Tyre and Pwyll: Galileo orbital remote sensing of mineralogy versus morphology at two selected sites on Europa

Abstract

Observational data from the Galileo Orbiter's remote‐sensing instruments, including solid‐state imaging (SSI), the Near‐Infrared Mapping Spectrometer (NIMS), and the ultraviolet spectrometer (UVS), are analyzed and interpreted in terms of the history of the Tyre and Pwyll impact sites and the nature of Europa's crustal zonal structure. Material characterized by asymmetric 1.4 and 2.0 μm bands and visible coloration characterizes each site where morphologic evidence suggests disruption of the topmost crust. The material is not H2O ice, is endogenic, and is common to the linea, impact basins, and dark trailing side terrain. Differences between Tyre and Pwyll are interpreted in terms of Europa's peculiar energy history, while other evidence may suggest flooding of the endogenic material as a liquid. The pure ice appearance of most of Europa's crust is probably superficial, while beneath a thin patina of sputtered H2O molecules the crust is everywhere laced with numerous generations of intrusions and extrusions of an aqueous phase. Our results, together with numerous laboratory experiments and theoretical analysis, suggest that the aqueous phase is dominated by the SO4= anion.