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Presence of amorphous water ice on Europa’s leading hemisphere at the equator and south pole

Presentation #106.02 in the session “Icy Galilean Satellites: Spectroscopy”.

Published onOct 26, 2020
Presence of amorphous water ice on Europa’s leading hemisphere at the equator and south pole

Europa’s surface composition and evidence for cryovolcanic activity can provide insight into the properties and composition of the subsurface ocean, allowing the evaluation of its potential habitability. One promising avenue for revealing the surface processing and subsurface activity is the relative fractions of crystalline and amorphous water ice observed on the surface, which are influenced by temperature, charged particle bombardment, vapor deposition from plumes, and cryovolcanic activity such as diapirs.

The crystalline-to-amorphous water ice fraction (“crystallinity”) observed on Europa’s leading hemisphere cannot be reproduced by thermophysical and particle flux modeling alone (Berdis et al. 2020), indicating there may be additional processes influencing the surface. In order to investigate the context for the discrepancy between modeled and observed crystallinity, we perform a spectral mixture analysis on hyperspectral image cubes from Galileo Near-Infrared Mapping Spectrometer (NIMS) to identify how surface crystallinity is influenced by physical processing at a high spatial resolution scale. We focus specifically on two image cubes, 15e015 (7.3° N, 114° W, 3.0 km/pixel) close to the equator, and 17e009 (63° S, 120° W, 1.5 km/pixel) close to the south pole, both on the leading hemisphere.

Since Europa’s surface is not purely water ice and contains other materials such as hydrated sulfuric acid, and sodium and magnesium chlorates, perchlorates, and chlorides (e.g., Dalton et al. 2012), we include these materials in our analysis and perform a Non-Negative Least Squares spectral mixture analysis to reveal both the salt composition and the crystalline-to-amorphous water ice fraction of the surface. Whereas Dalton et al. (2012) did not include amorphous water ice in their spectral mixture analysis of the NIMS observations we are analyzing, we calculate a lower X2 when amorphous water ice is included concurrently with crystalline water ice in the spectral mixture analysis. Near the equator, a minimal abundance of amorphous water ice is expected due to the thermal relaxation of water ice into the crystalline form within a few years at temperatures exceeding 115 K (Kouchi et al. 1994, Jenniskens et al. 1998). However, we identify non-negligible amounts of amorphous water ice at the equator and the south pole. We also contextualize the spatial distribution of the salt abundance and water ice crystallinity within each NIMS cube that is investigated in this study in order to identify possible sources for surface processing.

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