Chapter 11 - Alteration Processes in Gusev Crater, Mars: Volatile/Mobile Element Contents of Rocks and Soils Determined by the Spirit Rover

Abstract

The Alpha Particle X-ray Spectrometer instrument on the Mars exploration rover Spirit determined the compositions of 222 rock and soil targets in Gusev crater. The analyses show wide ranges of volatile/mobile element (S, Cl, Zn, and Br) contents that provide evidence constraining a variety of aqueous alteration processes. Rock coatings and soil indurations have correlations of SO3 and Zn with Cl and represent a recent alteration process that might be occurring in the present Martian climate. This process occurred at very low water/rock ratios and low temperatures. Pervasive alteration affected the older rocks and soils on the Columbia Hills and was mostly acid-sulfate alteration by fumarolic/hydrothermal activity, likely associated with volcanism. Water/rock ratios were quite variable, and temperatures were likely moderate—a few 10s to a few 100s of degrees C—leading to substantial differences in volatile/mobile element contents and alteration mineralogy. In the extreme case, either leaching by alteration fluids at high water/rock ratio or precipitation from solutions resulted in silica-rich rocks and soils (up to 91wt% SiO2). One outcrop contains abundant Mg-Mn-Fe-carbonates and might have been altered by near-neutral hydrothermal solutions; an alternative scenario—alteration at moderate- to low-temperatures in an ephemeral lake—cannot be excluded. In spite of prelanding predictions that Gusev crater had hosted a large body of standing water in the Late Noachian/Early Hesperian time frame, evidence garnered by Spirit shows that alteration was most commonly highly localized and done under conditions of low water/rock ratios, and must have occurred after draining of the Gusev crater lake.