AQUEOUS CORROSION TEXTURES OF SILICATE AND OTHER MINERALS ON MARS

Silicate minerals and their alteration products have been identified in Mars meteorites using laboratory petrographic and petrologic methods, and on Mars' surface using spectroscopy from orbital and lander/rover missions. Microscopy of Mars meteorites provides textural evidence establishing reaction relationships between reactant and product minerals. Recent missions to the surface of Mars have included various microscopic cameras among the spacecraft instrument packages, acquiring similar data from Mars' surface. Each Mars Exploration Rover (MER) is equipped with a Microscopic Imager (MI) among the instruments deployed at the ends of the rovers' robotic arms. The Mars Phoenix Lander (MPL) Microscopy, Electrochemistry, and Conductivity Analyzer (MECA) instrument package includes optical and atomic force microscopes. The MPL MECA atomic force microscope (AFM) is the highest magnification microscope flown to date.

Both Mars Rovers imaged moldic porosity in various rock types on Mars, implying removal by dissolution/weathering of euhedral minerals. In sedimentary strata at Meridiani Planum, MER Opportunity imaged cavities with parallelogram cross-sections several millimeters long, formed by removal (dissolution?) of earlier-formed soluble evaporite monoclinic or triclinic minerals. In weathered rinds on basaltic boulders at Gusev Crater, MER Spirit imaged submillimeter hexagonal cavities indicative of olivine removal during weathering of basalt. The MPL AFM recently imaged sawtooth features several microns in length, that strongly resemble denticles known to form by low-temperature aqueous corrosion of naturally weathered chain-silicate minerals (including pyroxenes) on Earth. Nearly identical corrosion features have been observed on pyroxenes, and similar corrosion textures have recently been recognized on olivines, in Mars meteorites. Mineral dissolution/corrosion textures have thus been identified using laboratory microscopy of Mars meteorites and in situ robotic microscopy from recent landed Mars missions.

Mars sample return will eventually allow the full range of microscopic techniques available in terrestrial laboratories to establish mineral reaction relationships in samples from known specific localities and geologic/geomorphic contexts on Mars.