Detection of the SO2 atmosphere on Io with the Hubble Space Telescope

Details Detection of the SO2 atmosphere on Io with the Hubble Space Telescope Detection of the SO2 atmosphere on Io with the Hubble Space Telescope

Sep 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994icar..111....2b&link_type=abstract

Icarus (ISSN 0019-1035), vol. 111, no. 1, p. 2-17

Astronomy and Astrophysics

Astronomy

67

Absorption Spectra, Io, Satellite Atmospheres, Spectroscopic Analysis, Sulfur Dioxides, Ultraviolet Astronomy, Ultraviolet Spectra, Atmospheric Temperature, Faint Objects, Hubble Space Telescope, Mathematical Models, Optical Thickness, Spatial Distribution, Spectrographs, Temperature Measurement

Scientific paper

Observations of the trailing hemisphere of Io made with the Faint Object Spectrograph of the Hubble Space Telescope (HST) in March 1992 have resulted in the first detection of atmospheric SO2 absorption bands in the ultraviolet. These observations represent only the third positive means of detection of what is widely believed to be Io's primary atmospheric constituent. Below approximately 2130 A the geometric albedo of the satellite is dominated by SO2 gas absorption band signatures, which have been analyzed using models that include the effects of optical thickness, temperature, and spatial distribution. The disk-intergrated HST data cannot resolve the spatial distribution, but it is possible to define basic properties and set constraints on the atmosphere at the time of the observations. Hemispheric atmospheres with average column density N = 6 - 10 x 1015/sq sm and T(gas) = 110 - 500 K fit the data, with preference for temperatures of approximately 200 - 250 K. Better fits are found as the atmosphere is spatially confined, with a limit of approximately 8% hemispheric areal coverage and N approximately equal to 3 x 1017/sq cm with colder 110 - 250 K temepratures. A dense (N greater than or equal to 1016/sq cm), localized component of SO2 gas, such as that possibly associated with active volcanoes, can generate the observed spectral constrast only when the atmosphere is cold (110 K) and an extended component such as Pele is included. The combination of a dense, localized atmosphere with a tenuous component (N less than 1016/sq cm, either patchy or extended) also fits the data. In all cases the best fit models imply a disk-averaged column density larger than exospheric but approximately 10 - 30 times less than the previous upper limit from near-UV observations.

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