Accessible H₂O on Mars — A Critical Review of Current Knowledge

Here, we review the search for H2O that is accessible for utilization by missions to Mars. In that connection, three important parameters that guide us are (1) the latitude, (2) the depth, and (3) the elevation of any H2O. “Accessible” in this review means within about 5 m of the surface, at an elevation at least 2 km lower than the MOLA Datum, and at a latitude within ±20°, possibly extendable to ±30°. This might not be widely shared by members of the community, especially regarding latitude. Twenty years ago, the neutron spectra and initial ice stability models suggested that ground ice on Mars was likely to be rare equatorward of about 50° latitude. Since then, observation after observation from orbit (using radar, photography and spectroscopy) revealed the likely presence of huge amounts of imbedded ice within the subsurface of Mars, at various depths mostly at so-called “mid-latitudes”. As a result, the pendulum has swung to the point that some enthusiastically suggest that ground ice occurs almost everywhere on Mars. We reviewed the various observations and analyses regarding H2O on Mars. Near-surface ice has been observed, spectra and radar have implied, and ground features have been interpreted to indicate that in wide areas of Mars, shallow ice apparently occurs at latitudes greater than 40° and at a few locations, persists into the 30s. The depths of such putative ice are not well known. Further study with much higher resolution might possibly reveal shallow ice at lower latitudes in unique locations. Mineral hydrates might offer a possible alternative as a supply of H2O but not enough is known about local high concentrations because current observation techniques do not penetrate the surface.