From the comments on the 2009 Fall AGU meeting abstracts post back in October, I think it might be useful to have an overview post on what is known about Io's composition and the volatile chemistry that takes place at Io's volcanoes and in its atmosphere. Much of the information I will present here is based on models of the kinds of chemical reactions that are thought to occur, but a few key measurements do underlie this discussion. This first is Io's bulk density, derived from measurements of Io's size and Io's effect on passing spacecraft and its fellow Jovian satellites. The latter measurement allows for an estimate of Io's mass. Second, spectroscopic measurements of Io's surface and atmosphere provide details on the sulfurous volatiles that are common at Io's volcanoes and cover the bulk of Io's surface. Finally, in situ and spectroscopic measurements of the composition of the Io Plasma Torus, a belt of charged particles co-orbital with Io, provide hints to the atomic breakdown of compounds that escape from Io and its atmosphere.
Io's Interior Composition
Io has the highest bulk density (3.53 g/cm3) of any object in the outer solar system. This high density suggests that Io is composed primarily of silicates with a metallic iron or iron sulfide core. Unlike nearly all the other moons in the outer solar system, very little water exists on the surface as Io formed inside the Jovian "snow line", where relatively little water condensed compared to those moons outside the snowline like Ganymede and Callisto. What little water Io did retain was later lost as Io's became a more active body. Thus, sulfurous compounds became the dominant volatiles on Io as the original metal sulfides in Io's interior became oxidized.
Based on Io's density and moment of inertia measurements (which allow for estimates of the size of Io's core), Io's bulk composition is thought to match that of ordinary L- and LL-chondrites, based on modeling work by Kuskov and Kronrod 2001. This suggest a low metallic iron content, with most of the iron and other metallic elements (like magnesium, aluminum, and titanium) tied up in oxides. Model runs by Keszthelyi et al. 2007 assumed a refractory composition of 36% SiO2/30% FeO/25% MgO bulk composition with the majority of the iron tied up in the core. The rest of the bulk refractory composition was taken up by additional oxides with potassium, calcium, sodium, and aluminum.
Io's core consists primarily of iron with some unknown percentage of iron sulfide (up to 37% by weight for the iron/sulfur eutectic. Because the amount of sulfur in the core is not known, the size of Io's core is only known as a range of sizes from 37% (assuming pure iron) to 52% (assuming an Fe-FeS eutectic mixture) of Io's radius.
Based on temperature estimates from Galileo and ground-based observations of active volcanoes and near-infrared imaging by Galileo, Io's dark lava flows, diffuse pyroclastic deposits, and lava lakes are thought to be mafic to ultramafic in composition, high in magnesium and iron oxides and low in silica. Minerals typically found in mafic basalt flows include plagioclase feldspar, olivine, and pyroxene. The identification of Io's lava flows with basalt (rather than sulfur, as presumed following Voyager) is based in part on the high temperatures measured by the SSI camera on Galileo. Initial temperature estimates in McEwen et al. 1998, based on the ratio of the observed brightness between the clear and 989 nm filters of Pillan during the summer 1997 eruption and other volcanic centers like Pele and Kanehekili, suggested ultramafic compositions for at least some of Io's lavas. However the lower limit of 1600°C was found to be an overestimate, as new cooling models taking lava fountains into account and reprocessing of the Galileo data, suggested lava temperatures between 1250 and 1350°C, more in line with models of Io's mantle and tidal heating and with ordinary mafic compositions. However, these estimates may underestimate the eruption temperature as the observed temperatures may be several hundred kelvin cooler after only a few seconds of cooling, so ultramafic compositions (less iron and more magnesium than regular mafic magmas) are not completely ruled out. In addition, the eruption temperature may not be reflective of the liquidus temperature of the magma due to super-heating of the magma as it ascends to the surface.
Another piece of evidence toward the composition of Io's lavas is the presence of an absorption band at 0.9 μm associated with dark regions on Io found in SSI images taken with the 889 nm filter (identified in Geissler et al. 1999). This absorption band has been associated with orthopyroxene, either the magnesium end member mineral enstatite (Mg2Si2O6) or the magnesium/iron mixture mineral (what used to be known as hypersthene). Either mineral is consistent with a mafic or ultramafic composition for Io's primary lavas. The model Io lithosphere used by Keszthelyi et al. 2007 (which would consist primarily of cooled lava flows) is similar in composition to terrestrial tholeiitic basalt, but with less silica (SiO2) and titanium oxide and more magnesium oxides.
