Shallow Radar (SHARAD), pedestal craters, and the lost Martian layers: Initial assessments

摘要

Since their discovery, Martian pedestal craters have been interpreted as remnants of layers that were once regionally extensive but have since been mostly removed. Pedestals span from subkilometer to hundreds of kilometers, but their thickness is less than —500 m. Except for a small equatorial concentration in the Medusae Fossae Formation, the nearly exclusive occurrence of pedestal craters in the middle and high latitudes of Mars has led to the suspicion that the lost units bore a significant fraction of volatiles, such as water ice. Recent morphological characterizations of pedestal deposits have further supported this view. Here we employ radar soundings obtained by the Shallow Radar (SHARAD) to investigate the volumes of a subset of the pedestal population, in concert with high-resolution imagery to assist our interpretations. From the analysis of 97 pedestal craters we find that large pedestals (diameter >30 km) are relatively transparent to radar in their majority, with SHARAD being able to detect the base of the pedestal deposits, and possess an average dielectric permittivity of 4 ± 0.5. In one of the cases of large pedestals in Malea Planum, layering is detected both in SHARAD data and in high-resolution imagery of the pedestal margins. We find that clutter is a major issue in the analysis of radar soundings for smaller pedestals, and tentative detection of the basal reflection occurs in only a few of the cases examined. These detections yield a higher average permittivity of —6. The permittivity value derived for the larger pedestals, for which a basal reflection is unambiguous, is higher than that of pure water ice but lower than that of most silicate materials. A mixture of ice and silicates or an ice-free porous silicate matrix can explain a permittivity of —4, and radar alone cannot resolve this nonuniqueness. Data from the Compact Reconnaissance Imaging Spectrometer (CRISM) tentatively confirms a mafic component in at least one pedestal in Malea Planum. Interpretation of SHARAD results can support either a mixture of ice and silicates or a porous silicate. The former is compatible with a model where nonpolar ice is periodically deposited in the midlatitudes as a result of obliquity variations. The latter is compatible with ash deposits, at least in where pedestals appear in volcanic centers such as Malea Planum.