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 midlatitudes. Its widespread presence poleward of 60° latitude was detected by remote neutron spectroscopy. Recent observations of exposed ice indicate that the near-surface ice layer extends to 35° latitude and exhibits pronounced spatial structure. However, previous models did not capture the observed spatial structure of the midlatitude ice layer. Here, using improved calculations using the Mars Planetary Climate Model, we show that mid-latitude buried ice could be the remnant of an 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 thousand years (4.18 million years) ago at 35° obliquity (40°), at latitudes of 40-55° North, would today reside at depths of 25-150 (41-255) centimeters, depending on regolith and ice properties. For ice emplaced 630 thousand years ago, the modeled burial depths align with observations and capture the observed longitudinal depth variations, in contrast to ice emplaced 4.18 million years ago. We therefore infer that the mid-latitude subsurface ice is younger than 4 million years.