The CRISM image shows the wall of Eos Chasma, part of the Valles Marineris canyon system. The steep wall exposes a layer of iron-magnesium (red) phyllosilicates just beneath a layer of aluminum phyllosilicates (cyan) that are exposed both in the wall and on the Noachian-aged basaltic surface. Phyllosilicates are clay and clay-like minerals formed during interaction between rock and liquid water. On Mars, iron-magnesium phyllosilicate is formed by reaction of igneous basaltic rocks with water, and is common in the southern highlands. The formation of aluminum phyllosilicates, however, implies a more sustained interaction and/or the involvement of a heat source. Pedogenesis, the weathering of volcanic materials into soils by rainfall or melted snow, would be expected to form a layered sequence of aluminum phyllosilicates over iron-magnesium phyllosilicates. The exposure in the Eos wall of aluminum phyllosilicates over iron-magnesium phyllosilicates therefore supports the hypothesis that there was once long-term overland (not subsurface) water in this region.
The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) is one of six science instruments on NASA's Mars Reconnaissance Orbiter. Led by The Johns Hopkins University Applied Physics Laboratory, the CRISM team includes expertise from universities, government agencies and small businesses in the United States and abroad.
CRISM's mission: Find the spectral fingerprints of aqueous and hydrothermal deposits and map the geology, composition and stratigraphy of surface features. The instrument also tracks seasonal variations in dust and ice aerosols in the Martian atmosphere, and water content in surface materials — leading to new understanding of the climate.