Mars ice caps

Minimum atmospheric temperatures required to prevent CO(2) condensatio in the Mars polar caps are higher than those obtained in a computer experiment to simulate the general circulation of the Mars atmosphere. This observation supports the view that the polar caps are predominantly solid CO(2). However, thin clouds of H(2)0 ice could substantially reduce the surface condensation rate.     

Behavior of carbon dioxide and other volatiles on Mars

We have found that a rather simple thermal model of the Martian surface, in combination with current observations of the atmospheric composition, points strongly toward the conclusion that the polar caps of Mars consist almost entirely of frozen CO(2). This study was based upon the following principal assumptions. 1) Carbon dioxide is a major constituent of the Martian atmosphere. 2) The blanketing effect of the atmosphere is small, and due principally to the absorption band of CO(2) near 15 microns. 3) Lateral and convective heat transfer by the atmosphere is negligible. 4) The far-infrared emissivity of the Martian soil and of solid CO(2) are near unity. 5) The reflectivities of the soil and of solid CO(2) in the visible part of the spectrum are about 0.15 and 0.65, respectively. 6) Values for soil conductivity, density, and specific heat are those characteristic of powdered minerals at low gas pressure. 7) Water is a minor constituent of the Martian atmosphere, the maximum total amount in the atmosphere being 10 to 30 X 1O(-4) g cm(-2). In addition, several simplifications were made, which might have significant effects but should not alter our principal conclusions. Among these are the following. 1) Local blanketing or snowfall effects due to clouds or polar haze were ignored. 2) Dark and light areas were not differentiated in this study, although Sinton and Strong (6) have observed temperature differences between such areas. 3) The effects of local topography and microrelief were neglected. We believe that these must have quite significant effects at the higher latitudes, especially in connection with the evaporation of the remanent south polar cap. 4) Variation of reflectivity with angle of incidence of the sunlight was neglected. 5) Temperature dependence of soil conductivity and specific heat was ignored. 6) Effects of saturation of the soil by ice upon the thermal properties of the soil were neglected. Although in our main investigation we used certain specific values for the various relevant parameters, we also tested the effects of moderate changes in these quantities. Specifically, the soil conductivity was varied by a factor of 3, the albedo and emissivity of the surface were changed by 15 to 20 percent, and the effects of a gross amount of atmospheric blanketing were studied, as described. Only the last of these variations had any significant effect on the model, and other results of the atmospheric blanketing were in disagreement with other physical observations of the planet. Consequently, we find it difficult to avoid the conclusion that CO(2) must condense in large amounts relative to H(2)0. The main conclusions indicated by this study are the following. 1) The atmosphere and frost caps of Mars represent a single system with CO(2) as the only active phase. 2) The appearance and disappearance of the polar caps are adequately explained on the presumption that they are composed almost entirely of solid CO(2) with perhaps an occasional thin coating of water ice. 3) If the currently reported water-vapor observations are correct, water-ice permafrost probably exists under large regions of the planet at polar and temperate latitudes. 4) The geochemically anomalous enrichment of CO(2) relative to N(2) in the present Martian atmosphere may be a result of selective trapping of CO(2) in the solid phase at and under the surface. 5) If the basic evaporation and condensation mechanisms for CO(2) and H(2)O discussed in this article are correct, the possible migration of volatile organic compounds away from the warm temperate regions of the planet and their possible accumulation in the polar regions need to be carefully considered.     

Annual heat balance of martian polar caps: viking observations

The Infrared Thermal Mappers aboard the two Viking orbiters obtained solar reflectance and infrared emission measurements of the Martian north and south polar regions during an entire Mars year. The observations were used to determine annual radiation budgets, infer annual carbon dioxide frost budgets, and constrain spring season surface and atmospheric properties with the aid of a polar radiative model. The results provide further confirmation of the presence of permanent CO(2)frost deposits near the south pole and show that the stability of these deposits can be explained by their high reflectivities. In the north, the observed absence of solid CO(2) during summer was primarily the result of enhanced CO(2) sublimation rates due to lower frost reflectivities during spring. The results suggest that the present asymmetric behavior of CO(2)frost at the Martian poles is caused by preferential contamination of the north seasonal polar cap by atmospheric dust.     

