Limb brightening on Uranus: The visible spectrum,II

New narrow-band (100 Å) photoelectric area-scanning photometry of the Uranus disk is reported. Observations were concentrated on the two strong CH₄ bands at λ 6190 and 7300 Å. Adjacent continuum regions at λ 6400 and 7500 Å were also measured for comparison. Both slit and pinhole scans were made in orthogonal directions. Disk structure in each waveband is apparent through lack of circular symmetry in the intensity distribution over the Uranus image. Polar brightening is especially prominent in the λ 7500-Å waveband. Coarse quantitative determinations of the true intensity distribution over the Uranus disk were made. For the λ 6190-Å CH₄ band, Uranus exhibits a disk of essentially uniform intensity except for a hint of polar brightening. For the λ 7300-Å CH₄ band, moderate limb brightening is apparent. Specifically, the true intensities at the center and limb of the planetary disk are approximately in the proportion 1:2. Extreme limb brightening, with a corresponding intensity ratio greater than 1:4, is not permitted by the observational data.     

Limb brightening on Uranus: An interpretation of the λ7300 Å methane band

Measurements of limb brightening on the Uranus disk within the λ7300 Å CH₄ band are interpreted using an elementary inhomogeneous radiative transfer model to describe the atmosphere. A two layer model which consists of a finite, optically thin, region of conservatively scattering particles overlying a semi-infinite clear H₂CH₄ atmosphere satisfactorily explains the observations. The maximum optical thickness of the upper layer appears to lie in the range 0.1 to 0.2. The CH₄/H₂ mixing ratio in the lower layer is larger than the corresponding solar value by a factor on the order of three or greater. The results are discussed briefly in terms of current models of the Uranus atmosphere.     

Spatially resolved reflectivities of Jupiter during the 1976 opposition

Spatially resolved absolute reflectivities of several regions of the Jovian disk in the wavelength region 3000 to 10760 Å are presented. Spectra were obtained of the central meridian and limbs of the Equatorial Region, North Equatorial Belt, and North Tropical Zone. Equivalent widths of several CH₄ and NH₃ bands are measured. The spatial variations of continuum reflectivity and absorption band profiles are shown in various ratio spectra.     

Interpretation of the 6818.9-Å methane line in terms of inhomogeneous scattering models for Uranus and Neptune

We have obtained high-resolution spectra of Uranus and Neptune in the methane transition near 6800 Å, and in particular, the 6818.9Å feature. Calculated equivalent widths for this line using recently proposed models of the atmospheres of these two planets indicate that the C/H ratio is greater than or equal to 5 × 10^?3 below the CH₄ saturation level. This value is 12 times the solar mixing ratio. The half-widths of the computed line profiles are in agreement with the observed half-widths. Therefore, it is unnecessary to introduce an unidentified constituent with an abundance comparable to H₂, postulated recently by Belton and Hayes, and by Bergstrahl, to account for the observed line broadening.     

An estimate of the temperature and abundance of CH4 and other molecules in the atmosphere of Uranus

We present a preliminary analysis of CH₄ absorptions near 6800 Å in new high resolution spectra of Uranus. A curve of growth analysis of the data yields a rotational temperature near 100 K and a CH₄/H₂ ratio that is 1 to 3 times that expected for a solar type composition. The long pathlengths of CH₄, apparently demanded by absorptions near 4700 Å, are qualitatively shown to be the result of line formation in a deep, predominantly Rayleigh scattering atmosphere in which continuum absorption is a strong function of wavelength. The analysis of the CH₄ also yields a minimum value for the effective pressure of line formation (～ 2 atm). This value is shown to be twice that expected on Uranus if the atmosphere were predominantly H₂. It is speculated that large amounts of some otherwise optically inert gas is present in the Uranus atmosphere. N₂ is suggested as a possible candidate since there are cosmogonic reasons why Uranus should contain large amounts of N relative to C, He, and H, and also because the pressure-induced pure rotation spectrum of N₂ could possibly account for the low brightness temperatures that have recently been observed at 33 and 350 μm. If N₂ is present the planet probably possesses a surface at the 10–100 atmosphere level.     

