Advances in Planetary Far-UV Imaging Spectroscopy: Applying lessons learned at the Moon to Europa, Ganymede, and other bodies

Far-ultraviolet reflectance measurements of the Moon, icy satellites, comets, and asteroids have proven useful for advancing our understanding of planetary surfaces, expanding upon the primary historical use of UV instruments to investigate planetary atmospheres and auroral processes. This new appreciation for planetary far-UV imaging spectroscopy is provided in large part thanks to over a decade of investigations with the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) and numerous advances in our understanding of Europa and other icy satellite surface features as a result of robust Hubble Space Telescope (HST) far-UV observing campaigns, Cassini UVIS surveys at Saturn, Rosetta-Alice comet observations, and HST asteroid observations. UV surface reflectance spectral mapping is capable of 1) constraining the composition of water ice and other surface volatiles, 2) assessing the diurnal variation of the hydration state of atmosphere-less rocky bodies, 3) determining relative space weathering effects via UV spectral slope, 4) characterizing the porosity of fairy-castle type surface regolith, 5) constraining the composition of anorthite and other mineral species, and 6) investigating nightside or poorly lit surfaces using starlight, interplanetary medium, and other illumination sources. These capabilities provide planetary scientists with newly appreciated tools for studying relative surface ages and various surface evolution processes, complementing infrared spectroscopy and visible imaging tools. Future investigations of the icy Galilean satellites with the ultraviolet spectrographs (UVSs) being developed for both Europa Clipper and the Jupiter Icy Moons Explorer (JUICE) missions will capitalize on these advances. We will discuss the need for enabling laboratory studies and opportunities for far-UV imaging spectrographs on numerous future science missions.