Changes in Io's Atmosphere in 2009: Atmospheric Inflation Near Perihelion?

Details Changes in Io's Atmosphere in 2009: Atmospheric Inflation Near Perihelion? Changes in Io's Atmosphere in 2009: Atmospheric Inflation Near Perihelion?

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.6603s&link_type=abstract

American Astronomical Society, DPS meeting #41, #66.03

Astronomy and Astrophysics

Astronomy

Scientific paper

We have been observing Io's atmosphere periodically since 2001, using the SO2 τ2 band at 19 µm, with the TEXES high-resolution mid-IR spectrometer on the NASA Infrared Telescope Facility. Disk-integrated spectra show strong longitudinal variations in band strength, with the absorption bands being strongest near longitude 200 W and weakest near longitude 330 W.
One goal of the long-term monitoring is to look for changes in atmospheric density correlated with changes in Io's heliocentric distance. An atmosphere supported by frost sublimation should expand near perihelion due to increased frost temperature and vapor pressure. For example, a frost temperature of 113.0 K at aphelion (R = 5.456 AU), yielding an SO2 vapor pressure of 0.69 nbar, would warm to as much as 118.6 K at perihelion (R = 4.948 AU), increasing SO2 vapor pressure nearly sevenfold to 4.7 nbar. Starting with our first observations in 2001 (R = 5.15 AU) and continuing through aphelion, SO2 band strength at most longitudes has remained remarkably constant, contrary to the expectation for a sublimation-supported atmosphere.
However, in June 2009, we observed Io with TEXES at its closest heliocentric distance so far (R = 5.06 AU), and saw an increase in band strength of up to 50% at most longitudes compared to previous years. If the increased band strength results from increased SO2 density, as is likely, we may be seeing the onset of a sublimation-controlled atmospheric spike associated with perihelion. Further observations as Io approaches perihelion, which occurs in early 2011, will provide a strong test of this hypothesis.
This work is supported by NASA grants NNX07AU40G and NNG06GI37G

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