Analysis of Voyager images of Europa: Plasma bombardment

Abstract

The photometrically corrected normal albedos of Europa, extracted from Voyager data, are converted into average single-scattering albedos and analyzed as a function of angular distance from the apex of orbital motion. The spectral effects associated with the textural property of the surface and impurity absorption are separated for the data from each Voyager filter for both “bright” and “dark” terrains. An ultraviolet absorption is found in the 0.35 ±0.03- μm filter data which directly correlate with the longitudinal distribution of ion implantation in both terrain types, confirming the magnetospheric modification of the surface. The data taken through all of the filters (0.35, 0.41, 0.48, 0.59 μm) exhibit a longitudinal dependence across the surface of Europa predicted by the grain size variation determined previously from the 1.04-μm water band depth. A generally “red” spectrum is found in both terrain types which is, very roughly, constant across the surface. This effect is separate from the UV absorption and may be endogenic or due to very small (<1 μm) dust particle or sputter redistribution of implanted sulfur. There is also a uniform increase in the absorption in the “dark” terrain over that in the “bright” indicative of a higher concentration of an absorbing material or larger average grain size. The approximate wavelength dependence of the average absorption coefficient of this “red” contaminant is given.