SHARAD observations for layered ejecta deposits formed by late-Amazonian-aged impact craters at low latitudes of Mars

Abstract

Frequent changes of the rotational inclination of Mars have driven global transitions of the cryosphere. Although glaciers of water ice may have extended to the equatorial region, current observations of surface deposits at latitudes lower than 30° show relative depletion in hydrogen. Thermal stability models predict that at these low latitudes, ancient water ice may be preserved below a desiccated regolith layer that is meters to hundreds of meters thick. However, confirmed detections of massive water ice and/or water ice remain sparse at low latitudes. Here we use sounding radar observations of late-Amazonian-aged impact craters with layered ejecta deposits at low latitudes to search for subsurface layering and interface properties to investigate the possible existence of subsurface water/water ice. Such craters are widespread at low latitudes, including the landing area of the Zhurong rover, Tianwen-1 mission. The layered ejecta deposits of selected craters have lateral sizes and thickness suitable for investigation by sounding radar. Compared to previously-reported, but rare, cases that contained obvious reflectors between the layered ejecta deposits and underlying materials, we detect no clear reflections in neither the layered ejecta deposits nor deeper materials, suggesting that these materials have similar dielectric properties across their interfaces. Referring to the surface permittivity of background terrains of the investigated craters, we derive a broad range of relative permittivity of ∼2–10 for materials in the layered ejecta deposits. We discuss the implications of these results in the context of past and present water ice at low latitudes of Mars.