Stratospheric Haze Research Articles

How will dust from Shoemaker‐Levy 9 alter Jupiter's stratospheric aerosol populations?

Jupiter's stratospheric haze is most abundant in the polar regions. This polar stratospheric haze has a distinctive signature: bright polar caps are visible in methane absorption band images at 0.89 µm. The upcoming Shoemaker‐Levy 9 impacts should significantly add to the stratospheric haze population, as a result of direct dust impact and perhaps as a result of a splash component of dust explosively ejected from the impact sites. The extra material will predominantly land in the southern hemisphere. Comet dust has a low albedo, and will generally lower Jupiter's brightness in continuum images, particularly near the limb. However, in the 0.89 µm methane absorption band images we expect Jupiter to brighten in dust impact regions, since the dust scatters light above most of the methane absorbers. Stratospheric sedimentation times are generally on the order of years, so this new haze component will be persistent.

Clouds, Hazes, and the Stratospheric Methane Abundance in Neptune

Analysis of high-spatial-resolution (∼0.8 arcsec) methane band and continuum imagery of Neptune's relatively homogeneous Equatorial Region yields significant constraints on (1) the stratospheric gaseous methane mixing ratio (ƒCH4,3), (2) the column abundances and optical properties of stratospheric and tropospheric hydrocarbon hazes, and (3) the wavelength-dependent single-scattering albedo of the 3-bar opaque cloud. From the center-to-limb behavior of the 7270-Å and 8900-Å CH4 bands, the stratospheric methane mixing ratio is limited to ƒCH4,3 < 1.7 × 10-3, with a nominal value of ƒCH4,3 = 3.5 × 10-4, one to two orders of magnitude less than pre-Voyager estimates, but in agreement with a number of recent ultraviolet and thermal infrared measurements, and largely in agreement with the tropopause mixing ratio implied by Voyager temperature measurements. Upper limits to the stratospheric haze mass column abundance and 6190-Å and 8900-Å haze opacities are 0.61 μg cm-2 and 0.075 and 0.042, respectively, with nominal values of 0.20 μg cm-2 and 0.025 and 0.014 for the 0.2-μm radius particles preferred by the recent Voyager PPS analysis of Pryor et al. (1992, Icarus 99, 302-316). The tropospheric CH4 haze opacities are comparable to that found in the stratosphere, i.e., upper limits of 0.104 and 0.065 at 6190 Å and 8900 Å, respectively, with nominal values of 0.085 and 0.058. This indicates a column abundance less than 11.0 μg cm-2, corresponding to the methane gas content within a well-mixed 3% methane tropospheric layer only 0.1 cm think near the 1.5-bar CH4 condensation level. Constraints on the single-scattering albedos of these hazes include (1) for the stratospheric component, 6190-Å and 8900-Å imaginary indices of refraction less than 0.047 and 0.099, respectively, with 0.000 (conservative scattering) being the nominal value at both wavelengths, and (2) CH4 haze single-scattering albedos greater than 0.85 and 0.50 at these two wavelengths, with conservative scattering again begin the preferred value. However, conservative scattering is ruled out for the opaque cloud near 3 bars marking the bottom of the visible atmosphere. Specifically, we find cloud single-scattering albedos of 0.915 ± 0.006 at 6340 Å, 0.775 ± 0.012 at 7490 Å, and 0.803 ± 0.010 at 8260 Å. Global models utilizing a complete global spectrum confirm the red-absorbing character of the 3-bar cloud. The global-mean model has ∼7.7 times greater stratospheric aerosol content then the Equatorial Region. An analysis of stratospheric haze precipitation rates indicates a steady-state haze production rate of 0.185-1.5 × 10-14 g cm-2 sec-1, in agreement with recent theoretical photochemical estimates. Finally, reanalysis of the Voyager PPS 7500-Å phase angle data utilizing the ƒCH4,3 value derived here confirms the Pryor et al. result of a tropospheric CH4 haze opacity of a few tenths in the 22-30°S latitude region, several times that of the Equatorial Region or of the globe. The factor-of-10 reduction in ƒCH4,3 below that assumed by Pryor et al. implies decreased gas absorption and consequently a decrease in the forward-scattering component of tropospheric aerosols.

