Jupiter: An inhomogeneous atmospheric model analysis of spatial variations of the H2 4-0 S(1) line

An analysis of the structure of the Jovian atmosphere, primarily based on center-to-limb variations (CTLV) of the equivalent width of the hydrogen quadrupole 4-0 S(1) line, is presented. These data require that the atmosphere have regions of both long- and short- scattering mean free paths. Two alternative cloud structures which fit the data are developed. The first is a two-cloud model (TCM) consisting of a thin upper cloud and a lower semi-infinite cloud, with absorbing gas between the clouds and above the upper cloud. The second model is a reflecting-scattering model (RSM), in which a gas layer lies above a haze consisting of scattering particles and absorbing gas. The cloud-scattering phase function in both models must have a strong forward peak. The CTLV data require, however, the presence of a backscattering lobe on the phase function, with the backscattering intensity about 4% of the forward scattering. The decrease in reflectivity of all regions from the visible to the ultraviolet is explained by the presence of dust particles mixed with the gas. Most of the ultraviolet absorption in the atmosphere must occur above the upper cloud layer. Particles with a uniform distribution of radii from 0.0 to 0.1 μm with a complex index of refraction varying as λ^?2.5 are used. The contrast in reflectivity between belts and zones may be explained by the larger concentration of dust in the belts than in the zones. Spatially resolved ultraviolet limb-darkening curves will help to determine the dust distribution of the Jovian atmosphere. The visible methane bands at λλ 6190, 5430, and 4860 Å are analyzed in terms of these models. We derive a methane-to-hydrogen mixing ratio of 2.8 × 10^?3, which is about 4.5 times the value for solar composition.     

The atmosphere of Saturn: An infrared perspective

A model of the radiative portion of the equatorial atmosphere of Saturn, constrained by the infrared data various observers, has been constructed using a technique which includes the variation of thermal flux with depth. The model has a high-altitude temperature inversion due to the absorption of ultraviolet sunlight. The inversion causes the observed infrared emission peaks at 8 ωm (methane) and 12 ωm (ethane). Mixing ratios of these gases to hydrogen are computed from these emission features. The bottom of the modeled region occurs at the radiative-convective boundary. At this level, an opaque cloud consisting of solid ammonia condensation particles is postulated. Above the cloud is a thin haze, also composed of ammonia particles. The haze is required to match infrared observations near 9.5 ωm and hydrogen quadrupole equivalent widths near 0.64 ωm. Predictions of the model are given for further observational tests.     

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.     

Airborne spectroscopy of Jupiter in the 100- to 300-cm−1 region: Global properties of ammonia gas and ice haze

A spectrum of the disk of Jupiter was obtained in January 1978 from the Kuiper Airborne Observatory, covering the 100- to 300-cm^?1 spectral range at a resolution corresponding to 1.65 cm^?1. Although taken more than a year before the Voyager 1 Jupiter encounter, this spectrum serves to extend the Voyager IRIS experiment coverage down from its lower limit of 200 cm^?1. Analysis of the spectrum provides information on global mean properties of ammonia gas and an ammonia ice haze. A vertical distribution indistinguishable from saturation equilibrium, with a sharp depletion near the temperature minimum, matches the observed shape of the rotational line absorption best. Constraints on the total optical thickness of the ammonia ice haze can be made, but other properties, such as particle size or vertical scale height, cannot be distinguished clearly from our data in this spectral region. Nevertheless, all models of the haze produce a “continuum” thermal emission between the NH₃ line manifolds which is much lower than that produced by the H₂ collision-induced dipole opacity.     

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.     

Long-term changes in Saturn's troposphere

We report the results of monitoring Saturn's H₂ quadrupole and CH₄ band absorptions outside of the equatorial zone over one-half of Saturn's year. This interval covers most of the perihelion half of Saturn's elliptical orbit, which happens to be approximately bounded by the equinoxes. Marked long-term changes occur in the CH₄ absorption accompanied by weakly opposite changes in the H₂ absorption. Around the 1980 equinox, the H₂ and CH₄ absorptions in the northern hemisphere appear to be discontinuous with those in the southern hemisphere. This discontinuity and the temporal variation of the absorptions are evidence for seasonal changes. The absorption variations can be attributed to a variable haze in Saturn's troposphere, responding to changes in temperature and insolation through the processes of sublimation and freezing. Condensed or frozen CH₄ is very unlikely to contribute any haze. The temporal variation of the absorption in the strong CH₄ bands at south temperate latitudes is consistent with a theoretically expected phase lag of 60° between the tropopause temperature and the seasonally variable insolation. We model the vertical haze distribution of Saturn's south temperature latitudes during 1971–1977 in terms of a distribution having a particle scale height equal to a fraction of the atmospheric scale height. The results are a CH₄/H₂ mixing ratio of (4.2 ± 0.4) × 10^?3, a haze particle albedo of $\text{ω}\text{= 0.995 ± 0.003}$, and a range of variation in the particle to gas scale-height ratio of 0.6 ± 0.2. The haze was lowest near the time of maximum temperature. We also report spatial measurements of the absorption in the 6450 Å NH₃ band made annually since the 1980 equinox. A 20 ± 4% increase in the NH₃ absorption at south temperate latitudes has occurred since 1973–1976 and the NH₃ absorption at high northern latitudes has increased during spring. Increasing insolation, and the resulting net sublimation of NH₃ crystals, is probably the cause. Significant long-term changes apparently extend to the deepest visible parts of Saturn's atmosphere. An apparently anomalous ortho-para H₂ ratio in 1978 suggests that the southern temperate latitudes experienced an unusual upwelling during that time. This may have signaled a rise in the radiative-convective boundary from deep levels following maximum tropospheric temperature and the associated maximum radiative stability. This would be further evidence that the deep, visible atmosphere is governed by processes such as dynamics and the thermodynamics of phase changes, which have response times much shorter than the radiative time constant.     

