No sulfur flows on Io

ICARUS 58, 197--226

(1984)

No Sulfur Flows on Io ANDREW T. YOUNG Astronomy Department, San Diego State University, San Diego, California 92182 Received March 28, 1983; revised January 10, 1984 Physical and chemical properties of elemental sulfur are incompatible with the suggestion that the colored flows associated with volcanoes on Io are quenched unstable allotropes of sulfur. Either the volcanic flows are not sulfur, or some mechanism other than quenching is required to produce colored forms of sulfur in them. The properties of sulfur are unsuited to the production and survival of colored unstable allotropes on Io. The color of this object is probably due to some other material, possibly iron compounds.

INTRODUCTION

Elemental sulfur has been a popular candidate for the yellow coloring of Io for the past decade. Nash and Fanale (1977) reviewed the early history of this suggestion, which Nelson and Hapke (1978) enlivened by suggesting that various rare and unstable, but vividly colored, allotropes might be responsible. Recently, Smith et al. (1979a), Sagan (1979), and others have argued that the colored flows seen around volcanic vents on Io by the Voyager cameras are elemental sulfur. However, because the $8 molecule is colorless at temperatures below about -50°C or 220°K, and the surface of Io is near 130°K, these authors must also invoke unstable allotropic forms of the element that are produced in the laboratory by quenching. Gradie et al. (1980) have offered similar arguments as a possible explanation of colors on Amalthea, suggesting that the sulfur comes from the Io torus instead of from volcanic flows. Sinton (1982) has investigated the thermal history of sulfur flows on Io, which he compares to flows of pahoehoe on Hawaii. I have spent several years studying the properties of sulfur, in hopes of using it to explain the yellowish color of the clouds of Venus. In the process, I have found that a number of assumptions commonly made by

planetary scientists about the properties of sulfur are incorrect. I believe that elemental sulfur is a much poorer candidate for the Io coloring than is generally supposed. The present paper adduces the relevant properties of the element and applies them to conditions on Io. COLORS ON IO

Because much of the argument for exotic sulfur allotropes on the inner Jovian satellites hangs on the brilliant colors displayed in the Voyager pictures (Smith et al., 1979b), some preliminary remarks about these colors are necessary. The colors in pictures described by Smith et al. (1979b) as having "normal color" are in fact already quite unrealistic and exaggerated, partly because of choices made by the Voyager experimenters, and partly because the lack of camera response functions similar to those of the human eye forces all such color reconstructions to be "false." In particular, the use of the Voyager violet filter band with an effective wavelength of about 0.41/~m for the " b l u e " channel of a trichromatic display which is perceived by the eye's receptors with peak response near 0.45/~m makes the pictures too red--especially those of Io, which is much darker at 0.41 than at 0.45 /zm (Soderblom et al., 1980).

197 0019-1035/84 $3.00 Copyright © 1984by Academic Press, Inc. All rights of reproduction in any form reserved.

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ces (B-V = + 1.2 for Io, but + 1.4 for Mars) bear this out, although Io is redder in U - B than Mars. But in the visually important green-red (V-R) color index, Mars is redder by nearly half a magnitude; that is why Mars looks red, but Io does not. The spectral reflectance of Io supports the impression of experienced visual observers that its average color is pale yellow. Thus, the most bathochromic regions of Io are slightly oranger than powdered orthorhombic sulfur at room temperature (see Fig. 2), so we need not try to explain " r e d " colored material on Io. Pale yellow will suffice. As will be shown later, even this presents serious problems when the thermochromism of all sulfur molecules is taken into account.

wavelength, microns COOLING OF LIQUID SULFUR FIG. 1. Average reflectance spectrum of the Io disk (Nelson and Hapke, 1978) and " r e d " regions on In (see Appendix A), compared to "typical" yellow, orange, and red materials (Hardy, 1936; MacAdam, 1981). Note that Io's reflectance falls below about half of its long-wavelength value only in the blue part of the visible spectrum, near 4500 ~., and is much richer in both blue and green light than even a typical yellow material. The " r e d " material on Io bears little resemblance to a "typical red" material. See Appendix A for more details on the colors on Io.

Figure 1 compares the reflectance spectrum of one side of Io (Nelson and Hapke, 1978) and that of " r e d " regions on Io (see Appendix A) with those of "typical" yellow, orange, and red materials (Hardy, 1936; MacAdam, 1981). Io is clearly both grayer (more neutral) and bluer than even a typical yellow material. Appendix A shows in detail that the average color of Io is a grayish, slightly greenish, pale yellow; the " r e d " regions are actually a moderate, grayish yellow that is certainly not reddish, and may be slightly greenish. Thus, Sagan's (1979) assertion that "Io has the reddest surface in the Solar System" is clearly an exaggeration, if visual color is intended. The standard color indi-

Meyer (1976) cautions that "great experimental care must be taken if a significant fraction of the high temperature species is to be trapped, because these particles recombine quickly and because sulfur is such a poor thermal conductor . . . . Slowly quenched polymeric sulfur is yellow. Polymer quickly quenched to 76 K is red, because it contains small molecules which recombine at -100°C. '' Slowly cooled sulfur reverts entirely to $8, which is yellow at room temperature and colorless at the temperatures common on Io. Even the thin strands of plastic sulfur produced by pouring boiling sulfur into cold water in the laboratory have a sufficient quenching gradient to revert to orthorhombic crystals from the center outward (Daguin, 1845; Kastle and Kelley, 1904). Evidently, any flow on Io extensive enough to be photographed by Voyager must be thick enough to have cooled slowly in its interior, and should have reverted entirely to (white) orthorhombic sulfur, except perhaps for a very narrow, thin region at the edges (which, being only a few centimeters wide at most, should be invisible in the flyby pictures).

NO SULFUR FLOWS ON IO

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FIG. 2. Chromaticity diagram for Io. Standard white illuminant C, intended to represent average daylight, lies near the left edge; greener colors lie above it, and reds to the right. The upper right corner of the figure just touches the spectrum locus at 584 nm. Heavy line: locus of true Io colors (see Appendix A). Crosses: false Io colors shown on Fig. 16 (the "pizza picture") of Smith et al. (1979b), estimated by comparison with the Munsell Book of Color, and converted to chromaticities by the tables of Newhall et al. (1943). Colors for green and purple sulfur were similarly converted from their Munsell designations given by Rice (1962). The open square marked "orthorhombic sulfur" shows the color of flowers of sulfur at room temperature, estimated on the Munsell system (TY 8.5/6.5) and likewise converted to chromaticity coordinates. The filled squares are the colors of different sizes of powdered orthorhombic sulfur at room temperature, converted to chromaticities from the data of Gradie and Veverka (1984).

Sagan (1979) has estimated the cooling time for a sulfur flow 10 m deep as at least 2 months. Sinton (1982) estimates the time for a melt to reach the freezing point of sulfur as several hours; but his calculation neglects convection in the pool of sulfur and so provides only a lower limit to the cooling time. These time scales are too long to permit significant amounts of colored sulfur allotropes to survive in the final solid material. Steudel and Mfiusle (1981) have shown that 12 hr suffice to establish thermodynamic equilibrium in liquid sulfur, even at its melting point. Steudel (1982) discusses the interconversion reactions of sulfur rings; he finds that the half-life for formation of the unstable colored species $6 and $7 from $8 is about 3 hr at 120°C and only 64

min at 150°C. But the reverse reaction is much faster: S 7 decomposes with a half-life of 31 min at 150°C. Thus, a slowly cooling mass of liquid sulfur will contain only the equilibrium trace amounts of the colored molecules if the time spent near the melting point is a few hours. Large bodies of liquid sulfur freeze from the bottom up, because monoclinic sulfur is about 5% denser than the liquid at the melting point. While a thin crust of solid sulfur can be supported from the edge of a small pool, its low tensile strength of 60 psi (Dale and Ludwig, 1965) makes a layer ! cm thick collapse of its own weight (based on standard beam-bending theory) if it is more than 3 m wide on Earth, or 7 m wide in the lower gravity of Io. The maximum width

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increases only with the square root of the Such large optical paths would be possithickness, so a slab 1 m thick can be at most ble if large transparent crystals existed in ten times these widths. (They are likely to sulfur which has largely reverted to the stabe overestimates, because both the fall of ble form. But the form that crystallizes the liquid level with freezing and the bend- from the melt is monoclinic sulfur, which ing of the slab by thermal gradients tend to itself is unstable; at about 96°C (369°K) it deprive it of the full buoyant support of the converts to the orthorhombic form. This liquid, which was assumed in this calcula- solid-state phase transition destroys the tion.) Hence this effect is negligible for the structure of the monoclinic crystals and kilometer-sized flows on Io. The sulfur left produces cryptocrystalline pseudomorphs at the surface of a large frozen pool thus that readily crumble into a fine powder. The was the last to freeze; any frozen-in high- orthorhombic crystallites are so tiny (and temperature molecules would be buried at the refractive index of sulfur so high) that the bottom and hidden from sight. the average optical path in the resulting maLiquid sulfur freezes at about 390°K, terial is a small fraction of a millimeter. In with a latent heat of about 9.2 cal/g. The fact, Wamsteker et al. (1974) have shown radiative cooling rate is about 320 c a l m -2 that the thermal inertia requires the surface sec -~, which increases the depth of frozen of Io to be a low-density powder if it is sulfur by about 18 /zrn/sec, or 6.6 cm/hr. made of sulfur. Matson and Nash (19~t3) The radiative cooling exceeds conductive have discussed the surface structure of Io cooling through the bottom of the pool if in detail, and suggest still lower surface the layer of solid at the bottom is thicker densities. than a few centimeters. Hence, pools of Furthermore, Sagan's (1979) statement sulfur a meter or more deep will take more that the color of liquid sulfur is "prethan 12 hr to freeze, which allows all of the served" on quenching is not strictly true. material visible at the surface to reach ther- The colored allotropes, like ordinary modynamic equilibrium. Thus, any sulfur orthorhombic sulfur, owe their colors to ulflow on Io deeper than a meter will have a traviolet absorption bands whose tails exsurface composed entirely of $8, which is tend into the visible; not, as stated by Nash colorless at Io's surface temperature. and Fanale (1977), to the fundamental abSagan (1979) has estimated the thickness sorption edge due to transitions into the of a flow large enough to be typical of those conduction band. [Emerald et al. (1976) in the Voyager pictures as 10 m. Sinton showed that this latter edge is located at (1982) estimates several times this. Such wavelengths shorter than 1000 A. The misflows have very long cooling times. Thus, taken "edge" terminology occurs widely in they cannot be "quenched" sulfur. both the planetary and the chemical literature.] These bands broaden with increasing COLORED S U L F U R MOLECULES temperature and narrow at low temperaIt might be supposed that even a trace of tures according to the classical "phononhighly colored sulfur molecules in ordinary broadening" mechanism well known to sulfur might impart significant color to the solid-state physicists [see, for example, Di bulk mixture, if the optical path in the mix- Bartolo, (1968)]. Thus, the color of all aUoture were large. This seems to be what Sa- tropes fades somewhat at low temperagan (1979) had in mind in his Table 1, which tures. lists "minimum t h i c k n e s s e s . . , of films of In fact, the rate of shift with temperature quenched liquid sulphur" as large as "tens of the visible tail of the absorption in "plasof centimeters" for yellow features on Io. tic" sulfur (produced by quenching hot liqEven for red features he suggests optical uid sulfur) was measured by Fukuda (1921) paths of millimeters. and found to be about three-sevenths as

