Spatial distributions and seasonal variations of features related to a venting process at high southern latitudes observed by the MOC camera
A distinctive terrain named cryptic region which is characterized by regions of low albedo and low temperature has been identified on the Martian south polar cap. In this zone, many fan- and spider-shaped features of km-scale appeared following the sublimation of the CO 2 frost layer. These peculiar features were apparently caused by a wind-blown system of dust-laden jets. During the warming period starting at Ls∼180°, the seasonal ice cap regresses and fans and spiders appear in sequence. These surface features are repeatable events that tend to occupy the same areas from year to year. In this study, we use the Mars Orbiter camera (MOC) narrow-angle images to produce a statistical study of the time distributions of the fans and spiders as functions of Ls and as functions of the topography. The time variations and spatial distributions of these features are further correlated with the CO 2 ice coverage measured by the Mars Orbiter laser altimeter (MOLA) instrument. We have documented that most of the fans are found in the early spring with Ls<230° and the fans and spiders coexist at Ls=250°±20°. It is also found that there is a strong dependence on latitude and altitude with fans and spiders most often observed at high latitude (>83°S) and high altitude (>2500 m). Our statistical result also indicates that the occurrence of fans is highly correlated with the thickness of the CO 2 frost thus providing support for the venting model.