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 wa ter on Mars(1-3). Near-surface ground ice is predicted to be stable at lati tudes higher than 40 degrees (ref. 4), where a number of geomorphologic fea tures indicative of viscous creep and hence ground ice have been observed(5 ). Mid-latitude soils have also been implicated as a water-ice reservoir(6) , the capacity of which is predicted to vary on a 100,000-year timescale ow ing to orbitally driven variations in climate(7). It is uncertain, however, whether near-surface ground ice currently exists at these latitudes, and h ow it is changing with time. Here we report observational evidence for a mi d-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 di stribution in the 30 degrees to 60 degrees latitude bands indicate a reserv oir of (1.5-6.0) x 10(4) km(3), 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 dimin ished.