EVAPORITE MINERALS, WATER, AND ORGANIC COMPOUNDS IN ASTEROID AND COMET SAMPLES

Water was widespread in the early solar system. Structural or trapped water remains a minor or trace phase in many asteroids, while water ice is a major constituent of comets. Meteorites and interplanetary dust particles (IDPs) are samples of asteroids and comets that have reached the Earth. A class of primitive meteorites - the carbonaceous chondrites - shows evidence for extensive water:rock interactions. In the CI-type and in many of the CM-type chondrites (two of several subgroups divided according to petrologic and compositional characteristics), the primary mineralogy has been nearly completely replaced with an assemblage of clay minerals, oxides, carbonates, and sulfates. Evaporite minerals observed in CI chondrites include calcite, dolomite-ankerite, magnesite-siderite, gypsum, epsomite, and blödite. The CM chondrite assemblage is similar except that they lack the highly soluble Mg-rich sulfates. The alteration occurred very quickly, within 50-100 Ma of the formation time (~4.5 Ga ago) of the parent bodies. The secondary minerals formed from brine-like fluids at temperatures below ~100-150C and at moderate water:rock ratios. Some ordinary chondrites (another major class of stony meteorites) also contain traces of evaporite minerals (halite and sylvite). Fluid inclusions in these salts also indicate precipitation from concentrated brines. I-Xe isotope systematics indicate that the ordinary chondrite salts formed within 2 Ma of the formation time of the parent asteroid (~4.6 Ga). Cometary IDPs show little to no evidence for aqueous interactions, while those from asteroidal sources show comparable levels of alteration to the CI chondrites. Siderite-magnesite solid solutions are a common, minor component of these asteroidal IDPs. Primitive carbonaceous chondrites contain up to 5% carbon, which was also affected by the early aqueous interactions. Over 400 pre-biotic organic compounds have been identified in CM chondrites. IDPs are even more carbon-rich and contain as much as ~50 wt. % carbon, much in the form of complex organic chemicals that survive entry through the Earth's atmosphere intact. Asteroids and comets have provided significant portions of the total water inventories and pre-biotic organic matter to the terrestrial planets, including Earth.