The structure, stability, and global distribution of Io's atmosphere

Abstract

Millimeter-wave observations of SO 2 have allowed the first groundbased direct detection of Io's neutral atmosphere. From observations of two SO 2 rotational lines, at 221.965 and 143.057 GHz, tentative detection of a third SO 2 line, at 346.652 GHz, and upper limits on two other lines, basic properties of Io's atmosphere are inferred. The SO 2 atmosphere appears to have global temporal stability and can be represented by a collisionally thick 10 11−10 12 cm −3 atmosphere ( p = 3−40 nbar) covering a limited fraction (5–20%) of Io's surface, with possibly larger pressures on the trailing side than on the leading. The horizontal distribution of gaseous SO 2 is best described as the result of discrete distribution of (equilibrium or volcanic) sources rather than by vapor pressure equilibrium over a smooth distribution of surface frosts. The lower atmosphere seems surprisingly hot, about 500–600 K at 40 km. A reanalysis of the IRIS/Voyager observation of the ν 3 SO 2 band at 7 μm over Loki, using a NLTE transfer model, suggests temperature/pressure conditions at Loki consistent with those derived for the global atmosphere from the millimeter-wave data. High temperatures in the lower scale height, however, are not accommodated by simple thermal models. Our results suggest that Io's atmosphere may be best described by a “volcanic source” atmospheric model, although some aspects of the “equilibrium” models, notably the temporal stability, are also present. While the primary problem remains the need to unambigously determine and explain the vertical thermal structure, it must be noted that if the atmosphere is hot, the concept of an atmosphere in dynamical equilibrium with one or more volcanic sources may provide a reasonable explanation to the Pioneer 10 occultation. Finally, new upper limits on atmospheric H 2S, SO, and CO were obtained.