels of both planets with notable differences: nighttime katabatic winds (drainage flow down sloping terrains) and daytime boundary layer convection (vertical growth of mixed layer over heated surfaces). While observations of katabatic events are difficult on Earth, except over vast ice sheets, intense clear-cut downslope circulations are expected to be widespread on Mars. Convective motions in the daytime Martian boundary layer are primarily driven by radiative contributions, usually negligible on Earth where sensible heat flux dominates, and exhibit turbulent variances one order of magnitude larger. Martian maximum heat fluxes are not attained close to the surface as on Earth but a few hundreds of metres above, which implies generalised definitions for mixing layer scales such as vertical velocity w_?. Measurements on Mars of winds in uneven topographical areas and of heat fluxes over flat terrains could be useful to assess general principles of mesoscale meteorology applicable to both terrestrial and Martian environments.     

Water distribution in Martian permafrost regions from joint analysis of HEND (Mars Odyssey) and MOLA (Mars Global Surveyor) data

We jointly analyze data from the High-Energy Neutron Detector (HEND) onboard the NASA Mars Odyssey spacecraft and data from the Mars Orbiter Laser Altimeter (MOLA) onboard the Mars Global Surveyor spacecraft. The former instrument measures the content of hydrogen (in the form of H₂O or OH) in the subsurface layer of soil and the latter instrument measures the surface albedo with respect to the flux of solar energy. We have checked the presence of a correlation between these two data sets in various Martian latitude bands. A significant correlation has been found between these data at latitudes poleward of 40° in the northern hemisphere and at latitudes 40°–60° in the southern hemisphere. This correlation is interpreted as evidence for the presence of stable water ice in these regions under a dry layer of soil whose thickness is determined by the condition for equilibrium between the condensation of water from the atmosphere and its sublimation when heated by solar radiation. For these regions, we have derived an empirical relation between the flux of absorbed solar radiation and the thickness of the top dry layer. It allows the burial depth of the water ice table to be predicted with a sub-kilometer resolution based on near-infrared albedo measurements. We have found no correlation in the southern hemisphere at latitudes >60°, although neutron data also suggest that water ice is present in this region under a layer of dry soil. We conclude that the thickness of the dry layer in this region does not correspond to the equilibrium condition between the water ice table and the atmosphere.     

Salten Skov I: A Martian magnetic dust analogue

A fine grained magnetic iron oxide precipitate found in Denmark has been studied with regard to grain size, magnetic properties, aerosol transport, grain electrification, aggregation and optical reflectance. It has shown itself to be a good Martian dust analogue. The fraction of the Salten Skov I soil sample <63 μm was separated from the natural sample by dry sieving. This fraction could be dispersed by ultrasonic treatment into grains of diameter ～1 μm, in reasonable agreement with suspended dust grains in the Martian atmosphere estimated from the Viking, Pathfinder and Mars Exploration Rover missions. Though mineralogical and chemical differences exist between this analogue and Martian dust material, in wind tunnel experiments many of the physical properties of the atmospheric dust aerosol are reproduced.     

Spectral detectability of Ca- and Mg-sulfates in Martian bright soils in the 4–5 μm wavelength range

Chemical analyses of soil samples performed at different landing sites on Mars suggest the presence of sulfate minerals. These minerals are also thought to be present in the globally mixed Martian bright soils covering large areas of the planet. However, remote soil spectra have so far provided only tentative identification of sulfates regarding mineral types and abundances. This paper concentrates on the detectability of four Ca- and Mg-sulfates (anhydrite, gypsum, kieserite, hexahydrite) in the 4–5 μm range of Martian remote soil spectra. This spectral range is important for sulfate detection as most fine-grained sulfates exhibit significant absorption bands between 4 and 5 μm, independent of the texture of the host soils (e.g., loose powdered or cemented soils). Furthermore, this is the spectral range for which the Planetary Fourier Spectrometer (PFS) and Observatoire pour la Minéralogie, l’Eau, les Glaces, et l’Activité (OMEGA) instruments onboard ESA/Mars Express mission provide high spectral and spatial resolution data. Laboratory near- and mid-IR reflectance spectra of the pure sulfates and their mixtures with a terrestrial Martian soil analog were acquired. The results show that even the smallest amount of admixed sulfate (∼5 wt%) generates significant absorption features in the portion of the 4–5 μm range not covered by the saturated Martian atmospheric CO₂ absorption band between 4.2 and 4.4 μm. Model calculations of the influence of emitted surface radiation on the detectability of sulfate features show that the depth of the features decreases strongly with increasing surface temperature of an observed area resulting in the fact that all sulfates are spectrally hidden at surface temperatures around 270 K even at ∼14 or ∼25 wt% sulfate content in the soils. Sulfates become increasingly detectable depending on the sulfate content if the surface temperature is below 260 K. The outcome of this work helps to constrain the conditions needed for remote detection of sulfates within Martian bright soils in the 4–5 μm range.     