NO SULFUR FLOWS ON IO large as that of freshly crystallized sulfur. Thus the colored allotropes---in particular, quenched sulfur--will be much less intensely colored at 130°K than at room temperature, and room-temperature colors should not be compared with colors seen on Io. Miller (1971) has observed the color of sulfur quenched from 190°C in liquid nitrogen. He found that "as the molten sulfur was quenched and approached the temperature of liquid nitrogen, the yellow color that was typically sulfur was bleached so that the sulfur sample appeared to be colorless and optically transparent. As the sulfur sample was warmed, the yellow color abruptly reappeared" near - 145°C (128°K). "The initial yellow color is a very faint canary-like color." This agrees with Meyer's (1976) remark quoted above. While Miller (1971) displays an amazing ignorance of the properties of sulfur, even confusing rhombic with rhombohedral sulfur, and many of his conclusions have been refuted by Currell and Williams (1974), I believe this simple observation is trustworthy. Miller's "faint canary-like color" is about the most intense color to be expected of sulfur on Io. Sagan (1979) claims that as much as 1% of frozen melt can be the highly colored $3 and $4 molecules, but he seems to have misread a paper by Meyer et al. (1972a), who gave abundances of 0.1 to 3% $3 and $4 for liquid sulfur itself, not the quenched melt. They demonstrated these molecules in a red glass at 77 K, produced by quenching a thin film of boiling sulfur in liquid nitrogen. These features do not appear in their spectra of liquid sulfur at 250°C (523°K), nor in their spectrum of polymeric sulfur in glycerol. They are unlikely to appear in naturally produced samples outside the laboratory because of their extreme reactivity: they are essentially frozen free radicals in the red glass described by Meyer et al. (1972a), which "fades at -90°C [183K] to deep yellow." In fact, Hopkins et al. (1973) found that

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S 3 disappears from quenched material on warming from 80 to 130°K. $4 was somewhat more durable, but disappeared somewhere between 130 and 180°K. Their identifications were based on Raman spectra, which provide much more specific and detailed information, and thus much more reliable identifications, than the broad bands in electronic spectra. Meyer and StroyerHansen (1972) found some disappearance of infrared bands they attributed to $4 on warming quenched sulfur vapor from 20 to 76°K, and gave a "lifetime" of 57.7 hr at 171°K for $4. [Their paper is somewhat confused by the attribution of bands at 483 and 270 cm -~ to $4; these are actually due to $7 (see Steudel and M~iusle, 1977), and the band Meyer and Stroyer-Hansen attributed to $4 at 320 cm -1 may be the $6 infrared band at 313 cm-l.] The survival of $4 on Io seems unlikely, and $3 appears to be impossible. Similarly, the suggestion by Gradie et al. (1980) that the greenish material on Amalthea can be explained by sulfur molecules such as linear $5 and $6 chains is unlikely. They relied heavily on the paper by Meyer et al. (1972a), and reproduced (with errors) a figure from this paper that purports to show the locations of band centers for open chain sulfur molecules. As this paper was also Sagan's (1979) only reference for the optical properties of sulfur, it deserves some discussion.

SOME DUBIOUS DIAGRAMS Gradie et al. (1980) reproduce a figure (their Fig. 10) that they attribute to Meyer (1972a), in which two points corresponding to $5 and $6 chains are indicated to be "calculated" by Gradie et al., instead of "observed" as Meyer et al. label them. The theoretical transition energies in this paper by Meyer et al. (1972a) are certainly wrong, as Miller and Cusachs (1969) had already demonstrated that "correlation of spectral properties to computed data makes no sense unless 3d orbitals are included in the

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basis set," which Meyer et al. (1972a) did not do. In a later paper, Meyer and Spitzer (1972) refer to these results as "preliminary," and the calculated points are considerably changed; presumably, this is due to the inclusion of 3d orbitals in the calculation of the excited states, which is necessary to obtain correct results for the spectra of molecules containing S-S bonds (Miller and Cusachs, 1969; Webb et al., 1973; Salaneck et al., 1975). Furthermore, the "observed" point for $5 chains has been deleted without explanation. The only evidence previously published by these workers for the sulfur absorption near 6200 A is Fig. 5d in a paper by Meyer et al. (1971), which shows a broad band in the spectrum of $4C12 photolysis product in isopentane-cyclohexane glass at 77°K, produced by exposure to a high-pressure mercury arc. They suggested that "the weak band at 6200 ,~ in photolysed 84C12m i g h t . . , belong to impurities, perhaps $5 or even higher sulfane derivatives." Subsequently, Meyer et al. (1972b) published a figure that shows several ill-resolved bands near 6200 ,~ produced by "annealing $2 in krypton at 20 K . . . . It is not certain whether these bands form two systems, and whether they belong to $6 and $8 chains, or whether they could be due to forbidden transitions of $3 and $4, or even due to intrinsic impurities." This is certainly shaky evidence on which to base any claim for "experimental" data on $5 or $6 chains. The same spectral region was investigated in liquid sulfur by Zanini and Tauc (1977), who saw no band structure at all in this region--only a weak, sloping continuum that appears to be the far wing of a very broad band centered in the blue or ultraviolet. They investigated liquid sulfur from its melting point up to about 200°C. From the slow growth of this broad feature in repeatedly reheated melts, it appears to be due to either $6 or $7 rings, which gradually accumulate in melted sulfur (Steudel and M~iusle, 1981). [Steudel's

recent papers indicate that the discussion by Zanini and Tauc (1977)is probably worthless.] However, a broad absorption feature near 9000 .A was observed by Weser et al. (1978), who found that its abundance seems to be closely proportional to that of diradical sulfur chains in high-temperature liquid sulfur, as indicated by electron spin resonance data. They attributed it to polymeric sulfur chains. In a later paper, Hensel (1980) shows spectra with a very broad feature in the 6000-~ region, but attributes it to the 5200-.A band of $4, broadened by temperature (II00°C = 1373°K) and pressure (dozens of bars). In these spectra, the longwavelength feature is only a shoulder and not a well-defined band. Only the very artificial laboratory techniques, which seem to require much lower temperatures than occur on the Galilean satellites, have produced a distinct feature, such as Gradie et al. (1980) require. The situation is further confused by a third publication by Meyer (1976), in which yet a third version of his figure appears. In this version, which resembles the "preliminary" version of Meyer et al. (1972a) in the placement of the points for sulfur chains, the labels for "observed" and "computed" points are interchanged, as in the Gradie et al. paper. (This may be the version actually used by Gradie et al.) This may be just a typographical error in the caption; but it is not a simple reproduction of the first version, because some of the points for large rings have been changed. Again there is no explanation of the changes; Meyer merely cites his earlier publications, in which the figure .is different. In this paper, Meyer (1976) also cites the unpublished thesis of his student Oommen (1970). The only suggestion in this thesis that colored allotropes absorbing in this part of the visible spectrum were observed is Oommen's Fig. 20, which shows the spectrum of a thin film of sulfur, estimated to be a few microns thick, at 77°K. The lowamplitude oscillations seen throughout the

NO SULFUR FLOWS ON IO visible part of this spectrum are clearly interference fringes in the sulfur film, and not, as Oommen believed, absorption bands (see Appendix B), Meyer and his co-workers considered only planar chains of sulfur. But Bergson et al. (1962) had shown that the dihedral angle at the sulfur-sulfur bond plays a crucial role in the absorption spectrum of the disulfide chromophore. Furthermore, later workers have failed to reproduce the theoretical calculations of Meyer and Spitzer (1972), even for the planar chains they assumed [see footnote 14 of Boyd (1974)]. Snyder and Carlsen (1977) have confirmed and extended Boyd's work. It is now clear that the dihedral angles of sulfur-sulfur bonds must be included in calculating the spectra of all the molecules discussed by Meyer et al. (1972a), so that the figure copied from their paper by Gradie et al. (1980) is useless in estimating the colors of sulfur molecules. Finally, the recent work of Steudel (1981) casts doubt on the classical polymerization theory of liquid sulfur. In particular, the discovery and separation of many large ring molecules in quenched melts by Steudel et al. (1981), and the nondetection of sulfur chains in quenched sulfur melts (Steuhel and M/iusle, 1977), support the view expressed by Semlyen (1968), Harris (1970), Schmidt (1973), and others that the viscous liquid sulfur between about 160 and 200°C is dominated by rings rather than by chains of any length. Only at much higher temperatures is there convincing evidence of unpaired electrons, and hence chain-type molecules. The chains postulated by the classical theory lead to a prediction of viscosities four orders of magnitude larger than are observed (Eisenberg, 1969). In short, there is no convincing evidence that $3 or $4 are observable in solids at the temperature of Io, that the spectrum of any form of $5 has ever been observed, or that $6 ,or longer chain molecules have been identified in visible spectra. As both the theoretical and the observational bases of Meyer's peculiar figure are exceedingly

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weak, it should not be used to speculate about the colors of Io or Amalthea. INSTABILITY OF QUENCHED SULFUR

