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 impacting into an ice-rich crust. The inference that some equatorial layered ejecta craters are Amazonian indicates that ice has persisted in the tropics. However, the detailed spatial and temporal distribution and evolution of this ice remain 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 that 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 examined 73 craters in a 30° × 30° area centered at 15°S, 355°E, with 44 layered and 29 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, along with the lack of trend with age, supports the suggestion that intermittent low-latitude surface ice—from excursions to high obliquity—could be responsible. Analysis also suggests an increasing proportion of layered ejecta craters with decreasing diameter for those older than 3.4 Ga. This trend would support the hypothesis of more ice being available in early martian history. Conversely, this could indicate that “armoring” preferentially preserves layered ejecta relative to radial ejecta.