Visibility of facular fields in Mgi b-lines

On the basis of photoelectric observations, the center-to-limb variations of the brightness of restricted areas (≈0.5″ × 7.0″) of unresolved facular granules were determined at different frequencies in the lines λ5183 Å and λ5172 Å of Mgi. It was found that the chromospheric plages reach maximum intensity in the central parts of the lines at the same position on the solar disk where photospheric faculae have maximum brightness. The floccular emission is conspicuous in the cores of the lines up to a distance Δλ = 0.02 Å. In the portion of the lines corresponding to 0.02 Å < Δλ < 0.18 Å the contrast of flocculi decreases to a minimum value and then increases again in the inner wings of these lines. In the far wings the contrast of facular areas systematically decreases to the continuum values.     

Spectrophotometry of carbon stars

The results of a spectrophotometric study of 13 carbon stars in the wavelength range from 4000 Å to 6800 Å with 2.75 Å resolution are given. The observed energy distributions for these stars relative to the flux at λ₀ = 5556 Å, represented in graphic form, were determined. Their color temperatures were determined from the [5710] ? [6680] color index. A dust shell is assumed to exist around U Cyg. The indices of the C_2’ MS, and CaCl molecular bands and the D_2,1 line were also determined.     

On inhomogeneous scattering models of Titan's atmosphere

The spectrum of Titan from 4800 to 11 000 Å has many CH₄ absorption bands which cover a range of intensities of several orders of magnitude. Yet even the strongest of these bands in Titan's spectrum has considerable residual central intensity. Some investigators have concluded that these strong CH₄ bands must be highly saturated, but recent laboratory measurements of the bands made at room temperature show that curve-of-growth saturation is very small. At the presumed low pressures and temperatures in Titan's atmosphere, we show that saturation is very dependent on the band model parameters. However, in either a simple reflecting layer model or in a homogeneous scattering model saturation cannot be the principal cause of the filling in of these strong CH₄ bands if our best estimates of the band model parameters are correct. We find that an inhomogeneous scattering model atmosphere with fine “Axel dust” above most ot the CH₄ gas is needed to fill in the band centers. The calculated spectrum of one particular model of this class is compared to observations of Titan. Our essential conclusion is that Titan does have most of its scattering particles above most of the CH₄ gas which has an abundance of at least 2 km-am. This large abundance of CH₄ is necessary to produce the 6420-Å feature recently discovered in Titan's spectrum.     

The spectra of Uranus and Neptune at 8–14 and 17–23 μm

An array spectrometer was used on the nights of 1985 May 30–June 1 to observe the disks of Uranus and Neptune in the spectral regions 7–14 and 17–23 μm with effective resolution elements ranging from 0.23 to 0.87 μm. In the long-wavelength region, the spectra are relatively smooth with the broad S(1) H₂ collision-induced rotation line showing strong emission for Neptune. In the short-wavelength spectrum of Uranus, an emission feature attributable to C₂H₂ with a maximum stratospheric mixing ratio of 9 × 10⁻⁹ is apparent. An upper limit of 2 × 10⁻⁸ is placed on the maximum stratospheric mixing ratio of C₂H₆. The spectrum of Uranus is otherwise smooth and quantitatively consistent with the opacity provided by H₂ collision-induced absorption and spectrally continuous stratospheric emission, as would be produced by aerosols. Upper limits to detecting the planet near 8 μm indicate a CH₄ stratospheric mixing ratio of 1 × 10⁻⁵ or less, below a value consistent with saturation equilibrium at the temperature minimum. In the short-wavelength spectrum of Neptune, strong emission features of CH₄ and C₂H₆ are evident and are consistent with local saturation equilibrium with maximum stratospheric mixing ratios of 0.02 and 6 × 10⁻⁶, respectively. Emission at 8–10 μm is most consistent with a [CH₃D]/[CH₄] volume abundance ratio of 5 × 10⁻⁵. The spectrum of Neptune near 13.5 μm is consistent with emission by stratospheric C₂H₂ in local saturation equilibrium and a maximum mixing ratio of 9 × 10⁻⁷. Radiance detected near 10.5 μm could be attributed to stratospheric C₂H₄ emission for a maximum mixing ratio of approximately 3 × 10⁻⁹. Quantitative results are considered preliminary, as some absolute radiance differences are noted with respect to earlier observations with discrete filters.     

