Subsurface Ice and Permafrost on Mars
Terrestrial permafrost varies widely in its physical and mechanical properties and behavior. Ice content, for example, may range from 0 to 100 % by volume. The types of subsurface ice are numerous and the crystal structure of terrestrial subsurface ice is variable. Most subsurface ice is hexagonal, Ice-I; clathrate structures are known, however. The ice content of permafrost is only a fraction, albeit the predominant one, of the water present. A significant portion of the water present exists in an unfrozen state and is distributed throughout the pore space and in interfacial areas. The proportion of ice to unfrozen water varies, in a characteristic manner, with temperature and solute concentration. These basic facts are important In determining the strength and deformation properties of permafrost and also its hydrological and electrical properties. Reliable relationships among these properties are derivable from basic thermodynamic theory and from empirical relationships recently established on the basis of laboratory and field data.
Permafrost exists at all latitudes on Mars and subsurface ice probably is abundant. The temperatures and pressures characteristic of each location or region determine, to a large extent, the depth and distribution of permafrost. Together with ground water salinity, they control the ice content, strength and deformation characteristics, in addition to other physical and electrical properties of local permafrost. Calculations based on the Viking Mission Data indicate that permafrost thicknesses range from about 3.5 km at the equator to approximately 8 km in the polar regions. The depths to the bottom of Martian permafrost are more than three times the depth characteristic of permafrost in terrestrial polar locations.
Martian permafrost, in general, is much colder than terrestrial permafrost. Consequently, the proportion of unfrozen water to ice generally is much lower. This, however, probably is somewhat offset by a significantly higher salinity of the Martian permafrost. The combination of low temperatures and great thicknesses of Martian permafrost, coupled with the low atmospheric pressure and very small snowfall, enhance the stability of the Martian surface. The “active layer” on Mars is extremely thin compared to that of terrestrial permafrost, making Martian permafrost more resistant to deformation and abrasion than is the case on Earth. The occurrence, quantities and behavior of subsurface ice, currently a matter of speculation and conjecture, is important in many respects. Its determination has been an objective of high priority in the exploration of Mars.
KeywordsUnconfined Compressive Strength Freeze Soil Unfrozen Water Martian Surface Unfrozen Water Content
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