Surface topographic impact of subglacial water beneath the south polar ice cap of Mars

Bright radar reflections observed in the Ultimi Scopuli region of Mars' south polar layered deposits(1-3) by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument have been interpreted as the signature of areas of subglacial water beneath it. However, other studies put forward alternative explanations, which do not imply the presence of liquid water(4-6). Here we shed light on the issue by looking at the surface topography of the region. On Earth, reduced or absent basal friction, and consequent ice velocity changes, cause a distinct topographic signature over subglacial lakes(7). Using Mars Orbiter Laser Altimeter data(8), we identify and characterize an anomaly in the surface topography of the south polar layered deposits overlying the area of the putative lakes, similar to those found above terrestrial subglacial lakes of similar size. Ice flow model results suggest that comparable topographic anomalies form within 0.5-1.5 Myr with locally elevated geothermal heating(9) or 2-5 Myr without elevated geothermal heating(2). These findings offer independent support for the presence of basal water beneath Ultimi Scopuli and suggest that surface topography could supplement radar returns to help identify other potential subglacial water bodies.