POSSIBLE CARBON DIOXIDE GLACIERS IN THE RECENT GEOLOGICAL HISTORY OF MARS

We consider a rare type of glacial features on Mars. In three localities on the planet, there are sets of narrow partly overlapping ridges with lobate planforms. The ridges are tens to ~100 m wide; the lobes have spatial dimensions of a few km. Detailed study of morphology of these features unambiguously suggests that these ridges are drop moraines left by several episodes of advance and retreat of cold-based glaciers. These features are very different from other morphologies produced by flowing ice on Mars. All three localities are at high northern latitudes: two are in craters (70.3°N, 266.5°E and 67.2°N 249.5°E) and one (74°N 96°E) is associated with a steep slope of an outlier of the polar layered deposits. Age of the glacier activity is poorly constrained, but it is certainly very young, on the order of millions of years. In the first of two craters, thickness of ice, when the glacier flow was active, can be inferred from morphological observations; it was 300 - 500 m. Although morphology unambiguously suggest ice flow as origin of these features, the inferred thickness of the glaciers, temperature regime at high northern latitudes in the recent past, and associations with other features are very difficult to reconcile with ice flow. Due to low temperatures, ice on Mars is much stronger (less plastic) than suggested by morphology of these unique features. We argue that these drop moraines can be formed by carbon dioxide glaciers. Perennial accumulation of carbon dioxide in preferable locations during recent periods of low obliquity of spin axis can lead to flow of solid carbon dioxide and can be responsible for formation of such glaciers. Solid carbon dioxide (dry ice) is softer (more plastic) than water ice; this explains high plasticity of flowing material, and also explains how high plasticity can coexist with purely cold-based morphology.