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[35.11] Millimeter--wave observations of Io's atmosphere: the IRAM 1999 campaign

E. Lellouch (Observatoire de Paris, France), G. Paubert (IRAM Granada, Spain), D.F. Strobel (JHU, Baltimore), M. Belton (NOAO, Tucson)

New millimeter-wave observations of Io were obtained on October 9-11 and November 25-27, 1999, using the 30-m IRAM radiotelescope at Pico Veleta, Spain, as part as the Earth-based campaign in support of the Galileo Io observations. Observations, similar in technique to our previous observations in 1990--1995 (e.g. Lellouch et al. Icarus, 98, 271, 1992), were mostly targeted at SO₂ lines. SO was also reobserved, and attempts to detect other species (H₂S, OCS, NaOH, S₂O, CS, ClO) were unsuccessful.

Compared to our previous observations, the SO₂ lines appeared somewhat stronger. Combined with very good sky conditions and improved instrumental performances, this allowed us to easily detect several SO₂ weak lines (e.g. at 146.605 and 104.239 GHz) that we had desperately looked for in the past. In addition, the high signal-to-noise obtained on the SO₂ strong lines (e.g. 221.965 GHz) allows us to monitor line characteristics as a function of orbital position, with a typical time resolution of about 1 hour.

Observational results will be presented, and the data will be interpreted in the context of static and volcanic atmospheres and in the light of recent HST and Galileo observations.