Fresh exposures of hydrous Fe-bearing amorphous silicates on Mars

Abstract

We 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.