DPS 2001 meeting, November 2001
Session 17. Io, Tori, and Satellite Atmospheres Posters
Displayed, 9:00am Tuesday - 3:00pm Saturday, Highlighted, Wednesday, November 28, 2001, 10:30am-12:30pm, French Market Exhibit Hall

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[17.03] Cl2SO2 deposits near the Marduk's volcanic center on Io

B. Schmitt (LPG, CNRS-UJF), S. Rodriguez (Obs. Bordeaux), NIMS/Galileo Team

An absorption feature at 3.92 \mum locally present in NIMS data near Marduk's volcanic center on Io has been tentatively identified by Cl2SO2 or possibly H2S (Schmitt and Rodriguez 2000: BAAS, 32, 1048).

Direct comparison between laboratory spectra and the Marduk NIMS spectrum, coupled with radiative transfer modeling, allows to propose as the surface material producing the 3.92 \mum absorption either:

- a (sub-)micron thin layer of pure solid H2S (phase II), (compact or sub-micron particles), at T > 103.6 K condensed on top of SO2 ice or,

- a millimeter thick layer of 1% Cl2SO2 mixed with solid SO2 covering a similar mixture strongly depleted in Cl2SO2 (< 0.1%).

Spectral constraints on the 3.92 \mum band alone, though favoring Cl2SO2 (better fit), are not able to unambiguously distinguish between these two possible identifications. Unfortunately, both molecules did not possess other bands strong enough to clearly appear in the NIMS data and firmly confirm one of these candidates.

On the chemical point of view the volcanic model of Fegley and Zolotov (2000) needs particular physical conditions in the vent to produce more than 10-5 mole fraction of Cl2SO2, an amount still not enough to account for the intensity of the band. On the other hand, it may produce enough H2S but our spectral modeling set an upper limit of 0.04 for the H2O / H2S abundance ratio in contradiction with the predicted value (about 10).

The sublimation rates of Cl2SO2 and SO2 have been measured in the laboratory at low temperature. We found that the relative volatilities of H2S, Cl2SO2 and SO2 coupled with their inferred physical states and the surface temperature conditions of Io strongly favor the incorporation of the chlorine molecule in solid SO2 at a concentration about 4 orders of magnitude larger than in the gas. On the other hand, the formation of a thin layer of pure solid H2S seems very improbable, or at least extremely ephemeral at the surface of Io. We conclude on some possible implications on Io's chemistry.

We acknowledge CNES and the French Programme National de Planétologie of CNRS (INSU) for their financial supports.

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