Global distribution of neutrons from Mars: results from Mars odyssey

Global distributions of thermal, epithermal, and fast neutron fluxes have been mapped during late southern summer/northern winter using the Mars Odyssey Neutron Spectrometer. These fluxes are selectively sensitive to the vertical and lateral spatial distributions of H and CO2 in the uppermost meter of the martian surface. Poleward of +/-60 degrees latitude is terrain rich in hydrogen, probably H2O ice buried beneath tens of centimeter-thick hydrogen-poor soil. The central portion of the north polar cap is covered by a thick CO2 layer, as is the residual south polar cap. Portions of the low to middle latitudes indicate subsurface deposits of chemically and/or physically bound H2O and/or OH.     

A sublimation model for martian south polar ice features

In their pioneering work, Leighton and Murray argued that the Mars atmosphere, which at present is 95% carbon dioxide, is controlled by vapor equilibrium with a much larger polar reservoir of solid carbon dioxide. Here we argue that the polar reservoir is small and cannot function as a long-term buffer to the more massive atmosphere. Our work is based on modeling of the circular depressions commonly found on the south polar cap. We argue that a carbon dioxide ice layer about 8 meters thick is being etched away to reveal water ice underneath. This is consistent with thermal infrared data from the Mars Odyssey mission.     

Massive CO₂ ice deposits sequestered in the south polar layered deposits of Mars

Shallow Radar soundings from the Mars Reconnaissance Orbiter reveal a buried deposit of carbon dioxide (CO(2)) ice within the south polar layered deposits of Mars with a volume of 9500 to 12,500 cubic kilometers, about 30 times that previously estimated for the south pole residual cap. The deposit occurs within a stratigraphic unit that is uniquely marked by collapse features and other evidence of interior CO(2) volatile release. If released into the atmosphere at times of high obliquity, the CO(2) reservoir would increase the atmospheric mass by up to 80%, leading to more frequent and intense dust storms and to more regions where liquid water could persist without boiling.     

Mars surface diversity as revealed by the OMEGA/Mars Express observations

The Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) investigation, on board the European Space Agency Mars Express mission, is mapping the surface composition of Mars at a 0.3- to 5-kilometer resolution by means of visible-near-infrared hyperspectral reflectance imagery. The data acquired during the first 9 months of the mission already reveal a diverse and complex surface mineralogy, offering key insights into the evolution of Mars. OMEGA has identified and mapped mafic iron-bearing silicates of both the northern and southern crust, localized concentrations of hydrated phyllosilicates and sulfates but no carbonates, and ices and frosts with a water-ice composition of the north polar perennial cap, as for the south cap, covered by a thin carbon dioxide-ice veneer.     

Martian north pole summer temperatures: dirty water ice

Broadband thermal and reflectance observations of the martian north polar region in late summer yield temperatures for the residual polar cap near 205 K with albedos near 43 percent. The residual cap and several outlying smaller deposits are water ice with included dirt; there is no evidence for any permanent carbon dioxide polar cap.     

Exposed water ice discovered near the south pole of Mars

The Mars Odyssey Thermal Emission Imaging System (THEMIS) has discovered water ice exposed near the edge of Mars' southern perennial polar cap. The surface H2O ice was first observed by THEMIS as a region that was cooler than expected for dry soil at that latitude during the summer season. Diurnal and seasonal temperature trends derived from Mars Global Surveyor Thermal Emission Spectrometer observations indicate that there is H2O ice at the surface. Viking observations, and the few other relevant THEMIS observations, indicate that surface H2O ice may be widespread around and under the perennial CO2 cap.     

