Paper: Mapping Io's Atmosphere with the Submillimeter Array

Another day, another Io atmosphere paper.  Today, we take a look at a paper by Arielle Moullet, Mark A. Gurwell, Emmanuel Lellouch, and Raphaël Moreno titled, "Simultaneous mapping of SO2, SO, NaCl in Io’s atmosphere with the Submillimeter Array." This paper was posted online Thursday in the Papers in Press section of the journal Icarus.  In press papers are articles which have gone through peer review, were revised, and have been approved for publication, but have not been published in the print journal yet.  This paper discusses results from sub-millimeter wavelength observations of Io's leading and trailing hemisphere.  Using these observations, the authors were able to map the distribution of three chemical species in Io's atmosphere, sulfur dioxide (SO₂), sulfur monoxide (SO), and sodium chloride (NaCl).  The authors then used these maps and the intensity of the observed emission lines to model the sources for these atmospheric components.  For additional information beyond this summary, check out Moullet's presentation from the Harvard-Smithsonian Center for Astrophysics SMA Science Symposium 2009.

Io has a very thin atmosphere (1-10 nanobars at the surface) composed mostly of sulfur dioxide, along with its disassociation products, sulfur monoxide and atomic sulfur and oxygen.  Other gases observed in Io's atmosphere or thought to exist in Io's atmosphere include sodium chloride, potassium chloride (KCl), diatomic sulfur (S₂), and sulfuryl chloride (Cl₂SO₂).  Io's atmosphere has significant density variations with time of day, being densest near the sub-solar point and thinnest on its night side, and with position on the surface.  The distribution of SO₂ gas was mapped across Io's surface using Hubble ultraviolet images of Io at the hydrogen Lyman-α line by Feaga et al. 2009, finding Io's sulfur dioxide gas in the atmosphere to be densest within 40 degrees of latitude from the equator and on Io's anti-Jupiter hemisphere.  Adaptive optics observations at Keck in 2002 showed SO gas at several volcanic centers, which de Pater et al. 2007 suggested was related to volcanic activity.  One of major questions these observations are trying to answer include identifying the dominant source of Io's atmosphere, whether it is sublimation of surface frosts, as atmospheric sulfur dioxide is thought to be in vapor-pressure equilibrium with the surface, or direct volcanic outgassing.

In this new paper, Moullet et al. 2010, the authors used observations taken at the Submillimeter Array (SMA) atop Mauna Kea in Hawaii, a set of eight, 6-meter-wide radio antennas acting as a radio interferometer.  The system observes at millimeter and sub-millimeter wavelengths between 0.3 and 1.7 millimeters (or frequencies between 180 and 700 GHz), at between the infrared and microwave portions of the electromagnetic spectrum.  For these observations, Moullet et al. observed Io in June 2006 and July 2008 at 338 and 346 GHz, at emission bands for sulfur dioxide, sulfur monoxide, and sodium chloride.  These observations were disk-resolved, allowing the authors to examine the hemispherical distribution of each gas species, and in the case of SO₂ and SO, the high spectral resolution and signal-to-noise ratio allowed for more detailed analysis of their total emission.

For SO₂, Moullet et al.'s SMA data at 346.523 and 346.652 GHz is consistent with the results from Fegea et al.'s Lyman-α data and Spencer et al. 2005's infrared data, further indicating that the sulfur dioxide in Io's atmosphere is predominantly located at equatorial latitudes and Io's anti-Jovian hemisphere.  Both constraints on the distribution of SO₂ are similar to the distribution of sulfur dioxide frost on Io's surface and volcanic plumes.  To see which source of SO₂ dominates, volcanic outgassing or sublimation of surface frosts, the authors modeled the appearance of Io's 346.652 GHz SO₂ line emission using a hydrostatic model of Io's sublimation atmosphere and another modeling emission purely from the volcanic plumes noted by Geissler et al. 2004.  Using an appropriate number of active plumes, the authors found that volcanism does not match the distribution and shape of SO₂ gas seen in the SMA data (except for the slight northward offset in emission seen over the anti-Jovian hemisphere in 2006).  In addition, to match the amount of emission observed, an unrealistic number of active volcanic plumes (40-230 Prometheus-type, or 5-20 Pele-type plumes, on each hemisphere) would be required.  Even if all 16 plumes noted by Geissler et al. were active, they would still only account for 5-11% of the emission on the leading side, and 13-18% on the trailing side.  The hydrostatic models of the frost sublimation component of Io's SO₂ atmosphere better match the distribution and emission flux seen in the SMA data. The derived SO₂ column densities (an average of 2.3-4.6×10¹⁶ cm⁻² for the leading hemisphere, and 0.7-1.1×10¹⁶ cm⁻² for the trailing side), gas temperatures, and variations with Io's distance from the Sun provide further evidence that the main source for sulfur dioxide, the dominant component in the satellite's atmosphere, is vapor-pressure equilibrium sublimation of SO₂ surface frosts.  A minor fraction of the SO2 also comes from volcanic gas plumes (though not all gas emissions for volcanoes form plumes).