A particular weakness of any attempt to explain the colors of the inner Jovian satellites with unstable sulfur altotropes is their instability to both light and X rays, which are produced in abundance in the Jovian radiation belts. This is a basic property of all colored sulfur compounds (polysulfides as well as sulfur itself), because it is a basic property of the disulfide bond. The predissociating nature of the characteristic disulfide band near 2600 A, whose long-wavelength tail is responsible for the yellowish color of these molecules, can be seen from the lack of vibrational structure in gas-phase spectra [see Figs. 9, 12, and 14 of Thompson et al. (1966); cf. Figs. IF and 2D of Ramakrishnan et al. (1965)]. Photochemical studies (Barltrop et al., 1954; Whitney and Calvin, 1955; Wallace et al., 1976) show that the disulfide bond is cleaved with nearly unity quantum efficiency, and that the same process occurs in elemental sulfur (Nishijima et al., 1976). These observations are-understandable theoretically through molecular-orbital calculations on disulfides [see, e.g., Boyd (1974), Snyder and Carlsen (1977), and references in these papers], which show that absorption in the band responsible for the visible color moves an electron from a nonbonding to an antibonding orbital. The photosensitivity of metastable sulfur molecules is well known. Even the most stable colored rings, S 6 and $7, are readily destroyed by visible light, and nearly every worker who has published on them has remarked on the need to keep them in the dark. Frondel and Whitfield (1950) found the rate of breakdown of $6 "relatively slow if the crystals are k e p t in the dark" but "markedly accelerated by exposure to Xrays." Schmidt and Wilhelm (1965) found it stable in the dark a t r o o m temperature, but converted to $8 by way of an intermediate polymerization through the action of light,

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heat, or bases. Steidel et al. (1978) remarked on "the rapid decomposition of the $6 crystals in the X-ray beam at 25°C, '' but found "no decomposition" at -90°C (183°K). Similarly, Schmidt et al. (1968) found that $7 "can be kept unchanged for weeks in the cold and the dark," but that "under the influence of light and also storage at room temperature it changes readily into $8 by way of a polymer," and that "X-rays rapidly decompose $7 at room temperature, but good single-crystal photographs were obtained at -80°C ''. The "plastic" sulfur produced by quenching the hot liquid is more directly relevant to the colors of Io and Amalthea. The macromolecular component of this can be stabilized by rapid stretching to form crystals of "fibrous sulfur"; even so, Meyer and Go (1934) had to replace their samples so that each point of the fiber was exposed to X rays for less than 15 sec to prevent its transformation. Likewise, although their material, produced by quenching from about 400°C, was quite stable in the dark, it partially converted to rhombic sulfur upon exposure to light. [The destruction of "plastic" sulfur by sunlight was first noticed by Daguin (1845).] Trillat and Forestier (1931) had noted that unstretched plastic sulfur is even more sensitive to X rays, an exposure of a few minutes sufficing to produce a transformation visible in polarized light. M~iusle and Steudel (1981) systematically investigated the stability of these and other forms of sulfur. They found that polymeric sulfur, produced by removing small rings from freshly quenched melts with CS2, was so unstable that even the red line of a krypton laser converted it to $8 at -100°C (173°K)! As a general rule (see Bartlett et al., 1958, 1961b; Kastle and Kelley, 1904; Smith and Holmes, 1905; Brownlee, 1907), impurities, especially hydrogen sulfide and sulfur dioxide, which is known to be present on Io, accelerate the conversion of unstable sulfur molecules to $8. These ob-

servations suggest that plastic sulfur would not last long on Io, where it would be exposed to both sunlight and X rays, even if it could be produced there. The redward shift of the first electronic transition of the disulfide chromophore with displacement of its dihedral angle from the minimum-energy value near 90° (Bergson et al., 1962; Boyd, 1972, 1974; Webb et al., 1973; Snyder and Carlsen, 1977) means that, in general, the more highly strained and unstable molecules have the deeper colors. Conversely, the more colored sulfur rings not only absorb more bond-breaking photons from sunlight, but are generally less stable thermally, and require lower temperatures to resist radiation damage. THERMALEFFECTS ON IO The evidence cited above leaves much to be desired, as the various experimenters have generally not specified the radiation doses that produce measurable decomposition of unstable sulfur molecules, nor have they quantified the rate of decomposition. None of the data refer to the temperatures reached on Io. We have only the qualitative relationship that the unstable forms are more stable at lower temperatures. Together with the outstanding uncertainties in the actual age of the surface we see on Io, which may be anywhere from a few months to many years old, we cannot yet say whether the lifetimes of particular sulfur molecules are long or short compared to the age of the visible surface. However, the pale yellow color of Io suggests that the more strongly colored, more unstable sulfur molecules are absent. Furthermore, one should expect the most unstable sulfur to be selectively bleached at low latitudes, where the maximum solar heating and maximum photodissociation should occur. However, the most intensely colored type 3 regions (see Appendix A) occur at the equator and show no preference for high latitudes. This argues against their

NO SULFUR FLOWS ON IO color being due to metastable sulfur molecules. Although the temperatures at which unstable sulfur molecules rapidly revert to $8 under visible or X-ray illumination are somewhat warmer than the 130°K infrared brightness temperature typical of Io (Hanel et al., 1979a,b; Sinton et al., 1980), the actual temperature of the visible surface must also be somewhat warmer if it is sulfur. Solid sulfur has an absorption depth of several millimeters throughout most of the thermal infrared (MacNeill, 1963). But Wamsteker et al. (1974) found that a lowdensity particulate sulfur layer was required to match the observed thermal inertia of Io. This layer had a bulk density four times lower than that of solid sulfur, and a thermal conductivity near 3 x 10-5 W cm -1 oK

1.

Because the refractive index of sulfur is near 2, Fresnel reflection alone reduces the emissivity of solid sulfur below 90%, so the physical temperature must be several degrees above the brightness temperature. Scattering in a particulate medium would substantially increase this effect. But the small absorption depth for blue light, combined with the much deeper level of re-radiation (about a centimeter for the material studied by Wamsteker et al.) and the low thermal conductivity, requires a substantial thermal gradient to conduct absorbed solar radiation from the surface to the depth from which it can be re-radiated. In round numbers, the solar constant at Io is about 50 W/m2; and about 15% of this lies below 4500 ]k, where Io rapidly changes from high to low albedo (Soderblom et al., 1980). Thus, if the surface is heated by 7.5 W/m2, and this must be conducted to a depth of 1 cm to be re-radiated, the surface must be about 20°K warmer than the infrared radiating level, and hence at least 20°K warmer than the infrared brightness temperature. [After these estimates were made, similar but more detailed calculations were published by Matson and Nash (1983).1 Quenched sulfur rapidly reverts to

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the (colorless) stable form at 180°K under the influence of light and/or X rays, and probably is bleached at 150°K as well, but more slowly. This calculation refers to the average surface of Io. Areas darker than average must also be warmer. Of course, dense, solid sulfur would require a much smaller temperature gradient; but it would not agree with the observed thermal inertia of Io, according to the results of Wamsteker et al. (1974). The substantial temperature gradient expected in a particulate sulfur surface means that the spectrum of Io should display the strong cluster of vibrational fundamentals near 470 cm -1 (MacNeill, 1963; Gautier and Debeau, 1974) in emission, with a temperature contrast of some ten degrees. "~hen the above paragraphs were first written, no such feature (which should easily have been seen by the Voyager infrared spectrometer) had been reported, and I took the lack of such report as evidence against any sulfur on Io. Subsequently, in response to a preprint of this paper, John Pearl informed me that in fact the 22-/xm sulfur feature, as well as infrared-active vibrations of other molecules, actually appears in emission in the IRIS spectra with a contrast of a few percent. Evidently, some sulfur exists on Io, but it is not the only material present at the surface. The Voyager data do not, however, show any molecule of sulfur but $8, though others were searched for. This agrees with the arguments given above. Besides the lack of direct evidence for unstable sulfur molecules, there is the problem of areas that are considerably warmer than average, but that still have colors typical of cooler areas on Io. Pearl and Sinton (1982) have found many areas tens to hundreds of kilometers across that are warmer than 200°K, which is so warm that arguments for preservation of unstable sulfur molecules within them are not plausible. Yet many of these areas contain the same dark and/or colored materials seen on the

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colder parts of Io. Pearl and Sinton point out the general association of warm areas with dark features, and the incompatibility of this observation with the notion that these could be quenched sulfur. OTHER WAYS TO MAKE COLORED SULFUR

Because sulfur molecules are so unstable to light and X rays, which are surely abundant on Io, one might imagine that these agents could produce colored sulfur molecules from $8 when the sulfur flows had cooled enough to preserve the unstable molecules. In fact, Radford and Rice (1960) found that sulfur irradiated at 77°K by y rays took on a "reddish" color, similar to the "red-brown" of sulfur prepared by condensing the products of a microwave discharge in sulfur vapor on a surface cooled by liquid nitrogen. Aside from the difficulty of reconciling red material with the greenish-yellow colors on Io, this mechanism for coloring the surface has the fatal disadvantage that it would cause the oldest surfaces to show the deepest colors, instead of the other way around as argued by Sagan (1979). Metastable colored sulfur molecules might be produced by charged-particle bombardment of the surface of Io, as suggested by Nash and Fanale (1977). However, they found that irradiation with 5-keV protons produced mainly hydrogen sulfide; they could not detect any change in the spectrum of their sulfur sample. Hydrogen sulfide is a powerful catalyst that rapidly destroys colored sulfur molecules such as $6, polymeric sulfur, and "photosulfur" (Bartlett et al., 1961a,b; Smith and Holmes, 1905). Also, if colored sulfur were produced by energetic-particle impacts on the surface from the Io torus, we would not expect to see any correlation of colors with flow features. This same problem arises if the colored sulfur were produced by sudden chilling of droplets of sulfur in the volcanic plumes, or condensation of sulfur vapor near vents. Furthermore, the low emissivity and aver-

age emission depth of several millimeters in liquid sulfur means that small droplets do not cool efficiently by radiation, but only by evaporation. While this mechanism might allow some slowly cooled sulfur to be deposited near volcanic vents, the color would not be very striking. Indeed, a sulfur droplet a few microns in diameter has about 5% emissivity, so it should assume an equilibrium temperature near 173°K if it is yellow enough to absorb below 4900 ,~. (At this temperature, evaporative cooling is negligible compared to radiation.) Genuinely red droplets would absorb even more sunlight (see Fig. 1) and be even warmer than 173°K. But red sulfur loses its color a t this temperature due to recombination of the $3 and $4 radicals (Meyer, 1976); so we have the paradox that any highly colored droplets will be too warm to remain colored in sunlight. Clearly, $3 and $4 cannot contribute significantly to the colors of droplets in the volcanic plumes. Only $6 and $7 may be stable enough, and weakly absorbing enough, to color such droplets. But, in general, the more colored molecules will be warm enough to be selectively bleached by sunlight, either in plumes or on the surface. This certainly rules out unstable allotropes of sulfur as the black material in calderas. There are a l s o difficulties in producing colored sulfur deposits by condensing the vapor directly on the surface. Meyer and Schumacher (1960a,b) found that no colored deposit could be produced unless the vapor were initially hotter than 500°K; the thermal measurements from Voyager (Hanel et al., 1979a,b; Pearl and Sinton, 1982) show that such hot spots are quite rare. Meyer and Schumacher (1960a,b) also found that the colored material produced from hotter vapor faded with a half-life of 57.7 hr at 169°K, which is not much warmer than the Io surface. Colored deposits formed from hot vapor should be only temporary, a t least at low latitudes. This mechanism may, however, account for the confinement of type 4 reflectance

