Free Astronomy Magazine January-February 2022

39 JANUARY-FEBRUARY 2022 on Europa’s trailing hemisphere there is no indication of H 2 O on the leading hemisphere of Europa. Space scientists working to under- stand these icy moons will soon be able to benefit from a close-up view. ESA’s JUpiter ICy moons Explorer (JUICE) mission is being prepared for a tour of Ganymede, Callisto and Eu- ropa, Jupiter’s three largest icy moons. JUICE is the first large-class mission in ESA’s Cosmic Vision 2015– 2025 programme and is expected to launch in 2022 and arrive at Jupiter in 2031. The probe will carry an ad- vanced suite of instruments — the most powerful remote sensing pay- load ever flown to the outer Solar System — and will spend at least three years making detailed obser- vations of the Jovian system. Europa will also be visited by a NASA mis- sion, Europa Clipper, which will per- form a series of flybys of the moon and investigate its habitability, as well as selecting a landing site for a future mission. “This result lays the groundwork for future science based on upcoming missions to the Jovian moons,” con- cluded Roth. “The more we can un- derstand about these icy moons before spacecraft like JUICE and Eu- ropa Clipper arrive, the better use we can make of our limited observ- ing time within the Jovian system.” This discovery and the insights from upcoming missions such as JUICE will improve our understanding of potentially habitable environments in the Solar System. Understanding the formation and evolution of Jupiter and its moons also helps as- tronomers gain insights into Jupiter- like exoplanets around other stars. Combined with observations from space telescopes such as the up- coming NASA/ESA/CSA James Webb Space Telescope, this could help as- tronomers determine if life could emerge in Jupiter-like exoplanetary systems elsewhere in the universe. T he puzzling, fascinating surface of Jupiter’s icy moon Europa. The scene shows the stunning diversity of Europa’s surface geology. Long, linear cracks and ridges crisscross the surface, interrupted by regions of disrupted terrain where the surface ice crust has been broken up and re- frozen into new patterns. This global color view consists of images ac- quired by the Galileo Solid-State Imaging (SSI) experiment on the spacecraft’s first and fourteenth or- bits through the Jupiter system, in 1995 and 1998, respectively. Image scale is 1 mile (1.6 kilometers) per pixel. North on Europa is at right. [NASA/JPL-Caltech/SETI Institute] ing ultraviolet observations of Eu- ropa 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 Spectro- graph (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. Previous observations of water vapour on Europa have been associ- ated with transient plumes erupting through the ice, analogous to gey- sers here on Earth but more than 100 kilometres high. The phenomena seen in these plume studies were apparently transient in- homogeneities or blobs in the at- mosphere. The new results, how- ever, show similar amounts of water vapour to be present spread over a larger area in observations spanning from 1999 to 2015. This suggests the long-term pres- ence of a water vapour atmosphere on Europa’s trailing hemisphere. De- spite the presence of water vapour standing of the atmospheres of icy moons,” commented Lorenz Roth of the KTH Royal Institute of Technol- ogy in Stockholm, Sweden, the au- thor of this study. “The detection of a stable H 2 O abundance on Europa is surprising because the surface temperatures are so low.” To make this discovery, Roth delved into archival Hubble datasets, select- !