Saturn: UBV photoelectric pinhole scans of the disk,II

During the 1980 Saturn apparition, calibrated UBV pinhole scans of the disk were obtained with a photoelectric area-scanning photometer. Point spread function data were also taken. Equatorial and polar scans were used to investigate the structure of the Satur atmosphere. The observational geometry was optimum. Not only was Saturn at opposition, but the ring system was essentially edge on to both the Sun and Earth. Our analysis indicates that the atmosphere of Saturn can be represented by a finite clear H₂ layer overlaying a semi-infinite absorbent aerosol haze. The extent of the clear H₂ region appears to be latitude dependent. The H₂ column density varies systematically from ～15 km-am over the equatorial and polar regions to ～ 31 km-am at temperate latitudes. The hemispheres of the planet are similar. Our earlier conclusion, that the aerosol haze is strongly absorbent in the ultraviolet, is confirmed; its effective U-band single-scattering albedo is ～0.4. Latitudinal disk structure at visual wavelengths appears to be the result of local variations in the volume density of absorbent particles in the aerosol layer.     

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.     

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.     

Interpretation of the 6818.9-Å methane feature observed on Jupiter,Saturn, and Uranus

High-resolution (0.1-Å) spectra of the 6818.9-Å methane feature obtained for Jupiter, Saturn, and Uranus by K. H. Baines, W. V. Schempp, and W. H. Smith ((1983). Icarus56, 534–542) are modeled using a doubling and adding code after J. H. Hansen ((1969). Astrophys. J.155, 565–573). The feature's rotational quantum number is estimated using the relatively homogeneous atmosphere of Saturn, with only J = 0 and J = 1 fitting the observational constraints. The aerosol content within Saturn's northern temperate region is shown to be substantially less than at the equator, indicating a haze only half as optically thick. Models of Jupiter's atmosphere are consistent with the rotational quantum-number assignment. Synthetic line profiles of the 6818.9-Å feature observed on Uranus reveal that a substantial haze exists at or above the methane condensation region with an optical depth eight times greater than previously reported. Seasonal effects are indicated. The methane column abundance is 5 ± 1 km-am. The mixing ratio of methane to hydrogen within the deep unsaturated region of the planet is 0.045 ± 0.025, based on an H₂ column abundance of 240 ± 60 km-am (W. H. Smith, W. Macy, and C. B. Pilcher (1980). Icarus43, 153–160), thus indicating that the methane comprises between one-sixth and one-half of the planet's mass. However, proper reevaluation of H₂ quadrupole features accounting for the haze reported here may significantly reduce the relative methane abundance.     

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.     

Jupiter and Saturn: Near-infrared spectral albedos

The near-infrared (0.65–2.5μm) spectral albedo of Jupiter and Saturn with 1.5% spectral resolution is presented for the center of disk and for the limb. There is a distinct difference in the continuum slope between Jupiter and Saturn which may be attributed to a difference in the dust content or composition of the two atmospheres. There is an indication of limb brightening in the deepest CH₄ bands on Saturn. No limb brightening is found for Jupiter.     

Long slit spectropolarimetry of Jupiter and Saturn

We present ground-based limb polarization measurements of Jupiter and Saturn consisting of full disk imaging polarimetry for the wavelength 7300 Å and spatially resolved (long-slit) spectropolarimetry covering the wavelength range 5200-9350 Å.For the polar region of Jupiter we find for λ = 6000 Å a very strong radial (perpendicular to the limb) fractional polarization with a seeing corrected maximum of about +11.5% in the South and +10.0% in the North. This indicates that the polarizing haze layer is thicker at the South pole. The polar haze layers extend down to 58° in latitude. The derived polarization values are much higher than reported in previous studies because of the better spatial resolution of our data and an appropriate consideration of the atmospheric seeing. Model calculations demonstrate that the high limb polarization can be explained by strongly polarizing (p ≈ 1.0), high albedo (ω ≈ 0.98) haze particles with a scattering asymmetry parameter of g ≈ 0.6 as expected for aggregate particles of the type described by West and Smith (West, R.A., Smith, P.H. [1991]. Icarus 90, 330-333). The deduced particle parameters are distinctively different when compared to lower latitude regions.The spectropolarimetry of Jupiter shows a decrease in the polar limb polarization towards longer wavelengths and a significantly enhanced polarization in strong methane bands when compared to the adjacent continuum. This is a natural outcome for a highly polarizing haze layer above an atmosphere where multiple scatterings are suppressed in absorption bands. For lower latitudes the fractional polarization is small, negative, and it depends only little on wavelength except for the strong CH₄-band at 8870 Å.The South pole of Saturn shows a lower polarization (p ≈ 1.0-1.5%) than the poles of Jupiter. The spectropolarimetric signal for Saturn decrease rapidly with wavelength and shows no significant enhancements in the fractional polarization in the absorption bands. These properties can be explained by a vertically extended stratospheric haze region composed of small particles <100 nm as suggested previously by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2005]. Icarus 179, 195-221).In addition we find in the V- and R-band a previously not observed strong polarization feature (p = 1.5-2.0%) near the equator of Saturn. The origin of this polarization signal is unclear but it could be related to a seasonal effect.Finally we discuss the potential of ground-based limb polarization measurements for the investigation of the scattering particles in the atmospheres of Jupiter and Saturn.     

