High-dispersion spectroscopic observations of Venus during 1968 and 1969 II. The carbon-dioxide band at 8689 Å

Thirty well-exposed photographic plates showing the spectrum of the carbon-dioxide band at 8689 Å in the atmosphere of Venus were obtained during 1968 and 1969. All spectra were obtained at a dispersion of 2 Å/mm for Venus phase angles varying from 10° to 126°. We find rotational temperatures ranging from 236 to 274 K. The average value of the rotational temperature is 246 ± 1 K (one standard deviation); for our 1967 observations, the rotational temperatures ranged from 222 to 248 K, with an average value of 238 ± 4 K. The variation of the equivalent width of the 8689 Å band, with Venus phase angle, was very similar for the two sets of observations (53 plates). The temporal variations, of approximately 30% were comparable with the phase variations over this limited range of phase angle.     

Absorption coefficients for the 727 nm band of methane at 77 K determined by intracavity laser spectroscopy

Methane spectral features in the visible to near-IR region are prominent in the spectra of the outer planets but laboratory data for the appropriate methane conditions are required to interpret the observational data. By use of the intracavity laser spectroscopy technique, a moderately high resolution (500,000) absorption spectrum of the 727 nm band of methane at 77 K is obtained. The methane absorption bands in the visible to near-IR region are very weak, but intracavity laser spectroscopy provides sufficient sensitivity to perform the measurements and to extract quantitative data for methane at low temperatures. Absorption coefficients are determined and are reported as averages at one Å intervals throughout the region 7127–7420 Å. By integrating over the band, an intensity of 753 cm^–1 km^–1 am^–1 is obtained. The results compare well with previous low resolution measurements on methane at room temperature, with gas phase results calculated using the absorption spectrum of liquid methane, and with absorption coefficients derived from methane features observed in the spectra of the outer planets and Titan.     

Voyager 1 imaging and IRIS observations of Jovian methane absorption and thermal emission: Implications for cloud structure

Images from three filters of the Voyager 1 wide-angle camera were used to measure the continuum reflectivity and spectral gradient near 6000 Å and the 6190-Å band methane/continuum ratio for a variety of cloud features in Jupiter's atmosphere. The dark “barge” features in the North Equatorial Belt have anomalously strong positive continuum spectral gradients suggesting unique composition, probably not elemental sulfur. Methane absorption was shown at unprecedented spatial scales for the Great Red Spot and its immediate environment, for a dark barge feature in the North Equatorial Belt, and for two hot spot and plume regions in the North Equatorial Belt. Some small-scale features, unresolvable at ground-based resolution, show significant enhancement in methane absorption. Any enhancement in methane absorption is conspicuously absent in both hot spot regions with 5-μm brightness temperature 255°K. Methane absorption and 5-μm emission are correlated in the vicinity of the Great Red Spot but are anticorrelated in one of the plume hot spot regions. Methane absorption and simultaneously maps of 5-μm brightness temperature were quantitatively compared to realistic cloud structure models which include multiple scattering at 5 μm as well as in the visible. A curve in parameter space defines the solution to any observed quantity, ranging from a shallow atmosphere and thin NH₃ cloud to a deep atmosphere with a thick ammonia cloud. Without additional constraints, such as center-to-limb information, it is impossible to specify the NH₃ cloud optical depth and pressure of a deeper cloud top independently. Variability in H₂ quadrupole lines was also investigated and it was found that the constancy of the 4-0 S(1)-line equivalent width is consistent with the constancy of the methane 6190-Å band equivalent width at ground-based resolution, but the much greater variability of the 3-0 S(1) line is inconsistent with either the methane band or 4-0 S(1) line. In hot spot regions the 255°K brightness temperature requires a cloud optical depth of about 2 or less at 5 μm in the NH₃ cloud layer. To be consistent with the observed 6190-Å methane absorption in hot spot regions, the NH₃ cloud optical depth in the visible is about 7.5, implying that aerosols in hot spot regions have effective radii near 1 μm or less.     

