Sodium at Io

Io, the innermost of Jupiter's four Galilean satellites, is anomalous in various respects. One curiosity is its atmosphere, and the 'plasma torus' of ionized matter through which the planet orbits. Using the 30-m IRAM (Institut de Radio-Astronomie Millimetrique) radio telescope, Lellouch et al. have now identified two spectroscopic lines of NaCl vapor, at wavelengths of 1.3 mm and 2.1 mm, in Io's atmosphere. They also detect three lines of SO2 and provide a tentative detection of KCl. The NaCl occurs only in patches in the atmosphere, consistent with there being just a few volcanic sources. An alternative, but much less likely, explanation is that the NaCl stems from evaporation or sputtering from material condensed on SO2 snowdrifts on Io's surface. Earlier observations of SO2 with the same telescope show that SO2 is also distributed patchily, and for this more volatile molecule both sources - volcanism and sputtering - are plausible. (CSA)