NO SULFUR FLOWS ON IO spectra (see Appendix A) to high latitudes. The "purple sulfur" produced by condensing $2 vapor on cold surfaces (Rice and Sparrow, 1953) shows an absorption band near 600 nm (Rice and Ingalls, 1959) that resembles the Io band at this wavelength. Chhtelain and Buttet (1965) show a similar spectrum of "purple sulfur" and place the band at about 590 nm. This band was attributed to $2 by Rice and Sparrow, but they also found the purple solid "stable indefinitely at liquid nitrogen temperatures," whereas Hopkins and Brown (1975) found that $2 "disappeared rapidly" at 31°K. Barletta and Brown (1971) found, from the Raman spectrum, that "purple sulfur" contains "very little" $8, but shows bands of "plastic" sulfur, which seems to be the matrix in which the green or purple absorber is embedded. They found no $2. The molecule responsible for the 600-nm band is clearly not $2 as stated by Nelson and Hapke (1978), but remains unidentified. Rice (1962) found that "purple sulfur" converted to "green sulfur" on warming to -150°C (123°K). This observation was confirmed by Barletta and Brown (1971). Thus "purple sulfur" cannot exist at low latitudes on Io, but might survive near the poles. Even there it could only be a minor component of the surface, as the polar regions are certainly not purple. His finding that 3' rays turn sulfur "bright red" at -200°C likewise suggests that sulfur is not a major component of Io's surface, given the intense radiation environment, and the lack of any considerable amount of red material on Io (cf. Appendix A). The suggestion that "purple sulfur" might somehow account for the polar material is weakened further by Meyer and Schumacher's (1960a,b) observations that (a) green and purple condensates appear to have complementary colors (see Fig. 2) and, in some cases, metallic luster; (b) both have apparently the same infrared spectrum; and (c) they differ primarily in the rate of deposition required to produce them. These facts suggest that "green" sul-

207

fur is merely a more dilute mixture of the same molecule that, at high concentrations, looks purple by reflected light. Meyer and Schumacher (1960a) found that their condensates appear "violet" only in reflection; in transmission, these are "yellow red." At low concentrations, the red (600 nm) and short-wavelength absorption bands would make the mixture o f this absorber with plastic sulfur (Barletta and Brown, 1971) semitransparent and green; in purer form, the absorption bands would approach metallic strength, the sample would be quite opaque, and the nearly metallic reflections at short and long visible wavelengths would make the reflected light purple. The pale greenish tint of the type 4 regions (see Appendix A) would then require a very dilute mixture of this molecule in something else. Although deposition of plume material looks like the most promising way to make slightly colored sulfur on Io, it can at most account for the diffuse shadings. There seems to be no way to produce the sharpedged colored features--especially, those whose morphology suggests liquid flowsm with sulfur. If the colors of the sharp features require some other substance, OCcam's Razor suggests that we should look to that same substance, instead of sulfur, to explain the diffuse features. OTHER PROBLEMS WITH S U L F U R

Soderblom et al. (1980) point out that the colored regions on Io are too bright in the ultraviolet to be p u r e sulfur; they "suggest that . . . SO2 frost may be ubiquitously mixed throughout all of Io's surface materials" to scatter enough light at short wavelengths. But Nash et al. (1980) find they cannot cover more than 20% of Io with such frost. They also call attention to the need for substantial areas covered neither by sulfur nor by SO2 but by some sodium compound, to supply ions for the plasma torus. This suggests that most of Io is covered by some material other than sulfur.

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ANDREW T. YOUNG

Lewis (1982) has shown that rather speOne should bear in mind that a yellowish cial conditions are required to produce ele- color, due to near-ultraviolet electronic mental sulfur instead of reduced or oxi- transitions, is characteristic of a wide varidized compounds on Io. One may consider ety of materials, and is hardly a unique these conditions as rather improbable. property of sulfur. As Gradie et al. (1980) Because any flows of sulfur large enough have emphasized for the similar case of to be visible in the Voyager pictures must Amalthea, broad electronic transitions with have cooled slowly, they must all have no vibrational structure are exceedingly depassed through the normal region of low ficient in diagnostic information. Both the viscosity just above the melting point, yellowish colors of Io and the reds and where sulfur is as runny as water, but sev- greens of Amalthea might be due to iron eral orders of magnitude less viscous than compounds, which are usually yellow or typical basalt flows. It would then be sur- red when oxidized and greenish in the +2 prising to find it building landforms so simi- oxidation state. The colors might then be lar to terrestrial basalts, which Sinton explained as due to a slight iron stain in the (1982) and others have remarked upon. SO2 frosts that are believed to cover parts The mechanical properties of solid sulfur of Io. also seem incapable of supporting the relief One has only to recall the wild variety of observed on Io, as Clow and Carr (1980) candidates proposed not long ago to explain point out. Thus at least some of the flow the similar pale yellow color of the clouds features on Io are not made of sulfur. Yet of Venus--water, " d u s t , " plastics, salts, they all display a similar gamut of colors; if silica, hydrocarbons, carbon suboxide, any are not sulfur, some other mechanism mercury chlorides, ice, ammonium chlothan "quenched sulfur flows" must be in- ride, iron chlorides, iron sulfate, metallic voked to account for their colors. This mercury, and even sulfur--to appreciate other mechanism then might equally well that the first thing that seems likely is not be responsible for the colors of flows that necessarily correct, even if apparently supare currently suggested to be sulfur on geo- ported by the lack of a better suggestion. morphological grounds. "What else can it be?" is not a compelling argument. The most natural interpretation of the reDISCUSSION semblance between some landforms on Io If sulfur were to form large flows on Io, it and terrestrial basalt flows would be that would have neither the forms nor the colors these flows on Io are also basalts. The dark of the features that have been ascribed to calderas would then naturally be explained such flows, but would form flat, colorless as bare basalt, where geothermal heating regions. Sulfur emitted from volcanic has evaporated the sulfur dioxide frost. vents, either as droplets or as vapor, might Hanel et al. (1979a) observed a "hot spot," be quenched to a pale yellow color, but but found that its thermal emission specwould form only diffuse features and could trum "rules out the possibility that all of the not account for the sharp-edged colored dark regions . . . correspond to molten flows. surfaces o f . . . sulfur," as suggested by Sulfur need not be absent on Io. It may Sagan (1979). They found the warm area to well be produced in small quantities by pho- have a temperature near 290°K. tolytic or charged-particle decomposition The recent enthusiasm for sulfur on Io of sulfur dioxide. But it cannot account for has eclipsed earlier suggestions for its surthe colored flow features. Sulfur flows on Io face composition. Pollack et al. (1978) should be mainly $8 and hence colorless at found that many of the visible and infrared 130°K. spectral features were consistent with fer-

NO S U L F U R F L O W S ON IO

rous salts; Nash and Fanale (1977) had previously found indications of ferric sulfate and hematite. These are still plausible suggestions. So are some of the defunct Venuscloud candidates. Whatever colors Io is likely to be something other than the bulk constituent of the volcanic flows. Volatile materials are likely to be good candidates, to account for the diffuse shadings. From this point of view, Sill and Clark's (1982) suggestion of colored polysulfur oxides is interesting; they are yellow at the highest temperatures on Io, but unfortunately are dark red, and hence the wrong color, at lower temperatures. This may be yet another wrong candidate suggested by the wrong colors of the published Voyager pictures (Smith et al., 1979b), which remain a red herring. Perhaps a step needed to redirect thinking about Io is the publication of pictures with realistic colors. APPENDIX A: COLORS ON IO

By "color," I mean the visual sensation of color, not just a spectral gradient. The visible color of planetary material is important in suggesting, by comparison with what one sees in the chemical or geological laboratory, the kinds of materials that are reasonable candidates for the planet under study. It is worse than useless to think about red materials if the planet is green. The specific question to be answered here is: if I had a piece of the Io surface (kept in cold storage, to avoid thermochromic shifts) lying on my desk near the window, what color would I see there? As a referee has mentioned Minton's (1973) ground-based color photographs, a cautionary remark is in order. The principles of color vision and color photography are very clearly explained in two books published by the Eastman Kodak Company: Kodak Publication E-74, Color as Seen and Photographed, and E-77, Kodak Color Films. Anyone who supposes that color films reproduce colors accurately

209

should read at least the first of these. " N o film reproduces all colors perfectly"; for commercial reasons, the overriding consideration is pleasing rendition of skin tones, as people will not buy films that make their friends' faces look peculiar. The result is that objects (like Io) whose reflectance spectra differ markedly from human skin are reproduced incorrectly. According to Kodak, "As a result . . . some other colors--such as shades of chartreuse, lime, pink, and orange--may reproduce less well." As we shall see, colors on Io fall into this range of poorly rendered pastels. Furthermore, normal manufacturing and processing variations can combine to shift colors as much as a Kodak CC20 color compensating filter. It happens (see Table I below) that the average color of Io is very nearly that of the light transmitted by the CC20Y filter. Thus normal tolerances in manufacture and processing alone could make a photograph of Io appear either white, or twice its normal saturation. Reciprocity failure in the long exposures necessary at the telescope will further distort the color balance, as may adverse storage and shipping conditions. (According to old Kodak data sheets, the old "High Speed Ektachrome" used by Minton required CCIOB compensation at exposure times of 0.1 and 1.0 sec, and a green filter for proper color balance at 10 sec. Thus, though Minton failed to mention his exposure times, his pictures were probably too red. The sixth edition of E-77 showed, on page 39, an example of a long exposure on another Ektachrome film, in which a yellow object appeared brown when no compensating filtration was used.) Additional effects occur if the film is " p u s h e d " by extra development to increase its effective speed, and when the photograph is photomechanically reproduced on a printed pag e. Also, Io's reflectance spectrum (see Fig. 3) rises between 600 and 700 nm, which produces the very objectionable problem of "anomalous reflectance": objects with high reflectance in the far red and near infrared

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ANDREW T. YOUNG

1.0

type 1

0.9

...............................................................

