Neutral Na, K, Cl, S, SO and ions of O, S, CI S02, SO, H2S, Cl, S ions

Primarily S02 S8(P) Hkely S4 most likely No olivine Ubiquitous feature, enhanced in southern polar region, depleted in Pele plume deposit Ubiquitous frost Production rate extremely small compared with resurfacing rate

Enhanced concentration in bright equatorial S02 snowfields Localized feature H2S unstable on Io Detailed analysis unpublished.

See Figure 9.4. Powerful method for Io composition studies

Upper limits for other species given by Na et al. (1998) H2S signal sporadic, noisy

In the vapor phase, between 473 K and 1,273 K, molecules with 2-10 atoms are formed and some of these molecules exist as two different isomers (S4 in particular). At low temperatures, the cyclic S8, S7, and S6 forms dominate, but at higher temperatures S3 and then S2 are the most abundant. S4 is most abundant at 900 K (Steudel et al, 2003).

Liquid sulfur at low temperature contains rings with 6 to at least 35 atoms and probably even larger rings and polymers, denoted SM. Above 523 K the small chains S2, S3, S4, and S5 are also present.

In the solid state many of these allotropes form one or more stable crystals. For example, the most stable form of sulfur is S8, the cyclo-octal form sometimes called ^-S, which can crystallize into orthorhombic and two different monoclinic forms, denoted S8(a), S8(p), and S8(^), respectively. High molecular weight sulfur molecules, polymeric sulfur, denoted S^ or SM, form long chains and probably contain large ringed molecules as well. The a-form of S8 is the only thermodynamically stable form below 367 K and assemblages of the more unstable molecules eventually revert to S8(a) at annealing rates that are not well established. S8(p) can be long-lived below 198 K. The polymeric form SM and S8(p) have been studied by Moses and Nash (1991), who showed that these forms can exist as long-lived metastable species on Io.

Solid sulfur phases are bound by Van der Waals forces so the crystals are friable and have low melting points. The melting points for S8(a) and S8(p) are about 388 K and 393 K, respectively (Eckert and Steudal, 2003).

Spectroscopic properties. The S2 molecule absorbs in the Schumann-Runge band B3£g ^ X3 £ ~ from the ultraviolet to 500 nm or greater, with the band origin occurring at 316 nm, and appears pale violet in the vapor state (Meyer et al., 1972; Eckert and Steudal, 2003). S3 has an absorption band with diffuse structure centered at 400 nm and extending to ^500 nm. S4 exhibits two continua, a band centered at about 530 nm and absorbing between ^460-590 nm that is attributed to the C2v isomer and a weaker band of the C2h form at 625 nm (Meyer et al., 1972; Eckert and Steudal, 2003). It is the 530-nm S4 (C2v) system that causes sulfur vapor to appear red (Meyer et al., 1972). The molar extinction coefficient for S3 is ~ 10 x that of S4. Weak bands around 750 nm are observed in the vapor but their origin is unknown (Meyer et al., 1972). S8 vapor shows a strong absorption band at ^280 nm, aminimum or inflection point at 245 nm, and a stronger band extending down to 210 nm and below (Bass, 1963).

The red absorbing tetrasulfur S4 can be formed during co-deposition of S2+Kr when S2: Kr > 1: 200, forming a red film. Films of S2 in Kr produced with lesser amounts of S2 yield S4 when irradiated with visible light (Meyer and Stroyer-Hansen, 1972). Annealing of S2 in a Kr matrix also produces the 530-nm feature of S4 (Meyer et al., 1972).

Liquid S8 at its melting point (393 K) is yellow due to the strong ultraviolet absorption and vibrational broadening that extends the wing of the absorption band into the blue end of the visible spectrum. As the temperature rises, the absorption shifts to longer wavelengths and other catena-S and chained-S contribute to the absorption. Between 573-973 K, the 400-nm absorption band of S3 becomes apparent, and this band and the 530-nm band of S4 (C2v) are found between 773-1,173 K. The deep red and red-brown color of liquid sulfur above 673 K has been attributed to a mix of greenish-yellow S3, the purple-red S4, and short-chains that can absorb at longer wavelengths. For temperatures >673 K, the increased density of chain radicals produces an absorption band at —950 nm due to excitation of the chain ends - dangling bonds (Hosokawa et al., 1994; Eckert and Steudal, 2003). The absorption can extend to 1.3 ^m and beyond.

Condensed sulfur vapor and quenched sulfur melts frozen at low temperatures exhibiting various colors ranging from black, green, or red that arise from small molecules and radicals trapped in the solid (Eckert and Steudal, 2003). Vapor condensed at 77 K is yellow for furnace temperatures of 415-475 K, green for temperatures of 475-550 K, olive green at 550-800 K, and purple for 800-1,200 K. The purple color changed to olive green when the sample's temperature was elevated from 77 K to 195 K. All films except the purple ones were stable at 195 K (Meyer et al., 1971; Eckert and Steudal, 2003; Radford and Rice, 1960; Chatelain and Buttet, 1965). Quenched red sulfur is metastable at 77 K and converts to yellow polymeric sulfur at 194K (Meyer et al., 1971).