Jupiter's ammonia clouds—localized or ubiquitous?

From an analysis of the Galileo Near Infrared Imaging Spectrometer (NIMS) data, Baines et al. (Icarus 159 (2002) 74) have reported that spectrally identifiable ammonia clouds (SIACs) cover less than 1% of Jupiter. Localized ammonia clouds have been identified also in the Cassini Composite Infrared Spectrometer (CIRS) observations (Planet. Space Sci. 52 (2004a) 385). Yet, ground-based, satellite and spacecraft observations show that clouds exist everywhere on Jupiter. Thermochemical models also predict that Jupiter must be covered with clouds, with the top layer made up of ammonia ice. For a solar composition atmosphere, models predict the base of the ammonia clouds to be at 720 mb, at 1000 mb if N/H were 4×solar, and at 0.5 bar for depleted ammonia of 10⁻²×solar (Planet. Space Sci. 47 (1999) 1243). Thus, the above NIMS and CIRS findings are seemingly at odds with other observations and cloud physics models. We suggest that the clouds of ammonia ice are ubiquitous on Jupiter, but that spectral identification of all but the freshest of the ammonia clouds and high altitude ammonia haze is inhibited by a combination of (i) dusting, starting with hydrocarbon haze particles falling from Jupiter's stratosphere and combining with an even much larger source—the hydrazine haze; (ii) cloud properties, including ammonia aerosol particle size effects. In this paper, we investigate the role of photochemical haze and find that a substantial amount of haze material can deposit on the upper cloud layer of Jupiter, possibly enough to mask its spectral signature. The stratospheric haze particles result from condensation of polycyclic aromatic hydrocarbons (PAHs), whereas hydrazine ice is formed from ammonia photochemistry. We anticipate similar conditions to prevail on Saturn.     

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.     

Vertical distribution of scattering hazes in Titan's upper atmosphere

Radial intensity scars of a Voyager 2 high phase angle image of Titan have been inverted to yield vertical extinction profiles at 1° intervals around the limb. A detached haze layer with peak particle number densities ～0.2 cm^?3 exists at all latitudes south of ～45°N, and at an altitude of 300–350 km. The optical depth 0.01 level lies at a radius of 2932 ± 5 km at the equator and at a radius of 2915 ± 10 km over the poles (altitudes of 357 ± 5 and 340 ± 10 km, respectively). In addition to the haze layer at 300–350 km, there is a small enhancement in the extinction at ～450 km which exists at all latitudes between 75°S and ～60°N.     

The upper Jovian atmosphere aerosol content determined from a satellite eclipse observation

The Galilean satellite eclipse technique for measuring the aerosol distribution in the upper Jovian atmosphere is described and applied using 30 color observations of the 13 May 1972 eclipse of Ganymede obtained with the 5-m Hale telescope. This event probes the South Temperate Zone. The observed aerosol lies above the visible cloud tops, is very tenuous and varies with altitude, increasing rapidly with downward passage through the tropopause. The aerosol extinction coefficient, κ_a (λ1.05 μm), is ～1.1 × 10^?9 cm^?1 in the lower stratosphere and ～1.1 × 10^?8 cm^?1 at the tropopause. The 1σ uncertainty in these values does not exceed 50% The observations require some aerosol above the tropopause but do not clearly determine its structure. The present analysis emphasizes an extended haze distribution, but the alternate possibility is not excluded that the stratospheric aerosol resides in a thin layer. The aerosol extinction increases with decreasing wavelength and indicates the particle radius to be ?0.2 μm. Larger radii are impossible. These overall results confirm Axel's (1972) suggestion of a small quantity of dust above the Jovian cloud tops and the optical depths are consistent with those required to explain the low uv albedo.     

