Urey prize lecture: Water on Mars

Geologic evidence indicates that the Martian surface has been substantially modified by the action of liquid water and that much of that water still resides beneath the surface as ground ice. The action of liquid water at the surface can be divided into an early epoch of valley system formation and a later epoch of outflow channel formation. The former might have required a climate somewhat warmer than the present one, while the latter did not. Layered deposits in the Valles Marineris may have been deposited in liquid water during the epoch of outflow channel formation, although the evidence that the deposition there was aqueous is not conclusive. Aqueous deposits were laid down in local topographic depressions during the epoch of valley system formation. Using the physics of heat transport in ice-covered lakes in the Dry Valleys of Antartica as a model, it appears that liquid water lakes could have persisted in these depressions under a protective ice cover for significant periods even if the early Martian climate were fairly cold. Early lakes on Mars could have served as sites for precipitation of carbonates, scavenging CO ₂ from the planet's atmosphere. Ancient aqueous sediments may contain a record of the climatic conditions that prevailed on the planet early in its history and are an important potential target for future sample return missions. Calculations of the thermodynamic stability of ground ice on Mars suggest that it can exist very close to the surface at high latitudes, but can persist only at substantial depths near the equator. A variety of observed Martian landforms can be attributed to creep of the Martian regolith abetted by deformation of ground ice. The most important of these, terrain softening, produces a pronounced rounding of sharp topographic features and conversion of concave or straight slope profiles to a convex shape. Finite element modeling of the flow process indicates that terrain softening is a near-surface phenomenon. Global mapping of creep features supports the idea that ice is present in near-surface materials at latitudes higher than ±30°, and suggests that ice is largely absent at lower latitudes. However, based on the rheology of terrestrial frozen ground, the Martian regolith must be thoroughly comminuted in order for significant creep to take place. Because regolith comminution is dominated by impacts, this condition is likely to be satisfied only relatively close to the surface. Substantial ground ice therefore may also exist at latitudes lower than ±30°, but at depths where materials are less thoroughly comminuted, preventing flow. It will be possible to investigate the geographic and vertical distribution of ice in the upper tens of centimeters of the Martian regolith using remote gamma-ray spectral data from the Mars Observer mission.