Volcanogenic sulfur on Earth and Io: Composition and spectroscopy

The causes of Io's variegated surface, especially the roles of sulfur, and the geochemical history of sulfur compounds on Io are not well understood. Suspecting that minor impurities in sulfur might be important, we have inve stigated the major and trace element chemistry and spectroscopic reflectanc e of natural sulfur from a variety of terrestrial volcanic-hydrothermal env ironments. Evidence suggests that Io may be substantially coated with impur e sulfur. On Earth, a few tenths of a percent to a few percent of chalcophi le trace elements (e.g., As and Se) comonly occur in sulfur and appear to s tabilize material of yellow, brown, orange, and red hues, which may persist even at low temperatures. Percentage levels of chalcophile impurities are reasonably expected to occur on Io in vapor sublimate deposits and flows de rived from such deposits. Such impurities join a host of other mechanisms t hat might explain Io's reds and yellows. Two-tenths to two percent opaque c rystalline impurities, particularly pyrite (FeS2), commonly produces green, gray, and black volcanic sulfur on Earth and might explain areas of Io hav ing deposits of these colors. Pyrite produces a broad absorption near 1 mu m that gradually diminishes out to 1.6 mu m-similar but not identical to th e spectrum of Io seen in Galileo NIMS data. Percentage amounts of carbonace ous impurities and tens of percent SiO2 las silicates) also strongly affect the spectral properties of Earth's sulfur. Io's broad absorption between 0 .52 and 0.64 mu m remains unexplained by these data but could be due to sod ium sulfides, as suggested previously by others, or to As, Se, or other imp urities. These impurities and others, such as P and Cl(which could exist on Io's surface in amounts over 1% that of sulfur), greatly alter the molecul ar structure of molten and solid sulfur. Minor impurities could impact Io's geology, such as the morphology of sulfur lava flows and the ability of su lfur to sustain high relief. We have not found any natural sulfur containin g significant Na beyond that attributable to silicate inclusions. In sum, t he unique physical-chemical properties of S-rich systems and the strong aff inity of certain elements for S may have broad implications for the appeara nce, spectroscopic interpretation, and geologic processes of Io. Identifica tion of impurities in sulfur may be helpful in tracing the geochemical evol ution of surface deposits on Io. Perhaps foretelling of new areas of invest igation, Cl has recently been reported in the Io torus (M. Kueppers and N. M. Schneider 1999, Eos Trans. 80, 5207), suggesting the presence on Io of e ither salts, such as halite, or sulfur chlorides. Further evidence of minor iogenic impurities should be sought in Io's neutral cloud and plasma torus as well as in further scrutiny of Io's reflectance spectra. (C) 1999 Acade mic Press.