Radar images of Mars

Full disk images of Mars have been obtained with the use of the Very Large Array (VLA) to map the radar reflected flux density. The transmitter system was the 70-m antenna of the Deep Space Network at Goldstone, California. The surface of Mars was illuminated with continuous wave radiation at a wavelength of 3,5 cm. The reflected energy was mapped in individual 12-minute snapshots with the VLA in its largest configuration; fringe spacings as small as 67 km were obtained. The images reveal near-surface features including a region in the Tharsis volcano area, over 2000 km in east-west extent, that displayed no echo to the very low level of the radar system noise. The feature, called Stealth, is interpreted as a deposit of dust or ash with a density less than about 0.5 gram per cubic centimeter and free of rocks larger than 1 cm across. The deposit must be several meters thick and may be much deeper. The strongest reflecting geological feature was the south polar ice cap, which was reduced in size to the residual south polar ice cap at the season of observation. The cap image is interpreted as arising from nearly pure CO(2) or H(2)O ice with a small amount of martian dust (less than 2 percent by volume) and a depth greater than 2 to 5 m. Only one anomalous reflecting feature was identified outside of the Tharsis region, although the Elysium region was poorly sampled in this experiment and the north pole was not visible from Earth.     

Seasonal erosion and restoration of Mars' northern polar dunes

Despite radically different environmental conditions, terrestrial and martian dunes bear a strong resemblance, indicating that the basic processes of saltation and grainfall (sand avalanching down the dune slipface) operate on both worlds. Here, we show that martian dunes are subject to an additional modification process not found on Earth: springtime sublimation of Mars' CO(2) seasonal polar caps. Numerous dunes in Mars' north polar region have experienced morphological changes within a Mars year, detected in images acquired by the High-Resolution Imaging Science Experiment on the Mars Reconnaissance Orbiter. Dunes show new alcoves, gullies, and dune apron extension. This is followed by remobilization of the fresh deposits by the wind, forming ripples and erasing gullies. The widespread nature of these rapid changes, and the pristine appearance of most dunes in the area, implicates active sand transport in the vast polar erg in Mars' current climate.     

Density of Mars' south polar layered deposits

Both poles of Mars are hidden beneath caps of layered ice. We calculated the density of the south polar layered deposits by combining the gravity field obtained from initial results of radio tracking of the Mars Reconnaissance Orbiter with existing surface topography from the Mars Orbiter Laser Altimeter on the Mars Global Surveyor spacecraft and basal topography from the Mars Advanced Radar for Subsurface and Ionospheric Sounding on the Mars Express spacecraft. The results indicate a best-fit density of 1220 kilograms per cubic meter, which is consistent with water ice that has approximately 15% admixed dust. The results demonstrate that the deposits are probably composed of relatively clean water ice and also refine the martian surface-water inventory.     

The Phase Composition of Triton's Polar Caps

Triton's polar caps are modeled as permanent nitrogen deposits hundreds of meters thick. Complex temperature variations on Triton's surface induce reversible transitions between the cubic and hexagonal phases of solid nitrogen, often with two coexisting propagating transition fronts. Subsurface temperature distributions are calculated using a two-dimensional thermal model with phase changes. The phase changes fracture the upper nitrogen layer, increasing its reflectivity and thus offering an explanation for the surprisingly high southern polar cap albedo (approximately 0.8) seen during the Voyager 2 flyby. The model has other implications for the phase transition phenomena on Triton, such as a plausible mechanism for the origin of geyser-like plume vent areas and a mechanism of energy transport toward them.     

Morphology and composition of the surface of Mars: Mars Odyssey THEMIS results

The Thermal Emission Imaging System (THEMIS) on Mars Odyssey has produced infrared to visible wavelength images of the martian surface that show lithologically distinct layers with variable thickness, implying temporal changes in the processes or environments during or after their formation. Kilometer-scale exposures of bedrock are observed; elsewhere airfall dust completely mantles the surface over thousands of square kilometers. Mars has compositional variations at 100-meter scales, for example, an exposure of olivine-rich basalt in the walls of Ganges Chasma. Thermally distinct ejecta facies occur around some craters with variations associated with crater age. Polar observations have identified temporal patches of water frost in the north polar cap. No thermal signatures associated with endogenic heat sources have been identified.     