Saturn's Upper Atmospheric Hazes Observed by the Hubble Space Telescope

We observed Saturn with the Hubble Space Telescope at wavelengths 0.30-0.89 μm for the purpose of determining the distribution of hazes. In the stratosphere, haze optical depths in the ultraviolet are essentially zero for midnorthern latitudes, are small (∼0.2) at low latitudes and mid-southern latitudes, but large (almost unity) above 70° north. The optical depth falls off sharply in the visible due to the small radii of the stratospheric aerosols (∼0.15 μm). The latitudinal distribution of tropospheric haze was found mostly consistent with previous investigations. It is completely different from the distribution of stratospheric haze since optical depths in the troposphere strongly increase from the north pole to the equator. In the ultraviolet, the stratospheric aerosols are darker than tropospheric aerosols. Latitudinal albedo and color variations in the visible, defining Saturn's belt and zone structure, can be explained by variations in the size of tropospheric aerosols (radii 1-2 μm). The unusual blue-green color of mid-southern latitudes in 1991 may be due to smaller radii (∼0.5 μm) in the troposphere. At wavelength 0.30 μm we found an indication of a gaseous absorption of ∼0.02 optical depth in the upper part of the stratosphere.

Polycyclic aromatic hydrocarbons in the atmospheres of Titan and Jupiter.

Polycyclic aromatic hydrocarbons (PAHs) are important components of the interstellar medium and carbonaceous chondrites, but have never been identified in the reducing atmospheres of the outer solar system. Incompletely characterized complex organic solids (tholins) produced by irradiating simulated Titan atmospheres reproduce well the observed UV/visible/IR optical constants of the Titan stratospheric haze. Titan tholin and a tholin generated in a crude simulation of the atmosphere of Jupiter are examined by two-step laser desorption/multiphoton ionization mass spectrometry. A range of two- to four-ring PAHs, some with one to four alkylation sites are identified, with net abundance approximately 10(-4) g g-1 (grams per gram) of tholins produced. Synchronous fluorescence techniques confirm this detection. Titan tholins have proportionately more one- and two-ring PAHs than do Jupiter tholins, which in turn have more four-ring and larger PAHs. The four-ringed PAH chrysene, prominent in some discussions of interstellar grains, is found in Jupiter tholins. Solid state 13C NMR spectroscopy suggests approximately equal to 25% of the total C in both tholins is tied up in aromatic and/or aliphatic alkenes. IR spectra indicate an upper limit in both tholins of approximately equal to 6% by mass in benzenes, heterocyclics, and PAHs with more than four rings. Condensed PAHs may contribute at most approximately 10% to the observed detached limb haze layers on Titan. As with interstellar PAHs, the synthesis route of planetary PAHs is likely to be via acetylene addition reactions.

Evidence for surface heterogeneity on Titan

UNLIKE all other planetary satellites, Saturn's moon Titan has a massive atmosphere1–8. At visible wavelengths, a thick stratospheric haze hides the surface from view. The emission from Titan in the infrared is largely from methane, nitrogen and hydrogen also in the stratosphere4. In the near-infrared, however, the extinction from haze decreases, and narrow windows exist in which the atmosphere absorbs only weakly9–14, and through which we might therefore catch a glimpse of the surface. Within two of these windows, Lemmon et a1.15,16 recently observed a difference in Titan's albedo when the satellite was at eastern and western elongation with respect to Saturn. Although these observations could be taken to imply that Titan's surface is heterogeneous (and therefore is not covered by a global methane–ethane ocean as predicted previously7), they could also be explained by transient clouds. Here I present observations from two more rotational periods which record the same albedo difference, indicating that the heterogeneity is most unlikely to be associated with transient features and must be intrinsic to the surface. These results also imply that Titan is locked in a synchronous orbit about Saturn.

Volcanoes May Warm Locally While Cooling Globally

The debris thrown into the stratosphere by the June 1991 eruption of Mt. Pinatubo was supposed to give the world a break from the record-shattering global warmth of the 1980s. But throughout North America and much of northern Eurasia those expectations were confounded as the following winter proved unusually mild. Normally frigid Minneapolis, for example, had its third warmest winter ever, with temperatures averaging a relatively sultry 4.3[degree]C above normal. So the obvious question is, what might have temporarily counteracted Pinatubo's chilling effects on the northern continents The stratospheric haze created by Mt. Pinatubo did, as expected, screen out some sunlight and cool the globe as a whole. But recent evidence also suggests that it may have had the counter-intuitive effect of raising winter temperatures in large regions of North America and northern Eurasia by altering the weather patterns in those areas.

