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

Abstract

We have observed about 16 absorption lines of the ν 2 SO 2 vibrational band on Io, in disk-integrated 19 µm spectra taken with the TEXES high spectral resolution mid-infrared spectrograph at the NASA Infrared Telescope Facility in November 2001, December 2002, and January 2004 These are the first ground-based infrared observations of Io's sunlit atmosphere, and provide a new window on the atmosphere that allows better longitudinal and temporal monitoring than previous techniques. Dramatic variations in band strength with longitude are seen that are stable over at least a 2 year period. The depth of the strongest feature, a blend of lines centered at 530.42 cm -1 , varies from about 7% near longitude 180 to about 1% near longitude 315 W, as measured at a spectral resolution of 57,000. Interpretation of the spectra requires modeling of surface temperatures and atmospheric density across Io's disk, and the variation in non-LTE ν 2 vibrational temperature with altitude, and depends on the assumed atmospheric and surface temperature structure. About half of Io's 19µm radiation comes from the sun-heated surface, and half from volcanic hot spots with temperatures primarily between 150 and 200 K, which occupy about 8% of the surface. The observations are thus weighted towards the atmosphere over these lowtemperature hot spots. If we assume that the atmosphere over the hot spots is representative of the atmosphere elsewhere, and that the atmospheric density is a function of latitude, the most plausible interpretation of the data is that the equatorial atmospheric column density varies from about 1.5 x 10 17 cm -2 near longitude 180 W to about 1.5 x 10 16 cm -2 near longitude 300 W, roughly consistent with HST UV spectroscopy and Lyman-α imaging. The inferred atmospheric kinetic temperature is less than about 150 K, at least on the anti-Jupiter hemisphere where the bands are strongest, somewhat colder than inferred from HST UV spectroscopy and millimeter-wavelength spectroscopy. This longitudinal variability in atmospheric density correlates with the longitudinal variability in the abundance of optically thick, near-UV bright SO 2 frost. However it is not clear whether the correlation results from volcanic control (regions of large frost abundance result from greater condensation of atmospheric gases supported by more vigorous volcanic activity in these regions) or sublimation control (regions of large frost abundance produce a more extensive atmosphere due to more extensive sublimation). Comparison of data taken in 2001, 2002, and 2004 shows that with the possible exception of longitudes near 180 W between 2001 and 2002, Io's atmospheric density does not appear to decrease as Io recedes from the sun, as would be expected if the atmosphere were supported by the sublimation of surface frost, suggesting that the atmosphere is dominantly supported by direct volcanic supply rather than by frost sublimation. However, other evidence such as the smooth variation in atmospheric abundance with latitude, and atmospheric ch...

First disk-resolved millimeter observations of Io's surface and SO2atmosphere

“…Large horizontal surface temperature variations will cause the effective emission size to decrease. The second surface temperature model we tested was the one used by Spencer et al (2005) (their Fig. 4), which matches 27 µm Galileo/PPR data.…”

“…For the surface temperature distribution of Spencer et al (2005), a disk-average brightness temperature of 93−99 K implies a mean emissivity of 0.79−0.84. This value is consistent with inferences by Muhleman & Berge (1991) who measured (at 2.6 mm) a brightness temperature of 101 ± 9 K for a diskaveraged physical temperature of 119 K. For a Fresnel emissivity law and surface sounding, the emissivity we determine would require a dielectric constant of ∼4.…”

“…The characteristic atmospheric temperature and its vertical structure are poorly constrained, with gas temperatures in the range 100−600 K claimed from different datasets (e.g. Lellouch et al 1992; Appendix A is only available in electronic form at http://www.aanda.org Jessup et al 2004;Spencer et al 2005). Several classes of models have been developed to describe the dynamical behavior of Io's atmosphere, including "meteorological" models (global and local, Ingersoll et al 1985;Ingersoll 1989;Moreno et al 1991), "unified" models coupling the effect of plasma interaction, horizontal transport, and chemistry on the overall distribution of the atmosphere (e.g.…”

“…As a consequence, the relative roles of volcanism and sublimation equilibrium in controlling the atmosphere remain uncertain, and arguments have been given in each direction (e.g. Spencer et al 2005;Lellouch et al 2006;Saur & Strobel 2004). Active volcanism is the ultimate source of Io's atmosphere, yet the question of its immediate source remains highly significant, in terms of vertical structure, lifetime, composition, and response to insolation variations (e.g.…”

“…In Sect. 5, data from both observatories are modeled in the framework of a hydrostatic atmosphere model, meant to represent a sublimation equilibrium atmosphere, by using newly proposed SO 2 distributions (Spencer et al 2005;Feaga 2005;Feaga et al 2008). Section 6 describes an initial approach for evaluating in a realistic way the contribution of volcanic plumes.…”