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R. W. Carlson (Jet Propulsion Laboratory, California Institute of Technology)
Europa's surface is bombarded by high fluxes of ionizing radiation (primarily high-energy electrons and protons) that destroy existing molecules and produce new species. The depth of direct radiolysis is ~ 1 mm, but the upper surface is being continuously overturned by micrometeroid impact ``gardening," burying exposed material and bringing material from depth to the surface. Continual exposure and redistribution produces a thick layer of radiolyzed matter whose composition is different than that of the unexposed, pristine material (Carlson et al., Science, 283, 2062, 1999; 286, 97, 1999). Biomarker molecules that could indicate biotic processes on Europa would be degraded and become less diagnostic. The cumulative energy deposition (fluence) and its variation with depth indicates the extent of radiolytic decomposition and the sampling depth that is necessary to obtain unmodified samples.
The fluence profile was computed using fluxes compiled by Cooper et al. (Icarus 149, 133, 2001), with the electron deposition concentrated on the trailing hemisphere (Paranicas et al., Geophys. Res. Lett. 28, 672, 2001)) and uniform proton irradiation. Two gardening models were used (Cooper et al. and Phillips and Chyba, LPSC 2001) and both synchronous and asynchronous rotations were considered. A surface age of 10 My was assumed, with the crust composed of fresh, unexposed material 10 My ago. The time development of the fluence in the optical layer was also computed.
The depth of significant fluence is ~ 1 meter, and the exposure is sufficient to destroy even the most radiation resistant molecules many times over (Carlson et al., Icarus 157, 456, 2002). The molecules within the upper meter will be in radiolytic equilibrium and different from the original parent mixture. The fluence levels are about 107 Mrad; a level of ~ 10 Mrad is fatal to Deinococcus radiodurans. Remote-sensing observations will sense radiolytically altered material except where very recent impacts have exposed material at meter depths, i.e. fresh craters with diameters > about 10 m. The time required to attain a fluence of 1000 eV per 16-amu molecule in a mm thick optical layer is about 1400 years on Europa's trailing side. An in-situ astrobiological mission should obtain samples from depths > 1 m.
This work was supported by NASA's Planetaty Geology and Geophysics Program.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.