Habitable conditions in the outer solar system : the space missions that changed our perception of what exists out there
Authors: Coustenis Athena 
Published: February 2023

Some of the satellites of the gas giants Jupiter and Saturn, at orbits beyond the snow-line and the traditional &#8220;habitable zone&#8221;, have been revealed by missions like Galileo, Cassini-Huygens and Juno as unique and extremely interesting bodies with a strong astrobiological potential [1]. Jupiter&#8217;s Europa and Ganymede show indications of harboring liquid water oceans under their icy crusts, which, in the case of Europa, may be in direct contact with a silicate mantle floor and kept warm through time by tidally generated heat. Ganymede, the largest satellite in our Solar System, is unique in possessing an induced magnetic field and probably harbours an undersurface liquid water ocean contained between two ice layers. Thanks to the fabulous international cooperation that came behind the Cassini-Huygens mission, Saturn&#8217;s system was revealed and Titan [2] and Enceladus [3], were found to possess organic chemistry, unique geological features and internal liquid water oceans. I will describe my personal experience of the Cassini-Huygens mission.The icy satellites provide a conceptual basis within which new theories for understanding habitability can be constructed. In view of the many questions remaining unanswered [4], these bodies will be further investigated in the future by new missions to the giant planets systems. Future space exploration towards the Galilean satellites will be performed by missions such as ESA&#8217;s JUpiter Icy moons Explorer (JUICE, [5]) (whose main target is Ganymede and will be launched in April 2023) and NASA&#8217;s Europa Clipper mission to launch in 2024. For a return to Titan, NASA has recently selected the Dragonfly mission [6], while other concepts are being studied for these and other icy moons, also around the ice giants.Future in situ measurements will be extremely useful in unveiling these worlds. In the meantime, Juno data and ground-based observations can help complement the space discoveries.I will discuss what we currently know and what we expect to learn about habitable conditions in the outer solar system and how our perception of these worlds has changed, along with the need to better protect their environments [7].References: 1. Coustenis, A., Encrenaz, Th., 2013. Life beyond Earth: the search for habitable worlds in the Universe. CUP. ISBN: 9781107026179.
2. Coustenis, A., 2021. The Atmosphere of Titan. In Read, P. (Ed.), Oxford&#160;Research Encyclopedia&#160;of Planetary Science.&#160; DOI: 10.1093/acrefore/9780190647926.013.120
3. Lunine, J., Coustenis, A., Mitri, G., et al., 2018. &#8220;Future exploration of Enceladus and other Saturnian moons&#8221;. In &#8220;Enceladus and the Icy Moons of Saturn&#8221;. LPI/UA/Space Science Series, P. Schenk, R. Clark, C.J.A. Howett, A. Verbiscer, J.H. Waite Eds., ISBN 9780816537075.
4. Nixon, C. A., et al., 2018. PSS, 155, 50-72.
5. Coustenis, A., Witasse, O., Erd, C., 2021. The JUICE mission: expectations and challenges. Fall issue of The Bridge on space exploration, Sept. 2021, Vol. 51, issue #3, pp. 41-50. https://www.nae.edu/260902/The-JUICE-Mission-Challenges-and-Expectations
6. Barnes, J. et al., The Plan. Sci. J., 4, 18.
7. Fisk, L., Worms, J.-C., Coustenis, A., et al., 2021. Introductory Note to the June 2021 and Update of the COSPAR Policy on Planetary Protection. Space Research Today 211, Aug. 2021, 9-25, https://doi.org/10.1016/j.srt.2021.07.009 and Policy : https://doi.org/10.1016/j.srt.2021.07.010
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