The Martian mid-latitude subsurface ice is the remnant of a past ice sheet

Abstract

On Mars, a relatively pure water ice layer lies beneath several centimeters of dry soil at mid-latitudes. Its widespread presence poleward of 60° latitude was detected by remote neutron spectroscopy and confirmed by the Phoenix lander at 68°N. 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, 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° obliquity (40°), at latitudes of 40-55°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.

Figures (4)

• Figure 1: Modeled annual zonal mean atmospheric humidity (expressed here by the column water vapor in pr. $\mu$m) as a function of time before present at latitude 45$^\circ$ N, Bottom: Obliquity as a function of timelaskar2004. Note that the humidity at 45$^\circ$N is found to vary by 50-200% depending on the longitude due to topography and winds.
• Figure 2: Example of subsurface ice loss rate at 45$^\circ$ N modeled by the 1D PCM as a function of depth and atmospheric humidity (expressed here by the column water vapor in pr. $\mu$m) and the above dry-soil layer thermal inertia, for present-day obliquity and eccentricity, and $L_p$ of 270$^\circ$. There is a strong and moderate decrease in flux with depth and atmospheric water content, respectively.
• Figure 3: Subsurface ice depth modeled evolution as a function of time at latitude 45$^\circ$ N for emplacement time of 630 kyr ago, no porosity, several initial dust fractions in the ice and lag layers thermal inertia. The ice depth increases as time progresses, with a burial depth of 45-102 centimeters for realistic initial dust fractions in the ice levels and thermal inertia values.
• Figure 4: Subsurface ice final depth interpolated map for latitudes 40-55 North, assuming ice was emplaced 630 kyr ago for 1% dust fraction scenario and no lag layer porosity. The red line represents MONS 7.5% Water-Equivalent Hydrogen contourPATHARE2018. The white circles and crosses are locations where ice-exposing craters and maybe ice-exposing craters are found, respectively. The black circles are locations where non-ice-exposing craters are found. The numbers next to the circles indicate the assumed crater depths Dundas2021.