Europa's icy bright plains and dark linea: Exogenic and endogenic contributions to composition and surface properties

Abstract

We compare linear spectral modeling solutions yielding surface abundance estimates and water ice grain size information from low‐noise Galileo Near‐Infrared Mapping Spectrometer (NIMS) observations for three widely spaced locations on Europa. Bright equatorial plains on the orbital leading side are dominated by fine‐grained (∼50–75 μm) water ice (>80 wt%). This area shows a low abundance of hydrated salts and statistically insignificant amounts of hydrated sulfuric acid. A midlatitude northern hemisphere location on the trailing side exhibits strikingly different surface composition and properties for similar terrain. The modeled abundance of hydrated sulfuric acid exceeds 40% here; large‐grained water ice (∼250 μm diameter) dominates the ice grain size distribution. A third location at high southern latitudes on the leading side exhibits high abundances of water ice (>67%) and no detectable sulfuric acid hydrate. Water ice grain sizes here are intermediate between those of the other locations, at 75–100 μm. We resolve compositional differences between the darkest materials (at visible wavelengths) and the most hydrated materials (as evidenced by the distortion of water ice absorption bands) on Europa's leading side. Distinctive water ice grain size distributions are associated with visibly dark materials in all three locations. Our results help distinguish between exogenic and endogenic influences on Europa's present‐day surface composition. The results are consistent with migration and redeposition of sputtered water ice molecules from trailing to leading side locations.