Martian craters viewed by the Thermal Emission Imaging System instrument: Double‐layered ejecta craters

Abstract

The Thermal Emission Imaging System (THEMIS) visible (VIS) images provide new insight into the nature and development of the unique ejecta deposits of Martian craters. This study focuses on double‐layered ejecta (DLE) craters. To date, over 100 DLE craters have been examined using mainly THEMIS VIS data. Our observations suggest that emplacement of DLE crater ejecta occurred in two stages, with the inner ejecta layer emplaced similar to single‐layered ejecta (SLE) crater ejecta. This may have involved both ballistic and flow processes. In contrast, the outer ejecta layer of DLE craters appears to have been emplaced through the high‐velocity outflow of materials from tornadic winds generated by the advancing ejecta curtain or base surge. Remarkably, DLE craters lack secondary craters, which suggests that the large ejecta blocks that normally produce such craters may have either been entrained and/or crushed by these winds or fragmented as a result of the presence of water in the target materials. These observations suggest that volatiles (either trapped in the subsurface or in the atmosphere) have played a key role in the emplacement of the ejecta of DLE craters and leaves open the question as to what role volatiles play in the emplacement of ejecta of other types of fluidized ejecta craters (i.e., SLE and MLE craters). Because DLE craters are found in many different regions of Mars, often in close proximity to other types of craters, conditions (e.g., atmospheric density) that produce DLE craters must fluctuate or the Martian crust must be unexpectedly heterogeneous (laterally and vertically). While the degree of heterogeneity has yet to be recognized, recent suggestions about possible Martian climate change raises the possibility of impact into target materials that are periodically wet or that a significantly higher atmospheric pressure may be periodically present.