NEW OBSERVATIONS OF ENIGMATIC LANDFORMS AND SURFACE TERRAINS ON THE NORTHERN PLAINS OF MARS

New observations and physical measurements of Martian surface features have been enabled by the instrumentation delivered to Mars on the Mars Reconnaissance Orbiter (MRO). In this work we are examining new measurements made by the High Resolution Imaging Science Experiment (HiRISE) and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) along with continuing to utilize existing data sets covering a set of enigmatic landforms and surface cover types on the northern plains of Mars. These landforms and terrains include pitted cones and domes, simple domes and mounds, troughs with medial ridges, flow-like terrains, and irregularly shaped terrains with thermal inertias higher than the background (“blebby terrain”). In previous work, we have described the pitted cones and domes as possible mud volcanoes or spring deposits based, in part, on their low thermal inertia relative to the background plains. Recently acquired HiRISE images support these interpretations showing that the features have mantles that show signs of soft sediment deformation, namely polygonal fracturing. Distinguishing between the mud volcano and spring deposit hypotheses will be aided when CRISM data of these features becomes available since spring deposits would characteristically consist of evaporite minerals. Recent interpretations have also been made of the separate class of features, the domes without summit craters or pits. These domes have recently been interpreted as volcanic intrusions. This interpretation seems at odds with the soft sediment features that are elsewhere present on the northern plains including the flow-like terrains. Also, the characteristic “surface type 2” spectral class identified from Thermal Emission Spectrometer (TES) data has been reinterpreted as the result of secondary silica-rich materials such as coatings. Amorphous silica could well be a part of soft sediment materials such as are postulated for other northern plains features. The evidence for the character of the “surface type 2” material will be considered in the light of the nature of the landforms and terrains examined here and also in the light of surface reflectance measurements made of northern plains targets by the CRISM imaging spectrometer.