An Unusual Spectral Unit in West Candor Chasma: Evidence for Aqueous or Hydrothermal Alteration in the Martian Canyons

Abstract

High spatial-resolution observations made by orbiting spacecraft from the Viking and Phobos missions suggest the local development of crystalline ferric oxides in a small area in Mars' Valles Marineris canyon system. Quantitative color analysis of Viking Orbiter multispectral images identifies two spatially coherent regions of unique hue among the interior layered deposits in western Candor Chasma. The anomalous color indicates a local compositional difference between these regions and the surrounding high-albedo materials. The unusual appearance of the unit in the Viking color observations could be explained by a small increase in the degree of crystallinity or abundance of hematite. Near-infrared spectra obtained by the ISM imaging spectrometer aboard Phobos 2 confirm a local enrichment in ferric oxides or oxyhydroxides in the region and suggest the presence of another phase in addition to hematite.In high resolution, the spectral unit can be seen to occur within two 20-km-long depressions on the margins of an Hesperian-aged layered deposit which forms a plateau on the chasma floor. Water is implicated in the formation of the iron oxides, since the same sedimentary layers seen in the depressions are exposed on steep surfaces elsewhere on the plateau but show no evidence of unusual coloration. This suggests that the mineralization occurred through secondary alteration of preexisting rocks and developed locally in association with the depressions, which could have ponded surface runoff or groundwater seepage.