HIGH-LATITUDE GROUND ICE ON MARS

Water on Mars is central to the planet’s geological, climatological, and geochemical histories. Ice plays no small role as a reservoir of water, as a driving force for geological processes, and as a signature of past and present hydrological processes. Mars high-latitude ground ice (water ice occurring in the shallow subsurface permafrost) has been predicted for decades and is now know to be wide spread (e.g., Feldman et al., in The Surface of Mars,ed. J. Bell, pg. 125-148, 2008). This ice persists as an ice-rich layer beneath a layer of ice-free frozen soil (or “dry” permafrost”) with a sharp “ice table” boundary between. On Earth only the Antarctic Dry Valleys exhibit a similar ice-rich and dry permafrost structure. On Mars the geographic distribution of this ice and the depth of the ice table agree remarkably well with predictions of ground-ice stability assuming that a diffusive equilibrium exists between the permafrost and the atmosphere in the modern martian climate. While the exact conditions of humidity and ground temperatures are still being deciphered, these conditions are clearly similar to the dry and deeply subfreezing climate that we observe today.

However, a major puzzle has emerged. Observations indicate that the ice is highly concentrated in excess of that expected within a porous soil in equilibrium with the present atmosphere and climate. In many places, the excess ice appears to be in concentrations >90% by volume. Observations from landed spacecraft and orbital imaging of fresh impacts and periglacial geomorphology indicate that the excess ice is also highly variable on a variety of spatial scales, from less than a meter to greater than 100’s kilometers. These observations also shed light on the depth extent of the ice and its age. Understanding the origin of the excess ice and the reasons for its variability will provide important clues and constraints for near-surface hydrological processes in the modern martian climate.