Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice

Ground ice in the crust and soil may be one of the largest reservoirs of water on Mars1,2,3. 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.