Soil Water Content on Mars as Estimated from Neutron Measurements by the HEND Instrument Onboard the 2001 Mars Odyssey Spacecraft

We present the results of 20 months of observations of Mars by the Russian HEND instrument onboard the NASA 2001 Mars Odyssey spacecraft. We show that there are two extended subpolar regions with a soil water content of several tens of percent in the northern and southern hemispheres of Mars. The southern subpolar region is well described by a two-layer model, according to which a soil with a water content of up to 55% by mass lies under a relatively dry soil with a water mass fraction of 2% and a thickness of 15–20 g/cm². The distribution of water in Martian regolith northern subpolar region is in good agreement with the homogeneous model and does not require invoking the more complex two-layer soil model. The water-ice content in the subsurface layer of the northern subpolar region reaches 53 % by mass. We show that there are two regions with a relatively high water content near the Martian equator. These are Arabia Terra and the Medusae Fossae formation region southwest of Olympus Mons. In these regions, a lower layer with 9–10% of water by mass may underlie the upper layer of relatively dry material 30 g/cm² in thickness. The moistest spot near the equator is at about 30° E and 10° N. Its lower-layer soil may contain more than 16% of water by mass.     

Physical properties of Martian meteorites: Porosity and density measurements

Abstract— Martian meteorites are fragments of the Martian crust. These samples represent igneous rocks, much like basalt. As such, many laboratory techniques designed for the study of Earth materials have been applied to these meteorites. Despite numerous studies of Martian meteorites, little data exists on their basic structural characteristics, such as porosity or density, information that is important in interpreting their origin, shock modification, and cosmic ray exposure history. Analysis of these meteorites provides both insight into the various lithologies present as well as the impact history of the planet's surface. We present new data relating to the physical characteristics of twelve Martian meteorites. Porosity was determined via a combination of scanning electron microscope (SEM) imagery/image analysis and helium pycnometry, coupled with a modified Archimedean method for bulk density measurements. Our results show a range in porosity and density values and that porosity tends to increase toward the edge of the sample. Preliminary interpretation of the data demonstrates good agreement between porosity measured at 100× and 300× magnification for the shergottite group, while others exhibit more variability. In comparison with the limited existing data for Martian meteorites we find fairly good agreement, although our porosity values typically lie at the low end of published values. Surprisingly, despite the increased data set, there is little by way of correlation between either porosity or density with parameters such as shock effect or terrestrial residency. Further data collection on additional meteorite samples is required before more definitive statements can be made concerning the validity of these observations.     

Analysis and survival of amino acids in Martian regolith analogs

Abstract— We have investigated the native amino acid composition of two analogs of Martian soil, JSC Mars‐1 and Salten Skov. A Mars simulation chamber has been built and used to expose samples of these analogs to temperature and lighting conditions similar to those found at low latitudes on the Martian surface. The effects of the simulated conditions have been examined using high‐performance liquid chromatography (HPLC). Exposure to energetic ultraviolet (UV) light in vacuum appears to cause a modest increase in the concentration of certain amino acids within the materials, which is interpreted as resulting from the degradation of microorganisms. The influence of low temperatures shows that the accretion of condensed water on the soils leads to the destruction of amino acids, supporting the idea that reactive chemical processes involving H₂O are at work within the Martian soil. We discuss the influence of UV radiation, low temperatures, and gaseous CO₂ on the intrinsic amino acid composition of Martian soil analogs and describe, with the help of a simple model, how these studies fit within the framework of life detection on Mars and the practical tasks of choosing and using Martian regolith analogs in planetary research.     

