Table of Contents at IgMin Research

Published: November 25, 2025 (275d)

Accessible H2O on Mars – A Critical Review of Current Knowledge

Donald Rapp^*1 and Vassilis Inglezakis²

Here, we review what is known about the distribution of accessible H2O on Mars. While “Accessible” in this review implies 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°, we have extended our review to cover observations of  H2O at moderate depth within the latitude range ±60°. 
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.