Did Valley Networks on Mars Form under Ancient Ice Caps?
Thousands of ancient valleys incise the southern hemispheric highlands of Mars, standing as proof that liquid water once flowed on its surface. Owing to their striking similarity to terrestrial rivers, the Martian valleys have historically been interpreted as fluvial systems, and this interpretation used to justify that Mars was one warmer and wetter. However, many aspects of valley morphology are hard to reconcile with fluvial erosion, e.g. along-valley profile uphill sections, lack of inner channels, lack of intervalley dissection, and channels that disappear and reappear. These puzzling characteristics are rather common indicators of channel incision at the base of ice caps. Modelling studies of fluvial and subglacial channel erosion on Mars, as well as field observations conducted in the former reaches of the Devon and Stacie ice caps (Canadian Arctic), show that the subglacial environment provides a plausible explanation for the formation, distribution, and shape of a considerable number of valley networks, and that clays and salt precipitates can form below ice masses. Detailed characterization of environments formed under ice caps, including subglacial geomorphology, geochemistry, and modeling of ice masses on Mars, is needed to assess the existence and extent of ancient glaciations on Early Mars.