[Previous] | [Session 35] | [Next]

[35.03] Detection of sodium chloride in Io's atmosphere

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.

If the author provided an email address or URL for general inquiries, it is as follows:

Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.