In the solid phase, cyclic and polymeric sulfur compounds absorb strongly in the ultraviolet with a wing extending into the visible due to thermal excitation of ground-state vibrational levels and, for S8, phonon-assisted indirect transitions (Eckert and Steudal, 2003). This absorption causes these molecules to appear yellow at room temperature and, if not exposed to ultraviolet radiation (see below), they become white (for S8, S12, S20) or light yellow (S6, S7, S10) at Io-like temperatures (Eckert and Steudal, 2003). Absorption spectra of S8 and polymeric S show an absorption maximum at —275-280 nm, an absorption minimum at 250 nm, and strong absorption at shorter wavelengths, similar to the absorption properties of S8 vapor (Nelson and Hapke, 1978; Sill and Clark, 1982). Sulfur is so absorbing below 400 nm that the reflection properties of most allotropes resemble metals, yielding a flat reflectance spectrum from Fresnel reflection. The 350-500-nm absorption profiles of S8(p) and SM differ somewhat from the S8 profile (Moses and Nash, 1991). Impurities in sulfur can also alter the absorption and spectral properties (see below).

Raman and infrared spectra are reviewed by Eckert and Steudal (2003) for many allotropes. The infrared-active lines of S8(a) include the bending transitions at 190200 cm-1 and 240 cm-1 and stretching transitions in the 465-480 cm-1 region. The infrared spectrum of S8(p) is not available. Polymeric sulfur SM exhibits a strong band at 460 cm-1 and a weaker one at 423 cm-1.

Photolytic and radiolytic properties. Under ultraviolet photolysis, white S8 at 77 K turns yellow (Steudel et al., 1986; Hapke and Graham, 1989), possibly due to generation of S3. Other allotropes become intense yellow (S7, S10), grayish-yellow (S12, S20, SM), or brownish-yellow (S6). S8 stays yellow while the allotropes revert to normal yellow upon heating to room temperature. The timescale at Io's illumination level is a few hours to establish color, and up to a few weeks to achieve equilibrium (Steudel et al., 1986). Photolysis of S8 in solutions produces bands at 325, 420, 530, and 600 nm (Casal and Scaino, 1985; Nishijima et al., 1976). These are likely from S3

(400-nm band) and S4 (530- and 600-nm bands), suggesting that S8 photolyzes to S3+ S5 and S4+ S4. The band at 325 nm may arise from the S5 molecule but its absorption spectrum is unknown (Eckert and Steudal, 2003).

Energetic electrons and ions bombarding Io's surface will initiate chemical reactions and produce new molecules. These radiolytic reactions are approximately independent of the specific type of ionizing radiation (e.g., electrons, ions, 7-rays, x-rays). Proton irradiation of S8 at 20 K produces multicolored samples that become black-brown-dark brown at 144 K (Moore, 1984). Under 7-ray irradiation S8 turns deep red or red-brown and this color remains stable only at low temperature, rapidly reverting to yellow upon warming to room temperature (Radford and Rice, 1960). Nelson et al. (1990) performed x-ray irradiations of S8 and found absorption bands at 420 and 520 nm, consistent with the formation of S3 and S4. The 420 -nm S3 feature disappears upon warming to 180 K, but the 520-nm feature remains, reduced somewhat in strength. S3 produced in an electric discharge disappears when warmed to 130 K while S4 disappears between 130 and 180 K, producing S8 (Hopkins et al., 1973). Photolytically produced S4 has a lifetime of ~60 hours at 171 K (Meyer and Stroyer-Hansen, 1972). Sputtering of S8 yields mainly S2 but atomic sulfur and all molecules up to S8 are present at the ^10% level (Boring et al., 1985; Chrisey et al., 1988).

Impurities in sulfur. As with ice, quartz, and many other minerals, optical transmission into, and scattering from, the interiors of sulfur crystals enables disseminated absorbers (impurities) to modify the effective reflectance spectra of dirty sulfur. This effect has been noted for natural sulfur samples (Kargel et al., 1999) and is further indicated in Figure 9.2 for the case of laboratory controlled disseminations of pyrite (FeS2) in sulfur (Kargel et al., 2000; MacIntyre et al., 2000). The pyrite imposes differing spectroscopic effects depending on both its grain size and its abundance, and also on the grain size of the sulfur.

Trace amounts of other types of impurities can have even more drastic effects on the spectral properties of sulfur if the impurity either ruptures the polymeric bonds in sulfur or tangles them. In general, elements close to sulfur in the periodic table of the elements have an affinity for sulfur, but unlike chalcophile transition metals (such as Fe, Ni, Cu, and Mn), these elements also have significant solubilities in molten sulfur. The strong chalcophile affinities of many elements has been noted in analyses of natural sulfur samples (Kargel et al., 1999). When these molten mixtures crystallize, the impurities commonly attach to the ends of polymer chains or intrude within them, thus modifying the polymeric state and other physical properties of the sulfur. Since polymer chain length can be large, even small amounts of these impurities can have a large effect on polymerization and spectral reflectivity. This is shown in the case of tellurium in sulfur in Figure 9.3 (Kargel et al., 2000; MacIntyre et al., 2000).

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