A common origin of oxygen at Jupiter’s icy moons and comets: Modeling thermal outgassing at Europa

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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