DETECTING EXTRASOLAR EARTHS ON THE GEOSCIENCE HORIZON

The technology to deploy a space observatory capable of directly detecting Earth-sized planets around other stars is being developed, and the NASA Terrestrial Planet Finder program is considering telescope designs and observing strategies. While the objective of these efforts is to detect habitable Earth-like planets around others stars, it seems likely that observations will uncover a diverse menagerie of planets: Earth's composition and habitability are the result of cosmochemical conditions that need not hold elsewhere. Compositional variations in protostellar nebulae and differences in planetary formation and evolution may have produced a diversity of silicate-metal planets that exceeds that of the Solar System (Gaidos 2000 Icarus 145, 637). Differences in composition and mineralogy will manifest themselves as variations in a planet's total volatile inventory and its mantle's rheology, melting properties, and dynamics. Understanding how differences in initial conditions and environments affect planetary evolution will be fertile scientific ground for future "geo"scientists. Here I use a simple model to examine these effects on a planet's water budget and water cycle. Earth's geologic water cycle is coupled to the mantle through partition of water into magmas, release at divergent margins and hotspots, and the subduction of hydrated oceanic lithosphere at convergent margins. The rate of the reduction of water to hydrogen in mantle material and its escape to space depends on the mantle oxidation state. Variation in these fluxes creates the possibility of dessication of the surface or dehydration of the mantle, with profound implications for the geology and habitability of the planet. Results of modeling of these processes that illustrate possible outcomes are presented. I also discuss how these planets may appear from our remote observing location.