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E. Lellouch (Obs. Paris), G. Paubert (IRAM, Granada), J.I. Moses (LPI, Houston), N.M. Schneider (LASP, Boulder), D.F. Strobel (JHU, Baltimore)
On January 14-18, 2002, we observed Io with the IRAM 30-m telescope and detected two rotational lines of NaCl at 143.237 and 234.252 GHz, obtaining the first detection of gaseous sodium chloride in Io's neutral atmosphere. The emission from SO2 was monitored by simultaneously observing SO2 lines at 143.057 and 234.187 GHz. Although intrinsically stronger by a factor ~ 300, the NaCl lines appear several times weaker than their SO2 counterparts, implying the NaCl is a minor compound. In the framework of an hydrostatic equilibrium atmosphere, the NaCl/SO2 is found to be (2.5-5)\times10-4, assuming NaCl and SO2 to be colocated. Improved fits are obtained for denser and more localized than SO2 NaCl atmospheres, with a preferred solution for NaCl/SO2 = 1.5 %, covering 8 % of Io's disk, and a NaCl column density of ~4\times1013 cm-2. \\
Because of its vanishingly small vapor pressure and short atmospheric lifetime, a sublimation equilibrium NaCl atmosphere is excluded, and direct volcanic output must be the main source of NaCl. A ~1 % mixing ratio is consistent with a CI chondritic composition for Io's erupting magmas. Based on simple gaseous plume models, the data indicate a volcanic emission rate in the range 1.7\times1027--8\times1028 NaCl molecules sec-1. NaCl is likely to be the common parent molecule of Na and Cl in Io's environment. Based on photochemical models predictions, the escape rate of molecular NaCl is typically 1000 times smaller that this surface rate; the escape rates of atomic Na and Cl are expected to be 10--20 times larger. While the direct NaCl escape is too small, the escape of Na and Cl appears consistent with the NaX+ production rate inferred from the fast sodium observations ((1--8)\times1026 sec-1). Thus, the production of fast sodium may be governed by reactions of atomic Na with molecular ions in Io torus.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.