High-phase-angle observations of Neptune at 2650 and 7500 Å: Haze structure and particle properties

Spatially resolved Voyager 2 PPS photometric and polarimetric observations of Neptune at 2650 and 7500 Å are presented and analyzed. A vertically homogeneous model with Rayleigh scattering and absorption fit the 2650 Å at 137° phase and below, but not the 159° phase data. The 159° phase data imply stratospheric particles of mean radius 0.20 ± 0.05 μm, with a best-fit imaginary refractive index at 2650 Å of 0.03. The optical depth of the stratospheric haze in the 5- to 100-mbar region is 0.19 ± 0.08 at 2650 Å. The 7500 Å data at latitude - 25° constrain the stratospheric haze mean radius to be less than 0.4 μm, with a best-fit value of 0.25 μm. Stratospheric haze optical depth at 7500 Å is 0.05 ± 0.02. A continuum absorber is abundant in the troposphere. If it is confined to the region below the methane haze layer, the optical depth of the methane haze can be no larger than about 0.8 (too much limb brightening is produced for larger optical depths). The methane haze optical depth at 7500 Å must be greater than 0.1. Models with this optical depth as large as 3 produced too much limb darkening if the continuum absorber is mixed in both the methane haze and deeper cloud. Tropospheric aerosols have phase functions with shalloow backward lobes, described by a double Henyey-Greenstein parameterization with g 2 (the asymmetry parameter for the backward lobe) in the range −0.07 to −0.22. The planetary phase integral (the ratio of the spherical albedo to the geometric albedo) at 7500 Å is 1.4 ± 0.1; at 2650 Å it is 1.26 ± 0.05.

Hydrocarbon nucleation and aerosol formation in Neptune's atmosphere

Photodissociation of methane at high altitude levels in Neptune's atmosphere leads to the production of complex hydrocarbon species such as acetylene (C 2H 2), ethane (C 2H 6), methylacetylene (CH 3C 2H), propane (C 3H 8), diacetylene (C 4H 2), and butane (C 4H 8). These gases diffuse to the lower stratosphere where temperatures are low enough to initiate condensation. Particle formation may not occur readily, however, as the vapor species become supersaturated. We present a theoretical analysis of particle formation mechanisms at conditions relevant to Neptune's troposphere and stratosphere and show that hydrocarbon nucleation is very inefficient under Neptunian conditions: saturation ratios much greater than unity are required for aerosol formation by either homogeneous, heterogeneous, or ion-induced nucleation. Homogeneous nucleation will not be important for any of the hydrocarbon species considered; however, both heterogeneous and ion-induced nucleation should be possible on Neptune for most of the above species. The relative effectiveness of heterogeneous and ion-induced nucleation depends on the physical and thermodynamic properties of the particular species, the abundance of the condensable species, the temperature at which the vapor becomes supersaturated, and the number and type of condensation nuclei or ions available. Typical saturation ratios required for observable particle formation rates on Neptune range from ∼3 for heterogeneous nucleation of methane in the upper troposphere to greater than 1000 for heterogeneous nucleation of methylacetylene, diacetylene, and butane in the lower stratosphere. Thus, methane clouds may form slightly above, and stratospheric hazes far below, their saturation levels. When used in conjunction with the results of detailed models of atmospheric photochemistry, our nucleation models place realistic constraints on the altitude levels at which we expect hydrocarbon hazes or clouds to form on Neptune.

Enhancement of atmospheric radiation by an aerosol layer.

The presence of a stratospheric haze layer may produce increases in both the actinic flux and the irradiance below this layer. Such haze layers result from the injection of aerosol-forming material into the stratosphere by volcanic eruptions. Simple heuristic arguments show that the increase in flux below the haze layer, relative to a clear sky case, is a consequence of "photon trapping." We explore the magnitude of these flux perturbations, as a function of aerosol properties and illumination conditions, with a new radiative transfer model that can accurately compute fluxes in an inhomogenous atmosphere with nonconservative scatterers having arbitrary phase function. One calculated consequence of the El Chichon volcanic eruption is an increase in the midday surface actinic flux at 20 degrees N latitude, summer, by as much as 45% at 2900 angstroms. This increase in flux in the UV-B wavelength range was caused entirely by aerosol scattering, without any reduction in the overhead ozone column.