Peculiarities of the UV continuum energy distribution for T Tauri stars

In the UV spectra of BP Tau, GW Ori, T Tau, and RY Tau obtained with the Hubble Space Telescope, we detected an inflection near 2000 Å in the F _λ ^c (λ) curve that describes the continuum energy distribution. The inflection probably stems from the fact that the UV continuum in these stars consists of two components: the emission from an optically thick gas with T<8000 K and the emission from a gas with a much higher temperature. The total luminosity of the hot component is much lower than that of the cool component, but the hot-gas radiation dominates at λ<1800 Å. Previously, other authors have drawn a similar conclusion for several young stars from low-resolution IUE spectra. However, we show that the short-wavelength continuum is determined from these spectra with large errors. We also show that, for three of the stars studied (BP Tau, GW Ori, and T Tau), the accretion-shock radiation cannot account for the observed dependence F _λ ^c (λ) in the ultraviolet. We argue that more than 90% of the emission continuum in BP Tau at λ>2000 Å originates not in the accretion shock but in the inner accretion disk. Previously, a similar conclusion was reached for six more classical T Tau stars. Therefore, we believe that the high-temperature continuum can be associated with the radiation from the disk chromosphere. However, it may well be that the stellar chromosphere is its source.     

The Raman signature of H2 in the uv spectra of Uranus and Neptune: Constraints on the haze optical properties and on the para-H2 fraction

The geometric albedos of Uranus and Neptune, inferred from archived Hubble Space Telescope observations and from the ground-based measurements of Karkoschka, 1994, are modeled in the wavelength range 2200–4200 Å. The radiative transfer model, which includes Rayleigh–Raman scattering and Mie scattering by haze particles, aims at reproducing the fine structure of the geometric albedos at a resolution of 2–10 Å. The steep variation of the total optical depth allows to investigate the influences of both the stratospheric and tropospheric haze layers and that of the deep tropospheric cloud, although their relative importance is difficult to estimate accurately. Using the haze models of Baines et al., 1995, the optical properties of the Mie scatterers are inferred. The haze material on Uranus is characterized by a slowly decreasing imaginary index of refraction: n_i varies from about 0.10 to 0.01–0.02 between 2200 and 4200 Å. Below 3000 Å, the absorptivity of Neptuness haze material is comparable to that on Uranus or slightly lower (n_i ∼ 0.03–0.10). Above 3000 Å, it exhibits a steeper decrease (from 0.30 to 0.003). The main source of uncertainty at longer wavelengths is the reflectivity of the underlying (H₂S ?) cloud. At shorter wavelengths, molecular scattering strongly dominates Mie scattering and the determination of the absorptivities is estimated to be accurate within a factor of 2. For Neptune, there is an additional uncertainty due to the inability of the initial haze model to provide a fit to the observed albedo. The Baines et al. model was modified by multiplying the number-densities of the hydrocarbons haze layers by a factor of 2.5–4.8, making it more consistent with the results of Pryor et al., 1992. For Uranus, these results suggest a darkening of the southern hemisphere since the Voyager epoch, in agreement with recent HST imaging. As a whole, the Neptunian haze seems to be more transparent than that of Uranus, possibly owing to the more turbulent dynamical state of the troposphere. Longwards of 3000 Å, the inferred absorptivities are consistent with laboratory measurements on tholins produced from CH₄–H₂ gas mixtures (Khare et al., 1987). The para-H₂ mole fraction on both planets is constrained from the strength of a prominent H₂ Raman feature at 2853 Å. On Uranus, at latitudes between 45 and 75°S and in the 50–500 mbar pressure range, the best agreement is obtained with an equilibrium para-H₂ distribution. On Neptune, there is an indication of a slight departure from equilibrium in the same pressure range at mid-southern latitudes. Although this new method is significantly less accurate, its results are consistent with those of previous investigations based on the analysis of H₂ quadrupole lines (Baines et al., 1995) and of the Voyager IRIS spectra (Conrath et al., 1998).     

