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Formation of Global-scale Thermal Anomalies on Europa due to Ion Sputtering and Sublimation

Presentation #210.04 in the session Ocean Worlds: Tectonics, Surfaces, and Ionospheres (Oral Presentation)

Published onOct 23, 2023
Formation of Global-scale Thermal Anomalies on Europa due to Ion Sputtering and Sublimation

Observations of Europa’s thermal emission by the Photopolarimeter Radiometer (PPR) onboard the NASA Galileo mission showed anomalously low nighttime temperatures centered near the equator [1]. These lower nighttime temperatures can be readily explained by ~2x lower thermal inertia than the mid-latitudes [2,3]. Importantly, the temperature patterns indicate a low thermal inertia in the subjovian hemisphere decreasing in the direction of the trailing hemisphere. A potential correlation of the strength of the observed thermal anomaly with lower albedo non-ice materials, which are thought to be formed through sulfur ion implantation and radiolytic chemistry [4], suggests that sputtering by jovian magnetospheric ions may play a role the observed reduction in Europa’s equatorial thermal inertia.

The model we employ includes the following effects: 1) removal of ice through ion sputtering and related change in thermal conductivity, 2) exospheric transport, 3) sublimation/recondensation and vapor diffusion in the regolith, 4) cold trapping at the high latitudes and poles. We use water sputtering rates (dominated by 100-keV Sn+ ions) from Cassidy et al. (2013) [5], yielding a peak water erosion rate on the trailing hemisphere ~10-12 kg m-2 s-1, which corresponds to removal of ~0.1 µm yr-1. We propose that since thermal conductivity is strongly dependent on ice concentrations in within the thermally active layer, removal of ice decreases the thermal inertia [6].

By comparing our model results to the PPR data, we will determine the possible effects of sputtering on thermal inertia and whether this exogenic process can adequately explain the equatorial thermal anomaly on Europa. We will also make comparisons to the icy Saturnian satellites, some of which exhibit similar thermophysical anomalies apparently caused by plasma bombardment [7]. Anticipated observations from NASA’s Europa Clipper mission, particularly from the Europa Thermal Emission Imaging System (E-THEMIS) may shed further light on the nature of Europa’s equatorial anomaly.

[1] Spencer, J. R., et al. (1999), Science, 284(5419), 1514-1516. [2] Rathbun, J. A. et al. (2010), Icarus, 210(2), 763-769. [3] Aberbook, S., & Hayne, P. O. (2022), AGU Fall Meeting, P55G-1666. [4] Carlson, R. W., et al. (2002), Icarus, 157(2), 456-463. [5] Cassidy, T. A., et al. (2013), Planet. Space Sci., 77, 64-73. [6] Mellon, M. T., et al. (1997), J. Geophys. Res., 102(E8), 19357-19369. [7] Howett, C. J. A., et al. (2012), Icarus, 221(2), 1084-1088.

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