Analytical investigations of quasi-circular frozen orbits in the Martian gravity field

Frozen orbits are always important foci of orbit design because of their valuable characteristics that their eccentricity and argument of pericentre remain constant on average. This study investigates quasi-circular frozen orbits and examines their basic nature analytically using two different methods. First, an analytical method based on Lagrangian formulations is applied to obtain constraint conditions for Martian frozen orbits. Second, Lie transforms are employed to locate these orbits accurately, and draw the contours of the Hamiltonian to show evolutions of the equilibria. Both methods are verified by numerical integrations in an 80 × 80 Mars gravity field. The simulations demonstrate that these two analytical methods can provide accurate enough results. By comparison, the two methods are found well consistent with each other, and both discover four families of Martian frozen orbits: three families with small eccentricities and one family near the critical inclination. The results also show some valuable conclusions: for the majority of Martian frozen orbits, argument of pericentre is kept at 270° because J ₃ has the same sign as J ₂; while for a minority of ones with low altitude and low inclination, argument of pericentre can be kept at 90° because of the effect of the higher degree odd zonals; for the critical inclination cases, argument of pericentre can also be kept at 90°. It is worthwhile to note that there exist some special frozen orbits with extremely small eccentricity, which could provide much convenience for reconnaissance. Finally, the stability of Martian frozen orbits is estimated based on the trace of the monodromy matrix. The analytical investigations can provide good initial conditions for numerical correction methods in the more complex models.     

Long-term characterization of the Martian atmosphere and soil from cosmic ray effects in returned samples

Analyses of cosmic ray effects in returned Martian soil and rock samples may fundamentally increase our understanding of: (a) past climatic variations, (b) the characteristic erosion rate of rocks, and (c) the average redistribution rate for soils. The reason is that the Martian atmosphere is a selective mass filter for galactic cosmic rays with the specific nuclear effects produced in surface materials being quite dependent on the atmospheric shielding. Cosmic ray VH nuclei produce particle tracks in crystalline materials, while the protons produce spallation products that are easily detected as isotopic anomalies in noble gases in both crystalline materials and weathering products. The ratio of tracks/spallation products changes by five orders of magnitude for a three order of the atmospheric shielding averaged over the exposure age of crystalline fragments within a soil sample. In addition, the absolute abundance of spallation gases in windblown soils can be used to infer the globally averaged ratio of exposure age to soil deposit thickness. A layered soil sample may provide data on temporal variations in atmospheric shielding. Spallation products in rock samples that have been in a chemically or mechanically erosive environment can be used to infer the average erosion rate if enough time for secular equilibrium has elapsed. Finally, cores from rocks (～10 cm deep) in an erosive environment can be used to infer the mean atmospheric shielding and the erosion rate over a time scale roughly two orders of magnitude less than the time scale for secular equilibrium of spallation gases, based on the decrease in nuclear particle track abundances with depth.     

Seasonal redistribution of water in the surficial Martian regolith: Results from the <Emphasis Type="Italic">Mars Odyssey</Emphasis> high-energy neutron detector (HEND)

The seasonal variation of neutron emissions from Mars in different spectral intervals measured by the HEND neutron detector for the entire Martian year are analyzed. Based on these data, the spatial variations of the neutron emissions from the planet are globally mapped as a function of season, and the dynamics of seasonal variation of neutron fluxes with different energies is analyzed in detail. No differences were found between seasonal regimes of neutron fluxes in different energy ranges in the southern hemisphere of Mars, while the regime of fast neutrons (with higher energies) during the northern winter strongly differs from that during the southern winter. In winter (L _s = 270°–330°), the fast neutron fluxes are noticeably reduced in the northern hemisphere (along with the consecutive thickening of the seasonal cap of solid carbon dioxide). This provides evidence of a temporary increase in the water content in the effective layer of neutron generation. According to the obtained estimates, the observed reduction of the flux of fast neutrons in the effective layer corresponds to an increase in the water abundance of up to 5% in the seasonal polar cap (70°–90°N), about 3% at mid-latitudes, and from 1.5 to 2% at low latitudes. The freezing out of atmospheric water at the planetary surface (at middle and high latitudes) and the hydration of salt minerals composing the Martian soil are considered as the main processes responsible for the temporary increase in the water content in the soil and upper layer of the seasonal polar cap. The meridional atmospheric transport of water vapor from the summer southern to the winter northern hemisphere within the Hadley circulation cell is a basic process that delivers water to the subsurface soil layer and ensures the observed scale of the seasonal increase in water abundance. In the summer northern hemisphere, the similar Hadley circulation cell transports mainly dry air masses to the winter southern hemisphere. The point is that the water vapor becomes saturated at lower heights during aphelion, and the bulk of the atmospheric water mass is captured in the near-equatorial cloudy belt and, thus, is only weakly transferred to the southern hemisphere. This phenomenon, known as the Clancy effect, was suggested by Clancy et al. (1996) as a basic mechanism for the explanation of the interhemispheric asymmetry of water storage in permanent polar caps. The asymmetry of seasonal meridional circulation of the Martian atmosphere seems to be another factor determining the asymmetry of the seasonal water redistribution in the “atmosphere-regolith-seasonal polar caps” system, found in the peculiarities of the seasonal regime of the neutron emission of Mars.     