Detection of a CH4 atmosphere on Pluto

Using a low-resolution spectrograph and a CCD array, a spectrum of Pluto from 0.58 to 1.06 μm was obtained. The spectrum had a resolution of $\text{～25}\text{A}\text{?}$ and a signal-to-noise ratio of ～300. It showed CH₄ absorption bands at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 Å. The strongest of these bands was at 8900 Å with an absorption depth of 0.23. This band was heavily saturated, compared to the weaker bands, providing proof for the gaseous origin of the observed absorptions. By applying CH₄ band model parameters to our data, a total CH₄ abundance of 80 ± 20 m-am was derived. This translates into a one-way abundance of 27 ± 7 m-am and a CH₄ surface pressure of 1.5 × 10^?4 atm. An upper limit to the total pressure of ～0.05 atm could be set. First-order calculations on atmospheric escape showed that this methane atmosphere would be stable if the mass of Pluto is increased 50% over its current value and its radius is 1400 km. Alternatively a heavier gas mixed with the CH₄ atmosphere would aid its stability. The relatively large amount of gaseous CH₄ observed implies that the absorption bands recently reported at 1.7 and 2.3 μm are likely due to atmospheric CH₄ absorptions rather than surface frost as interpreted earlier.     

Uranus: Narrow-waveband disk profiles in the spectral region 6000 to 8500 Angstroms

New narrow-band (100 Å) photoelectric slit scan photometry of Uranus has been obtained in the spectral region 6000 to 8500 Å. Coarse radial intensity profiles in seven wavebands are presented. Measurements of the point spread function have been used to partially remove the effects of atmospheric seeing. Restoration of the Uranus image, with a spatial resolution limit ～0″.5 arc, has been achieved by means of analytical Fourier-Bessel inversion. Results of the investigation confirm earlier studies of limb brightening on the Uranus disk. But not all strong CH₄ absorption bands are found to exhibit limb brightening. Specifically, the CH₄ bands at 8000 and 8500 Å show pronounced apparent limb darkening. Polar brightening may be responsible for the phenomenon. If so, an aerosol haze with a local optical thickness ～0.5 or greater would be required. Visibility of the dense cloud layer located deep in the atmosphere might also cause apparent limb darkening. If so, the maximum permitted [CH₄/H₂] mixing ratio in the visible atmosphere would correspond to ～3 times the solar value.     

The abundance of CH3D in the atmosphere of Titan,derived from 8- to 14-μm thermal emission

The 8.6-μm emission feature of Titan's infrared spectrum was analyzed using the Voyager temperature-pressure profile. Although both C₃H₈ and CH₃D have bands at that wavelength, we show that CH₃D dominates the observed emission on Titan. We derived a CH₃D/CH₄ mixing ratio using this band and the strong CH₄ band at 7.7 μm. The corresponding D/H ratio is 4.2_?1.5⁺² × 10^?4, neglecting deuterium fractionation with other molecules. The main uncertainty in this value comes from the continuum emission characteristics. The D/H ratio is apparently significantly enhanced on Titan with respect to published values for Saturn.     

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.     

Measurement of stratospheric aerosol on Saturn using an eclipse of Titan

An eclipse of Titan by Saturn was observed on December 20, 1979, to measure the aerosol content in the atmosphere of Saturn. The measurements were made with the 74-in. telescope of the Helwan Observatory, Egypt, in the bandpass 6300–7300 Å and extend to ～5 magnitudes of eclipse darkening. The faint portion of the lightcurve unambiguously requires the presence of aerosol in the lower stratosphere of Saturn. The aerosol extends to at least 20 km above the tropopause and has a one-way stratospheric vertical optical depth of 0.4_?0.02^+0.04 at 6700 Å. The results apply to the sunset terminator at a cronographic latitude of 23°S.     

