Two Martian Winters at Gusev Crater and Meridiani Planum: New Results From the MER Mossbauer Spectrometers

After sol 511 of its mission in Gusev Crater, Spirit traversed from the top of Husband Hill to its current Winter Haven on Low Ridge. M ssbauer analyses of several rock and soil targets along the traverse yielded further evidence for the wide-spread occurrence of aqueous processes in the Columbia Hills. The rock Independence was found on the flank of Husband Hill. It has low total Fe with about 24-30 % of its iron in ilmenite. This assemblage implies alteration under aqueous conditions; some phases were altered and elements such as Fe were leached out, while less soluble Fe-bearing phases such as ilmenite remain. The soil target Dead_Sea_Samra was found in subsurface soil revealed when the wheels dug into soil during the traverse from Husband Hill to Home Plate. Its M ssbauer spectrum shows a high abundance of ferric sulfate, similar to the Paso Robles soil targets found on Husband Hill. At its current location at Winter Haven Spirit investigated the target Halley which appears to be part of a wider-spread indurated layer underlying basaltic soil. This target shows the highest abundance of hematite in all Gusev soil and rock targets investigated to date. Opportunity at Meridiani Planum traversed from the 300 m diameter buried Erebus Crater towards 800 m Victoria Crater. The main components of Meridiani Planum jarosite-bearing outcrop rocks, basaltic soil, and a hematite lag remain remarkably constant in M ssbauer spectra throughout the traverse. Cobbles (rock fragments greater than 1 cm) show variability however. A meteorite (Barberton) has been identified based on kamacite peaks in the M ssbauer spectrum. Other cobbles show Mossbauer spectra similar to jarosite-bearing outcrops, or to basaltic rock, or mixtures thereof, suggesting an origin as impact breccias. Some cobbles were investigated at the edge of the annulus of Victoria Crater from which they may have been excavated. Mossbauer spectra reveal a basaltic signature, dominated by olivine and pyroxene. In general for both rovers the radioactive Mossbauer source became naturally weaker, but both instruments are still able to perform good quality measurements.