The Effect of Magnetospheric Ion Bombardment on the Reflectance of Europa's Surface

A series of laboratory experiments has been performed to investigate the role of sulfur ion implantation on the differences in the reflectivity of Europa's "leading" and "trailing" hemispheres in the UV and visible. The lower reflectivity of Europa's trailing hemisphere at around 0.28 /xm can be attributed to a SO band which is caused by either the implantation of sulfur ions from Jupiter's magnetosphere into the water-ice surface of Europa's trailing hemisphere or by the preferential condensation of SO 2 from volcanism on the trailing side. Laboratory measurements of the strength of this absorption band allow an estimate of the column density of 2 × 1017 c m -2 SO bonds within the sampling depth of the photons reflected from Europa's trailing hemisphere. Under the conditions tested in the laboratory, sulfur implantation or SO 2 deposition cannot account for the general "reddening" in the UV of the trailing hemisphere relative to the leading hemisphere, a feature which we previously showed can be produced by fast penetrating ions. ,~ 1992 Academic Press, Inc.