A Galileo Multi-Instrument Study of Europa's Color Heterogenieties

J.C. Granahan, F.P. Fanale, T.B. McCord (STI and Univ. of Hawaii), G. Hansen (Univ. of Hawaii), R. Carlson, L. Kamp, D. Matson, A. Ocampo, W. Smythe (JPL), A.R. Hendrix, C.A. Barth (LASP), F. Leader, R. Mehlman (UCLA), R. Greeley, R. Sullivan (ASU), B.E. Clark, P. Helfenstein, J. Veverka (Cornell), P. Geissler (LPI), M.J.S. Belton (NOAO), K. Becker, T. Becker, D. Cook (USGS), Galileo NIMS, SSI, and UVS Teams

The present study combines data from the the Galileo Near Infrared Mapping Spectrometer (NIMS), the Solid State Imager (SSI), and the UltraViolet Spectrometer (UVS) to investigate the compositional diversity of the surface of Europa. Three of the terrains studied here (mottled terrain, linea, and trailing hemisphere dark terrain) were known from Voyager results. Galileo revealed an additional terrain (infrared bright plains) that represents bright Europan icy plains materials with unusually large near infrared (890 and 990 nm) albedos (Greeley et al. 1996).Our G1 orbit SSI/NIMS composite spectra show that the mottled terrain possesses spectra which offset the dominant water ice bands to slightly shorter wavelengths. Observations of the linea taken during a NIMS E6 observation and a G1 SSI clear filter image indicate that the linea and some dark dome-like features have a spectral signature similar to that of the mottled terrain. This suggests that the same darkening agent is the water ice contaminant for both mottled terrain and the linea. The best spectral matches for this component are evaporite, hydrated minerals which are produced as aqueous weathering products. The trailing hemisphere dark terrain of Europa has been studied using a G2 SSI color observation, 3 E4 NIMS , and 3 E4 UVS observations . The dark terrain on the trailing hemisphere appears to mask the indigenous Europa surface albedo and may be due to a form of sulfur material (a finding consistent with previous Voyager and IUE studies). The infrared bright plains were best observed in the G1 NIMS and SSI northern high latitude observations. The infrared bright plains are interpreted to contain purer water ice and/or water ice with larger grain sizes than the immediate surrounding regions.