1. Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA

Correspondence to: Correspondence and requests for materials should be addressed to J.F.M. (e-mail: Email: john_mustard@brown.edu).

Abstract

Ground ice in the crust and soil may be one of the largest reservoirs of water on Mars^{1, 2, 3}. Near-surface ground ice is predicted to be stable at latitudes higher than 40° (ref. 4), where a number of geomorphologic features indicative of viscous creep and hence ground ice have been observed⁵. Mid-latitude soils have also been implicated as a water-ice reservoir⁶, the capacity of which is predicted to vary on a 100,000-year timescale owing to orbitally driven variations in climate⁷. It is uncertain, however, whether near-surface ground ice currently exists at these latitudes, and how it is changing with time. Here we report observational evidence for a mid-latitude reservoir of near-surface water ice occupying the pore space of soils. The thickness of the ice-occupied soil reservoir (1–10 m) and its distribution in the 30° to 60° latitude bands indicate a reservoir of (1.5–6.0)  10⁴ km³, equivalent to a global layer of water 10–40 cm thick. We infer that the reservoir was created during the last phase of high orbital obliquity less than 100,000 years ago, and is now being diminished.