Presentation #215.01 in the session “Icy Satellites: Surface and Below”.
The dusk-over-dawn asymmetry observed and simulated in Europa’s radiolytically produced O2 atmosphere (1, 2), as imprinted onto its oxygen aurorae (3), led to the suggestion that thermal outgassing is the primary source of the O2 atmosphere (1,4). This was surprising, since for decades the direct source of atmospheric O2 was thought to be magnetospherically sputtered O2. Here, we note a similarity to the enigmatic O2 identified at comets. That is, comets 67 P C/G and Halley, if assembled from material originating in the interstellar medium, should have an O2 to H2O fraction of ∼10-5, whereas spectroscopic observations of the coma indicate that the ratio is nearly ∼3×10-2. Surprisingly, this is within a factor of two of the amount of trapped O2 indicated by the dimer absorption band seen in the icy satellite surfaces (5,6). That band suggests the radiolytically produced O2 can be stably trapped in bubbles or cages in the irradiated ice matrix (7). This comparison would appear to indicate that the origin of the exorbitant O2 trapped in the cometary ice prior to being driven into the coma is consistent with the cometary material being embedded in a highly irradiated, energetic plasma environment (8). Here we discuss the role of the formation, trapping and outgassing of O2 at both the icy satellites and comets based on observations and recent experiments on co-deposition and subsequent thermal desorption of O2 and H2O (e.g., 9-11).
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