Nitrogen on Triton

The near-infrared spectrum of Triton is characterized by strong absorption bands of methane, probably in the solid state. An additional absorption band at 2.16 μm is tentatively identified as the density-induced (2-0) band of molecular nitrogen in the liquid state. The fundamental overtones of this band system cannot presently be observed because of limitations of the terrestrial atmosphere or spectral signal precision. Using the absorption coefficient for this band derived from laboratory observations and from the literature, it is calculated that Triton must have a layer of nitrogen at least tens of centimeters deep over much of its surface; this quantity is plausible in terms of the cosmic abundance of nitrogen and by comparison with Titan where a massive atmosphere of nitrogen exists. The Triton spectrum has been modeled with liquid nitrogen and solid methane, and it is found that the shape of the continuum in two spectral regions can be properly accounted for by adding a spectral component corresponding to fine-grained water frost. It is speculated that yet another component, a dark, solid, photochemical derivative of methane, may occur as a trace contaminant of the surface materials. If much of the surface of Triton is liquid, the radiometric observations of the satellite must be reinterpreted to derive the radius and surface albedo. If there is liquid nitrogen exposed on the surface, the atmosphere of Triton is probably dominated by nitrogen rather than methane because of the much higher vapor pressure of the former. At the calculated subsolar temperature of Triton, the vapor pressure of nitrogen implies a surface atmospheric pressure in the range 0.13 to 0.30 atm.     

The albedo of Titan

The irradiance of Titan has been measured from 0.50 to 1.08μ in 30 Å band-passes spaced 0.01–0.02μ apart. Geometric albedos have been computed at the wavelenghts of measurement using a standard solar flux distribution after Labs and Neckel. The maximum value of p_λ(0) is 0.37 at 0.68, 0.75, and 0.834μ, the minimum value, in the centers of the strongest methane absorption bands, is 0.10 at 0.887 and 1.012μ.The brightness of Titan at the time of the present measurements has been compared with that of previous modern photoelectric measurements. Within the apparent consistency of the different photoelectric systems, the brightness of Titan appears to undergo changes with time.A provisional curve of the geometric albedo from 0.30 to 4.0μ has been made by combining the present results with those of other authors, i.e., relative measurements of Titan from 0.30 to 0.50μ, and measurements of Jupiter and Saturn from 1.08 to 4.00μ. The latter are used to estimate the strengths of the methane absorption bands of Titan in that spectral range. The bolometric geometric albedo, $\text{p}{}^1\text{(0)}$, is computed to be 0.21. A variety of current measurements of Titan indicate a substantial atmosphere, suggesting a value of the phase integral $\text{q}\text{= 1.30 ± 0.20}$. The bolometric Bond albedo, $\text{A}{}^1$, is then 0.27 ± 0.04, giving an effective radiative temperature T_e= 84 ± 2°K.The absorption band contours of Titan have been compared with those of Jupiter and Saturn at the same resolution. The bands of the planets are known to be due primarily to methane, and they show a very regular relationship, with those of Saturn being consistently deeper and wider. For Titan, the strengths of the bands are equal or less than those of Jupiter in the band centers, while the wings are stronger than those of Saturn.Previous photoelectric and photographic spectra have been examined for evidence of temporal variation of the methane path length in the atmosphere of Titan. Differences in measurement techniques prohibit detection of small differences. The only potential differences beyond experimental uncertainties are those of Kuiper (1944) and Harris (mid-fifties). Taking Kuiper's results at face value, Titan appears to have a shorter methane path length in 1972. Harris's results can be reconciled only by the doubtful hypothesis of an almost complete absence of methane at that time.     

The Temperature-Dependent Spectrum of Methane Ice I between 0.7 and 5 μm and Opportunities for Near-Infrared Remote Thermometry

New infrared absorption coefficient spectra of pure methane ice I were measured at temperatures between 30 and 90 K, over wavelengths from 0.7 to 5 μm, along with spectra of methane ice II at 20 K and liquid methane at 93 K. The spectra were derived from transmission measurements through monocrystalline samples grown in a series of closed cells having interior dimensions ranging from 100 μm to 1 cm. The thicker samples permitted measurement of extremely weak absorption bands, with absorption coefficients as small as 0.003 cm⁻¹. We report 14 new absorption bands, which we tentatively assign to specific vibrational transitions. Two of the new bands are attributed to CH₃D. Measurements of the weaker CH₄ bands are particularly needed for interpreting spectral observations of Pluto and Triton, where a number of weak CH₄-ice absorption bands have been observed. The data presented in this paper complement studies of spectral transmission by thin films of methane ice, which are most suitable for measuring the stronger absorption bands. Temperature-dependent spectral features revealed by the new data offer the opportunity to determine CH₄-ice temperatures remotely, via near-infrared reflectance spectroscopy. This approach could prove particularly valuable for future spacecraft exploration of Pluto.     

