Water ice at mid-latitudes on Mars

Mars’s mid-latitudes, corresponding approximately to the 30°–60° latitude bands in both hemispheres, host abundant water ice in the subsurface. Ice is unstable with respect to sublimation at Mars’s surface beyond the polar regions, but can be preserved in the subsurface at mid-to-high latitudes beneath a centimeters-to-meters-thick covering of lithic material. In Mars’s mid-latitudes, water ice is present as pore ice between grains of the martian soil (termed “regolith”) and as deposits of excess ice exceeding the pore volume of the regolith. Excess ice is present as lenses within the regolith, as extensive layers tens to hundreds of meters thick, and as debris-covered glaciers with evidence of past flow. Subsurface water ice on Mars has been inferred indirectly using numerous techniques including numerical modeling, observations of surface geomorphology, and thermal, spectral, and ground-penetrating radar analyses. Ice exposures have also been imaged directly by orbital and landed missions to Mars. Shallow pore ice can be explained by the diffusion and freezing of atmospheric water vapor into the regolith. The majority of known excess ice deposits in Mars’s mid-latitudes are, however, better explained by deposition from the atmosphere (e.g., via snowfall) under climatic conditions different from the present day. They are thought to have been emplaced within the last few million to 1 billion years, during large-scale mobilization of Mars’s water inventory between the poles, equator, and mid-latitude regions under cyclical climate changes. Thus, water ice deposits in Mars’s mid-latitudes probably host a rich record of geologically recent climate changes on Mars. Mid-latitude ice deposits are leading candidate targets for in situ resource utilization of water ice by future human missions to Mars, which may be able to sample the deposits to access such climate records. In situ water resources will be required for rocket fuel production, surface operations, and life support systems. Thus, it is essential that the nature and distribution of mid-latitude ice deposits on Mars are characterized in detail.