High resolution spectrophotometry of selected features in the 1.1 μm spectrum of Comet Kohoutek (1973f)

Fabry-Perot interferometry of Comet Kohoutek (1973f) at 1.1 μm with a resolution of 1.2 Å showed emission features identified as OH and CN lines in addition to a strong Fraunhofer continuum. Central intensities have been derived for three cases (uniform, gaussian, and gaussian plus ?^?1 law) of brightness profiles in the comet coma. Limits for CH₄, H₂O, HeI, SiL and CrI are also derived.     

Axel dust on Saturn and Titan

The scattering and absorption properties of Axel dust were investigated by means of Mie theory. We find that a flat distribution of particle radii between 0 and 0.1 μm, and an imaginary part of the index of refraction which varies as λ^?2.5 produce a good fit to the variation of Titan's geometric albedo with wavelength (λ) provided that τ_ext, the extinction optical depth of Titan's atmosphere at 5000 Å, is about 10. The real part of the complex index is taken to be 2.0. The model assumes that the mixing ratio of Axel dust to gas is uniform above the surface of Titan. The same set of physical properties for Axel dust also produces a good fit to Saturn's albedo if τ_ext = 0.7 at 5000 Å. To match the increase in albedo shortward of 3500 Å, a clear layer (containing about 7 km-am H₂) is required above the Axel dust. Such a layer is also required to explain the limb brightening in the ultraviolet. These models can be used to analyze the observed equivalent widths of the visible methane bands. The analysis yields an abundance of the order of 1000 m-am CH₄ in Titan's atmosphere. The derived CH₄/H₂ mixing ratio for Saturn is about 3.5 × 10^?3 or an enhancement of about 5 over the solar ratio.     

New observational constraints on the temperature inversions of Uranus and Neptune

We present 20-μm photometry of Uranus and Neptune which confirms the presence of a temperature inversion in the lower stratospheres in both planets. We find the brightness temperature difference between 17.8 and 19.6 μm to be 0.8 ± 0.5°K for Uranus and 1.8 ± 0.6°K for Neptune. These results indicate that the temperature inversions on both planets are weaker than previously thought. Comparison to model atmospheres by J. Appleby [Ph.D. thesis, SUNY at Stony Brook 1980] indicates that the temperature inversions can be understood as arising from heating by the absorption of sunlight by CH₄ and aerosols. However, the stratospheric CH₄ mixing ratio on Neptune must be higher than that at the temperature minimum.     

Near-infrared spectrophotometry of Titan

Detailed analysis of the R(5) manifold of Titan's 3ν₃ CH₄ band confirms that the column abundance of Titan's spectroscopically visible atmosphere is greater than 1.6 kmamagats. This agrees with the value estimated from the strength of Titan's 3ν₃ CH₄Q branch and is at least 25 times the value for the column abundance of Mars' atmosphere. Moreover, the enhanced strength of the weaker CH₄ lines in Titan's spectrum relative to Saturn's spectrum suggests that CH₄ constitutes a significant fraction of this bulk.Recently discovered strong, unidentified absorptions in Titan's spectrum at 1.05–1.06 μm have been compared with laboratory spectra of a number of gases including CH₄, C₂H₄, C₂H₆, and C₃H₈ with negative results. These comparisons, however, have not excluded the possibility that these features arise from a very large quantity of CH₄ or from an isotope of CH₄. The fundamental transition of the responsible molecule may affect the interpretation of Titan's 8–14 μm spectrum since its wavelength may lie in this window. Comparison with Uranus' spectrum suggests that the visible abundance of this molecule in Titan's atmosphere may be much greater than in Uranus' relatively clear, deep atmosphere.Spectra of features at λ8150.7 and λ8272.7 attributed possibly to H₂ have been obtained at high resolution also during the apparitions of 1971, 1972, and 1973. These are presented for comparison with the results of the 1970 apparition. The existence of the λ8150.7 feature is established definitively but further observations are needed to establish whether the λ8272.7 feature exists beyond doubt.