The evolution of the martian elastic lithosphere and implications for crustal and mantle rheology

Estimates of the martian elastic lithosphere thickness Te imply that Te increased from around 20 km in the Noachian to about 70 km in the Amazonian period. A phase of rapid lithospheric growth is observed during the Hesperian and we propose that this elastic thickness history is a consequence of the martian crustal rheology and its thermal evolution. A wet crustal rheology is found to generate a mechanically incompetent layer in the lower crust during the early evolution and the rapid growth of Te during the Hesperian results from the disappearance of this layer due to planetary cooling. The incompetent layer and the related rapid lithospheric growth are absent for a dry basaltic crustal rheology, which is therefore incompatible with the observations. Furthermore, we find that the observed elastic thickness evolution is best compatible with a wet mantle rheology, although a dry mantle cannot be ruled out. It therefore seems likely that rheologically significant amounts of water were retained in the Martian crust and mantle after planetary accretion.     

Impact craters in the northern hemisphere of Mars: Layered ejecta and central pit characteristics

Abstract— Mars Global Surveyor (MGS) and Mars Odyssey data are being used to revise the Catalog of Large Martian Impact Craters. Analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of Mars. This report focuses on the ejecta morphologies and central pit characteristics of these craters. The results indicate that single‐layer ejecta (SLE) morphology is most consistent with impact into an ice‐rich target. Double‐layer ejecta (DLE) and multiple‐layer ejecta (MLE) craters also likely form in volatile‐rich materials, but the interaction of the ejecta curtain and target‐produced vapor with the thin Martian atmosphere may be responsible for the large runout distances of these ejecta. Pancake craters appear to be a modified form of double‐layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions. Pedestal craters are proposed to form in an icerich mantle deposited during high obliquity periods from which the ice has subsequently sublimated. Central pits likely form by the release of vapor produced by impact into ice‐soil mixed targets. Therefore, results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the Martian northern hemisphere.     

Water undersaturated mantle plume volcanism on present‐day Mars

Based on meteorite evidence, the present‐day Martian mantle has a combined abundance of up to a few hundred ppm of H₂O, Cl, and F, which lowers the solidus and enhances the magma production rate. Adiabatic decompression melting in upwelling mantle plumes is the best explanation for young (last 200 Myr) volcanism on Mars. We explore water undersaturated mantle plume volcanism using a finite element mantle convection model coupled to a model of hydrous peridotite melting. Relative to a dry mantle, the reduction in solidus temperature due to water increases the magma production rate by a factor of 1.3–1.7 at 50 ppm water and by a factor of 1.9–3.2 at 200 ppm water. Mantle water also decreases the viscosity and increases the vigor of convection, which indirectly increases the magma production rate by thinning the thermal boundary layer and increasing the flow velocity. At conditions relevant to Mars, these indirect effects can cause an order of magnitude increase in the magma production rate. Using geologic and geophysical observations of the Late Amazonian magma production rate and geochemical observations of melt fractions in shergottite meteorites, present‐day Mars is constrained to have a core–mantle boundary temperature of ~1750 to 1800 °C and a volume‐averaged thermal Rayleigh number of 2 × 10⁶ to 10⁷, indicating that moderately vigorous mantle convection has persisted to the present day. Melting occurs at depths of 2.5–6 GPa and is controlled by the Rayleigh number at the low pressure end and by the mantle water concentration at high pressure.     

