A possible perchlorate-enabled mechanism for forming thick near surface excess ice layers; in the Amazonian regolith of Mars

Abstract

We use the freezing point depressing magnesium and calcium perchlorates in Martian regolith to redistribute ground ice by residual liquid water migration following the initial emplacement of ground ice by vapour deposition. This residual liquid water is moved by forces generated by periodic surface temperatures that decay with depth in conjunction with the geothermal vertical temperature gradient. We examine the period means of the bulk water speeds with depth and the mean divergence of the bulk water speeds, which relates to the rate of change in ice content in the regolith. Silt and clay rich regoliths behave differently. In silty regolith, for the short 1.88 a period and for longer 50 ka (precession) and 120 ka (obliquity) temperature cycles, there is a mean movement of liquid perchlorate aqueous solution that results in formation of near surface excess ice layers. The excess ice formed by the seasonal 1.88a period is confined at high latitudes to the upper meter of regolith. For the longer periods, there is a well-defined surface temperature region where near surface thick (≥40 m) excess ice layers form; (from 192 K to 210 K). For mean surface temperatures <192 K no near surface thick excess ice layers formed, but deeper layers are predicted and followed. The formation of excess ice layers in silt near the surface is controlled by the relationship between the temperature cycles, geothermal gradient and the eutectic temperature of the perchlorate (eg. ~198 K for Ca and ~ 205 K for Mg perchlorate). At a given depth if the periodic temperature is below the eutectic then there is nearly no liquid water left and what there is has much higher viscosity. The sudden change in the liquid water amount and viscosity with temperature generates net average water speeds in silts that are two orders larger than in clays.