0.8

type 2

0.7

Nelson & Hapke

0,6 • o

~

0.5

2

0.4

o.a

0.2

0.1 0.0 . . . . . . 0.35 0.40

, 0.45

~ 0.50

, 0.55

, 0.60

, 0.65

, 0.70

, 0.75

0.80

Jk,microns

FIG. 3. Reflectancespectra of Io. Heavy line: disk average (Nelson and Hapke, 1978). Thin solid lines: averages from Voyagerdata for regions of types 1 to 4 of Soderblomet al. (1980). Dotted lines showadopted extrapolations to the red of the Voyagerorange filter. "reproduce poorly because color films are much more sensitive to the far red than the e y e , " according to Kodak. (The most familiar example of this is the purple rendition of blue flowers like ageratums and morning glories by color films; an example is shown on p. 51 of the eighth edition of E77.) Perhaps the most damning possible indictment of photographic colors is the warning note below every color plate in Kelly and Judd (1976): "CAUTION: Since all the colors in this Figure are photographs of the colors, they are approximations of them and so should never be used in place of the actual samples." Thus, one can be certain that Minton's pictures of Io show the wrong colors; probably, they are too red. In any case, color photography cannot help us. But this does not mean that the accurate determination of Io's colors is beyond reach. For half a century, color science has been able to relate reflectance spectra (which are known for Io) to visually perceived colors. Filter

transmission spectra, as well as the spectral irradiances of colored light sources, can also be translated into standard color notations. This appendix applies these well-established methods to Io. As is evident to anyone who has ever seen Jupiter through a telescope, the colors displayed on Voyager pictures are false. The question is, what are the true colors? This cannot be answered by using the Voyager data alone, because they do not extend as far to the red as does the response of the human eye. However, by combining the Voyager data with the spectral reflectance of Io measured over a wider spectral range from the ground, one can reach a fairly close approximation to the truth. I shall use the reflectance spectrum measured by Nelson and Hapke (1978), because it is detailed, reasonably accurate, and particularly because it was used as a reference spectrum by Soderblom e t al. (1980). This spectrum extends well beyond the limits of human vision, and represents the average reflectance of one hemisphere of Io (see Fig. 3). The Voyager data for several types of colored regions on Io at four wavelengths also appear in Fig. 3; by themselves, these severely undersample the spectrum. However, as the Voyager data sample all the major regional types of reflectance, it must be possible to obtain the Nelson and Hapke (1978) average spectrum by a suitably weighted mean of the individual (unknown) regional reflectance spectra. This condition, together with the Voyager data points, places rather narrow constraints on the possible regional spectra. The matter is further simplified by the observation that the average spectrum shows only two absorption features in the visible spectrum: a strong band centered somewhere in the near ultraviolet, which evidently varies in strength from region to region, and a weaker band near 600 nm. The breadth of these features is typical of electronic transitions in solids. The various regions were denoted by

NO SULFUR FLOWS ON IO their apparent (i.e., false) colors on the Io color pictures in the work of Soderblom et al. (1980); but, to avoid prejudicing the matter, I shall refer to the regions by the numbers of the regions marked in their Figs. 3 and 4. Thus, for example, the regions they called " r e d " will simply be called "type 3 regions" here, to avoid confusion when the true colors are discussed. Figure 3 shows the average and the range of the narrow-band reflectance measurements for each type of region. Even the brightest region shows some selective absorption at short wavelengths, so it is not quite white. However, as the NelsonHapke average spectrum shows a steady rise to longer wavelengths, and it is unlikely that such a nearly white material would have appreciable electronic absorptions in the red without showing more absorption in the blue, I think the most reasonable extrapolation is essentially flat to the red of the Voyager orange filter. This extrapolation is shown as a dashed line in Fig. 3. The regions of type 4 are quite similar to the disk average at all Voyager wavelengths, so their most plausible extrapolation runs parallel to the average. Regions of type 2 appear similar at short wavelengths, though they seem to have a bit less than average of the ultraviolet absorber, while type 4 regions have a bit more. But, at the longest Voyager wavelengths, there is no indication of the 600-rim absorption in the type 2 regions. I have therefore drawn the type 2 spectrum slightly above the disk average, but without the 600-nm band. Again the extrapolations are drawn dashed in Fig. 3. As the very dark type 5 regions cover only a small fraction of the disk, they cannot contribute much to lowering the average reflectance below those of the type 1 and type 2 regions. (This also means that the redward extrapolation of type 5 is essentially indeterminate; these areas will be ignored here.) Type 4 lies only a little below the average. Thus, the only major type of region responsible for countering the abnor-

211

mally high reflectances of types 1 and 2 is type 3, which covers an appreciable fraction of the total area, and is appreciably darker than average. Type 3 evidently contains even more of the ultraviolet absorber than type 1, 2, and 4 regions. Because it lies below the disk average at 590 nm, type 3 may contain the 600-nm band, so its reflectance may rise appreciably at longer wavelengths. However, it must always lie below the disk average, to balance the high reflectances of the type 1 and 2 regions. The disk-averaged spectrum is therefore an upper limit to the range of possible redward extrapolations. On the other hand, the type 3 reflectance spectrum might remain as flat toward longer wavelengths as those assumed for types 1 and 2, which would leave type 4 as the only region with the 600-nm band. Then it would be difficult to account for the strength of the 600nm band in the disk-averaged spectrum. Thus, I believe that these two possibilities are extremes, between which the true spectrum of type 3 must lie. The "flat" extrapolation is indicated in Fig. 3 by a dashed line; the other extreme possibility rises rapidly beyond the Voyager point at 590 nm, and coincides with the average spectrum beyond 620 nm. Given these reflectance spectra, one can determine the colors they would show to a person with normal color vision. To do so, one multiplies the reflectance spectrum by the spectral irradiance distribution of an illuminating source and by a set of three effective response functions. These functions take account of psychological effects such as opponent mechanisms in the human visual system, as well as the physical absorption spectra of the three visual pigments involved in color vision. They were well established more than 50 years ago, and standard values adopted by the Commission Internationale de l'Eclairage (CIE) in 1931 have been employed since then in color science and technology; they are described in great detail by Wyszecki and Stiles (1982), who also give convenient ta-

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ANDREW T. YOUNG

bles for performing the necessary numerical integrations. The standard illuminant used in most work is CIE illuminant C, which was intended to represent average daylight. Although improved daylight distributions have been adopted recently, I shall need to refer to a large body of work based on illuminant C; so I adopt it for the present purpose. The numerical integrations produce three numbers, X, Y, and Z, which form a three-color system somewhat like the wideband systems used in astronomical photometry. But instead of converting the three responses to magnitudes and color indices, color scientists divide both the X response and the Y response (which has been arranged to coincide with the brightness channel of the visual system, and thus resembles the "visual" band of astronomical broadband systems) by the sum (X + Y + Z), to produce normalized "chromaticity coordinates" x and y. Notice that, apart from the logarithms involved in astronomical magnitudes and color indices, the ratio of responses is the basic information in a color index. Thus the chromaticity coordinates x and y are essentially color indices, and the Y response is like a visual magnitude. The coordinate x may be thought of as a blue-red color index, and y as a purple-green color index. The response functions have been transformed mathematically to avoid any negative values, and are effective rather than actual physical response functions; but the essential point is that they retain all the color information seen by a human with normal color vision. The chromaticity coordinates x and y and the measure of brightness Y completely specify the apparent color of a material under a given illumination. In particular, with illuminant C, they closely represent the color seen under average daylight conditions. Thus, the coordinates calculated from the reflectance spectra in Fig. 3 allow one to specify a standard color chip (in any of a number of common systems) that would

have the same visual color as the corresponding piece of Io. [Note that the projection of the Hilbert space of all possible spectral reflectance functions into the three-dimensional visual color space maps many points (i.e., reflectance spectra) into one (color), which throws away a great deal of detailed information, so that many different reflection functions appear identical to the eye. Such visually equivalent but spectrally distinct functions are called metamers of one another. The standard color chips are merely metamers of the corresponding patches on Io, and may have quite different spectral reflectances.] Once the color of a region on Io is specified by its coordinates x, y, and Y, it can be compared with the standard colors shown in the Munsell Book of Color, which is a widely used reference standard. The transformation between the CIE system and the Munsell system is given by Newhall et al. (1943). The Munsell colors, in turn, have been mapped into a standard set of color names, first set up by the Inter-Society Color Council and the National Bureau of Standards (Judd and Kelly, 1939; Kelly and Judd, 1976). These names are in accord with common usage, and allow fairly accurate verbal descriptions of colors in ordinary language. A more precise, but less accessible, description is provided by the Munsell system, or by the computed coordinates themselves. The colors can also be described by referring to the printed color plates in the Maerz and Paul (1930) Dictionary of Color, which is widely used. I have tried to specify the Maerz and Paul color nearest each of the Io colors in Table I; these were determined by reference to both color chips and the ISCCNBS names of the Maerz and Paul colors listed in Kelly and Judd (1976). However, they are less accurate than the CIE or Munsell specifications. Yet another description of visible colors specifies their dominant wavelength (that monochromatic light which, added to the white illuminant, would visually match the

213

NO S U L F U R FLOWS ON IO TABLE I COLORS ON Io

Region Disk avg

1

2

3

4

Flat

Avg

CIE coordinates x y Y (%)

0.337 0.357 62

0.323 0.332 88

0.349 0.366 71

0.376 0.400 54

0.381 0.394 54

0.342 0.361 57

Munsell hue value/chroma

10Y 8.2/2.1

2GY 9.4/0.9

6Y 8.6/3.0

5.5Y 7.7/4.8

4Y 7.7/4.9

9Y 7.8/2.6

Maerz and Paul

11B 1

9B 1

9D 1

IIG2

llH3

llC1

Dominant wavelength (A)

