Mixtures of clays and sulfates within deposits in western Melas Chasma, Mars

Draping across some wallrock slopes is a light-toned deposit that sometimes overlies a blocky, layered, and spectrally bland medium-toned deposit. The light-toned deposit exhibits spectra consistent with mixtures of materials that may include jarosite, acid-leached clays, silica, poorly crystalline clays, and polyhydrated sulfates. Hesperian-age valleys dissect the medium-toned deposit along some wallrock slopes and may be partially filled in by light-toned deposits. The draping nature of these deposits can best be explained by airfall, either volcanic ash or atmospheric dust, with ice/snow accumulation perhaps controlling sedimentation along the wallrock. An examination of other regional hydrated deposits around southwestern Melas indicates similar draping deposits occur farther east and west along the wallrock, whereas layered sulfates to the southeast consist of monohydrated and polyhydrated sulfates that are similar to interior layered deposits found in many of the chasmata. Small opal and jarosite exposures within the Melas basin and along the nearby plateau are most likely the result of younger localized alteration and deposition. The diversity of hydrated minerals and fluvial features indicates multiple episodes of aqueous activity under distinct environmental conditions occurred throughout southwestern Melas Chasma during the Hesperian to Amazonian that may be contemporaneous with late aqueous activity elsewhere on Mars.