Evidence of Exposed Dusty Water Ice within Martian Gullies

Midlatitude slopes on Mars are mantled by deposits proposed to contain H₂O ice and dust, overlaid by a desiccated lag. However, direct evidence of their volatile content is lacking. Here we present novel evidence of light-toned materials within midlatitude gully alcoves eroded into these mantles. The appearance and Lambert albedo of these materials suggests that they are either dust or H₂O ice. We interpret them to be H₂O ice because it is unlikely for a short-term, localized dust deposit to form only within the mantle walls. The temperatures are generally too warm (>∼240 K) for the ice to be a frost in equilibrium. Therefore, this ice is likely similar to the dusty ice documented within midlatitude scarps, but with more dust, and exposed in smaller patches by slumping. It has been proposed that CO₂ frosts remove the overlying lag, causing the exposed H₂O ice to sublimate, liberate dust within the ice for transport, and erode gullies in the mantle. But we observe gullies eroded in wall rock that continue into the mantle, implying that the same process erodes both substrates. H₂O ice melt can explain gullies eroded in the wall rock and the mantle. Numerical models show that relatively dense H₂O snow on Mars melts only when it contains small amounts of dust. The observed exposure of dusty ice provides a mechanism for it to melt under some conditions and form some gullies. Access to liquid water within this ice could provide potential abodes for any extant life.