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Observations by the NASA/ESA Hubble Space Telescope recently revealed water vapour in the atmosphere of Ganymede, one of Jupiter’s moons. A new analysis of archival images and spectra has now revealed that water vapour is also present in the atmosphere of Jupiter’s icy moon Europa. The analysis found that a water vapour atmosphere is present only on one hemisphere of the moon. This result advances our understanding of the atmospheric structure of icy moons, and helps lay the groundwork for upcoming science missions which will explore Jupiter’s icy moons.
Europa — one of Jupiter’s 79 moons — is both the sixth closest moon to Jupiter and the sixth largest moon in the Solar System. It is an icy orb larger than the dwarf planet Pluto with a smooth, icy surface scarred by cracks and fissures. The surface of the moon is a bleak environment with an average temperature of −170 °C and only a tenuous atmosphere. However, astronomers suspect that Europa harbours a vast ocean underneath its icy surface, which some scientists speculate could host extraterrestrial life [1]. Now, for the first time, an astronomer has discovered evidence for persistent water vapour in the atmosphere of Europa.
Using a technique that recently resulted in the discovery of water vapour in the atmosphere of Jupiter’s moon Ganymede, an astronomer has found evidence of water in Europa’s trailing hemisphere — the portion of the moon that is always opposite to its direction of motion [2]. The asymmetric distribution of water vapour was predicted by previous studies based on computer simulations, but had not previously been detected observationally.
“The observation of water vapour on Ganymede and on the trailing side of Europa advances our understanding of the atmospheres of icy moons,” commented Lorenz Roth of the KTH Royal Institute of Technology in Stockholm, Sweden, the author of this study. “The detection of a stable H2O abundance on Europa is surprising because the surface temperatures are so low.”
To make this discovery, Roth delved into archival Hubble datasets, selecting ultraviolet observations of Europa from 1999, 2012, 2014 and 2015 while the moon was at various orbital positions. These observations were all taken with one of Hubble’s most versatile instruments — the Space Telescope Imaging Spectrograph (STIS). These ultraviolet STIS observations allowed Roth to determine the abundance of oxygen — one of the constituents of water — in Europa’s atmosphere, and by interpreting the strength of emission at different wavelengths he was able to infer the presence of water vapour.
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