A physical model of Titan's aerosols

Microphysical simulations of Titan's stratospheric haze show that aerosol microphysics is linked to organized dynamical processes. The detached haze layer may be a manifestation of 1 cm sec −1 vertical velocities at altitudes above 300 km. The hemispherical asymmetry in the visible albedo may be caused by 0.05 cm sec −1 vertical velocities at altitudes of 150 to 200 km, we predict contrast reversal beyond 0.6 μm. Tomasko and Smith's (1982, Icarus 51, 65–95) model, in which a layer of large particles above 220 km altitude is responsible for the high forward scattering observed by Rages and Pollack (1983, Icarus 55, 50–62), is a natural outcome of the detached haze layer being produced by rising motions if aerosol mass production occurs primarily below the detached haze layer. The aerosol's electrical charge is critical for the particle size and optical depth of the haze. The geometric albedo, particularly in the ultraviolet and near infrared, requires that the particle size be near 0.15 μm down to altitudes below 100 km, which is consistent with polarization observations (Tomasko and Smith 1982, West and Smith 1991, Icarus 90, 330–333). Above about 400 km and below about 150 km Yung et al.'s (1984, Astrophys. J. Suppl. Ser. 55, 465–506) diffusion coefficients are too small. Dynamical processes control the haze particles below about 150 km. The relatively large eddy diffusion coefficients in the lower stratosphere result in a vertically extensive region with nonuniform mixing ratios of condensable gases, so that most hydrocarbons may condense very near the tropopause rather than tens of kilometers above it. The optical depths of hydrocarbon clouds are probably less than one, requiring that abundant gases such as ethane condense on a subset of the haze particles to create relatively large, rapidly removed particles. The wavelength dependence of the optical radius is calculated for use in analyzing observations of the geometric albedo. The lower atmosphere and surface should be visible outside of regions of methane absorption in the near infrared. Limb scans at 2.0 μm wavelength should be possible down to about 75 km altitude.

Properties of scatterers in the troposphere and lower stratosphere of Uranus based on Voyager imaging data

We have used photometrically and geometrically corrected Voyager images of Uranus to define spatially resolved specific intensities over a wide range of phase angles for two latitude bands and have modeled these data with scalar and vector radiative transfer and microphysical models. Our scattering model included photo-chemically produced hydrocarbon ices in the stratosphere and upper troposphere, a methane ice cloud from 1.2–1.3 bar, and an optically thick hydrogen sulfide cloud at 3 bars. We find that the methane cloud has an optical depth of 0.7 at 22.5°S and an optical depth of 2.4 at 65°S. The mean particle size in the methane cloud is ⪡10 μm for the most likely particle shapes and is probably about 1 μm. The volume absorption coefficient of the methane cloud particles is about 50% higher at 22.5°S than at 65°S, assuming the mean cloud particle size is the same at both latitudes. The mass production rate of the stratospheric hydrocarbon ice components is about 10 −16 g cm −2 sec −1, and the average particle charge in the stratosphere is around 10 electrons per micrometer diameter. The imaginary part of the stratospheric haze refractive index is ∼0.01–0.001, and there are ∼10 precipitable nanometers of non-volatile absorbing haze residue per kilometer-amagat of gas between 1.3 and 3 bar.

The atmospheric structure and dynamical properties of Neptune derived from ground-based and IUE spectrophotometry

