Galileo's multiinstrument spectral view of Europe's surface composition

We have combined spectral reflectance data from the Solid State Imaging (SS I) experiment, the Near-Infrared Mapping Spectrometer (NIMS), and the Ultra violet Spectrometer (UVS) in an attempt to determine the composition and im plied genesis of non-H2O components in the optical surface of Europa, We ha ve considered four terrains: (1) the "dark terrains" on the trailing hemisp here, (2) the "mottled terrain," (3) the linea on the leading hemisphere, a nd (4) the linea embedded in the dark terrain on the trailing hemisphere. T he darker materials in these terrains exhibit remarkably similar spectra in both the visible and near infrared. In the visible, a downturn toward shor ter wavelengths has been attributed to sulfur. The broad concentrations of dark material on the trailing hemisphere was originally thought to be indic ative of exogenic sulfur implantation. While an exogenic cause is still pro bable, more recent observations by the UVS team at higher spatial resolutio n have led to their suggestions that the role of the bombardment may have p rimarily been to sputter away overlying ice and to reveal underlying endoge nic non-H2O contaminants, If so, this might explain why the spectra in all these terrains are so similar despite the fact that the contaminants in the linea are clearly endogenic and those in the mottled terrain are almost ce rtainly so. In the near infrared, all these terrains exhibit much more asymmetrical ban ds at 1.4 and 2.0 mu m at shorter wavelengths than spectra from elsewhere o n Europa. It has been argued that this is because the water molecules are b ound in hydrated salts. However, this interpretation has been challenged an d it has also been argued that pure coarse ice can exhibit such asymmetric bands under certain conditions. The nature of this controversy is briefly d iscussed, as are theoretical and experimental studies bearing on this probl em. (C) 1999 Academic Press.