Evidence for locally-derived, ultramafic intracrater materials in Amazonis Planitia, Mars

Abstract

We studied a dark, intracrater feature in Amazonis Planitia using visible, thermal, and spectra1 data from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), MGS Mars Orbiter Camera (MOC), and 2001 Mars Odyssey Thermal Emission Imaging System (ODY THEMIS) instruments. Visible and thermal data indicate that there are heterogeneities within the dark feature at meter to kilometer scales, and suggest that it represents eroded terrain containing inactive ripple-like bedforms (possibly armored or indurated) as well as loose, possibly actively saltating, sediment In addition, part of the dark intracrater material may be covered with dust, either as an optically thin (few microns thick) layer, or as thicker discontinuous patches. The thermal properties of the dark material are consistent with a mixture of sand, rock, and bedrock, whereas the rest of the crater floor has thermal characteristics of fine (~35 pm) particulates. Spectra1 data indicate that the crater floor is covered by a layer of dust that is spectra1ly and compositionally similar to the globally homogenized surface dust in other high albedo regions on Mars. The mineralogy of the dark materials is mafic (dominated modally by pyroxene and olivine); however, unlike other lithologies common in low albedo regions (i.e., surface types I and II), its derived bulk chemistry indicates that it is an ultramafic lithology with the lowest silica content detected on Mars to date. Sheet silicates and glasses are not identified above detection limits, and as such, the materials do not display evidence for significant chemical weathering. The geomorphology and visible distribution of the dark, intracrater materials combined with the lack of an obvious source in the region outside the crater suggest they are the product of erosion of a local source