Probing Ice Crystallinity with Far-ultraviolet Spectroscopy: Implications for Condensed Volatiles in Lunar Permanently Shadowed Regions and Outer Solar System Icy Satellites

Water ice exists on the surfaces of many planetary bodies in either amorphous or crystalline phase, depending on their thermal, radiation, and impact history. Despite being ubiquitous in the Solar System, discerning ice properties such as abundance, phase, and porosity from remote-sensing data has been primarily achieved through infrared spectroscopy. The paucity of optical constants in the far-ultraviolet has hindered the interpretation of existing observations. For instance, the absence of the distinct 165 nm ice edge in the Hubble reflectance spectra of the Galilean satellites (Becker et al. 2018, Molyneux et al. 2020) remains a mystery yet to be solved. Similarly, constraining estimates of lunar hydration and its diurnal variation from LRO-LAMP (Lunar Reconnaissance Orbiter-Lyman Alpha Mapping Project) measurements (Gladstone et al. 2012, Hendrix et al. 2012, 2019) has proven challenging in the absence of reliable optical constants of water ice films. We address these difficulties by deriving optical constants of vapor-deposited amorphous and crystalline water ice films from new, robust laboratory spectra obtained over 10–140 K, but more importantly spanning from 115 nm to 16 μm. These optical constants will be used to model remote sensing spectra of ice-bearing objects across the Solar System to constrain their abundance, spatial distribution, possible variations, and phase (amorphous vs. crystalline) fraction.