Oxygen Trapped in Europa's Icy Surface May Offer Clues to Moon's Habitability.

NEW ROCHELLE, N.Y. -- The production of chemically reactive oxidants on the surface of Mars and icy moons such as Europa may provide clues to their habitability and offers new avenues for future space exploration and the analysis of surface soil and ice shells, according to three Special Papers published in the most recent (Volume 6, Number 3) issue of Astrobiology, a peer-reviewed journal published by Mary Ann Liebert, Inc.

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In a Special Paper entitled, "Clathrate Hydrates of Oxidants in the Ice Shell of Europa," Kevin Hand, Christopher Chyba, Robert Carlson, and John Cooper present the following hypothesis: that the formation of mixed clathrate compounds could make up 12% to 53% of the moon's ice shell and stably trap O2 and other gaseous oxidants within the ice. The clathrate compounds, which consist of a lattice, or cage, made of H2O molecules, could trap a variety of gas molecules including O2, CO2, and SO2 produced by radiation-induced chemistry at Europa's surface. Mixed gas clathrates are more stable, yet denser, at Europa's outer surface than pure O2 clathrates and could more easily sink through the thick ice crust to the subsurface ocean.

In regions of pure H2O ice, oxygen produced by radiation escapes from the relatively unstable pure O2 clathrate to form Europa's extremely thin oxygen atmosphere. Earlier Hubble Space Telescope measurements indicate that the atmosphere appears denser over the pure water-ice regions than over sulfate-rich regions where the mixed gas clathrates form. This finding may be further confirmed next March during the New Horizons spacecraft flyby through the Jovian system en route to Pluto.

The authors, from Stanford University, the SETI Institute, Princeton University, NASA Jet Propulsion Laboratory and NASA Goddard Space Flight Center, also describe the usefulness of Raman spectroscopy for studying clathrates and detecting life forms. They propose a role for this instrument on future spacecraft landers for studying the characteristics of the moon's icy surface and the habitability of Europa.

"The Hand et al. hypothesis is intriguing for a couple of reasons," says journal Editor-in-Chief, Sherry L. Cady, Ph.D., Associate Professor in the Department of Geology at Portland State University. "The presence of O2 enclathrating compounds not only explains the paradoxical occurrence of solid O2 on the europan surface, but it provides a plausible mechanism for delivering O2 to the europan ocean, which may be oxidant limited. This work has important implications for habitability assessments of Europa."

In a second Special Paper in this issue of the journal, Gregory Delory, William Farrell, Sushil Atreya, et al. conclude that electrically charged dust could greatly increase the production of the oxidant hydrogen peroxide on Mars, and this could explain the inability of the Viking spacecraft to detect organic materials and signs of possible life.

The authors propose that dust devils and dust storms on Mars generate electrostatic fields--similar to the electric fields produced by thunderstorms on Earth--capable of transforming carbon dioxide and water into the precursors of H2O2. In their paper entitled, "Oxidant Enhancement in Martian Dust Devils and Storms," the authors, from the University of California, Berkeley, NASA Goddard Space Flight Center, the University of Michigan, Ann Arbor, Duke University, the University of Alaska, Fairbanks, the SETI Institute, Southwest Research Institute, the University of Washington, Seattle, and the University of Bristol (U.K.), used a plasma physics model to determine that the strong electric fields produced by dust storms can drive atmospheric chemical reactions that enhance oxidant production.

In a companion paper, Sushil Atreya, Ah-San Wong, Nilton Renno, et al. propose that hydrogen peroxide--formed by chemical reactions induced by electrostatic fields generated by sand and dust in martian dust devils and storms--or another superoxide formed from hydrogen peroxide may be responsible for scavenging organic material from Mars and could accelerate the loss of methane from the martian atmosphere.

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