Spatial and Temporal Heterogeneity of Martian Tropical Water Ice Through Analysis of Radial and Layered Ejecta Craters

Martian layered ejecta craters are theorized to form by tapping into water ice. The inference that some equatorial layered ejecta craters are Amazonian indicates ice has persisted in the tropics. However, detailed spatial and temporal distribution and evolution of this ice remains unknown; which is critical to constraining Mars’ global water cycle and climate change over eons. Here we estimate absolute model formation ages for layered and radial (ballistic) ejecta craters to constrain the spatial and temporal distribution of equatorial ice. The assumption is radial ejecta form where volatiles are not present in significant quantities. Ages are derived from the density of smaller craters superposed on the ejecta blankets. We examine 73 craters in a 30° x 30° area centered at 15ºS, 355ºE, with 44 layered and 29 radial ejecta. Analysis suggests an increasing proportion of layered ejecta craters with increasing diameter. This trend is amplified when considering younger (<3.4 Ga) craters only andwould be in agreement with deeper tropical subsurface ice and a receding ice table. Conversely, it could indicate “armoring” preserves layered over radial ejecta. Layered and radial ejecta craters are mixed over distances comparable to their diameters, which represents an unreasonably short length scale for ground-ice emplacement. This supports intermittent low-latitude surface ice—from excursions to high obliquity—could be responsible. The combination of increasing proportion of layered ejecta with crater size and random spatial distribution may suggest a hybrid model in which buried ice and intermittent, but extensive, tropical glaciers both contribute to layered ejecta crater formation.