A TRAVERSE THROUGH HESPERIAN RIDGED PLAINS ON MARS: EXTENDING THE WORK OF RON GREELEY IN GUSEV CRATER

The Spirit rover landed in Gusev crater to search for evidence of an ancient lake that was suggested by the termination of Ma’adim Vallis at the crater rim and a possible eroded delta at its mouth. The ~2.5 km traverse by Spirit across the floor of Gusev revealed olivine-rich basaltic rocks known as Adirondack-class that are the result of “impact gardening” of lava flows. Greeley et al. [1] presented evidence that the flows were likely low viscosity, comparable to mare lavas on the Moon. The presence of well-developed mare-like ridges (“wrinkle ridges”) was also noted and used to support the hypothesis that massive basalt flows extended across the floor of Gusev crater and perhaps buried sediments delivered by Ma’adim Vallis. Crater counts from large portions of these ridged plains indicate an emplacement age of 3.65 Ga [1], which coincides with the Early Hesperian period on Mars. Here we combine these results with comparisons to other similar surfaces elsewhere to support our hypothesis that the floor of Gusev crater represents an example of the widespread Hesperian ridged plains unit that covers as much as 30% of the Martian surface [2].

If the floor of Gusev crater investigated by Spirit represents Hesperian ridged plains, then information obtained along its traverse may serve as “ground truth” for other examples of this global unit. However, the rocks observed by Spirit comprise only a small fraction (<20%) of the surface, which is dominated by soil (the fine fraction of the regolith). And both of these surface units are variably covered by airfall dust. Nevertheless, the thermal infrared (TIR) spectral characteristics of all three units have been measured with the Miniature Thermal Emission Spectrometer (Mini-TES) and can be used for investigations beyond Gusev crater. We will present initial results comparing Mini-TES spectra to TIR spectra from the Mars Global Surveyor Thermal Emission Spectrometer for other Hesperian ridged plains in order to assess possible compositional similarities.

[1] Greeley, R., et al. (2005), J. Geophys. Res., 110, E05008, 10.1029/2005JE002401.

[2] Head, J. W., et al. (2002), J. Geophys. Res., 107 (E1), 5003, doi: 10.1029/2001JE001831.