Abstract
AbstractWe have discovered relatively fresh exposures of a hydrated, amorphous material along the wall rock slopes in Coprates Chasma, Mars. Visible and near‐infrared reflectance spectra extracted from the deposits exhibit broad absorptions around 1.42, 1.94, and 2.25 µm that are most consistent with laboratory spectra of nanophase hydrated Fe‐rich allophane and Fe‐rich opal. The three absorptions, especially the 1.4 µm band, have the strongest hydration signatures yet detected on Mars by orbital data, suggesting high‐water content material that is relatively fresh and has not altered or lost water since its formation or exposure. Age dating from crater size‐frequency distributions of the Fe‐rich allophane/opal deposits yields ages of ~50–100 Myr, consistent with a young exposure time and minimal time for dehydration. Either the Fe‐rich allophane/opal represents an older material already contained within the wall rock that has been more recently exposed, or it represents a younger material formed during more recent aqueous activity.
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