The predominate volatiles, i.e. chemicals that can be sublimated or condensed at normal Io temperatures, are sulfur and sulfur dioxide (SO2). In fact, SO2, the SO2 photolysis product sulfur monoxide, and the various allotropes of sulfur are the only volatiles that have been definitely identified on Io's surface and in its atmosphere and volcanic plumes. These two are also largely responsible for Io's colorful appearance. Course grained sulfur dioxide is responsible for the white-gray regions seen across Io's surface, including the large Colchis and Bosphorus regions seen in the color C21 mosaic. Finer grained sulfur dioxide is more transparent at visible wavelengths, but can be identified using near-infrared absorption bands. Band depth and width maps using near-infrared spectral data from Galileo has been used to create maps of SO2 abundance and grain size across Io in paper such as Doute et al. 2001. Sulfur dioxide is also the dominant chemical species in Io's plumes (from re-volatilized surface frost) and atmosphere and the deposition of which can produced bright regions surrounding volcanic plume vents. Finally, sulfur dioxide maybe a primary lava in some areas, such as the bright floor of Balder Patera, during the early stages of patera formation as terrain above a sill starts to melt.
Sulfur in various forms can be seen across Io's surface as red, red-brown, orange, and yellow region across its surface. Diatomic sulfur (S2) is outgassed from Io's interior during volcanic eruptions, in some cases forming large plumes (along with condensing sulfur dioxide) such as those at Pele or Tvashtar. S2 is quickly reorganized into reddish S4, by photolysis, when it is deposited on the surface, helping to create the large red rings seen around some active volcanoes on Io. Over time, continued photolysis builds sulfur into the stable cyclic S8 form, which is yellowish in color. This is why plume deposits from briefly active volcanoes eventually fade back to the earlier appearance from before the eruption (like at Grian Patera). At Io's poles, where charged particles can more easily reach the surface, cyclic sulfur can be broken back down into S4 form, producing Io's dark, reddish-brown polar regions.
Additional volatiles have been suggested for Io based on models of Io's volcanic gas chemistry, tentative identification of absorption bands in near-infrared spectra of Io's surface, and spectra of the neutral cloud that surrounds Io. For example, additional sulfur oxides are likely in Io's atmosphere, such as sulfur monoxide (SO) and polysulfur oxide (SxO) based on models of Io's gas chemistry. Additional thermochemical models of Io's volcanic gases suggest that sodium chloride would be a dominant salt in Io's plumes, and this is support by the identification of Na+ and Cl- in the Io Plasma Torus and NaCl in the dust streams that radiate out from Jupiter and have been associated with Io. Potassium chloride is also likely. Sulfuryl chloride (Cl2SO2) was tentatively identified at 3.92 μm within the reddish plume deposit at Marduk by Schmitt and Rodriguez 2003. Those authors also suggested that Cl2S might be the cause for the red color of the deposit, though how this fits with the ability for other reddish deposits to fade rather quickly is not certain. Kargel et al. 1999 attributed Io's reddish material to impurities in Io's volcanogenic sulfur, such arsenic and selenium, which can drastically change the color of sulfur even at very low concentrations (~1%). They also suggested that the green color of some paterae on Io, like Chaac Patera, may result from the interaction between sulfur and cooling, iron-rich lavas, forming pyrite.
Finally, water or at least hydroxyl may have been identified on Io by way of a 3.15 μm absorption band and a broad one found at 3 μm in the low spectral resolution NIMS data from the flybys. The 3.15 μm band was initially found in ground-based data by Salama et al. 1990 and identified with either H2O or H2S. However the lack of a corresponding 2.97 μm feature suggests another culprit for this absorption band, perhaps HCl. The 3 μm band is observed in high-spatial, but low-spectral, resolution data at several mountain structures, such as Gish Bar Mons, Tvashtar Mensae, and Tohil Mons. One possible explanation is that these features maybe the result of water ice or hydrous minerals deposited on Io by small cometary impacts in the last million years that have been brought back to the surface by the uplift of these mountains. However the lack of other water ice absorption bands at 1.48 and 2.0 μm led Granahan in 2004 to look for another compound that might create the observed band at 3 μm. He identified pyrite (FeS) or pyrrhotite as possible compounds responsible for the absorption bands.
Of course many of the chemical identifications on Io (save sulfur and sulfur dioxide) are either tentative or are based on chemical models of Io's volcanic gases or photolysis of gases in its atmosphere and plumes. Additional spectroscopic studies with much higher spectral resolution than what was obtained by Galileo during its Io flybys will be needed to settle many of our questions about Io's surface composition. Information on the eruption temperature of Io's lavas, in situ mass spectroscopy of its atmosphere and plumes, and gravity estimates of its interior structure will also be needed to refine our knowledge of Io's bulk composition. The measurements will hopefully await us in the 2020s with IVO and JEO.