PERIGLACIAL EVIDENCE (USING HIRISE IMAGES) OF VERY RECENT PONDING IN UTOPIA & WESTERN ELYSIUM PLANITIA

On Earth, thermokarst ponds occur in ice-rich regolith whose thermal equilibrium has been disturbed, often by climate change and accompanying rises in diurnal, seasonal or annual mean temperatures. Using Hi-RISE, THEMIS and MOC images, we have identified scalloped depressions in Utopia and western Elysium Planitia that are consistent in scale, morphology and possible origin with drained thermokarst ponds (alases), present in the Canadian Arctic. Pingo-like mounds and thermal contraction (possibly ice-wedge) polygons also are widespread in the region. By analogy with terrestrial polar regions, the assemblage of landforms suggests that periglacial processes could have played an important role in the development of this regional landscape on Mars. We focus on the possible alases and show that two of their characteristics – internal terraces and small orthogonal polygons – are uniquely consistent with ponded water and then water loss by episodic evaporation or slow drainage. The characteristics are not consistent with water loss by sublimation. In terms of age, the alas-like features could be very young. In some instances, they cross-cut gullies and are superposed on them. As the gullies themselves are thought to be amongst the youngest geological features on Mars, this points to the depressions being younger still. We propose that the scalloped depressions are youthful features whose evolution is a product of very recent shifts in obliquity, when boundary conditions could have allowed for water to pond and episodically to recede slowly. As for the origin of the ice-rich regolith itself, we invoke high obliquity once again, suggesting that a sequence of events, beginning with atmospheric transportation and deposition of water ice in the mid-latitudes, could be responsible.