JUVENTAE CHASMA/MAJA VALLES, MARS: IMPLICATIONS OF MOC CRATERING DENSITIES AND GEOMORPHOLOGY

MOC data indicate a marked difference in cratering densities between the Juventae Chasma, the channel areas of Maja Valles, and the debouchment area in Chryse basin, possibly due to Amazonian-age melting of ground ice and (volcanism-induced?) floods. Cumulative densities for craters with diameters >50 m/106km2 indicate the chasma floor/chaotic areas (167K ± 48K) have a paucity of impact craters with respect to other areas. The transition area from chaos to channel (700K ± 74K) and channels directly north of the chasma (543K ± 243K) are also sparsely cratered. However, from about 300 km north of the chasma, the Maja channel area is more heavily cratered (4,524K ± 383K), as is the Chryse basin floor (6,224K ± 1,118K). The highest counts on the floor of the Chryse basin might be due to a combination of wide, thin, sediment dispersal and deposition upstream in Maja Valles. The heavily-cratered north channel area could be accounted for if (1) few young floods reached these extents, (2) the floods in these wide channel areas lacked the power to remove craters, (3) craters were protected from erosion by ice infills, or (4) target rocks were less friable. The young age at the channel head may be due to recent flood activity and/or eolian mantling (however channel features are observable through the thin dune cover). The very young age of the Juventae chasma and chaos area could be explained by a combination of friable target rocks, some material (subsequently removed) that protected the area from impacts, recent eolian activity, disruption by melting of late-stage ground ice, and/or young volcanic activity. If the interior mounds are sub-ice volcanoes, at least 2 periods of flooding may be supported by 2 stages of mound formation; i.e. the largest mound postdates chaos, but a very small interior mound, shown on M0804669, is partly buried by, therefore older than, chaos. This image may show evidence of late-stage ground ice (possible thermokarst pits on an impact crater and brittle fracture of channel-confined, dune-covered material).