Spectral variation and alteration across the Cumberland Ridge outcrops, Columbia Hills, Mars

During sols 407-420 and 461-514, the Mars Exploration Rover Spirit investigated three neighboring outcrops along Cumberland Ridge in the Columbia Hills. The rocks comprising these outcrops have strikingly similar elemental abundances according to the rover's APXS instrument. Moessbauer measurements of their iron mineralogy, however, show great variety, with the ratio of ferric to total iron ranging from 0.43 (Keystone target on the Methuselah outcrop, whose iron mineralogy is composed of nearly 50% pyroxene, with 10% magnetite, 15% hematite, and nearly 20% nanophase oxide) to 0.94 (Paros target on the Larry's Lookout outcrop, with over 65% of the iron present as nanophase oxide, nearly 20% as hematite, and over 10% as goethite). The variation of iron mineralogy across outcrops separated by a few tens of meters points to highly localized alteration; the presence of goethite in particular indicates aqueous process(es) were involved. The isochemical nature of the alteration is consistent with a low water-to-rock ratio. Previous work has focused on comparisons of the Cumberland Ridge rocks with other Gusev rocks. In contrast, here we examine the variation across each outcrop and among the three outcrops, taking advantage of the full Athena remote sensing capability by examining both thermal infrared and visible/near-infrared spectra. For the former, we make use of the many Mini-TES spectra taken across each outcrop; for the latter, we use Pancam multispectral data from comparatively dust-free regions on the Larry's Lookout, Jibsheet, and Methuselah outcrops that were imaged with all 13 of Pancam's geology filters. Outcrops are relatively rare in the Columbia Hills; Cumberland Ridge provides an opportunity to examine three outcrops that appear to be genetically and stratigraphically related. By looking at the progression of alteration across continuous rock faces, we hope to provide constraints on the style of aqueous alteration in this region, and contribute to the known history of the Columbia Hills.