Detection and characterization of Io's atmosphere from high-resolution 4-μm spectroscopy

We report on high-resolution and spatially-resolved spectra of Io in the 4.0μm region, recorded with the VLT/CRIRES instrument in 2008 and 2010, which provide the first detection of the ν₁+ν₃ band of SO₂ in Io's atmosphere. Data are analyzed to constrain the latitudinal, longitudinal, and diurnal distribution of Io's SO₂ atmosphere as well as its characteristic temperature. Equatorial SO₂ column densities clearly show longitudinal asymmetry, but with a maximum of ~1.5×10¹⁷cm^-2 at central meridian longitude L=200-220 and a minimum of ~3×10¹⁶cm^-2 at L=285-300, the longitudinal pattern somewhat differs from earlier inferences from Ly α and thermal IR measurements. Within the accuracy of the measurements, no evolution of the atmospheric density from mid-2008 to mid-2010 can be distinguished. The decrease of the SO₂ column density towards high latitudes is apparent, and the typical latitudinal extent of the atmosphere found to be ±40° at half-maximum. The data show moderate diurnal variations of the equatorial atmosphere, which is evidence for a partially sublimation-supported atmospheric component. Compared to local noon, factor of 2 lower densities are observed ~40° before and ~80° after noon. Best-fit gas temperatures range from 150 to 220K, with a weighted mean value of 170±20K, which should represent the column-weighted mean kinetic temperature of Io's atmosphere. Finally, although the data include clear thermal emission due to Pillan (in outburst in July 2008) and Loki, no detectable enhancements in the SO₂ atmosphere above these volcanic regions are found, with an upper limit of 4×10¹⁶cm^-2 at Pillan and 1×10¹⁷cm^-2 at Loki.