5720

5752

5741

5750

5774

5727

18

8

24

39

40

21

121 Pale yellow green

92 Yellowish white

89 Pale yellow

89 (Pale yellow)

153 (Greenish white)

---

Excitation purity (%) ISCC-NBS No. and color name (nearest neighbor)

given color) and purity (the fraction of monochromatic light in the matching mixture). These are treated in detail by MacAdam (1981), who reproduces charts from the Handbook of Colorimetry (Hardy, 1936) that show the dominant wavelength and purity of each point in (x,y) space. These, together with the Munsell specifications, chromaticity coordinates, and ISCCNBS names of the corresponding colors, are given for each of the Io regional reflectance spectra of Fig. 3 in Table I. Table I shows that even the most saturated colors on Io are rather pale, and that even type 3 regions, described as " r e d " by Soderblom et al. (1980) on the basis of the false-color reproductions, lie to the green side of pure yellow. Type 4 regions, which they called "yellow-brown," are too light to be brown, as well as too green: " b r o w n " refers to colors of Munsell value 6.5 or less,

90 Grayish yellow

90 Grayish yellow

90 Grayish yellow

105 (Grayish greenish yellow)

87 (Moderate yellow)

121 (Pale yellow green)

corresponding to a mean visual reflectance nearly two times smaller than that of type 4; but a surface that dark, having the same hue and chroma as the type 4 reflectance spectrum, would be called "light grayish olive" in the ISCC-NBS naming system. The average color of the side of Io measured by Nelson and Hapke (1978) is as well determined as their spectrum, which extends well outside the wavelength interval to which the eye is sensitive. This color lies near the point in color space where the ISCC-NBS names "pale yellow," "pale greenish yellow," "grayish yellow," and "grayish greenish yellow" meet. It happens to lie very near the color of the light transmitted by the Kodak CC20Y filter (Xc = 0.3369, yc = 0.3597), according to Kodak Publication B-3, Kodak Filters for Scientific and Technical Uses." More exactly, the average color of Io is close to that of the

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A N D R E W T. YOUNG

C C 2 0 Y filter plus a b o u t 0.15 neutral density. T h e neutral density is important, b e c a u s e the p e r c e i v e d c o l o r s in the region o c c u p i e d b y Io (Table I) are r e m a r k a b l y sensitive to lightness and d a r k n e s s (Munsell value). Light colors a p p e a r yellow, but d a r k e r ones o f the s a m e hue and c h r o m a a p p e a r greenish. This p h e n o m e n o n is vividly a p p a r e n t in the 10Y page o f the Munsell B o o k , or on c o m p a r i n g the top r o w s (pure yellows) o f plates 9 to 16, or 17 to 24, o f M a e r z and Paul (1930). Kelly and J u d d (1976) explain:

042 0.41 0.40

0.39 0.38

y 0.37 038 0.35

0.34

Unlike the terms green and blue, which are hue names applying to all lightnesses and saturations, the term yellow is commonly used to designate not only a certain hue range but also a high lightness range within this hue range. Dark colors of the same hue as yellow are commonly called olive or olive brown. Common usage limits the term orange even more strictly; it is taken to refer not simply to a range of yellow-red hues but also to a medium-lightness range and a high saturation range. Colors of the same hue but of lower lightness and saturation than the orange range are called browns. B e c a u s e I o reflects less than two-thirds o f visible light, it w o u l d a p p e a r greenish yellow r a t h e r than the pale y e l l o w o f the C C 2 0 Y filter alone, if w e could c o m p a r e it with a white standard. H o w e v e r , w h e n we see it telescopically f r o m E a r t h , the brightest a r e a in the field o f v i e w is usually on Jupiter, w h i c h the e y e and brain a d o p t as " w h i t e . " I f w e c o u l d put a large white surface next to Jupiter, b o t h the planet and the satellite, w h i c h h a v e similar colors, w o u l d be visibly d a r k e r and a p p e a r greenish. (I a m i n d e b t e d to G o d f r e y Sill for pointing this o u t to me.) This is o c c a s i o n a l l y realized at occultations o f Jupiter b y the M o o n . W h e n conditions are suitable, visual o b s e r v e r s find the g r e e n i s h c o l o r o f J u p i t e r quite striking (see A p p e n d i x C). T h e w e l l - k n o w n difficulty o f seeing I o against the disk o f t h e planet s h o w s that I o t o o w o u l d look greenish if seen in the p r o p e r c i r c u m s t a n c e s .

0.33 0.33

0.34

0.35

0.36

0.37

0.38

0.39

0.40

x

FIG. 4. A small part of the CIE chromaticity diagram enclosing the two most strongly colored Io regions, types 3 and 4. The ISCC-NBS color name boundaries are plotted for Munsell value 7.75, near that of these regions (see Table I). The two points plotted for type 3 regions correspond to the "flat" extrapolation, shown in Figs. 1 and 3, and an extrapolation that coincides with the average Io spectrum. These extremes almost certainly bracket the true color of type 3 regions; in either case, their color name is "grayish yellow," though the redder extrapolation almost reaches "moderate yellow." Note that the color of type 4 regions is very nearly on the boundary between "grayish yellow" and "pale yellow green". This is also close to the disk average (cf. Fig. 3). The numbers of the name classes are also given for convenience in using NBS SP 440.

T h e greenish tint o f I o is w e a k , but definite. Figure 4 s h o w s the b o u n d a r i e s o f s o m e I S C C - N B S n a m e d regions in the c h r o m a t i c i t y d i a g r a m at Munsell value 7.75, w h i c h is close to the m e a n reflectance o f the t w o m o s t d e e p l y c o l o r e d regions, t y p e s 3 and 4. T h e t w o e x t r e m e possibilities are s h o w n f o r t y p e 3; the r e d d e s t possible c o l o r for these regions is a m e d i u m y e l l o w that is neither greenish n o r orangish. T h e colors o f t y p e s 1, 2, and 4 are likely to be less u n c e r t a i n t h a n the range s h o w n f o r t y p e 3. T h e general g r e e n i s h tint is absolutely

NO SULFUR FLOWS ON IO certain, unless the Nelson-Hapke average reflectance spectrum is seriously incorrect. Indeed, the spectra shown for the Voyager regions in Fig. 3 cannot reproduce the prominent knee in the Nelson-Hapke average near 500 nm. To reproduce this feature correctly in the average, at least one of the regional spectra must be higher at 500 nm than the curves drawn. This would raise the green reflectance of the area, making its color greener in hue and less saturated in chroma. Thus, the regional colors in Table I are already biased too far to the red. The statements of Sagan (1979) and others suggest that the Voyager investigators took their false colors literally, and thus were seriously misled to suggest "red sulfur" and similar materials. In fact, Io is various pale shades of greenish yellow; there is no evidence for any substantial amount of red material at all. One could, of course, manipulate the long-wavelength extrapolations in Fig. 3 to make some area yellow-orange (though " r e d " appears to be out of the question: arbitrarily increasing the reflectance of type 3 areas to 0.92 at all wavelengths longer than 610 nm only changes the color to "pale orange yellow"); but, to maintain the disk-averaged spectrum, some other area would have to become still greener than the colors in Table I. This seems quite improbable; all the hues in Table I plot nearly along a radial line in the CIE chromaticity diagram (Fig. 2), at dominant wavelengths confined to about a 5-nm interval, just to the green side of the mercury yellow doublet. They all lie between 4Y and 2GY in the Munsell system, but have different saturations. This is what one expects, because (apart from the small effect of the weak band at 600 nm) the colors are mainly due to varying amounts of the same ultraviolet absorption feature, and hence probably result from different concentrations of the same chemical substance. A second material is required to account for the 600-nm band, whose strength is uncorrelated with the ultraviolet band. If

215

further large variations occurred in the red, yet a third material would be required. While Sill (1973) has described cold orthorhombic $8 as " a faint pale green," which might be consistent with the type 1 areas, there remains little room for the more strongly colored sulfur allotropes on Io. The yellowish type 3 regions may be consistent with the $7 molecule, but the orange-red of $6 is inconsistent with most of the planet. Some of the colored forms of sulfur might be hidden in the dark type 5 regions, but these cover only a small fraction of the total area. Clearly, red materials are inappropriate candidates for surface materials on Io, which is greenish-yellow, not red. This conclusion is implicit in the work of Soderblom et al. (1980) and many others, who have found that sulfur is too dark at short wavelengths, and too bright at long wavelengths, to match Io. In order to incorporate any form of sulfur in Io surface models, these workers have been forced to dilute it with other materials. This compounds the problem of nonuniqueness emphasized by Gradie et al. (1980): electronic spectra contain too little information to determine surface composition with any confidence. APPENDIX B: OOMMEN'S SPURIOUS BANDS

The colors of soap bubbles and other thin films have been known for thousands of years, and were investigated by Boyle, Hooke, and Newton. They were first explained correctly with the wave theory of light by Thomas Young in 1801. The next year he reported the first observations of the channeled spectra reflected from thin plates. He later (Young, 1807) described clearly how "every part of a thin plate, which exhibits the appearance of colour, affords . . . a divided spectrum, when viewed through a prism." Brewster (1842) also studied the channeled spectra produced by thin plates. No colors are seen in thicker films, where the path difference between rays reflected at the two faces of the plate is large. How-