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.     

Greenhouse models of the atmosphere of titan

The greenhouse effect is calculated for a series of model atmospheres of Titan containing varying proportions of methane, hydrogen, helium, and ammonia. The pressure induced transitions of hydrogen and methane are the major sources of infrared opacity. For each model atmosphere we first computed its temperature structure with a radiative-convective equilibrium computer program and then generated its brightness temperature spectrum to compare with observed values. This comparison indicates that the methane-to-hydrogen ratio is 1_?.67⁺², the surface pressure is at least 0.4atm, and the surface temperature at least 150°K. In addition, except possibly close to the surface, the amount of ammonia is far less than the saturation vapor value. Large amounts of helium may also be present. Many of the successful model atmospheres have methane condensation clouds in the upper troposphere, which help reconcile spectroscopic gas abundances and the observed ultraviolet albedo of Titan with the gas amounts required for the greenhouse effect. The occurrence of large amounts of hydrogen may be a prerequisite for the occurrence of large amounts of methane in the atmosphere and vice versa. This hypothesis may help explain why Titan is the only satellite in our solar system known to have an atmosphere.     

Saturn: UBV photoelectric pinhole scans of the disk

UBV pinhole scans of the Saturn disk have been made with a photoelectric area-scanning photometer. Limb profiles, spaced parallel to the equator, were obtained over the entire southern hemisphere of the planet. Saturn was found to exhibit strong limb brightening in the ultraviolet, moderate limb brightening at blue wavelengths, and strong limb darkening in the visual region of the spectrum. Latitudinal variations in the disk profiles were found. In general, the degree of limb brightening decreases towards the polar region. Pronounced asymmetry is apparent in the disk profiles in each color. The sunward limb is significantly brighter than the opposite limb. This asymmetry depends on phase angle; approaching zero at opposition, it reaches a maximum near quadrature. Our observations are interpreted using an elementary radiative transfer model. The Saturn atmosphere is approximated by a finite homogeneous layer of isotropically scattering particles overlying a Lambert scattering haze or cloud layer. The reflectivity of the haze or clouds is a strongly dependent function of wavelength. Our best-fitting model consists of a clear H₂ layer of column density ～31 km-am above the haze or clouds; the maximum permitted H₂ column density is ～46 km-am. The H₂ column density above the equatorial region appears to be less than at temperate latitudes. The phase-dependent asymmetry in the disk profiles is a natural consequence of the scattering geometry. Our results are consistent with current knowledge of the Saturn atmosphere.     

A model for the dust envelope of V1853 Cygni

We computed a model for the dust envelope of the protoplanetary nebula V1853 Cyg by using data on its fluxes from the ultraviolet to the far infrared. The spherically symmetric envelope was assumed to be composed of silicate grains with the standard MRN size distribution; their number per unit volume is inversely proportional to the distance squared. The optical depth of the envelope, whose inner boundary lies at a distance of 7.6×10¹⁶ cm from the central star, is 0.18 at a wavelength of 0.55 μm. The grain temperature at the inner boundary of the envelope is 110 K. The distance to V1853 Cyg is estimated to be 4.1 kpc. The current mass-loss rate of the object was found by solving a self-consistent problem of radiative transfer and dust motion in the envelope to be 2.2 × 10^?5M_⊙ yr^?1.     

Ammonia photolysis and the greenhouse effect in the primordial atmosphere of the earth

Photochemical calculations indicate that in the prebiotic atmosphere of the Earth ammonia would have been irreversibly converted to N₂ in less than 40 years if the ammonia surface mixing ratio were ≤ 10^?4. However, if a continuous outgassing of ammonia were maintained, radiative equilibrium calculations indicate that a surface mixing ratio of ammonia of 10^?5 or greater would provide a sufficient greenhouse effect to keep the surface temperature above freezing. With a 10^?4 mixing ratio of ammonia, 60 to 70% of the present day solar luminosity would be adequate to maintain surface temperatures above freezing. A lower limit to the time constant for accumulation of an amount of nitrogen equivalent to the present day value is 10 my if the outgassing were such as to provide a continuous surface mixing ratio of ammonia ≥ 10^?5.     

Does Io have an ammonia atmosphere?

An atmosphere containing 0.5 cm atm of ammonia is assumed on Io. Such an atmosphere will be frozen at the unilluminated pole during the solstices, but will evaporate at the equinoctial seasons. The ammonia atmosphere will explain: (1) the posteclipse brightenings and their observed times of occurrence and nonocurrence; (2) the observed departure from a two-layer model beating curve upon emergence from eclipse; (3) the discordant temperatures obtained at 10 and 20 μm; and (4) discordant temperatures obtained at 10 and 20 μm during the total phase of an eclipse by Jupiter.In order to explain items 3 and 4 above, a proton flux in Jupiter's magnetosphere of 1.1 × 10⁹ cm^?2s^? at an energy of 0.5MeV at io's distance from Jupiter is assumed. This flux is 40 times the flux in Divine's (1972) “upper-limit” model of the Jovian radiation belts, while the proton energy is eight times less. The proton flux, plus the solar ultraviolet and infrared flux absorbed by the ammonia, will heat the atmosphere to 245 ± 10°K. At this temperature the occultation atmospheric upper limit allows the addition of 4 cmatm of nitrogen.     