Methane abundance in the atmosphere of Uranus

Modeling of the geometric albedo of Uranus in and near prominent methane absorption bands between 0.5 and 0.9 μm indicates that the visible atmosphere probably consists of a thin aerosol haze layer (τ_scat ? 0.3?0.5; ω_H ? 0.95) above an optically thick, semi-infinite Rayleigh scattering atmosphere. A significant depletion of methane gas above the haze layer is indicated. The mixing ratio of methane in the lower atmosphere is consistent with a value of CH₄/H₂ ? 3 × 10^?3, comparable to those derived for Jupiter and Saturn.     

Optical reflectance polarimetry of Saturn's globe and rings: IV. Aerosols in the upper atmosphere of Saturn

From 1958 to 1976 the degree and direction of polarization of the light at Saturn's disk center were measured in orange light over 74 nights and at five wavelenghts over 19 nights. Measurements were also recorded at limb, terminator, and pole. In addition, extensive regional polarization measurements were collected over Saturn's disk and several polarization maps were produced. These data were analyzed on the basis Mie scattering theory and of transfer theory in planetary atmospheres. A model of the Saturn upper atmosphere aerosol structure is derived in which the top part of the the main cloud layer is composed of spherical transparent particles of radius 1.4 μm and refractive index 1.44. Above this layer, a fine haze of submicron-sized grains was detected by its production of a component of polarization which is always directed poleward; this upper haze is interpreted as having nonspherical particles which are systematically oriented. This upper haze layer covers approximately the whole planet uniformly but varies in thickness from year to year. The clear gas above the cloud layer has an optical thickness of around 0.1.     

Neptune: Limb brightening within the 7300-Angstrom methane band

Narrow-waveband (100 Å) photoelectric slit-scan photometry of the Neptune disk is reported. Observations were concentrated within the strong CH₄ band at λ7300 Å. For comparison, measurements were also made within a continuum waveband at λ6800 Å. Point spread function data were obtained in both colors. Qualitative estimates of the true intensity distribution over the Neptune disk were made. Within the λ6800-Å continuum band, Neptune appears as an essentially uniform disk. Within the λ7300 Å CH₄ band, the planet exhibits strong limb brightening. Our results appear to require the presence of an optically thin layer of brightly scattering aerosol particles high in the Neptune atmosphere.     

The structure and microphysical properties of the Venus clouds: Venera 9, 10, and 11 data

