The Martian mid-latitude subsurface ice is the remnant of a past ice sheet
On Mars, a relatively pure water ice layer lies beneath several centimeters of dry soil at mid-latitudes. Its widespread presence poleward of 60{\deg} latitude was detected by remote neutron spectroscopy and confirmed by the Phoenix lander at 68{\deg}N. Recent observations of exposed ice indicate that the near-surface ice layer extends to 35{\deg} latitude and exhibits pronounced spatial structure. However, previous models did not capture the observed spatial structure of the midlatitude ice layer. Here, on the basis of improved calculations using the Mars Planetary Climate Model, we show that mid-latitude buried ice could be the remnant of a ice layer deposited on the surface when the obliquity was higher than today. Assuming that the ice subsequently sublimated and became buried beneath a sublimation lag, we estimate that surface ice emplaced 630 kyr (4.18 Myr) ago at 35{\deg} obliquity (40{\deg}), at latitudes of 40-55{\deg}N, would today reside at depths of 25-150 (41-255) cm, depending on the regolith and ice properties. For ice emplaced 630 kyr ago, the modeled burial depths align with observations and capture the observed longitudinal depth variations, in contrast to ice emplaced 4.18 Myr ago. We therefore infer that the mid-latitude subsurface ice is younger than 4 Myr.