COLD-CLIMATE WEATHERING OF BASALT ON MARS AND OPPORTUNISTIC PERIGLACIATION

Possible landscape revisions by periglaciation on Mars today are explained by two key assumptions: 1) icy mantles developed in the very Late Amazonian Epoch when obliquities and eccentricities were favourable at the mid- to high latitudes in both hemispheres, by the atmospheric precipitation and surface accumulation of relatively (light-toned) dusty ice; and, 2) current boundary-conditions exhibit extreme aridity, low atmospheric-pressure and mean temperatures largely below the triple point of water. As such, sublimation is the only process consistent with periglaciation wherever the latter is thought to be current.

On the other hand, possible Martian periglacial landscape-assemblages composed, for example, of thermokarst-like depressions, (clastically) sorted polygons, and low/high centred (and clastically) non-sorted polygons show similarities of form, features, scale and spatial association to periglacial assemblages on Earth, i.e. the Lena Peninsula of eastern Russia. At such locations water is abundant, freeze-thaw cycling to depth is commonplace and excess ice is ubiquitous. Excess ice, i.e. ice comprised of veins, lenses, wedges and larger consolidated masses, exceeds the pore volume of the frozen ground in which it occurs and requires relatively fine-grained sediments such as weathered basalts to form. By contrast, dust is insufficiently permeable to host excess ice let alone the freeze-thaw cycling of water within its pore space.

Within the rim materials of the Argyre impact-crater, (dark-toned) basaltic terrain has broken down into blockfields with clastic constituents that are shattered, imbricated and show apparent polygonised self-organisation or sorting. Non-sorted polygons and thermokarst-like depressions also are observed in close spatial-association with the sorted polygons. Similar spatial associations amidst weathered basalt are observed in eastern Utopia Planitia (UP), at the distal reaches of the Elysium Mons shield volcano, and in southern UP.

Sublimation may be the process by which excess ice currently is removed from a landscape; however, the origin of excess ice and associated landforms requires: 1) liquid water at/near the surface in the recent past; 2) the freeze-thaw cycling of this water; and, 3) appropriately permeable sediments such as weathered basalts.