Data processing and interpretation of the nephelometer measurements made in the Venus atmosphere aboard the Venera 9, 10 and 11 landers in the sunlit hemisphere near the equator are discussed. These results were used to obtain the aerosol distribution and its microphysical properties from 62 km to the surface. The main aerosol content is found in the altitude range between 62 km (where measurements began) and 48 km, the location of the cloud region. Three prominent layers labeled as I (between 62 and 57 km), II (between 57 and 51 km) and III (between 51 and 48 km), each with different particle characteristics are discovered within the clouds. The measured light-scattering patterns can be intrepreted as having been produced by particles with effective radii from 1 to 2 μm depending on height and indices of refractivity from 1.45 in layer I to 1.42 in layer III. These values do not contradict the idea that the droplets are made of sulfuric acid. In layers II and III the particle size distribution is at least bimodal rather than uni-modal. The index of refraction is found to decrease to 1.33 in the lower part of layer II, suggesting a predominant abundance of larger particles of different chemical origin, and chlorine compounds are assumed to be relevant to this effect. In the entire heightrange of the Venera 9–11 craft descents, the clouds are rather rarefied and are characterized by a mean volume scattering coefficient σ ～ 2 × 10^?5 cm^?1 that corresponds to the mean meteorological range of visibility of about 2 km. The average mass content of condensate is estimated to be equal to 4 × 10^?9 g/cm³, and the total optical depth of clouds to τ ～ 35. Near the bottom of layer III clouds are strongly variable. In the subcloud atmosphere a haze was observed between 48 and 32 km; that haze is mainly made of submicron particles, r_eff ～ 0.1μm. The atmosphere below that is totally transparent but separate (sometimes possibly disappearing) layers may be present up to a height of 8 km above the surface. A model of this region with a very low particle density (N ? 2–3 cm^?3) strongly refractive large particles (r_eff ? 2.5 μm; 1.7 < n < 2.0) provided satisfactory agreement. The optical depth of aerosol in the atmosphere below the subcloud haze does not exceed 2.5.     

Venus: Mesospheric hazes of ice,dust, and acid aerosols

We speculate on the origin and physical properties of haze in the upper atmosphere of Venus. It is argued that at least four distinct types of particles may be present. The densest and lowest haze, normally seen by spacecraft, probably consists of a submicron sulfuric acid aerosol which extends above the cloud tops (at ～70 km) up to ～80 km; this haze represents an extension of the upper cloud deck. Measurements of the temperature structure between 70 and 120 km indicate that two independent water ice layers may occasionally appear. The lower one can form between 80 and 100 km and is probably the detached haze layer seen in high-contrast limb photography. This ice layer is likely to be nucleated on sulfuric acid aerosols, and is analogous to the nacreous (stratospheric) clouds on Earth. At the Venus “mesopause” near 120 km, temperatures are frequently cold enough to allow ice nucleation on meteoric dust or ambient ions. The resulting haze (which is analogous to noctilucent clouds on Earth) is expected to be extremely tenous, and optically invisible. On both Earth and Venus, meteoric dust is present throughout the upper atmosphere and probably has similar properties.     

Inhomogeneous models of the atmosphere of Saturn

Analyses of ultraviolet, visible, and near-infrared spectra of Saturn lead to an inhomogeneous atmospheric model, having a clear gas layer which lies above an absorbing particle layer which lies above an ammonia haze layer. The boundary between the clear layer and the absorbing particle layer is at a pressure of 0.2 atm in the equatorial region and 0.3 atm in the temperate region. The boundary between the absorbing particle layer and the haze layer is at the radiative-convective boundary. Observations of ammonia absorption lines indicate that sunlight penetrates the haze to the ammonia sublimation level at a depth of 1.1 atm. Absorbing particles cause the observed decrease in reflectivity from visible to ultraviolet wavelengths. Consideration of the wavelength variation of Mie scattering parameters leads to an upper limit of about 0.2 μm for the particle radii and a particle number density of 10³ cm^?3. Some possible particle compositions are discussed. Comparison of computed 3-0 and 4-0 band hydrogen quadropole line equivalent widths with observed values leads to a haze layer optical thickness above the ammonia sublimation level of approximately 10. Equivalent widths computed for an equilibrium distribution of states agree better with observed values than those computed for a normal distribution. Methane 3ν₃ band manifold equivalent widths are in best agreement with measured equivalent widths for a CH₄/H₂ abundance ratio of 2 × 10^?3, which is 4.5 times the solar C/H ratio.     

