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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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