A new Io-related paper was posted online on the journal Icarus's Article-in-Press page, "The global distribution of Sulfur Dioxide ice on Io, observed with OSIRIS on the W.M. Keck telescope" by Conor Laver and Imke de Pater. This paper covers K-band (1.97-2.38 μm), high-spatial and spectral resolution observations of Io's leading and trailing hemisphere acquired by the OSIRIS near-infrared spectrometer at Mauna Kea's Keck II telescope. This is the third in a series of papers by Laver and de Pater based on observations acquired in April and June 2006. The first paper, published in Icarus in 2007, covered an outburst eruption at Tvashtar Patera in April 2006. The second paper, published in Icarus in 2008, covered observations acquired in April 2006 of SO2 frost distribution across Io's leading hemisphere.
In their most recent paper, Laver and de Pater explore data acquired on June 1 and 2, 2006 to look at SO2 frost/ice distribution across the leading and trailing hemisphere of Io. In particular, they look at two weak SO2 absorption bands at 1.981 μm (3ν1 + ν3 vibrational mode) and 2.126 μm (ν1 + 3ν3). In addition to including coverage over the trailing hemisphere, the data from June have a higher spatial resolution (65 km at Io's equator) than the April data that was the subject of last year's paper. Like the NIMS and VIMS instruments on Galileo and Cassini, respectively, the OSIRIS instrument produces a reflectance spectrum for each of the nearly 2000 pixels that cover the disk of Io. From these reflectance spectra and following some calibration procedures described in the paper, the authors calculated the equivalent width (a method of both measuring the strength of an absorption band and normalizing it to the surrounding continuum) and the Full-width at half-maxium (FWHM; basically the wavelength width of the absorption band in nanometers) for each of the two absorption bands. Based on these calculations, the authors can then measure the fractional coverage of SO2 for each pixel in their map based on the equivalent width and a lower-limit grain size of 700 μm. Theoretically, differences in the FWHM can be used the look at differences in grain size across Io, but the spectral resolution was not high enough to explore this. Based on the observed FWHM and the depths of these bands, a grain size lower limit of 700 μm was determined.
The authors found an enhancement of SO2 frost within Io's bright equatorial regions, such as Bosphorus Regio, Colchis Regio, Tarsus Regio, and Media Regio and a significant deficiency in SO2 frost west and southwest of Pele on Io's trailing hemisphere. The authors compared these results to similar studies conducted using Voyager ISS data (McEwen 1988) and Galileo NIMS data (Carlson et al. 1997 and Douté et al. 2001). The maps created by Laver and de Pater agree fairly well with McEwen and Carlson's weaker band results, but disagree strongly with Douté and Carlson's stronger band results. The differences observed are likely due to the latter maps showing the distribution of small grained SO2 while the results from weaker bands, like those observed by Laver and de Pater, are sensitive to larger grained ice.
The maps by Douté, for example, show higher concentrations of small-grained SO2 at mid-latitudes, particularly north and south of Pele. Laver and de Pater theorize that this difference in the distribution of small- and large-grained SO2 frost results from frost grains annealing in the warmer, equatorial regions (similar to aging snowfields on Earth, just imagine the difference between walking on freshly fallen snow and snow that's been on the ground for a few days, though some of that is due to shrinking pore space between ice grains and diurnal freeze/thaw cycles). At mid-latitudes, frost grains are cooler, and don't anneal to the degree they do at the equator.
Overall, an interesting paper that was quite easy to follow, despite being a spectroscopy paper ;) The authors hope to acquire additional observations of these regions to improve the signal-to-noise ratio of their maps.
Link: The global distribution of Sulfur Dioxide ice on Io, observed with OSIRIS on the W.M. Keck telescope [dx.doi.org]