An acousto-optical imaging spectrophotometer for astrophysical observations

We present the results of our tests of an acousto-optical imaging spectrophotometer with a CCD detector for astronomical observations. The tunable acousto-optical filter, based on a paratellurite single crystal with a 13 Å pass band operates in the wavelength range 6300–11000 Å. We obtained image spectra for the planetary nebula NGC 7027 in the Hα line and for Saturn in the methane absorption band, as well as Hα and continuum images for the nuclear region of the Seyfert galaxy NGC 1068.     

Improved near-infrared methane band models and k-distribution parameters from 2000 to 9500 cm and implications for interpretation of outer planet spectra

The band model fits of Sihra [1998. Ph.D. Thesis. University of Oxford], subsequently reported by Irwin et al. [2005. Icarus 176, 255-271], to new measurements of low-temperature near-infrared self-broadened methane absorption spectra combined with earlier warmer, longer path measurements of both self- and hydrogen-broadened methane spectra measured by Strong et al. [1993. J. Quant. Spectrosc. Radiat. Transfer 50, 363-429], have been found to contain severe artefacts at wavelengths of very low methane absorption. Although spectra calculated from these new band data appear to be reliable for paths with low to medium absorption, transmissions calculated for long paths of high methane absorption, such as for Uranus, Neptune and Titan are severely compromised. The recorded laboratory transmission spectra of Sihra [1998. Ph.D. Thesis. University of Oxford] and Strong et al. [1993. J. Quant. Spectrosc. Radiat. Transfer 50, 363-429] have thus been refitted with a more robust model and new k-distribution data for both self- and hydrogen-broadened methane absorption derived. In addition, a new model of the temperature dependence of the absorption has been employed that improves the quality of the fit and should also provide more accurate extrapolations to low temperatures.     

Methane absorption variations in the spectrum of Pluto

Absolute spectrophotometry of Pluto in the wavelength range of 5600 to 10,500 Å was obtained on 4 nights covering lightcurve phases of 0.18, 0.35, 0.49, and 0.98. The four phases included minimum light (0.98) and one near maximum light (0.49). The spectra reveal significant variations in the absorption depths of the methane bands at 6200, 7200, 7900, 8400, 8600, 8900, and 10,000 Å. The minimum amount of absorption was found to occur at minimum light. This variation would imply a 30° change in the column abundance of methane within 3 days. A model employing an anisotropic surface distribution of methane frost and a clear layer of CH₄ gas was developed to explain the variation in absorption strength with rotational phase. The fit to the overall spectrum requires the presence of a frost with particle sizes on the order of a few millimeters. An upper limit of 5.5 m-am is derived for the one-way column abundance of CH₄ gas. An equally good fit to the variation of the 7200-Å band is obtained if the atmosphere is removed from the model entirely.     

Interpretation of the lunar microwave brightness temperature spectrum: Feasibility of orbital heat flow mapping

A detailed model of the lunar regolith is analyzed to examine the feasibility of an orbital mapping of heat flow using microwave radiometers. For regolith thermal and electrical properties which are representative of Apollo findings, brightness temperature observations in the bandλ = 5–30 cm would be required for heat flow analysis. Spectral variations shortward of 5 cm are controlled primarily by the temperature dependencies of the thermal conductivity and electrical absorption within the diurnal-varying layer. For wavelengths longer than 30 cm, unwanted emission from high impedance subregolith layers can be significant and size limitations on spacecraft radiometers is a factor. Over the 5- to 30-cm band, lunation-averaged brightness temperature increases of 2–10°K are predicted for heat flow values representative of the Apollo measurements. The magnitude of this increase depends directly on the value of regolith microwave absorption. For absorption values consistent with Apollo laboratory measurements, a spectral increase of 5°K is predicted. This value is considered marginally sufficient for an orbital heat flow measurement. However, important non-heat flow effects must be accounted for. Spectral variations can occur due to surface topography and subsurface scattering. For nadir viewing, surface roughness effects are not expected to be significant and topographic effects are nearly constant with wavelength for λ > 5cm. Volume scattering due to subsurface rock fragments can cause emission darkening of 1–6°K. However, spectral variations will not be large unless the distribution of scatterer sizes is sharply skewed. For the Moon, the most serious spurious effect appears to be emissivity variations due to the near-surface density gradient. A brightness temperature decrease of 10°K is predicted from centimeter to decameter wavelengths. If the transition from porous surface fines to compacted regolith soil occurs rapidly (within the upper 3–5 cm), most of the emissivity decrease will occur in the 5- to 30-cm wavelength band. It is recommended that complementary radar measurements be utilized to augment constraints on regolith emissivity and scattering properties.     

