SO_2-rock interaction on Io 2. Interaction with pure SO_2

Abstract

A Na-S mineral on the surface of Io is required to be the source of the famous atomic cloud. SO_2 is a confirmed atmospheric and surface constituent, and because of the rapid volcanic resurfacing rate, the SO_2 is buried within the crust, where at least occasionally, over many cycles of burial and eruption, it must contact silicate materials at midlevel crustal temperatures. Surface interaction experiments were performed for a wide variety of silicate compositions showing that interaction products of these with SO_2 could be observed at 1123 K on laboratory timescales, even in the absence of external redox agents. Not all experiments produced deposits that could be studied by scanning electron microscopy; some required the greater sensitivity of photoelectron spectroscopy (XPS). Characterization of the alteration products by XPS showed that both oxidized and reduced sulfur species were formed, indicating that a disproportionation mechanism producing a sulfate and a reduced S species although smaller amounts of interaction leading to Na_2SO_3 formation cannot be ruled out. The reduced sulfur species is best explained as elemental S which was independently documented for two compositions. Scanning electron microscopy studies for those compositions where reaction was extensive enough to be observed showed (1) Na_2SO_4 for a soda-lime composition, (2) a mixed Na-Ca-sulfate liquid and CaSO_4 for AbAnDi and a chondrule glass composition, and (3) Fe-sulfate for a natural obsidian. Infrared spectroscopy for the soda-lime glass composition showed peaks best explained by Na_2SO_4. We conclude that SO_2 disproportionation as well as direct formation from SO_3 under oxidizing conditions can produce Na_2SO_4 by interaction of SO_2 with silicates on Io, but Ca and Fe sulfates may form preferentially in more basaltic compositions. As highly oxidizing conditions may be unlikely for Io, the disproportionation mechanism may be more competitive on Io than it is in laboratory experiments. Very low rates of Na_2SO_4 production are required to supply the Io atomic cloud, so the interaction processes can be very inefficient.