These impacts started out as normal bowl-shaped craters, and over millions of years the ground ice sublimated away and caused the craters to expand. Mapping efforts locate them are mostly at high- and mid-latitudes. Studying the secondary impacts- because they came from a primary crater with a known age-could constrain the age of the ice.

ID: ESP_056953_2160
date: 20 September 2018
altitude: 291 km

https://uahirise.org/hipod/ESP_056953_2160
NASA/JPL-Caltech/University of Arizona
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