Constraints on the atmospheric structure and dynamical properties of Neptune are derived from a broad range of recently acquired, high-quality, full-disk spectral observations. The analysis, based on detailed modeling of a broadband (7 Å) geometric albedo spectrum from 3500 to 10,000 Å, an IUE geometric albedo spectrum from 2000 to 3200 Å, and high-resolution (∼0.1 Å) observations of H 2 4-0 quadrupole and 6818.9 Å CH 4 lines, determines the abundances of spectrally active gas species, such as the deep-atmosphere methane molar fraction, f CH 4 , and the mean ortho/para hydrogen ratio in the visible atmosphere, as well as stratospheric and tropospheric aerosol properties. Limits include: 0.023 < f CH 4 < 0.037, and 0.85 < f eH 2 < 1.00, where f eH 2 denotes the fraction of H 2 in the equilibrium state. Ultraviolet and near-infrared spectral observations require the existence of stratospheric aerosols, the vertical distribution of which is consistent with recently reported thermodynamic and photochemical calculations of hydrocarbon condensate abundances. Both spectroscopic and dynamical modeling indicate that the stratospheric haze is much less vertically extended than on Uranus, with the bulk of the observable hydrocarbon material residing at pressures less than 20 mbar. The stratospheric haze column density, m s, mean modal radius, r m, and mean-wavelength imaginary index of refraction, n i, of particulates above the 10-mbar level are 4.0 < m s < 8 μg cm −2, 0.09 < r m < 1.0 μm, and 0.004 < n i < 0.008, implying that high-altitude haze material is one to two orders of magnitude more abundant and individual particles are at least several times more massive and marginally brighter than those found near the 10-mbar region on Uranus. Some 6–14% of the average incident solar flux in the ultraviolet and visible spectral regions is deposited in stratospheric aerosols. Consideration of particulate fall speeds leads to an estimate for the stratospheric aerosol mass production rate of 3 × 10 −8 to 6 × 10 −7 μg cm −2 sec −1, some three orders of magnitude greater than on Uranus. Compared to Uranus, the greater abundance and shorter lifetimes of Neptunian particulates in the stratospheric region irradiated by the solar ultraviolet flux suggests that such radiation is the darkening agent of stratospheric aerosols on both of these planets. Moreover, such UV-induced solid-state chemistry accounts for the correlation between the solar ultraviolet flux and the blue-yellow planetary geometric albedo of Neptune observed by G. W. Lockwood and D. T. Thompson (1986, Science 234, 1543–1545). Quantitative modeling of the effect yields an ∼0.002 temporal variation in n i, indicating that stratospheric aerosol absorption varies by 33–100% over a solar cycle. In the troposphere, an optically thin haze layer, comparable in opacity, mass, and mean particle size to the stratospheric haze, is derived for the methane condensation region near 1 bar. Specifically, the methane haze opacity at 6190 Å is limited to 0.08 < τ H (6190 A ̊ ) < 0.23 , consistent with that previously reported and less than half that derived for Uranus. The wavelength-dependence of τ H obeys an inverse power law greater than unity, indicating that particulates average less than 1 μm in radius. Finally, we derive an optically infinite cloud at a pressure level P Cld, with 3.2 < P Cld < 3.80 bars, some 0.5–1.2 bars deeper than the bottom of the Uranian atmosphere. The single-scattering albedo of deep-cloud particles decreases sharply between 5890 and 6040 Å, similar to Uranian aerosols.

The ultraviolet continuum albedo of Uranus

A radiative transfer code explicitly treating the Raman scattering of solar protons by H2 is presently used to analyze the Uranus geometric albedo in the 2000-5000 A range. The Baines and Bergstralh (1986) baseline model used reproduces the geometric albedo peak produced by Raman scattering filling of solar absorption line cores, but is found to be excessively bright for wavelengths below 2400 A. This discrepancy is resolvable through inclusion of an absorbing stratospheric haze layer, and results are thereby obtained which are consistent with the Pollack et al. (1987) model, in which aerosols are generated stratospherically through photochemical effects on hydrocarbons.

Vertical aerosol structure of Neptune: Constraints from center-to-limb profiles

We present and analyze center-to-limb (CTL) profiles of the equatorial region of Neptune, extracted from images with good spatial resolution. Models of the quiescent atmosphere of Neptune are consistent with at least two distinct cloud layers, a thin stratospheric haze and a deep cloud. The 8900-Å CTL profile requires the presence of some material above the 5-mbar level. Assuming isotropic scattering, the optical depth of the material is less than 0.15 for hazes with single scattering albedos larger than 0.6. CTL profiles at three methane bands could not be fitted simultaneously with a single scattering layer, implying that there may be multiple haze layers. The methane-band data tolerate thin scattering hazes in the pressure level from 0.4 to 1.5 bars, but the optical depths must be less than 0.05 (again, assuming isotropic scattering). Such a thin haze does not provide sufficient reflected flux at 6340 Å (a continuum region), indicating the presence of another scattering layer deeper than 1.5 bars. A bright cloud layer (single scattering albedo near 1) with optical depth 3.0 at 2.6 bars provides a good fit to 6340-Å reflectivity. Superposed on the quiescent atmosphere are discrete bright features. The lack of detectable disk-integrated variability at shorter wavelengths helps constrain the vertical aerosol structure of these bright regions. Specifically, the difference of the bright feature from its surroundings in 1986 was probably caused by an increase in optical depth or height of a stratospheric haze layer.