Photometry and polarimetry of Jupiter at large phase angles: I. Analysis of imaging data of a prominent belt and a zone from pioneer 10

Limb-darkening curves are derived from Pioneer 10 imaging data for Jupiter's STrZ (?18 to ?21° latitude) and SEBn (?5 to ?8° latitude) in red and blue light at phase angles of 12, 23, 34, 109, 120, 127, and 150°. Inhomogeneous scattering models are computed and compared with the data to constrain the vertical structure and the single-scattering phase functions of the belt and the zone in each color. The very high brightness observed at a 150° phase angle seems to require the presence of at lleast a thin layer of reasonably bright and strongly forward-scattering haze particles at pressure levelsof about 100 mbar or less above both belts and zones. Marginally successful models have been constructed in which a moderate optical thickness (τ ≥ 0.5) of haze particles was uniformly distributed in the upper 25 km-amagats of H₂. Excellent fits to the data were obtained with models having a thin (optical depths of a few tenths) haze conentraated above most of the gas. Following recent spectrospcopicanalyses, we have placed the main “cloud” layer or layers beneath about 25 km-amagats of H₂, although successful fits to our continuum data probably could be achieved also if the clouds were permitted to extend all the way up to the thin haze layer. Similarly, below the haze level our data cannot distinguish between models having two clouds separated by a clear space as suggested by R. E. Danielson and M. G. Tomasko and models with a single extensive diffuse cloud having an H₂ abundance of a few kilometer-amagats per scattering mean free path as described by W. D. Cochran. In either case, the relative brightness of the planet at each phase angle primarily serves to constrain the single-scattering phase functions of the Jovian clouds at the corresponding scattering angles. The clouds in these models are characterized by single-scattering phase functions having strong forward peaks and modest backward-scattering peaks, indicating cloud particles with dimensions larger than about 0.6 μm. In our models, a lower single-scattering albedo of the cloud particles in the belt relative to the zone accounts for the contrast between these regions. If an increased abundance of absorbing dust above uniformly bright clouds is used to explain the contrast between belts and zones at visible wavelengths, the limb darkening is steeper than that observed for the SEBn in blue light at small phase angles. The phase integral for the planet calculated for either the belt or the zone model in either color lies in the range 1.2 to 1.3. If a value of 1.25 is used with D.J. Taylor's bolometric geometric albedo of 0.28, the planet emits 2.25 or 1.7 times the energy it absorbs from the Sun if it effective temperature is 134 or 125°K, respectively—roughly as expected from current theories of the cooling of Jupiter's interior.     

The mesosphere and lower thermosphere of Titan revealed by Cassini/UVIS stellar occultations

Stellar occultations observed by the Cassini/UVIS instrument provide unique data that probe the mesosphere and lower thermosphere of Titan at altitudes between 400 and 1400 km. This region is a site of complex photochemistry that forms hydrocarbon and nitrile species, and plays a crucial role in the formation of the organic hazes observed in the stratosphere, but has yet to be adequately characterized. We analyzed publicly available data obtained between flybys Tb in December 2004 and T58 in July 2009, with an emphasis on two stable occultations obtained during flybys T41 and T53. We derived detailed density profiles for CH₄, C₂H₂, C₂H₄, C₄H₂, HCN, HC₃N and C₆H₆ between ∼400 and 1200 km and extinction coefficients for aerosols between 400 and 900 km. Our analysis reveals the presence of extinction layers in the occultation data that are associated with large perturbations in the density profiles of the gaseous species and extinction profiles of the aerosols. These relatively stable features vary in appearance with location and change slowly over time. In particular, we identify a sharp extinction layer between 450 and 550 km that coincides with the detached haze layer. In line with recent images obtained by Cassini/ISS, the altitude of this layer changes rapidly around the equinox in 2009. Our results point to unexpected complexity that may have significant consequences for the dynamics and physical processes taking place in the upper atmosphere of Titan.     

On the clouds of uranus

Several models for the atmosphere of Uranus are considered. If the H₂ abundance is less than 250 km-am and the internal heat source is only a few percent of the total emitted energy then the cloud at the base of the atmosphere may be composed of solid CH₄ particles, while if the H₂ abundance is greater than 250 km-am or if the internal heat source is near the current upper limit of 35% of the total emitted energy the cloud at the base of the atmosphere may be composed of either solid NH₃ or H₂S particles.     