Thermal conductivity modelling of terrestrial soil media—A comparative study

The paper describes prediction of thermal conductivity in terrestrial soil media. The model operates statistically by probability of occurrence for contacts between particular fractional compounds. It combines physical properties, specific to particular compounds, into one apparent conductance specific to the mixture. The concept of substituting grain compounds by hypothetical spheres is an essential tool to control porosity by the number of spheres, their radii and probability of contacts between them. The spheres are equal in radii. The spheres substitute compounds, regardless of the phase state. Control of particular phase states is possible by means of specific properties assigned to the spheres, at the input to the model. Performance of the model is successfully proved for many diverse terrestrial soil media in a wide range of bulk density, composition, water and water vapour content. Only the compounds of sand decline from the expected values and require introducing a correction to the thermal conductivity of sand grains. One possible explanation is that the thermal conductivity of sand is uncertain. Nevertheless, the model is useful and worth extending beyond terrestrial purposes.     

The dawn polar cap boundary at high altitude

Comparison of hot plasma data from ATS-6 and GEOS-1 when the satellites were near dawn L.T. conjunction reveals the presence of strong gradients separating plasmas differing by more than two orders of magnitude in keV particle fluxes. These gradients are observed at off-equatorial geomagnetic latitudes of 25–30° on field lines outside the synchronous orbit. They are associated with magnetic storms and are distinct from magnetopause crossings. Interpretation of these events in terms of a boundary between magnetospheric and polar-cap plasma leads to the following conclusions: (1) the polar cap/lobe region is essentially devoid of keV plasma at these times; (2) the field lines defining this boundary are significantly distorted from a dipolar to a more stretched form consistent with the presence of a storm-ring current, (3) smaller substorm-scale motions are superposed on the gross motion of the boundary with some evidence present for structure in the plasma spatial profile, and (4) magnetosheath-like plasma finds access to the inner magnetosphere at dawn L.T., much as it does near noon, along polar-cap boundary-layer field lines which close through the low latitude magnetospheric boundary layer.     

Model Estimates of Non-Hydrostatic Stresses in the Martian Crust and Mantle: 2−Three-Level Model

Regions of maximum shear and tension-compression stresses in the Martian interior have been revealed using the three-level compensation model. Nonequilibrium relief, density anomalies at the crust?mantle boundary, and density anomalies at the base of the lithosphere are the sources of the anomalous gravitational field. The thickness of elastic lithosphere positioned on a weak layer that has partially lost its elastic properties varies from 150 to 500 km. The weakening of the layer under the lithosphere is simulated by a tenfold lower value of the shear modulus down to the core boundary. In general, the stresses for the threelevel compensation model differ from the values obtained for the two-level model (nonequilibrium relief and density anomalies at the crust?mantle boundary are the sources of the anomalous gravitational field) by 5?10%. Considerable differences between the models of two-level and three-level compensation are revealed beneath Hellas and Argyre regions.     

Model Estimates of Non-Hydrostatic Stresses in the Martian Crust and Mantle: 1—Two-Level Model

Regions of maximum shear and tension–compression stresses in the Martian interior have been revealed using two types of models: the elastic model and the model with an elastic lithosphere of varied thickness (150–500 km) positioned on a weak layer that has partially lost its elastic properties. The weakening is simulated by a ten-fold lower value of the shear modulus down to the core boundary. The numerical simulation applies Green’s functions (load number method) with the step of 1 × 1 grade along latitude and longitude down to a depth of 1000 km. The boundary condition is the expansion of the latest data on Martian topography and the gravitational field (model MRO120D) in spherical harmonics up to the degree and order of 90 in relation to the reference surface that is assumed an equilibrium spheroid. The considered two-level compensation model assumes nonequilibrium relief and density anomalies at the crust–mantle boundary to be the sources of the anomalous gravitational field. Calculations are performed for two test models of Martian internal structure with the crust mean thicknesses of 50 to 100 km and mean density of 2900 kg/m³. Considerable tangential and simultaneously compressive stresses occur under the Tharsis region. The main regions of high shear and simultaneously extentional stresses are located in the Hellas region crust and in the lithosphere of the following regions: Argyre Planitia, Mare Acidalium, Arcadia Planitia and Valles Marineris. The zone of high maximum shear and extentional stresses has been found at the base of the lithosphere under the Olympus volcano and that under the Elysium rise.     