Stratospheric aerosols from CH4 photochemistry on Neptune

We have used a combined photochemical‐condensation model to study hydrocarbon ices produced from CH4 photolysis in the stratosphere of Neptune. We predict a total stratospheric haze production rate of 4.2 × 10−15 grams cm−2s−1 (75% ethane, 25% acetylene, trace diacetylene). The total production rate is insensitive to within a factor of two to order of magnitude changes in the eddy diffusion coefficient and methane mixing ratio, which is within our estimate of uncertainty for this number. The condensation temperatures are 97 K for C4H2, 71 K for C2H2, and 64 K for C2H6. Voyager 2 images of Neptune will be able to confirm the presence of stratospheric aerosols and provide constraints on their production rate and location.

The thermal structure of Titan's atmosphere

We have developed a radiative-convective model of the thermal structure of Titan's atmosphere. The model computes the solar and infrared radiation in a series of spectral intervals with vertical resolution. Sources of opacity in the visible and near infrared include stratospheric haze particles, methane cloud particles, and gaseous methane; sources of opacity in the thermal infrared include the pressure-induced opacity of N 2, CH 4, and H 2, the permitted transitions of C 2H 2 and C 2H 6, and particulate opacity. The haze properties are determined with a simple microphysics model. The model contains a minimum of free parameters and we try to determine these by fits to independent data sets. We find that gas and haze opacity alone, with the temperatures fixed by Voyager observations, produces a model that is within a few percent of radiative convective balance everywhere in the atmosphere. In a self-consistent computation of temperatures, we find that our model calculation for the surface temperature is, in general, colder than the observed value by 5–10°K. The presence or absence of methane condensation clouds only slightly alters the results. Good agreement can be obtained by adjusting the parameters in the model. The model parameters in these optimized cases are typically within 15% of the baseline values and within the limits allowed by observations. We conclude that the most important factors controlling Titan's thermal structure are absorption of sunlight by the stratospheric haze and the pressure-induced gas opacity in the infrared. Within the uncertainties of the model, these effects can explain the observed temperature profile. Condensation clouds play a minor role, if any.

Discrete cloud structure on Neptune

Discrete cloud structure on Neptune is discussed, emphasizing information derived from recent CCD imaging. At methane-band wavelengths, discrete cloud features were obvious in the southern hemisphere, but no cloud features have been visible in the planet's northern hemisphere since 1986. This is a distinct change from earlier imaging and implies that some substantial change has occured in the northern hemisphere. A review of past literature shows that this is not a unique occurrence; at least twice in the past 10 years, the reflected light from the planet at methane-band wavelengths has come from a single discrete feature. In the images, the brightest clouds were always found at midlatitudes (−30° to −50°), but fainter discrete clouds were sometimes seen further south. From the fact that the bright clouds were not spatially resolved, and from an analysis of the flux reflected by the bright cloud in 1987, approximate limits on the cloud diameter were obtained: 4000 km < d cld < 10,700 km. Disk-integrated photometry derived from the imaging demonstrated that in 1986 and 1987, a bright cloud feature was responsible for the diurnal variation observed at 8900 Å. The amplitude of this rotational modulation also varied on time scales of days, providing evidence for the development of the cloud features. The lack of detectable variability at shorter wavelengths helped constrain the vertical aerosol structure of bright regions. Specifically, the difference of the bright cloud from its surroundings in 1986 was probably caused by a change in the stratospheric haze layer.

Analytical electron microscope studies of size-segregated particles collected during AGASP-II, flights 201?203

Cascade impactor samples were collected over the Alaskan Arctic during the first three research flights of AGASP-II. These samples were analyzed using analytical electron microscopy to determine the morphology, mineralogy and elemental composition of individual particles. For analytical considerations, a typical impactor sample was run for approximately 20 min, thus giving excellent time resolution of discrete events. Samples collected during flights 201 and 202 consisted of stratospheric aerosol and lower-altitude haze samples. Stratospheric samples were characterized by moderate loadings of H2SO4 droplets with relatively few particles of other types. Samples collected in tropospheric haze layers generally exhibited light-to-moderate particle loadings. H2SO4 was again the most prevalent species, with crustal and anthropogenic particles also observed. One sample taken over south-central Alaska near the end of flight 203 showed high concentrations of solid crustal particles, with relatively little associated H2SO4. Giant particles larger than 5 μm were occasionally observed in this aerosol. The composition of this material closely matches that of bulk ash from the Mt. Augustine volcano, which erupted 9–13 days before collection of this sample. This brings forth the possibility that pockets of ash-rich aerosol existed over parts of south and central Alska during the AGASP-II field mission. There is no evidence that these volcanic aerosols were present in the AGASP study area north of the Brooks Range.