Moullet et al. also mapped the distribution of sulfur monoxide and sodium chloride in Io's atmosphere for the first time.  Both gases were predominately found on Io's anti-Jupiter hemisphere, as SO₂ was.  The signal-to-noise ratio for the SO data was sufficient for the authors to perform an analysis on the source of that gas, similar to the one they performed on sulfur dioxide. Two sources were considered: photolysis of SO₂, when sulfur dioxide is broken up by solar ultraviolet photons into SO and O, and direct volcanic emission.  In the case of volcanic activity, they found that, using a reasonable level of plume activity, volcanism is the source of 2.5% of the SO observed in the SMA data, using a model that included four volcanic plumes.  Using more plumes, or Pele plumes, would result in models that was more extended spatially than the emission maps from the data acquired.  Even using a favorable, 16 plume model, volcanic plumes account for 13-40% of the total SO emission, assuming a 10% mixing fraction in the plumes.  This at least suggests though, that volcanism is a more significant source of sulfur monoxide than sulfur dioxide.  The dominant source, however, is photolysis from SO₂, as the modeling performed by Moullet et al. suggests.  The authors note that the "relative contribution is not easy to assess in the absence of precise estimates on SO lifetime in the atmosphere."

The signal-to-noise ratios of the NaCl maps were much lower than those for the SO2 and SO data, but radiative transfer modeling of NaCl in Io's atmosphere shows that the data is consistent with a volcanic origin. This is particularly the case since NaCl has a short lifetime in Io's atmosphere due loss from condensation on dust particles and photolysis into atomic sodium and chloride.  The amount of NaCl emission observed is consistent with a lower-bound, mixing ratio in volcanic plumes of 0.6% on the trailing hemisphere, and 2.5% on the leading side, similar to thermochemical modeling done by Fegley and Zolotov 2000.  Moullet et al. could not rule out other sources for the NaCl emission, such as sputtering.

This paper seems to provide further evidence that most of the gases in Io's atmosphere result from the sublimation of sulfur dioxide surface frosts, with direct volcanic emission playing a minor role.  Other major atmospheric components such as sulfur, oxygen, and sulfur monoxide result from the photo-disassocation of sulfur dioxide into its elemental components by solar ultraviolet photons, though a larger percentage of the sulfur monoxide in the atmosphere may come from direct volcanic outgassing.  Finally, some atmospheric components not related to sulfur chemistry, such as sodium chloride, could be entirely explained by volcanic outgassing, though other sources such as sputtering are also possible, but weren't modeled by the authors as the distribution of surface deposits of minerals other than sulfur and sulfur dioxide are very poorly constrained.  According to their presentation from April 2009, the authors stated they planned to map the distribution of other species such as potassium chloride, silicon oxide, and disulfur monoxide in the summer of 2009 using the APEX antenna in Chile, and will submit a proposal to observe Io's atmosphere at much higher resolution using the ALMA telescopes when they are finished in 2012.

Link: Simultaneous mapping of SO2, SO, NaCl in Io’s atmosphere with the Submillimeter Array [dx.doi.org]
Link: Presentation - Mapping of SO2, SO and NaCl emission in Io's atmosphere [www.cfa.harvard.edu]