Periods of active permafrost layer formation during the geological history of Mars: Implications for circum-polar and mid-latitude surface processes

Permafrost is ground remaining frozen (temperatures are below the freezing point of water) for more than two consecutive years. An active layer in permafrost regions is defined as a near-surface layer that undergoes freeze-thaw cycles due to day-average surface and soil temperatures oscillating about the freezing point of water. A “dry” active layer may occur in parched soils without free water or ice but significant geomorphic change through cryoturbation is not produced in these environments. A wet active layer is currently absent on Mars. We use recent calculations on the astronomical forcing of climate change to assess the conditions under which an extensive active layer could form on Mars during past climate history. Our examination of insolation patterns and surface topography predicts that an active layer should form on Mars in the geological past at high latitudes as well as on pole-facing slopes at mid-latitudes during repetitive periods of high obliquity. We examine global high-resolution MOLA topography and geological features on Mars and find that a distinctive latitudinal zonality of the occurrence of steep slopes and an asymmetry of steep slopes at mid-latitudes can be attributed to the effect of active layer processes. We conclude that the formation of an active layer during periods of enhanced obliquity throughout the most recent period of the history of Mars (the Amazonian) has led to significant degradation of impact craters, rapidly decreasing the steep slopes characterizing pristine landforms. Our analysis suggests that an active layer has not been present on Mars in the last ∼5 Ma, and that conditions favoring the formation of an active layer were reached in only about 20% of the obliquity excursions between 5 and 10 Ma ago. Conditions favoring an active layer are not predicted to be common in the next 10 Ma. The much higher obliquity excursions predicted for the earlier Amazonian appear to be responsible for the significant reduction in magnitude of crater interior slopes observed at higher latitudes on Mars. The observed slope asymmetry at mid-latitudes suggests direct insolation control, and hence low atmospheric pressure, during the high obliquity periods throughout the Amazonian. We formulate predictions on the nature and distribution of candidate active layer features that could be revealed by higher resolution imaging data.     

Comparison between infrared Martian disk spectra and optical properties of terrestrial analogs

Medium spectral resolution (20 cm^?1) infrared measurements of the Martian disk made between 2900 and 5600 cm^?1 from the NASA Lear Airborne Observatory have been successfully compared with predictions derived from a model of the Martian soil and atmosphere. Modeling of the Martian atmosphere permitted the extraction of Martian soil reflectance in the CO₂ bands centered at 3657 cm^?1. Three previously considered acceptable Martian soil analogs, limonite, montmorillonite, and basalt, were analyzed to determine the optical complex indices of refraction in the same range as the airborne observations, for mathematical modeling. A characteristic surface particle size ～1 to 3 μm diameter is indicated. It is concluded that the Martian soil surface near-infrared optical properties are consistent with a soil composition similar to montmorillonite or limonite, mixed with a basalt.     

Evaporation of ice in planetary atmospheres: Ice-covered rivers on mars

The evaporation rate of water ice on the surface of a planet with an atmosphere involves an equilibrium between solar heating and radiative and evaporative cooling of the ice layer. The thickness of the ice is governed principally by the solar flux which penetrates the ice layer and then is conducted back to the surface. These calculations differ from those of Lingenfelter et al. [(1968) Science161, 266–269] for putative lunar channels in including the effect of the atmosphere. Evaporation from the surface is governed by two physical phenomena: wind and free convection. In the former case, water vapor diffuses from the surface of the ice through a lamonar boundary layer and then is carried away by eddy diffusion above, provided by the wind. The latter case, in the absence of wind, is similar, except that the eddy diffusion is caused by the lower density of water vapor than the Martian atmosphere. For mean Martian insolations the evaporation rate above the ice is ～ 10^?8 g cm^?2 sec^?1. Thus, even under present Martian conditions a flowing channel of liquid water will be covered with ice which evaporates sufficiently slowly that the water below can flow for hundreds of kilometers even with quite modest discharges. Evaporation rates are calculated for a wide range of frictional velocities, atmospheric pressures, and insolations and it seems clear that at least some subset of observed Martian channels may have formed as ice-choked rivers. Typical equilibrium thicknesses of such ice covers are ～ 10 to 30 m; typical surface temperatures are 210 to 235°K. Ice-covered channels or lakes on Mars today may be of substantial biological interest. Ice is a sufficiently poor conductor of heat that sunlight which penetrates it can cause melting to a depth of several meters or more. Because the obliquity of Mars can vary up to some 35°, the increased polar heating at such times seems able to cause subsurface melting of the ice caps to a depth which corresponds to the observed lamina thickness and may be responsible for the morphology of these polar features.