Excess Ejecta Craters Record Episodic Ice-Rich Layers at Middle Latitudes on Mars

Abstract

We infer Amazonian climate change events from the presence of fresh craters with excess volumes of ejecta. Using the Mars Orbiter Laser Altimeter data, the geometrical properties of 572 fresh impact craters with rim diameters between 2.5 and 102 km were compiled in lowland and highland plains. The data reveal a class of fresh craters with anomalously high ejecta volumes preferentially located in Utopia Planitia. These fresh, "excess ejecta'' craters have volumes of material above the preimpact surface larger than the crater cavity volumes by factors of 2.5 to 5.8. The excess volume corresponds to an excess thickness of ejecta of about 20 to 100 m averaged over the continuous ejecta blanket. The excess material cannot be accommodated by ejecta bulking alone and requires an external process to increase the apparent volume of the ejecta and/or the uplifted surface. On the basis of the geologic setting, ejecta morphology, and calculations of increased ice stability with burial, we conclude that the most likely origin of the excess ejecta volume is the presence of an ice-rich layer tens of meters thick at the time of impact. The icy layer or a lag deposit is partially preserved beneath the ejecta blanket today. In this scenario, the icy layer has since been removed from areas unprotected by ejecta blankets, creating an apparent preimpact surface lower than the original elevation. The statistical occurrence of excess ejecta craters is consistent with climate model predictions of recent glacial periods on Mars.