Io’s atmosphere: Constraints on sublimation support from density variations on seasonal timescales using NASA IRTF/TEXES observations from 2001 to 2010

Abstract

We present an analysis of 19 μm spectra of Io’s SO 2 atmosphere from the TEXES mid-infrared high spectral resolution spectrograph on NASA’s Infrared Telescope Facility, incorporating new data taken between January 2005 and June 2010 and a re-analysis of earlier data taken from November 2001 to January 2004. This is the longest set of contiguous observations of Io’s atmosphere using the same instrument and technique thus far. We have fitted all 16 detected blended absorption lines of the ν 2 SO 2 vibrational band to retrieve the subsolar values of SO 2 column abundance and the gas kinetic temperature. By incorporating an existing model of Io’s surface temperatures and atmosphere, we retrieve sub-solar column densities from the disk-integrated data. Spectra from all years are best fit by atmospheric temperatures <150 K. Best-fit gas kinetic temperatures on the anti-Jupiter hemisphere, where SO 2 gas abundance is highest, are low and stable, with a mean of 108 (±18) K. The sub-solar SO 2 column density between longitudes of 90–220° varies from a low of 0.61 (±0.145) × 10 −17 cm −2, near aphelion in 2004, to a high of 1.51 (±0.215) × 10 17 cm −2 in 2010 when Jupiter was approaching its early 2011 perihelion. No correlation in the gas temperature was seen with the increasing SO 2 column densities outside the errors. Assuming that any volcanic component of the atmosphere is constant with time, the correlation of increasing SO 2 abundance with decreasing heliocentric distance provides good evidence that the atmosphere is at least partially supported by frost sublimation. The SO 2 frost thermal inertias and albedos that fit the variation in atmospheric density best are between 150–1250 W m −2 s −1/2 K −1 and 0.613–0.425 respectively. Photometric evidence favors albedos near the upper end of this range, corresponding to thermal inertias near the lower end. This relatively low frost thermal inertia produces larger amplitude seasonal variations than are observed, which in turn implies a substantial additional volcanic atmospheric component to moderate the amplitude of the seasonal variations of the total atmosphere on the anti-Jupiter hemisphere. The seasonal thermal inertia we measure is unique both because it refers exclusively to the SO 2 frost surface component, and also because it refers to relatively deep subsurface layers (few meters) due to the timescales of many years, while previous studies have determined thermal inertias at shallower levels (few centimeters), relevant for timescales of ∼2 h (eclipse) or ∼2 days (diurnal curves).