Methane absorption in the atmosphere of Saturn : Rotational temperature and abundance from the 3ν30 band

Three high-dispersion spectra of Saturn, in the methane 3ν3 band at 1.1 μm, were obtained during September and October, 1970. Tracings of these spectra have been measured, and reduced by a curve of growth technique which assumes a reflecting-layer model and Lorentzian line profiles. The reductions yield a range of rotational temperatures from 122 to 142°K, and methane line-of-sight abundances, ηN, from 86 ± 14 to 51 ± 11 m amagat, depending on the value of the Lorentz halfwidth, a, used in computation of the curves of growth.     

Observations of Ganymede,Callisto, Ceres,Uranus, and Neptune at 3.33 mm wavelength

We have measured the 3.33 mm wavelength disk brightness temperatures of Ganymede (136 ± 21°K), Callisto (95 ± 17°K), Ceres (137 ± 25°K), Uranus (125 ± 9°K), and Neptune (126 ± 9°K). Our observations of Ganymede are consistent with the radiation from a blackbody in solar equilibrium, whereas Callisto's microwave spectrum indicates a surface similar to that of the Moon. The disk temperature for Ceres agrees with that expected from a rapidly rotating blackbody. The millimeter temperatures of Uranus and Neptune greatly exceed solar equilibrium values, implying atmospheres with large temperature gradients.     

High-resolution observations of the 6815-Å band of methane in the major planets

High-resolution (0.1-Å) spectra of the 6815-Å band of methane are presented for Jupiter, Saturn, Uranus, and Neptune. Spectra for Uranus, Neptune, and the equatorial region of Saturn were acquired with the SPIFI (W. H. Smith, T. R. Hicks, and J. P. Born (1978). Proceedings of the 4th International Colloquium on Astrophysics, Triest, July 3–7, 1978. pp. 593–599) at the 2.2-m telescope of the Mauna Kea Observatory during May and June 1980. Additional spectra were obtained for Jupiter and the northern temperate and polar regions of Saturn in December 1980 and January 1981 from Kitt Peak National Observatory's McMath Solar Telescope. The spectra show a dichotomy in strength of methane absorption between Jupiter-Saturn and Uranus-Neptune. A simple model analysis, based on homogeneous scattering models, is unable to resolve whether this dichotomy is due to an actual increase in the methane mixing ratio with solar distance or to the temperature dependence of line strengths and absorption pathlengths in these atmospheres. If the rotational quantum number for the prominent 6818.9-Å feature is J < 4, then significant aerosol extinction must exist within the visibly accessible portion of Uranus' atmosphere for the methane mixing ratio to be greater than the solar value.     

Near-IR methane absorption in outer planet atmospheres: Improved models of temperature dependence and implications for Uranus cloud structure

Near-IR absorption of methane in the 2000-9500 cm⁻¹ spectral region plays a major role in outer planet atmospheres. However, the theoretical basis for modeling the observations of reflectivity and emission in these regions has had serious uncertainties at temperatures needed for interpreting observations of the colder outer planets. A lack of line parameter information, including ground-state energies and the absence of weak lines, limit the applicability of line-by-line calculations at low temperatures and for long path lengths, requiring the use of band models. However, prior band models have parameterized the temperature dependence in a way that cannot be accurately extrapolated to low temperatures. Here we use simulations to show how a new parameterization of temperature dependence can greatly improve band model accuracy and allow extension of band models to the much lower temperatures that are needed to interpret observations of Uranus, Neptune, Titan, and Saturn. Use of this new parameterization by Irwin et al. [Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Bowles, N., Calcutt, S.B., 2005b. Icarus. In press] has verified improved fits to laboratory observations of Strong et al. [Strong, K., Taylor, F.W., Calcutt, S.B., Remedios, J.J., Ballard, J., 1993. J. Quant. Spectrosc. Radiat. Trans. 50, 363-429] and Sihra [1998. Ph.D. Thesis, Univ. of Oxford], which cover the temperature range from 100 to 340 K. Here we compare model predictions to 77 K laboratory observations and to Uranus spectra, which show much improved agreement between observed and modeled spectral features, allowing tighter constraints on pressure levels of Uranus cloud particles, implying that most scattering contributions arise from pressures near 2 bars and 6 bars rather than expected pressures near 1.25 and 3.1 bars. Between visible and near-IR wavelengths, both cloud layers exhibit strong decreases in reflectivity that are indicative of low opacity and submicron particle sizes.     

