Metallic Iron in Meteorites as a Sensitive Tracer of Surface-volatile Interactions on Mars

Introduction: Meteorites on Mars (martian finds) are more than curiosities. They may provide a means to gauge the past and recent extent of water exposure across the martian latitudes. The many roles of water in solid, liquid, and vapor phases pose a set of outstanding questions relating to the geologic history and habitability of Mars. Because we are looking for evidence of trace amounts of water in many cases, a sensitive water indicator is desirable. Reduced (metallic) and ferrous iron in meteorites provides such an indicator because it will oxidize readily in the presence of liquid water, water vapor, and possibly even water ice [e.g. 1]. Weathering effects may thus be recorded by oxidation/hydration states and secondary mineralogy on rock surfaces and interiors [2]. Background: Iron-nickel metal is found in some 88 percent of meteorite falls on Earth [3]. The percent fraction is likely to be similar (if not identical) in near-Mars space [4]. The meteoritic flux on Mars could be relatively high as well. Studies indicate that meteoritic nickel contributions to martian soil may be as high as 3% [5]. Because of their high relative reactivities, a martian find might exhibit subtle signs of alteration where a martian basalt would not, depending on the extent of exposure. Martian finds therefore represent the most sensitive indicators of water exposure available on the martian surface. A hypothetical global distribution map showing locations and weathering intensity of meteorites on Mars would help constrain models of ice extent during periods of high martian obliquity, and possibly address more long-term paleoclimatic trends as well. Both Mars Exploration Rover (MER) vehicles are located in near-equatorial latitudes, where water exposure under current obliquity cycles is anticipated to be negligible [6]. A single “rusty” meteorite with measurable oxyhydroxides at either location would therefore be evidence of either 1) more drastic paleoclimatic trends than previously modeled, or 2) widespread surface water at some time in the history of the planet. Such a finding would be significant irrespective of meteorite residence time. A lack of oxyhydroxides in meteorites is also insightful, but questions of residence time become more important in such cases. The difference between irons and stony meteorites becomes significant here because alteration products may still exist within stony meteorite interiors while they may be readily abraded by wind erosion from the surfaces of irons. This work encompasses three phases, including 1) identifying reduced iron-bearing meteorites on Mars; 2) assessing their weathering intensity; and 3) conducting laboratorybased meteorite studies to assess weathering in terrestrial desert environments, and to refine the techniques used in steps 1 and 2. This abstract reports progress on studies 1 and 3, conducted in parallel, which address the detection of meteorites on Mars using the MER science instruments. Meteorite spectral library: Vibrational spectroscopy using the Miniature Thermal Emission Spectrometer (MiniTES) on the rovers is ideal for recognizing meteorites at a distance. Therefore, a comprehensive library of thermal emission spectra for meteorites of all relevant types and in various stages of weathering is being prepared. The library may also be useful for independent assessment of weathering intensity (Table 1).