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ANDREW T. YOUNG

ever, the same interference phenomena still appear when the light is dispersed spectroscopically. Fizeau and Foucault (1849) could see interference bands at path differences up to 4000 wavelengths. A few years later, Esselbach (1856) used a channeled spectrum produced in transmission to extend his wavelength calibration into the ultraviolet. Later still, Edser and Butler (1898) sharpened the bands in the spectrum by partially placing a FabryPerot etalon over the slit, and used them as fiduciary marks to interpolate prism spectra between lines of known wavelength. In the English literature, the bands are still often called Edser-Butler bands; and the technique continues to be used, most recently to provide a wavenumber scale for tunable diode lasers, with an uncoated germanium plate as the etalon. These bands have also been widely used to determine the thickness and optical constants of thin samples, especially of those that absorb strongly. After Kellner (1929) derived the formula for the transmission and reflection of absorbing films, Czerny and his students published a long series of papers in Zeitschrift far Physik on infrared dispersion curves of a variety of materials, determined from channeled spectra. After World War II, the interference bands began to be used to. measure the thickness and refractive index of thin evaporated films [see, for example, Abel,s (1950)]. This requires taking account of the refractive index of the substrate. In the early 1960s, the bands were used to measure optical constants of thin films of molecular solids condensed on substrates cooled by liquid nitrogen [see, for example, Hollenberg and Dows (1962); Friedrich and Person (1963); and subsequent papers in Journal of Chemical Physics]. Randall and Rawcliffe (1967) cited many earlier references. At about the same time, Szymanski (1964) published a textbook on spectroscopic technique for chemists, in which he mentioned the utility of the bands in mea-

suring the thickness of absorption cells. [Other textbooks of the time for chemists also mention this technique; e.g., Potts (1963).] He also warned that measurements on thin films could encounter difficulties, because "if the film thickness is a multiple of the wavelength . . . . interference fringes may appear in the spectrum." Despite both the long history of the Edser-Butler bands and their widespread applications in chemical spectroscopy at the time, Oommen (1970) makes no mention of these interference phenomena in his thesis. Oommen's (1970) Fig. 20 shows spectra of a film of purple sulfur evaporated onto a cold sapphire substrate. The Edser-Butler bands are prominent; but Oommen calls them absorption bands. To show they are not, I shall demonstrate that these features are well explained by interference phenomena in a sulfur film a few microns thick. As we know neither the thickness nor the refractive index of Oommen's condensed film, it is useful to make rough estimates of these from published data. Then standard thin-film techniques can be used to estimate their values from the Edser-Butler bands in Oommen's spectra directly. I shall take agreement of the independent estimates with those derived from Edser-Butler analysis of Oommen's spectra as evidence that the bands are, in fact, interference bands and not absorption bands. Meyer and Schumacher (1960a,b) reported that similar cold films about 10/xm thick were deeply colored; the 590-nm band is saturated in their spectra, and the films were all red in transmitted light. On the other hand, Oommen's films were quite transparent through most of the visible spectrum, with no more than a weak suggestion of the 590-nm absorption band. This suggests that his films were, at most, only a few microns thick. Oommen's (1970) films "crumbled into yellow powder resembling orthorhombic sulfur" at room temperature, and became "almost white" on cooling back to 77°K. Similarly, about one-fourth of the films of

NO SULFUR FLOWS ON IO Barletta and Brown (1971) gave "Residue I , " indistinguishable from orthorhombic sulfur, at room temperature. They reported that "in all of these cases the film was extremely thin," which agrees with the conclusion above. Because Barletta and Brown (1971) found their "purple" films had a Raman spectrum very similar to that of their ~'Residue III," which they concluded was "actually the so-called 'plastic' form of sulfur," and because Oommen (1970) described his films as having " a plastic texture," it appears that all these films were mostly "plastic" sulfur. One would therefore expect the density and refractive index of these films to be similar to those of "plastic" sulfur. There is considerable variation among samples of "plastic" sulfur, which is not a well-defined material. But the term is usually applied to sulfur that has been quenched from a high temperature. We can thus try to infer the likely properties of Oommen's film from data on hot liquid sulfur, whose composition might resemble that of his quenched hot vapor. Both the density and refractive index of liquid sulfur were measured by Mondain-Monval and Schneider (1928a,b). They found that the Lorenz-Lorentz refractivity increases with temperature, due to the changing composition of the liquid. At the highest temperature they measured, 200°C, R = 0.271. There are no refractive-index measurements for plastic sulfur at low temperatures, so we must assume a constant refractivity and use density data to estimate the likely refractive index of Oommen's films. Densities of quenched liquid samples vary, but 1.92 may be taken as a typical value at room temperature. This, with R = 0.271, corresponds to n = 2.063 at room temperature for plastic sulfur. To estimate the likely refractive index at 77°K, I assume the plastic form increases in density by the same factor of 1.036 found for orthorhombic sulfur by Sapper and Biltz (1931), on cooling with liquid nitrogen. This predicts a refrac-

217

tive index of 2.123 for Oommen's films at liquid nitrogen temperature. The possible uncertainty in this estimated refractive index may be inferred from the change in refractivity of liquid sulfur, due to changes in composition, over the range measured by Mondain-Monval and Schneider; this is about 0.01 in refractivity, or roughly 0.1 in refractive index. Another indication of uncertainty is the variation in refractive index with the direction of the electric vector in orthorhombic sulfur; this amounts to nearly 0.2 in refractive index. The molecules that form from condensed vapor on a cold substrate may well have a preferred orientation, so this effect may be important. The best we can say from independent data is that the expected refractive index of Oommen's film is near 2.1, with an uncertainty of about 0.2. Now we can estimate the thickness and the refractive index of Oommen's films from the amplitude and spacing of the Edser-Butler bands in his Fig. 20. The simplest method is that of Abel,s (1950), who uses the reflection maxima to determine the refractive index. We do not have a reflection spectrum, but can assume the reflection to be 1 minus the transmission in nonabsorbing regions. As this method requires a nonabsorbing medium, and Oommen's spectra indicate a weak absorption between 550 and 600 nm, in agreement with the spectra of purple sulfur published by Rice and Ingalls (1959) and by Chfttelain and Buttet (1965), I restrict my attention to wavelengths longer than 600 nm. The two density peaks at 625 and 680 nm average 0.12 in density, corresponding to a refractive index of 2.27 (if one adopts n = 1.767 for the sapphire substrate). This seems quite reasonable. However, one may fear that the optical densities are too high because of scattering in the film. In this case, a safer estimate might be derived from the method of Goodman (1978), which uses the contrast between fringe maxima and minima. From Oommen's Fig. 20b I adopt an average fringe contrast of 1.07,

218

ANDREW T. YOUNG 2.4

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. /

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./

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FIG. 5. Frequency (in reciprocal microns) of the Edser-Butler bands in Oommen's (1970) Fig. 20, as functions of interference order. Transmission maxima (density minima) occur at integral orders of interference, and are plotted as open circles; half-integral orders are transmission minima (density maxima) and are plotted as solid dots. Upper line: extrema (or inflections) read from Oommen's Fig. 20b, for "purple" sulfur at 77°K. Lower line: density maxima in his Fig. 20c, a spectrum of the same film warmed up; plotted from wavelengths listed on p. 87 of his thesis. Note the good fit to the leastsquares lines; this equal spacing in frequency is necessary if the bands are due to interference, but would be most unusual for unrelated electronic absorption bands.

which corresponds to n = 2.07, again, a most reasonable value. Figure 5 compares the expected positions of the Edser-Butler bands with the maxima and minima in Oommen's spectra. The bands are very nearly equally spaced in frequency, as they should be. The analysis used here ignores dispersion, which is considerable for sulfur. However, to first order, the effect of dispersion is to leave the bands equally spaced, but shift the line through them so that it no longer passes through the origin. The least-squares lines in Fig. 5 miss the origin by about one-fourth of a fringe. A further complication, also neglected, is due to the birefringence of sapphire; but it cannot be taken into account, because Oommen did not specify the orientation of his substrate. This introduces an additional uncertainty of about 0.008 in the derived refractive index.

Considering all the evidence, I adopt a refractive index of 2.10 for Oommen's cold film. This, together with the average fringe spacing of about 1084 cm -1 (see Fig. 5), indicates a film thickness of 2.17 /zm. This thickness is also entirely plausible, considering the transparency arguments given above. Oommen's Fig. 20c shows the spectrum of the same film at some unspecified temperature between liquid nitrogen and room temperature. The Edser-Butler bands in this spectrum are almost exactly complementary to those in the original spectrum. The very similar spacing is to be expected if the thickness and refractive index have changed only a little; the disappearance of the 580-nm "purple sulfur" band shows that the composition is definitely somewhat different. The change in composition should change the dispersion curve, and

NO SULFUR FLOWS ON IO hence the offset of the line in Fig. 5 from the origin, however. Oommen estimated the positions of all the peaks in this spectrum, so I use his wavelengths (plotted in Fig. 5) to calculate the thickness of the warmed-up film. The average density at the peaks is now reduced to 0.085, corresponding to a transmission of 82%, or a refractive index of 2.082. The average fringe contrast is almost unchanged, but because more fringes can be used (owing to the loss of the 580-nm absorption band and some absorption at shorter wavelengths), a more precise estimate can be given: 1.074. Goodman's method then gives n = 2.079. Here the two methods are in very good agreement, not only with each other but with the 2.06 estimated for plastic sulfur at room temperature. Adopting n = 2.08, I find from the mean fringe spacing (see Fig. 5) a film thickness of 2.25/zm. These figures suggest that, on warming, the film expanded slightly, and reduced its refractive index. Qualitatively, these changes are plausible; however, the amounts are rather larger than should be expected from thermal expansion alone, and must involve the change in composition as unstable low-temperature molecules reacted on warming. A decrease in refractive index is to be expected from dispersion theory, due to the fading of the absorption bands in the visible. But, because of the crudeness of the numbers, little confidence should be placed on the amounts of the derived changes. The essential point is that interpreting the oscillations in both spectra as Edser-Butler bands gives closely similar film thicknesses and refractive indices at the two temperatures. Indeed, one would have expected fringes of about the observed amplitude and spacing in Oommen's spectra, simply from the rough estimates of refractive index and thickness made without reference to the spectra. Then it would have been difficult to explain their absence, if they had not been observed. Evidently the weak features in these spectra are just the Edser-Butler

219

bands and (apart from the much broader absorption around 580 nm, observed by others in purple sulfur) are not absorption bands at all. The disappearance of the interference bands after the film warmed up to room temperature and reverted to orthorhombic sulfur is easily understood, because Oommen reported that it became almost opaque. (This is undoubtedly due to scattering rather than absorption; the conversion of any unstable form of sulfur to orthorhombic generally produces a cryptocrystalline material of high scattering power, even in thin layers.) In any case, a high opacity due to either scattering or absorption would prevent the formation of the standing waves within the film that are necessary to produce interference. However, Oommen claimed that some of the bands had been observed in the spectrum of sulfur at 270°C. The spectrum he mentions to support this claim (his Fig. 5d) was made by partly moving the absorption cell out of the measuring beam. He attempted to justify this anomalous procedure with the fallacious argument that "if we have a 200-~m cell raised by 50% in the optical path, the absorption should be lowered by 50% which is equivalent to using a 100-/~m cell in the full beam." (But this changes the transmittance, and not its logarithm, the absorbance, by a factor of 2 in regions of high absorption.) The spectrum is further compromised by being that of a liquid film with a gas bubble in it. From a comparison of the noisy shortwavelength region of this spectrum with others in Oommen's thesis that were taken by inserting filters as dense as 4 density units into the comparison beam of his Cary 14 spectrophotometer, it is evident that all of the " b a n d s " mentioned are due to the wavelength distribution of stray light in the instrument, and have nothing to do with sulfur. For example, he found "the maximum absorption was less than 5 0 . D . " In his 100-t~m-thick ceil, even though his own calculations showed that the optical density

