IN-SITU EVIDENCE FOR ALTERATION BY ACID FOG ON HUSBAND HILL, GUSEV CRATER, MARS

In 2004, Tosca et al. (“Acid-sulfate weathering of synthetic Martian basalt: The acid fog model revisited”, doi:10.1029/2003JE002218) reported the results of laboratory experiments reacting sulfuric and hydrochloric acids with synthesized martian basalts; the compositions were based on rock and soil targets analyzed by the Mars Pathfinder Alpha Proton X-ray Spectrometer. Among the resulting alteration phases were amorphous silica, Ca and Fe sulfates, and Fe oxides. The authors suggested that interactions between basaltic rocks and acid fog on the martian surface would result in phases with poor crystallinity.

The Mars Exploration Rover Spirit observed evidence of this process in the Columbia Hills of Gusev Crater. Spirit examined a set of geochemically similar but mineralogically distinct outcrops on Cumberland Ridge and the Husband Hill summit. The iron oxidation state (Fe³⁺/Fe_total) of the “Watchtower Class” exposures on Cumberland Ridge range from 0.43 to 0.94, across a distance of only ~30m. The outcrops’ microtextures exhibit knobby protuberances, whose size correlates directly with the iron oxidation state and abundance of nanophase Fe³⁺ oxides as measured by the Mössbauer Spectrometer, and the abundance of amorphous phases modeled through the deconvolution of Mini-TES thermal emission spectra. We propose that the knobby protuberances are agglomerations of preexisting grains, formed from the interaction of acid fog and the host rock. In this alteration scenario, acid fog condensed on the outcrop surfaces, dissolving material at the condensation-surface interface and forming a gel, which desiccated as the adsorbed water evaporated. Differences in condensation residence time, due to amount of insolation and/or winds, resulted in the oxidative, mineralogical, and microtextural variations Spirit observed on these targets.