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R. W. Carlson, M. S. Anderson, W. D. Smythe (Jet Propulsion Laboratory, California Insitute of Technology), R. E. Johnson (University of Virginia), L. A. Soderblom (US Geological Survey, Flagstaff), T. B. McCord, G. B. Hansen (University of Hawaii), B. Dalton, R. N. Clark (US Geological Survey, Denver), A. C. Ocampo, D. L. Matson (Jet Propulsion Laboratory, California Institute of Technology)
Galileo Near Infrared Mapping Spectrometer (NIMS) measurements show an absorption feature at 3.5 microns for Europa's icy, leading-side surface (central longitude ~ 120 degrees). In the initial report of this feature (Smythe, et al., LPSC, 1998), the identification of the species producing this feature was not established, but ammonia compounds and hydrocarbons were possible candidates. Recent laboratory measurements, and comparison to prior experimental work, suggest that this absorption band is caused by the \nu2 + \nu6 absorption band of condensed hydrogen peroxide, H2O2. The strength of the band in Europa's spectrum, compared to laboratory spectra of mixtures of hydrogen peroxide in water ice (at ~ 80 K), indicate relative concentrations of ~ 0.3%. This concentration applies to at least the upper surface layer, of order 30 microns thick, which is optically sensed. Hydrogen peroxide was predicted to occur on the surfaces of the icy Galilean satellites due to ultraviolet photolysis and radiolysis by magnetospheric particles (Johnson and Quickenden, J. Geophys. Res, 102,10985-10996, 1997), and the concentrations are consistent with radiolysis estimates.