There is direct evidence for the existence of caves on Mars. These subterranean cavities can prove useful for habitats on Mars since they offer natural protection from radiation, insulation from temperature cycling and sealability to contain breathable atmospheres. Many of these Martian caves can also serve as repositories with perennial water ice deposits. Such caves are termed ice caves and besides the advantages that normal caves offer, ice caves provide direct access to water which make them particularly attractive. Water ice in ice caves can accumulate via a number of mechanisms which include freezing of accumulated water, wind-blown snow and deposited frost.
A large ‘skylight’ entrance into a cave on the slopes of the Pavonis Mons volcano. The entrance is about 35m in diameter and 20m deep, and the floor of the cave is illuminated. (Credit: NASA/JPL/University of Arizona)
In order for an ice cave to exist, it is not necessary that the external ambient air temperature must be below freezing for most of the year. This is possible as long as the geometry of the cave allows it to function as a “cold trap”. A typical geometry for an ice cave is a subterranean cavity with a single entrance leading up to the surface. In winter, air in the cave tends to be warmer than the outside air. Since cold air is denser than warm air, cold air settles into the cave and displaces the warmer air inside. In summer, air in the cave tends to be cooler than the outside air and so continues to remain in the cave. The ceiling thickness of the cave also needs to be sufficient to suppress the seasonal temperature variations that occur on the Martian surface. In this case, a cave ceiling thickness of a metre or more will be enough to thermally isolate the cave.
Since air is only exchanged when the outside air is cooler than air in the cave, the “cold-trapping” effect of an ice cave allows for the accumulation and preservation of water ice over long timescales. Water ice in an ice cave is loss through a process of ice sublimation as the atmospheric pressure on Mars is too low for water ice to melt. Ice sublimation in an ice cave is expected to be a slow process. Additionally, the condensation of carbon dioxide ice over water ice and the deposition of frost produced from the cooling of air that is humidified through ice sublimation can further slow the ice sublimation process. Even in the absence of further accumulation, water ice in an ice cave can persist for tens of thousands of years. It is possible for ice caves on Mars to contain water ice from a pervious epoch in Mars’ recent history about 100 thousand years ago. Back then, the planet had a much greater axial tilt which supported a different global climate that favoured the widespread accumulation of water ice at mid-latitudes.
1. K.E. Williams, et al., “Do ice caves exist on Mars?” Icarus 209 (2010) 358-368
2. D.L. Murphy, et al., “Human utilization of subsurface extraterrestrial environments,” Gravitational and Space Biology Bulletin 16(2), pg 121-131, June 2003