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Keck II Observations of Hemispherical Differences in H2O2 on Europa

We present results from Keck II observations of Europa over four consecutive nights using the near-infrared spectrograph (NIRSPEC). Spectra were collected in the 3.14--4.0 micron range, allowing detection and monitoring of the 3.5 micron feature due to hydrogen peroxide. Galileo Near-Infrared Spectrometer (NIMS) results first revealed hydrogen peroxide on Europa in the anti-jovian region of the leading hemisphere at an abundance of 0.13+/-0.07% by number abundance relative to water. We find comparable results for the two nights over which we observed the leading hemisphere. Significantly, we observed a small amount of hydrogen peroxide (~0.04%) during observations of Europa's anti- and sub-Jovian hemispheres. Almost no hydrogen peroxide was detected during observations of just the trailing hemisphere. We conclude that the Galileo observations likely represent the maximum hydrogen peroxide concentration, the exception potentially being the cold water ice regions of the poles, which are not readily observable from the ground. Our mapping of the peroxide abundance across Europa requires revisions to previous estimates for Europa's global surface abundance of oxidants and leads to a reduction in the total oxidant delivery expected for the sub-surface ocean, if exchange of surface material with the ocean occurs.