MARTIAN CRATER EJECTA: DECOUPLING CRATER AGE AND EJECTA PRESERVATION
The youngest craters on a planetary surface are often identified by the presence of easily modified deposits such as crater rays and impact melt deposits, in addition to a lack of overprinting craters and a high depth/diameter ratio. Young Martian craters appear to preserve a thermophysical contrast between crater ejecta and the underlying surface, which can manifest as rays visible in infrared data that are not apparent in visible images due to a lack of albedo contrast. An examination of both visible and infrared datasets, however, suggests that crater age and preservation of ejecta are not necessarily correlated. Decoupling these criteria suggests that Martian crater deposits may fall into four general categories:
- "pristine" ejecta / young craters (with preserved rays and/or melt deposits and an ejecta blanket that exhibits a thermophysical contrast with the underlying surface)
- "modified" ejecta / young craters (craters that appear young but lack thermophysical contrasts)
- "well-preserved" ejecta / older craters (lacking preserved melt deposits but still presenting a thermophysically contrasted ejecta blanket)
- "degraded" ejecta / older craters (where both crater and ejecta are visibly modified).
An examination of the typical visible morphology and thermophysical character of craters in each of these classes should serve as a first step to identifying the characteristics associated with pristine ejecta and those affected by later modification.