Eclipse Spectroscopy of Io's Atmosphere

Mid-infrared detection of large longitudinal asymmetries in Io's SO atmosphere

“…Lower temperatures, and local abundances of about 1 × 10 17 cm -2 , more consistent with the UV results, are possible if mm line widths are due to rapid motions associated with plumes or supersonic winds ( (Ballester et al 1994, Lellouch 1996. In addition to SO 2 , lesser amounts of SO , NaCl (Lellouch et al 2003) and S 2 (Spencer et al 2000a) have also been detected, as have neutral atomic emissions from S, O, and Na (Ballester et al 1987, Geissler et al 1999a, 2001a, Feaga et al 2002, Roesler et al 1999, Bouchez et al 2000, and molecular emissions from SO (dePater et al 2002) and, probably, SO 2 and/or S 2 (Geissler et al 1999a, 2001a, Jessup et al 2004). The emission data are more difficult to interpret in terms of bulk atmospheric properties than the absorption data, as emission strength depends on excitation and dissociation mechanisms as well as atomic or molecular abundances.…”

Alkali and Chlorine Photochemistry in a Volcanically Driven Atmosphere on Io

“…Several recent observations indicate that active volcanoes might play a larger role in controlling atmospheric properties than was previously thought. The apparent patchiness of SO 2 vapor across Io's surface, the correlation of observed atomic and molecular emission features with known volcanic centers on Io, and the observed red-shifted and broadened shape of the SO 2 and SO lines at millimeter wavelengths all suggest that volcanoes are important atmospheric drivers (e.g., Lellouch et al 1990Lellouch et al , 1992Lellouch et al , 1994Lellouch et al , 1996Ballester et al 1990Ballester et al , 1994Sartoretti et al 1994Sartoretti et al , 1996Lellouch 1996;McEwen et al 1998;Geissler et al 1999;Hendrix et al 1999;Bouchez et al 2000;; see also the theoretical models of Ingersoll 1989, Moreno et al 1991, Strobel and Wolven 2001. In addition, theoretical modeling of the equilibrium chemistry of ionian volcanic vapors suggests that a variety of S-, O-, Na-, K-, and Cl-bearing gases may be emitted from high-temperature silicate magmas on Io (see Zolotov and Fegley 1998a, 1998bFegley and Zolotov 2000).…”

Photochemistry of a Volcanically Driven Atmosphere on Io: Sulfur and Oxygen Species from a Pele-Type Eruption

“…The possibility that electron impact could be occurring at night is clearly demonstrated in Galileo eclipse and nighttime images of Io (e.g., McEwen et al 1998a, Geissler et al 1999b in which visible gas emissions are observed both in localized regions associated with known volcanic centers and in broad regions of diffuse emission across the whole disk. The emission is believed to be caused by electron impact of SO 2 and/or SO (prominent at blue wavelengths), atomic oxygen (prominent at red and green wavelengths), and atomic sodium (prominent at green wavelengths) (see Bouchez et al 2000, Geissler et al 1999b; see also the theoretical descriptions of the "equatorial spots" and other emissions, e.g., Retherford et al 2000a, Saur et al 2000, Michael and Bhardwaj 2000. The fact that Io's disk-integrated brightness decreases with time after Io has passed behind Jupiter's shadow in these images suggests that SO 2 might be condensing (and the atmosphere collapsing) as the surface temperature drops (Geissler et al 1999b).…”

“…Recent observations of SO 2 , a molecule that has long been known to be present in the ionian atmosphere (Pearl et al 1979), have helped constrain the surface pressure and the degree of atmospheric "patchiness" on Io (Lellouch et al 1990, Ballester et al 1990, Sartoretti et al 1994, Lellouch 1996, Trafton et al 1996, Hendrix et al 1999. Other molecular constituents such as SO and S 2 have recently been identified on the satellite , and atomic and molecular emissions from neutral O, S, Na, Cl, H, SO 2 (and/or SO) have been mapped on and about Io's disk from Galileo Orbiter, Hubble Space Telescope (HST), and ground-based observations (e.g., Belton et al 1996;Geissler et al 1999b;Roesler et al 1999;Ballester et al 1999;Bouchez et al 2000;McGrath et al 2000;Feldman et al 2000a;Trafton 2000;Retherford et al 2000aRetherford et al , 2000b; see also the Voyager observations of Cook et al 1981). Continuing Io plasma torus and neutral cloud observations, including the recent detection of Cl + and Cl ++ in the torus (Küppers and Schneider 2000, Feldman et al 2000b, confirm that Na, K, and Cl in atomic and/or molecular form must be present in Io's atmosphere (e.g., Spencer and Schneider 1996).…”

“…Second, the observed red shifts and the degree of broadening of the millimeter-wave SO 2 and SO lines suggest that energetic outgassing of gases from volcanic plumes may be responsible for these aspects of the microwave emission (Lellouch et al 1994; see also Ballester et al 1994, Lellouch 1996. Third, visible-wavelength emissions from what is presumed to be neutral atomic oxygen, atomic sodium, and molecular SO 2 and/or SO were observed on Io when the satellite passed into Jupiter's shadow (e.g., McEwen et al 1998a, Geissler et al 1999b, Bouchez et al 2000; see also the ultraviolet observations of Roesler et al 1999). The correlation of many of the emission features in these eclipse images with known volcanic plumes or other centers of volcanic activity on Io suggests that volcanoes may be a major source of these neutral species (Geissler et al 1999b).…”