220

A N D R E W T. Y O U N G

At the previous occultation on 22 July 1856, Baron Dembowski (1856) reported, "About 18 minutes before the beginning of the occultation, when I saw them simultaneously in the field of the telescope, I was struck by the difference in color of the two planets. Compared to each other, the color of the Moon was a beautiful straw yellow, very delicate--that of Jupiter, a slightly dirty mixture of yellow and green, tending to pistachio. This difference between the APPENDIX C: JUPITER IS GREENISH colors became more and more perceptible Not all occultations of Jupiter provide a as the discs approached, and when Jupiter brighter reference, because some occur far had begun to disappear behind the illumifrom full Moon, or near the lunar termina- nated limb of the Moon, I saw its outline tor, where the Moon is no brighter than Ju- perfectly sharp in yellow on the light green piter himself. However, a suitable occulta- of Jupiter . . . . This difference between the tion was observed on 8 November 1856, by colors always remained the same for the some of the most illustrious visual observ- whole duration of the observation." At the third occultation in the series, ers who ever lived. Their reports appear in the 14 November issue of Monthly No- Challis (1857) reported that Jupiter "was tices--a speed of publication rarely ap- pale compared to . . . the moon, and had proached today. De la Rue (1856) observed the slightest possible tinge of olive green." the immersion with a small refractor, and The same occultation was observed by Laswrote, " I was much struck with the pale sell, the discoverer of Triton; he wrote: and evidently greenish hue which the planet "The planet was certainly much paler than presented in contrast with the moon's con- the moon, inclining by contrast to a greenspicuously warmer tints, and also with the ish hue" (Lassell, 1857). Hartnup (1857) remuch greater brightness of most portions of ported that "the dim copper colour of the the lunar surface," and was "able to con- planet contrasted strongly with the bright firm most satisfactorily the observations in limb of the m o o n " - - c o u l d he have been regard to the relative tint and brilliancy of color-blind? At Oxford, Johnson saw "The Jupiter" with his larger Newtonian reflec- colour of Jupiter on emersion was seator at emersion. In response to a query green," and Pogson (the inventor of the from De la Rue, the Rev. Dawes (1856) re- modem magnitude scale) considered it plied--in a letter dated 20 November (publi- "dull bluish-white," noting that "Jupiter cation really was fast in those days)--"I was less bright than most part of the was struck, as you were, with the differ- moon's surface." At Ventnor, Mann reported that at emersion, "the difference of ence of tint of Jupiter and the moon." The relative brightness of the Moon and the planetary and lunar lights became very Jupiter was also attested by a drawing by remarkable, the hue of the planet being an William Huggins (1856), who noted, exceedingly soft greenish gray," and also "Dusky appearance of Jupiter as con- remarked on "the superior brilliancy of the trasted with the moon very marked." lunar light." Dawes (1857) observed this event also. Grove (1856) noticed that "The light of Jupiter was notably less brilliant than that of He made no comment about the color of Io the Moon; I should say hardly equal to half at immersion. He reported that the fourth the intensity of the latter. It was of a much satellite at emersion "did not differ much f r o m . . , the moon, perhaps a trifle redder; bluer tint than the Moon . . . . "

of this thickness of sulfur should exceed 500 at short wavelengths. In summary, Oommen's thesis provides no valid information on the spectra of any form of sulfur at short wavelengths, and no credible evidence for such molecules as linear $5 or $6. His spectra of quenched sulfur vapor show no true absorptions beside those published by other investigators.

NO SULFUR FLOWS ON IO but it was decidedly fainter." (This shows that the color of Jupiter is not due to brightness contrast alone.) But "Jupiter was of a pale-green at its reappearance." Io on reappearance was "not so pale as the third satellite," which he had called " a pale green."

Grove remarked that "The difference of light and colour between the planet and the moon was the same" as at the previous occultation. Watson also said that "the greenish tint, noticed by Mr. De la Rue in November, was very perceptible on the face of Jupiter." Slatter found the planet "had a slight inclination to green"; Peto noted "The pale greenish light of Jupiter contrasted strongly with the golden colour of the moon." Hodgson found that "At emersion the light of Jupiter appeared pale, and slightly tinged with blue as compared with the moon." All these reports are collected in the pages of Monthly Notices immediately following Lassell's (1857) paper, and so are not cited here individually. The general impression of a greenish color is not confined to the occultations of 1856-1857. On 7 August 1889, another favorable event was observed by Ki~stner (1889) in Bedim He made no remarks about color at ingress at the dark limb; but at the bright-limb egress, he commented: "The planet appeared strikingly dim and palebluish in comparison with the intense light of the Moon . . . . The part of the planet's surface next to the Moon seemed (probably because of the light contrast) considerably more weakly illuminated than the distant part; this also continued a long time after the egress, so was independent of the contact of the two bodies. The dimmer illuminated part of Jupiter shone in greenish gray light." Von Engelhardt (1889) also observed this occultation, and found the "greenish color" of Jupiter "very striking" compared to the "white-bluish" color of the Moon at egress. Similarly, Lorenzoni (1890) saw the light of Jupiter "considerably fainter than that of the moon and had a light greenish tint," when he was able to

221

see both objects in the field together. Safarik (1890) also found the brightness of Jupiter on egress "significantly feebler than that of the Moon, and the color strikingly dirty greenish." Peal (1893) observed the next occultation, at which he carefully studied both the colour and albedo . . . . The first thing which struck me after getting the planet and limb into the f i e l d . . , was the m u c h Colgreater d a r k n e s s of the disk of Jupiter . . . . our contrast w a s more c o n s p i c u o u s t h a n I had expected, the p l a n e t ' s surface . . . presenting a distinctly greenish tinge . . . . By rapidly comparing the planet's disk with the floor of Plato the latter h a d a purplish tint, and the brighter portions of the m o o n a rosy flush, no doubt due to contrast, i.e., c o m p l e m e n t a r y colour.

Most of these reports have in common (a) a lack of comment on color if Jupiter is not juxtaposed to the bright limb of the Moon; (b) a pale greenish color, often very similar to that found for Io in Appendix A (grayish, dirty, olive green, etc.), when Jupiter is seen with the limb of the Moon; (c) association of the greenish color of Jupiter with comments on its paleness compared to the Moon; and (d) expressions of surprise, often in very similar wording in English ("struck": De la Rue and Dawes; "strikingly beautiful": Challis; "very remarkable": Mann), French ("je fus frapp6": Dembowski), and German ("auffallend": Ktistner, Engelhardt, and Safarik). Although several observers argued that the unexpected greenish color of Jupiter was due to contrast, this would require the same appearance at occultations by the dimmer parts of the Moon, near the terminator (which, by the reciprocity theorem, must have the same color as the limb). But I have found no such reports: the greenish color appears only when the Moon's limb is appreciably brighter than Jupiter. The argument offered by Safarik (1890), That the often o b s e r v e d greenish color of Jupiter at the bright lunar limb is only a contrast-effect" which " o n e can easily d e m o n s t r a t e if one takes the shade off a study-lamp, fixates the brightly shining Moon, and with outstretched arm slowly

222

A N D R E W T. Y O U N G brings the flame toward the Moon: already at a distance of several degrees the Moon becomes greenish, right next to the flame beautifully intense verdigris,

is falsified by the low color temperature of the flame. In this case, a great difference in hue is involved, which suffices to explain the effect by the usual simultaneous-contrast mechanism. But the Moon does not have nearly so saturated a color as a lamp flame. The chromaticity coordinates (cf. Appendix A) of the standard Mare Serenetatis region 2 of McCord et al. (1972) are xc = 0.3343, Yc = 0.3382; assuming an albedo of 0.11, we have Munsell color 2Y 3.8/0.8, whose NBS name is "brownish gray" (near "olive gray"). The very low chroma here means a less saturated color than even the "white" type 1 regions on Io. The Moon is neutral enough to serve quite well as a white reference for either Io or Jupiter. Of course, when the Moon is the brightest thing in the field of view, it appears "white" instead of "brownish gray." More precisely, it appears to be an object of similar Munsell hue and chroma, but high value, and appears "yellowish white." Because of its low chroma, the more saturated colors reported for Jupiter cannot possibly be due to simultaneous .contrast, because such contrast-induced colors are generally much less saturated than the inducing color. Furthermore, any contrast-induced hue in Jupiter should be complementary to the Moon's 2Y, and hence blue, not green. Hence, the colors reported are consistent with the effect discussed in Appendix A: greenish-yellow surfaces like Io and Jupiter appear yellow when bright, but olive-green when dark. The effect simply requires a " w h i t e " reference brighter than the surface of Jupiter or Io to produce the greenish impression. These visual observations confirm Sill's suggestion that, if Io is really greenish, the similarity in color of Jupiter and Io implies that Jupiter too is greenish. (Jupiter's green

tint is partly due to the methane and ammonia bands at the red end of its spectrum; the stronger bands of Uranus and Neptune make them visibly green in the telescope, even without a white reference.) Conversely, the observed greenish color of Jupiter confirms that Io, too, would look greenish if compared to a brighter neutral surface. ACKNOWLEDGMENTS I thank Jonathan Gradie, Godfrey Sill, Carl Sagan, and an anonymous referee for pointing out weak spots in the original manuscript. John Pearl kindly communicated unpublished Voyager IRIS results. Bob Nelson supplied his Io spectrum in digital form. Dave Pieri introduced me to the geomorphology of Io and discussed his reasons for accepting the sulfur-flow hypothesis. Jonathan Gradie also discussed the Cornell measurements on quenched sulfur samples at length. Discussions with all of these people at the Ithaca DPS meeting were extremely helpful in clarifying the issues concerning sulfur on Io. Carl F. Gerlee provided invaluable assistance with color names, by lending me his copy of NBS SP 440. Parker Pace, of Frazee Paint and Wallcoverings, kindly supplied spectral-reflectance data on a number of Io-colored paint samples. This work was supported by Planetary Atmospheres Grant NAGW-250 from the National Aeronautics and Space Administration.

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