Phase reddening on near-Earth asteroids: Implications for mineralogical analysis,space weathering and taxonomic classification

Phase reddening is an effect that produces an increase of the spectral slope and variations in the strength of the absorption bands as the phase angle increases. In order to understand its effect on spectroscopic observations of asteroids, we have analyzed the visible and near-infrared spectra (0.45–2.5 μm) of 12 near-Earth asteroids observed at different phase angles. All these asteroids are classified as either S-complex or Q-type asteroids. In addition, we have acquired laboratory spectra of three different types of ordinary chondrites at phase angles ranging from 13° to 120°. We have found that both, asteroid and meteorite spectra show an increase in band depths with increasing phase angle. In the case of the asteroids the Band I depth increases in the range of ～2° < g < 70° and the Band II depth increases in the range of ～2° < g < 55°. Using this information we have derived equations that can be used to correct the effect of phase reddening in the band depths. Of the three meteorite samples, the (olivine-rich) LL6 ordinary chondrite is the most affected by phase reddening. The studied ordinary chondrites have their maximum spectral contrast of Band I depths at a phase angle of ～60°, followed by a decrease between 60° and 120° phase angle. The Band II depths of these samples have their maximum spectral contrast at phase angles of 30–60° which then gradually decreases to 120° phase angle. The spectral slope of the ordinary chondrites spectra shows a significant increase with increasing phase angle for g > 30°. Variations in band centers and band area ratio (BAR) values were also found, however they seems to have no significant impact on the mineralogical analysis. Our study showed that the increase in spectral slope caused by phase reddening is comparable to certain degree of space weathering. In particular, an increase in phase angle in the range of 30–120° will produce a reddening of the reflectance spectra equivalent to exposure times of ～0.1 × 10⁶–1.3 × 10⁶ years at about 1 AU from the Sun. This increase in spectral slope due to phase reddening is also comparable to the effects caused by the addition of different fractions of SMFe. Furthermore, we found that under some circumstances phase reddening could lead to an ambiguous taxonomic classification of asteroids.     

Far-infrared brightness temperature of Saturn's disk and rings

We present far-infrared observations of Saturn in the wavelength band 76–116 μm, using a balloon-borne 75-cm telescope launched on 10 December 1980 from Hyderabad, India, when B′, the Saturnicentric latitude of the Sun, was 4°.3. Normalizing with respect to Jupiter, we find the average brightness temperature of the disk-ring system to be 90 ± 3° K. Correcting for the contribution from rings using experimental information on the brightness temperature of rings at 20 μm, we find T_D, the brightness temperature of the disk, to be 96.9 ± 3.5° K. The systematic errors and the correction for the ring contribution are small for our observations. We, therefore, make use of our estimate of T_D and earlier observations of Saturn when contribution from the rings was large and find that for wavelengths greater than 50 μm, there is a small reduction in the ring brightness temperature as compared to that at 20 μm.     

High-dispersion spectroscopic observations of Venus near superior conjunction: I. The carbon dioxide band at 7820 Å

Nine plates of the 7820 Å CO₂ band were taken in 1971. A curve-of-growth analysis of the CO₂ lines indicates a rotational temperature of 241 ± 2°K, with an average slope to the curve of growth of 0.60 ± 0.03. The Venus phase angle ranged from 7.2 to 10.7°. The equivalent widths of the 1971 data fall on a smooth curve fit through the 1969 data for this band; there does not appear to be any discontinuity in the phase curve at small phase angles.     

