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Water Ice-Vapor Evolving over Europa and Ganymede’s ~85 and 172 hour Orbits

Presentation #501.08 in the session Icy Galilean Satellites: Magnetosphere and Exosphere (Oral Presentation)

Published onOct 23, 2023
Water Ice-Vapor Evolving over Europa and Ganymede’s ~85 and 172 hour Orbits

Europa and Ganymede’s near-surface atmospheres are sourced primarily by magnetospheric and thermal radiation of water ice. Radiolytic sputtering triggers the formation and evolution of predominantly O2 atmospheres observed and simulated to be at least ~3 x 1014 O2/cm2. Given the previous characterization of orbital variability in Europa and Ganymede’s O2 atmospheres (Leblanc et al. 2017; Oza et al. 2019) we now seek to understand the behavior of their water ice-vapor atmospheres with exosphere general model (EGM) simulations. Our 3-D model includes satellite revolution about Jupiter for Europa (τorb~85 h) and Ganymede (τorb~172 h), so that one can explicitly study the orbital evolution of water product species. By sputtering and sublimating a water vapor atmosphere, with components of hydroxide (OH), hydrogen (H), peroxide (H2O2) and a recently identified thermally-driven molecular component (H2 and O2), we seek to determine the ambient exospheric state of a pure ice surface.

EGM column densities are roughly consistent with recent ground-based and space-based observations. Despite the expectation of Europa being far more cryovolcanically active, Ganymede’s global exosphere harbors ~10x more water exceeding 1015 H2O/cm2. This is primarily due to the fact that Europa is smaller, and far colder than Ganymede on average. We find that when water ice sublimation is significant compared to direct sputtering, a water vapor atmosphere builds up by noon, Europa local time, and collapses during eclipse/midnight, akin to Ganymede. Furthermore, we examine several physical processes on both satellites which appear to be critical during egress, upon exit from Jupiter’s shadow.

Our simulations provide specific orbital phases that would be amenable to distinguishing a transient, localized atmosphere by Europa Clipper, JUICE, and future landers. At Europa for instance, simulations suggest that if line-of-sight gas phase observations detect > ~1014 H2O cm-2 at orbital longitudes when the colder, leading hemisphere begins to be illuminated Φorb~0-90°, a geologic source may be favored over a sublimated or sputtered exosphere. Since the onset of the 4:2:1 Laplace Resonance with Io, Europa & Ganymede’s ices have of course experienced a unique regime of tidal heating over geological timescales, whose contributions to the exosphere are poorly understood at present. The column density maps presented here serve as a background exosphere which enable the search for and characterization of trace species (Na, K, SO2) with both ground and in-situ spacecraft measurements.

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