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. Plain Language Summary Midlatitude, Martian pole‐facing slopes are covered by smooth deposits (“mantles”) thought to contain dusty ice, covered by desiccated material. However, there is a lack of evidence that these deposits contain dusty ice. We present evidence of light‐toned materials being exposed within gullies eroded into these smooth deposits. Based on the materials' brightness, we interpret them to be water ice because it is unlikely for a short‐term, localized dust deposit to form only within these smooth deposits. It is too warm for the ice to be water frost, so the ice is likely dusty ice exposed when the overlying desiccated material slumps down. Previous studies propose that evaporating Martian carbon dioxide frosts can mobilize the overlying desiccated material to expose dusty ice, free up dust for transport and erode gullies by evaporating the water ice. But we observe gullies eroded in rock that continue into the smooth deposits, which are readily explained by ice melt. Computer simula