Airglow O2(1Σ) atmospheric band at 8645 Å and the rotational temperature observed at 23°S

Regular observations of theO₂(¹Σ), 0–1) atmospheric band at 8645 Å [O₂A(0, 1)] and the rotational temperature, together with the OH(9,4) band and OI 5577 Å airglow emissions, using multichannel tilting filter type photometers, have been carried out at Cachoeira Paulista (22.7°S, 45.0°W), Brazil, since February 1983. The O₂A(0, 1) band intensities occasionally vary from 200 to 1000 R during a night. Covariations in nocturnal and seasonal variation with the OI 5577 A emission were observed. The temperatures determined from the P branch of the O₂A(0, 1) band vary between 180 and 230 K. The amplitude of the nocturnal temperature variation is sometimes larger than that determined from the OH emission, and the phase of the variation, on some occasions, leads that of the OH.     

The atmosphere of Titan: An analysis of the Voyager 1 radio occultation measurements

Two coherently related radio signals transmitted from Voyager 1 at wavelengths of 13 cm (S-band) and 3.6 cm (X-band) were used to probe the equatorial atmosphere of Titan. The measurements were conducted during the occultation of the spacecraft by the satellite on November 12, 1980. An analysis of the differential dispersive frequency measurements did not reveal any ionization layers in the upper atmosphere of Titan. The resolution was approximately 3 × 10³ and 5 × 10³ electrons/cm³ near the evening and morning terminators, respectively. Abrupt signal changes observed at ingress and egress indicated a surface radius of 2575.0 ± 0.5 km, leading to a mean density of 1.881 ± 0.002 g cm^?3 for the satellite. The nondispersive data were used to derive profiles in height of the gas refractivity and microwave absorption in Titan's troposphere and stratosphere. No absorption was detected; the resolution was about 0.01 dB/km at the 13-cm wavelength. The gas refractivity data, which extend from the surface to about 200 km altitude, were interpreted in two different ways. In the first, it is assumed that N₂ makes up essentially all of the atmosphere, but with very small amounts of CH₄ and other hydrocarbons also present. This approach yielded a temperature and pressure at the surface of 94.0 ± 0.7°K and 1496 ± 20 mbar, respectively. The tropopause, which was detected near 42 km altitude, had a temperature of 71.4 ± 0.5°K and a pressure of about 130 mbar. Above the tropopause, the temperature increased with height, reaching 170 ± 15°K near the 200-km level. The maximum temperature lapse rate observed near the surface (1.38 ± 0.10°K/km) corresponds to the adiabatic value expected for a dry N₂ atmosphere—indicating that methane saturation did not occur in tbis region. Above the 3.5-km altitude level the lapse rate dropped abruptly to 0.9 ± 0.1°K/km and then decreased slowly with increasing altitude, crossing zero at the tropopause. For the N₂ atmospheric model, the lapse rate transition at the 3.5-km level appears to mark the boundary between a convective region near the surface having the dry adiabatic lapse rate, and a higher stable region in radiative equilibrium. In the second interpretation of the refractivity data, it is assumed, instead, that the 3.5 km altitude level corresponds to the bottom of a CH₄ cloud layer, and that N₂ and CH₄ are perfectly mixed below this level. These assumptions lead to an atmospheric model which below the clouds contains about 10% CH₄ by number density. The temperature near the surface is about 95°K. Arguments concerning the temperature lapse rates computed from the radio measurements appear to favor models in which methane forms at most a limited haze layer high in the troposphere.     

Photometric properties of powdered sulfur

The spectrophotometric (0.39 < λ < 0.7 λm) properties of three particle-size fractions (diameters <10 λm, <150 λm, and 420–850 λm) of sulfur have been investigated in the laboratory. Particle size, temperature, thermal history, and scattering geometry are all shown to influence the spectral reflectance of the normal (S₈) sulfur samples and an “orange-colored” S₈ sample produced by quenching molten sulfur. A scattering law consisting of a linear combination of lunar-like and Lambertian terms adequately describes the data for all particle sizes. Where sulfur is darkest (λ < 0.45 λm), the reflectance decreases with increasing particle size, whereas where sulfur is brightest (λ > 0.45 λm) the reflectance increases with decreasing particle size. In reflected light, the long wavelength edge of the strong ultraviolet absorption retreats smoothly to shorter wavelengths with decreasing temperature at ～1.6Å/°K, a value lower than the 2.2Å/°K value previously reported for transmitted light. Near opposition, sulfur powders are found to follow closely a Minnaert limb darkening law except where the reflectance is low, i.e., in the strong ultraviolet absorption band of the larger particle size fractions. It is clear from our data that quantitative comparisons between disk-integrated observations of Io and laboratory measurements of flat samples of sulfur are not adequate unless temperature effects and changes in scattering geometry are included.