Detection of Callisto's oxygen atmosphere with the Hubble Space Telescope

摘要

We report the result of a search for evidence of an O-2-dominated atmosphere on Callisto, using the high far-ultraviolet sensitivity of the Hubble Space Telescope Cosmic Origins Spectrograph (COS). Observations of Callisto's leading/Jupiter-facing hemisphere show, for the first time, variable-strength atomic oxygen (O I) emissions with brightness up to 4.7 +/- 0.7 Rayleighs for the O I 1304 A triplet and 1.9 +/- 0.4 Rayleighs for the O I 1356 angstrom doublet, averaged over the 2.5 arcsec. diameter COS aperture. Because the observations were made in Earth's shadow, and are brighter than expected emission from nighttime geocoronal airglow or other plausible sources, we are confident that they originate from Callisto or its immediate vicinity. In addition, COS's limited (similar to 1 arcsec) spatial resolution implies a 2 sigma detection of excess 1356 A emission concentrated on the disk of Callisto itself, with brightness 3.2 +/- 1.6 Rayleighs. The (O I 1356 angstrom)/(O I 1304 angstrom) emission ratio from Callisto's disk favors dissociative excitation of O-2, suggesting that O-2 is the dominant atmospheric component rather than other possible oxygen-bearing alternatives. Photoelectrons, rather than magnetospheric electrons, are the most likely source of the dissociative excitation. This detection yields an O-2 column density of similar to 4 x 10(15) cm(-2) on the leading/Jupiter facing hemisphere, which implies that Callisto's atmosphere is collisional and is the fourth-densest satellite atmosphere in the Solar System, in addition to being the second-densest O-2-rich collisional atmosphere in the Solar System, after Earth. Longitudinal variations in published densities of ionospheric electrons suggest that O-2 densities in Callisto's trailing hemisphere, which we did not observe, may be an order of magnitude greater. The aperture-filling emissions imply that there is also an extended corona of predominantly O I 1304 angstrom emission around Callisto, with observed strength of 1-4 Rayleighs, likely due to solar resonance scattering from sputtered atomic O, with a density of up to 10(4) cm(-3) at the exobase. (C) 2015 Elsevier Inc. All rights reserved.