Viking: Mars atmospheric water vapor mapping experiment--preliminary report of results

Observations made from the Viking I orbiter show very little water vapor in the Mars atmosphere in the southern hemisphere (0 to 3 precipitable micrometers) with a gradual increase across the equator to northern latitudes. Maximum amounts between 20 and 30 micrometers have been observed in the short period covered by the observations to date. The season, northern midsummer, corresponds to the beginning of the water vapor cycle in that hemisphere. A strong repetitive diurnal cycling between the solid and vapor phases is observed at a site to the east of the Tharsis Ridge at 10 degrees north latitude; the vapor lies close to the martian surface and is most probably in saturation equilibrium with a surface haze or fog throughout much of the day.     

Climatic change on Mars

The equatorial sinuous channels on Mars detected by Mariner 9 point to a past epoch of higher pressures and abundant liquid water. Advective instability of the martian atmosphere permits two stable climates-one close to present conditions, the other at a pressure of the order of 1 bar depending on the quantity of buried volatiles. Variations in the obliquity of Mars, the luminosity of the sun, and the albedo of the polar caps each appear capable of driving the instability between a current ice age and more clement conditions. Obliquity driving alone implies that epochs of much higher and of much lower pressure must have characterized martian history. Climatic change on Mars may have important meteorological, geological, and biological implications.     

Mars: northern summer ice cap--water vapor observations from viking 2

Observations of the latitude dependence of water vapor made from the Viking 2 orbiter show peak abundances in the latitude band 70 degrees to 80 degrees north in the northern midsummer season (planetocentric longitude approximately 108 degrees ). Total column abundances in the polar regions require near-surface atmospheric temperatures in excess of 200 degrees K, and are incompatible with the survival of a frozen carbon dioxide cap at martian pressures. The remnant (or residual) north polar cap, and the outlying patches of ice at lower latitudes, are thus predominantly water ice, whose thickness can be estimated to be between 1 meter and 1 kilometer.     

Mariner 9 ultraviolet spectrometer experiment: seasonal variation of ozone on Mars

Ozone is observed to be present in the polar regions of Mars and to have a seasonal variation. In the summer, the amount present in the polar atmosphere is less than 3 micrometer-atmospheres. In the fall, ozone increases in amount and is found in association with the formation of the polar hood. In winter, the maximum amount of ozone is present, 57 micrometer-atmospheres over the polar hood and 16 over the polar cap. In spring, the amount over the polar cap decreases monotonically until by the beginning of summer the ozone disappears. Ozone is not observed in the equatorial region during any season.     

Carbon dioxide clathrate in the martian ice cap

Measurements of the dissociation pressure of carbon dioxide hydrate show that this hydrate (CO(2) . 6H(2)O) is stable relative to solid CO(2) and water ice at temperatures above about 121 degrees K. Since this hydrate forms from finely divided ice and gaseous CO(2) in several hours at 150 degrees K, it is likely to be present in the martian ice cap. The ice cap can consist of water ice, water ice + CO(2) hydrate, or CO(2) hydrate + solid CO(2), but not water ice + solid CO(2).     

Infrared spectroscopy experiment on the mariner 9 mission: preliminary results

The Mariner 9 infrared spectroscopy experiment has provided goodquality spectra of many areas of Mars, predominantly in the southern hemisphere. Large portions of the thermal emission spectra are significantly affected by dust with a silicon oxide content approximately corresponding to that of an intermediate igneous rock, thus implying that Mars has undergone substantial geochemical differentiation. Derived temperature profiles indicate a warm daytime upper atmosphere with a strong warming over the south polar cap. Atmospheric water vapor is clearly observed over the south polar area and less strongly over other regions.