Article,
Galileo's multiinstrument spectral view of Europe's surface composition
Icarus, 139 (2): 179--188 (1999)
Abstract
We have combined spectral reflectance data from the Solid State Imaging
(SSI) experiment, the Near-Infrared Mapping Spectrometer (NIMS),
and the Ultraviolet Spectrometer (UVS) in an attempt to determine
the composition and implied genesis of non-H2O components in the
optical surface of Europa, We have considered four terrains: (1)
the "dark terrains" on the trailing hemisphere, (2) the "mottled
terrain," (3) the linea on the leading hemisphere, and (4) the linea
embedded in the dark terrain on the trailing hemisphere. The darker
materials in these terrains exhibit remarkably similar spectra in
both the visible and near infrared. In the visible, a downturn toward
shorter wavelengths has been attributed to sulfur. The broad concentrations
of dark material on the trailing hemisphere was originally thought
to be indicative of exogenic sulfur implantation. While an exogenic
cause is still probable, more recent observations by the UVS team
at higher spatial resolution have led to their suggestions that the
role of the bombardment may have primarily been to sputter away overlying
ice and to reveal underlying endogenic 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 certainly so. In the
near infrared, all these terrains exhibit much more asymmetrical
bands at 1.4 and 2.0 mu m at shorter wavelengths than spectra from
elsewhere on Europa. It has been argued that this is because the
water molecules are bound in hydrated salts. However, this interpretation
has been challenged and it has also been argued that pure coarse
ice can exhibit such asymmetric bands under certain conditions. The
nature of this controversy is briefly discussed, as are theoretical
and experimental studies bearing on this problem. (C) 1999 Academic
Press.
