High Ice Purity of Martian Lobate Debris Aprons at the Regional Scale: Evidence From an Orbital Radar Sounding Survey in Deuteronilus and Protonilus Mensae

Lobate Debris Aprons (LDA) are martian landforms with a strong morphologic resemblance to rock glaciers and debris-covered glaciers. While the Shallow Radar (SHARAD) sounder has confirmed that a handful of LDA are composed of >80% water ice, viscous flow morphology can also be produced by as little as 30% ice. To distinguish between these endmembers, we conducted a comprehensive regional SHARAD survey of LDA, in Deuteronilus Mensae. We found that the majority of LDA are penetrated by SHARAD and determined that they are composed of a material with epsilon' = 3 and tan delta approximate to 0.002 < 0.005. These LDA are thus consistently composed of >80% water ice, which constrains the regional sequestered ice budget to a minimum of 0.9 - 1.0 x 10(5) km(3) or a 62-69-cm Global Equivalent Layer. Plain Language Summary Lobate debris aprons (LDA) are dust and rock-covered Martian landforms thought to contain buried water ice due to their resemblance to creeping permafrost and debris-covered glaciers. Their ice content has been the subject of debate; it may be as low as 30% or nearly pure ice. Understanding LDA composition and its variability across Mars is important for exploring climate history and investigating the feasibility of these features as water reservoirs for potential manned missions. The Shallow Radar (SHARAD) instrument on Mars Reconnaissance Orbiter can probe the interior of LDA, constraining their ice content. This has been demonstrated for individual LDA, which were shown to be composed of >80% water ice; however, no regional survey has been made to determine if these LDA are outliers or the norm. We present a survey of LDA composition constrained by SHARAD for the region of Deuteronilus Mensae-a region with abundant, widespread LDA. We found that 87% of LDA were penetrated by SHARAD and that these all had radar properties broadly consistent with high purity (80-90%) water ice. Combining these results with previous LDA volume mapping constrains their water ice content to 4 times the combined volume of the Great Lakes.