Voyager 2 imaging eclipse observations of the Jovian high altitude haze

Images of Jupiter's northern hemisphere dawn and dusk limbs taken by the Voyager 2 wide angle camera were analyzed to explore the nature of the high stratospheric haze layer. At most latitudes the scattered intensities at 0.43- and 0.60-μm wavlengths are smaller than those for a conservatively scattering molecular atmosphere due to extinction by aerosols in the upper 100-mbar region. Considerable latitudinal structure is evident, with intensities in the latitude range 40°N to 50°N being higher, at 0.60 μm only, than those for a conservatively scattering molecular atmosphere. Asymmetry in the dawn and dusk limbs is also observed at high latitude. These variations are evidence for a dynamically active stratosphere and may be the most visible indicators of processes such as diabatic circulation, latitudinally dependent eddy mixing, or wave dissipation and breaking. Two kinds of aerosols models were compared with the observations. In the first, free parameters of the model were the particle radius, constant with altitude, the pressure level of the top of the haze, the haze optical depth at 0.43 μm, and the haze scale height. Although the solutions are multiple, the top of the haze must be in the range of a few millibars to a few tenths of a millibar, the optical depth above the 100-mbar level can be either small ( τ ∼ 0.1) or large (many times that value) at low latitudes but is always large at high latitudes, and the most frequent particle radii found were in the range between 0.01 and 0.05 μm. In the second type of model, the haze radius and number density were prescribed by a microphysical model which has particle radius increasing with depth. The free parameters were the total column density and the pressure of the haze top. Sensitivity studies were performed to assess the influence of particle size in the top layers and the refractive index at 0.60-μm wavelength. Again the solutions are not unique, but the most economical solution can explain the variations at latitudes less than 50°N by a haze with optical depth (above the 100-mbar level) near 0.1 at 0.43 μm, with a minimum particle radius, r min, of 0.05 μm and with variations in haze top pressure, from 7 mbar at 5.9°N to 0.5 mbar at 39.7°N, producing most of the variation in limb intensity. Simplistic calculations of particle transport suggest that, at low latitudes, hydrazine mixed upward from the tens of millibars region where it is produced can account for the inferred particle number densities. At high latitudes the same may be true, pending the results of a realistic microphysical model.

Longitudinal variations in the Saturnian atmosphere: I. Equatorial region

Ground-based and Voyager observations in and out of methane and ammonia bands are analyzed to search for longitudinal variations in Saturn's equatorial region. A model with a reflecting layer at 2.1 bars, an extended haze to 170 mbar, and an overlying thin stratospheric haze is adopted. Two sets of data are analyzed, a set of ground-based observations covering the 6000-Å spectral region and a set of Voyager 1 images obtained with the orange and methane filters. The spectral variations are not consistent with a variation in the height of the reflecting layer. They are modeled by variations in the single scattering albedo of the haze and in the specific abundance of gas in the haze. The ground-based spectra, having a spatial resolution of 21,000 km, are consistent with a specific abundance of gas in the haze of 14 ± 1 km-am per mean free path and a haze single scattering albedo in the continuum at 6055 Å of 0.0090 (±0.006) with a longitudinal variation of ±0.003. The single scattering albedo derived from the ground-based observations at 6475 Å is 0.993 (±.006) with a longitudinal variation of ±.003. The Voyager data, having a spatial resolution of ∼500 km, are consistent with a specific abundance of gas in the haze between 10 and 24 km-am. The larger variation in the specific abundance derived from the Voyager data set is due to the larger relative uncertainty in the intensity (2%) for Voyager as compared to 1% for the ground-based spectra. We derive a methane mixing ratio of 2.2 (+0.8/−0.2) × 10 −3, representing a C/H ratio which is enhanced by a factor of 2.3 over the solar value. Our estimate of the ammonia mixing ratio, 4.5 × 10 −4, is a lower limit due to our assumption that ammonia exists at its saturation vapor pressure everywhere above the reflecting layer. There is no conclusive evidence that there are longitudinal variations in the structure of the Saturnian atmosphere in the Equatorial Region on a scale greater than 600 km.