Vibration rotation bands of NO in sunspots

The equivalent widths of R ₁(26.5) lines, belonging to the 1-0, 2-1 and 3-2 vibration-rotation bands of NO near 5.3 m, have been computed for two different sunspot models at four positions on the disc. The computed equivalent widths in both models suggest a possible presence of these NO bands in the sunspot spectrum.     

Observations of the phase variation of Venus CO2 equivalent widths

We present equivalent widths of Venus CO₂ scans of the P branch (P8–P32) of the 5ν₃ band at 8689 Å, the P16 line of the 5ν₃ band, and the P14 line of the ν₁ + 5ν₃ band at 7820 Å covering phase angles between 5°.1 and 170°. The equivalent widths reach a minimum at 10°, in agreement with a phase function with a backward lobe at 160° which is caused by a single internal reflection within the cloud particles. This is evidence that Venus cloud particles are composed of liquid droplets. Maximum equivalent widths are observed at ～60°, a value which is closer to the maximum of single-layer Mie scattering models than to that of two-layer models. At high phase angles we observe equivalent widths greater than those computed from homogeneous scattering models, indicating that at high altitudes the mixing ratio of scattering particles to CO₂ increases with depth. At all phase angles, particularly at large phase angles, the temporal and spatial variations in the observed equivalent widths confuse the phase variation.     

The mean Jovian temperature structure derived from spectral observations from 105 to 630 cm−1

New far-infrared observations of the NH₃ rotation-inversion manifolds in the spectrum of Jupiter have been inverted with the use oftthe detailed ammonia line opacity. A temperature of 160°K at a 1-bar pressure level and a temperature of 105°K for the minimum temperature of the inversion level at 0.15 bars have been derived for gaseous absorption due to NH₃, H₂, and He. The overall fit to the brightness temperature as a function of frequency σ is within ±1°K for 100 ≤ σ ≤ 400 cm^?1 except for the centers of the NH₃ rotation-inversion manifolds where for J ≥ 7 the fit is about 5°K too high. In the continuum for 400 ≤ σ ≤ 630 cm^?1 the fit is within 2.5°K. Consideration of an ammonia ice haze, photodissociation of NH₃ by uv radiation, NH₃ abundance variation, different He/H₂ ratios, and uncertainties in the data effect the temperatures at 1 bar and the temperature at the inversion layer by <7°K. The presently derived temperature at 1 bar of 160°K is consistent with Jovian interior models which can match the gravitational moment, J₂.     

Clouds, haze, and CH4, CH3D, HCN, and C2H2 in the atmosphere of Titan probed via 3 μm spectroscopy

Using synthetic spectra derived from an updated model atmosphere together with a continuum model that includes contributions from haze, cloud and ground, we have re-analyzed the recently published (Geballe et al., 2003, Astrophys. J. 583, L39-L42) high-resolution 3 μm spectrum of Titan which contains newly-detected bands of HCN (in emission) and C₂H₂ and CH₃D (in absorption), in addition to previously detected bands of CH₄. In the 3.10-3.54 μm interval the analysis yields strong evidence for the existence of a cloud deck or optically thick haze layer at about the 10 mbar (∼ 100 km) level. The haze must extend well above this altitude in order to mask the strong CH₄ lines at 3.20-3.50 μm. These cloud and haze components must be transparent at 2.87-2.92 μm, where analysis of the CH₃D spectrum demonstrates that Titan's surface is glimpsed through a second cloud deck at about the 100 mbar (∼ 50 km) level. Through a combination of areal distribution and optical depth this cloud deck has an effective transmittance of ∼ 20%. The spectral shape of Titan's continuum indicates that the higher altitude cloud and haze particles responsible for suppressing the CH₄ absorptions have a largely organic make-up. The rotational temperature of the HCN ranges from 140 to 180 K, indicating that the HCN emission occurs over a wide range of altitudes. This emission, remodeled using an improved collisional deactivation rate, implies mesospheric mixing ratio curves that are consistent with previously predictions. The stratospheric and mesospheric C₂H₂ mixing ratios are ∼¹⁰⁻⁵, considerably less than previous model predictions (Yung et al., 1984), but approximately consistent with recent observational results. Upper limits to mixing ratios of HC₃N and C₄H₂ are derived from non-detections of those species near 3.0 μm.