Paleoenvironment implications of layered ejecta craters in the Chryse Planitia, Mars

The formation of martian layered ejecta craters (LECs), which have one or multiple distinctive fluidized layered ejecta deposits, is generally hypothesized to be directly related to subsurface volatiles. Using a high-resolution (5 m/pixel) context camera (CTX) mosaic as a base map, 525 LECs (including 157 newly identified) were catalogued in the Chryse Planitia and divided into three types according to the number of layered ejecta, i.e., single layer ejecta (SLE), double layer ejecta (DLE), and multiple layer ejecta (MLE). It is found that the minimum crater diameter (here called the onset diameter) decreases with increasing latitude, and could be as small as 1 km. The average ejecta mobility (EM) values usually increase from the low to the high latitude interval for each LEC type and gradually increase with the number of layered ejecta within each latitude interval. The lobateness values of the SLE craters have a decreasing trend with increasing latitude, however, such a trend is not obvious for the DLE and MLE craters. The absolute model ages (AMAs) of 135 LECs were determined from the crater size-frequency distribution (CSFD) method to constrain the timing of the fluvial activities of the circum-Chryse outflow channels and to investigate the local temporal-spatial evolution of the subsurface volatile-rich layer. The superposition relationship between the LECs and circum-Chryse channels suggests fluvial activity might have extended into the early Amazonian. Floods that carved the Ares, Tiu, and Simud Valles likely ended before -mu 2.7 Ga, while those relating to theVedra, Manmee, and Maja Valles ceased between -mu 2.2 Ga and -mu 1.9 Ga. The fluvial resurfacing activities in the Kasei Valles likely did not last past -mu 2.8 Ga. The dating results and excavation depths of LECs (diam. < 6 km) suggest that the minimal roof depth of the volatile-rich layer has remained relatively stable at least around 560- 590 m since the early Amazonian, and regional differences indicate that it could be as shallow as-440 m at high latitudes and-540 m at low latitudes.