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Radiation Darkening of Europa’s Cryoplume Fallouts

Presentation #315.06 in the session Icy Satellites: Surfaces, Ice Shell, and Interior (Poster)

Published onOct 23, 2023
Radiation Darkening of Europa’s Cryoplume Fallouts

Europa’s plumes seem episodic (Roth et al., 2014; Sparks et al., 2017; Paganini et al., 2019), and the frequency and scale of cryovolcanic eruptions are uncertain. Without direct plume observations by the Clipper and JUICE missions, deposits emplaced on the surface may be the best indicators of recent geological activity (Quick and Hedman, 2020). While visible and infrared imagery by Clipper’s Europa Imaging System (EIS) and Mapping Imaging Spectrometer for Europa (MISE) offer indirect plume detection methods (Quick and Hedman, 2020), far-ultraviolet (FUV) surface observations by Europa-UVS and JUICE-UVS present a unique approach to identifying and characterizing fallouts. Europa is surprisingly dark in the FUV with albedos < 2% (Becker et al., 2018). Plume deposits, mainly composed of spectrally bright H2O recondensed vapor and ice grains, will contrast starkly against Europa’s dark UV surface, especially > 165 nm. Observing such localized UV bright spots of may constitute a novel approach for detecting plume fallouts with Europa-UVS and JUICE-UVS.

But how long will plume fallouts remain UV-bright following processing by the Jovian plasma? Radiation darkening, as postulated for the Saturnian moons (Hedman et al., 2020), prompts new experiments to quantify FUV darkening rates of water ice films under electron irradiation in the lab. We report significant FUV darkening of water ice films with trace amounts of CO2 during keV electron irradiation. Europa’s plumes may contain CO2, organics, and salts in trace amounts, like Enceladus (Waite et al., 2006). These species could recondense with plume H2O vapor/grains and be subject to energetic processing. We will measure the dependence of darkening rate on (1) CO2 abundance, (2) ice temperature, and (3) radiation dose and combine these findings with the energetic particle precipitation flux (Nordheim et al., 2022) to estimate the age of a cryoplume deposit based on its observed brightness.

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