Solar infrared occultation observations by SPICAM experiment on Mars-Express: Simultaneous measurements of the vertical distributions of H2O, CO2 and aerosol

The infrared AOTF spectrometer is a part of the SPICAM experiment onboard the Mars-Express ESA mission. The instrument has a capability of solar occultations and operates in the spectral range of 1-1.7 μm with a spectral resolution of ∼3.5 cm⁻¹. We report results from 24 orbits obtained during MY28 at Ls 130°-160°, and the latitude range of 40°-55° N. For these orbits the atmospheric density from 1.43 μm CO₂ band, water vapor mixing ratio based on 1.38 μm absorption, and aerosol opacities were retrieved simultaneously. The vertical resolution of measurements is better than 3.5 km. Aerosol vertical extinction profiles were obtained at 10 wavelengths in the altitude range from 10 to 60 km. The interpretation using Mie scattering theory with adopted refraction indices of dust and H₂O ice allows to retrieve particle size (reff∼0.5-1 μm) and number density (∼1 cm⁻³ at 15-30 km) profiles. The haze top is generally below 40 km, except the longitude range of 320°-50° E, where high-altitude clouds at 50-60 km were detected. Optical properties of these clouds are compatible with ice particles (effective radius reff=0.1-0.3 μm, number density N∼10 cm−3) distributed with variance νeff=0.1-0.2 μm. The vertical optical depth of the clouds is below 0.001 at 1 μm. The atmospheric density profiles are retrieved from CO₂ band in the altitude range of 10-90 km, and H₂O mixing ratio is determined at 15-50 km. Unless a supersaturation of the water vapor occurs in the martian atmosphere, the H₂O mixing ratio indicates ∼5 K warmer atmosphere at 25-45 km than predicted by models.     

Polarization Models for Rayleigh Scattering Planetary Atmospheres

We present Monte Carlo simulations for the polarization of light reflected from planetary atmospheres. We investigate dependencies of intensity and polarization on three main parameters: single scattering albedo, optical depth of a scattering layer, and albedo of a Lambert surface underneath. The main scattering process considered is Rayleigh scattering, but isotropic scattering and enhanced forward scattering on haze particles are also investigated. We discuss disk integrated results for all phase angles and radial profiles of the limb polarization at opposition. These results are useful to interpret available limb polarization measurements of solar system planets and to predict the polarization of extra-solar planets as a preparation for VLT/SPHERE. Most favorable for a detection are planets with an optically thick Rayleigh-scattering layer. The limb polarization of Uranus and Neptune is especially sensitive to the vertically stratified methane mixing ratio. From limb polarization measurements constraints on the polarization at large phase angles can be set.     

The thermal radiation of Saturn and its rings

Infrared observations of Saturn from 5 to 40 μm are described. There is intense limb brightening at 12.35 μm over the southern polar cap. The C ring is anomalously bright at 10 and 20 μm and has bluer (hotter) colours than the A and B rings. The ring spectra have been extrapolated beyond 40 μm and subtracted from low-resolution far-infrared measurements to show that the far-infrared spectrum of the disk of Saturn is qualitatively similar to that of Jupiter and that Saturn radiates 2.5 ± 0.6 times the energy it absorbs from the Sun.     

Spatially resolved methane band photometry of Jupiter: III. Cloud vertical structures for several axisymmetric bands and the Great Red Spot

We present cloud structure models for Jupiter's Great Red Spot, Equatorial Zone, North Tropical Zone, North and South Temperate Zones, North and South Polar Regions, and North and South Polar Hoods. The models are based on images of Jupiter in three methane bands (between 6190 and 8900 Å) and nearby continuum. Radiative transfer calculations include multiple scattering and absorption from three aerosol layers, the topmost of which is a high thin haze and the lower two are called clouds. All models are computed relative to a similar model for the South Tropical Zone which fits methane absorption data and Pioneer photometry data well. Outstanding features suggested by the model results are the transition in the upper-cloud altitude to about 3 km lower altitude from the tropical zones to temperate zones and polar regions, a N/S asymmetry in cloud thickness in the tropical and temperate zones, the presence of aerosols up to about 0.3 bar in the Great Red Spot and Equatorial Zone, the need for a significant (τ ～ 0.75 to 1.0) aerosol content in this region in the Equatorial Zone, and perhaps an even higher and thicker cloud in the South Polar Hood. The haze layer above both polar hoods may exhibit different scattering properties than the haze which covers lower latitudes. In comparing the present results with models derived from polarization and infrared observations we conclude that polarization data are sensitive to aerosols in and above the upper cloud layer but insensitive to deeper cloud structure, and the converse is true for infrared data.