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The Imaging Photopolarimeter (IPP) experiment aboard the Pioneer spacecraft measured the linear polarization of red and blue sunlight scattered from Saturn's atmosphere at phase angles from 9 to 150°. This paper presents the observations, discusses their reduction, and summarizes the reduced data. Detailed tables are given for a bright zone (7°S–11°S), a darker belt (15°S–17°S) and a north-south scan. In blue light the minimum polarization on the disk rises from near zero at small phase to a maximum of > 10% near 100° phase in the zone (> 20% in the belt) and decreases toward zero at larger phase, suggestive of Rayleigh scattering. In red light the polarization is small and positive (maximum electric vector perpendicular to scattering plane) at small and large phase, and negative (maximum electric vector parallel to scattering phase) at intermediate phase angles in both regions, suggesting dominance by relatively large cloud particles. In both colors the polarization at all latitudes increases steeply from the center of the map toward both limb and terminator at phase angles near 90°. The data in the belt and zone have been compared with multiple scattering models containing a single type of cloud particle with various single-scattering phase matrices and vertical distributions in Rayleigh scattering gas. Unless there is some minimum optical depth of positively polarizing material (such as the gas) above a negatively polarizing or nonpolarizing cloud, the steep increase in polarization toward the limb is not reproduced. If the gas optical depth above the cloud top is too great, the cloud particles must be extremely negatively polarizing to compensate—unlike aboratory measurements of ammonia crystals. Models in which the cloud top ( τ = 0.4 level) is located at a pressure level of 150 ± 50 mb in the zone and 270 ± 80 mb in the belt fit the blue data with a relatively nonpolarizing base cloud. These cloud-top locations are consistent with scattering models in methane absorption bands for a CH 4 /H 2 mixing ratio of 2.5 × 10 −3 . Assuming the single-scattering properties of the cloud particles are similar at other locations, the variation in cloud-top pressure has been derived for latitudes from 25°S to 55°N from the blue polarimetry. Near the equator, the cloud top rises to ∼150 mb, while at latitudes > 20° the cloud top falls to pressures as great as ∼400 mb. This variation in cloud height is also seen qualitatively in the methane band data, and the higher cloud top at equatorial latitudes coincides with the high-speed equatorial jet seen by Voyager. The steep increase in polarization toward the limb in red light requires an additional optical depth of ∼0.02 of small, highly polarizing aerosols (probably photochemically produced) above both the belt and zone clouds. If these aerosols have radii of ∼0.1 μm, they do not strongly affect the blue polarimetry, while they can provide an absorption optical depth of a few tenths at pressure levels of 70 mb or less as required by Voyager observations in the ultraviolet.

The imaging photopolarimeter aboard Pioneer 10 produced hundreds of red and blue images of Jupiter covering a wide range of phase angles and having good linearity and signal-to-noise characteristics. In this preliminary analysis the limb darkening across two of the red images (at phase angles of about 34 and 109 deg) in both a prominent dark belt and a bright zone are compared with multiple-scattering models. Of the simple models tried, the smallest deviations from the observations result for ones consisting of a thin absorbing layer above a semiinfinite atmosphere of particles scattering according to the Henyey-Greenstein phase function. The asymmetry parameter for the best fits to both the belt and the zone data is in the range g = 0 to 0.25, corresponding to particles small in comparison with the wavelength of red light. The phase integral derived from the models lies in the range of about 1.5 to 1.6, implying a substantial internal heat source for Jupiter.

The imaging photopolarimeter (IPP) experiment on the Pioneer 10 and 11 missions to Jupiter measured the intensity and linear polarization of red and blue sunlight reflected from the planet over a range of phase angles inaccessible from the Earth. We give an overview of the polarization data obtained in the two Jupiter encounters at phase angles from 43° to 117° and briefly describe the photometry data from the Pioneer 11 encounter at phase angles between 34° and 80° which partially fill a gap in the phase coverage from Pioneer 10 (M. G. Tomasko, R. A. West, and N. D. Castillo, 1978, Icarus 33, 558–592). The polarimetry and photometry of the South Tropical Zone (STrZ), the north component of the South Equatorial Belt (SEBn), and a north-south cut extending to the south pole are given in detailed tables. Comparison of the data to multiple-scattering models yields several details of the distribution and single-scattering properties of the clouds and aerosols on Jupiter. The observed polarization in blue light at latitudes less than about 40° shows only small variations between belts and zones. Simple models indicate that the tops of the belt and zone clouds are reached at nearly the same pressure level of about 320 mb and that the polarization differences are a result of the lower cloud albedo in the belt. The optical thickness of the belt as well as the zone clouds at this level must be at least 1.5 to prevent the polarization produced by underlying gas from being seen in the data. The polarization rises abruptly toward the limb and terminator in red light, indicating a haze of positively polarizing particles with an optical thickness of a few tenths at a pressure level of about 120 mb. The polarization in both colors increases abruptly from latitudes north of 40°N and south of 48°S to values as high as 60% at high latitudes. This effect is not due to a longer slant path but must be due to a large increase in the optical thickness of the polarizing haze at high latitudes. There is some indication that the size of the haze aerosols grows with increasing latitude as well. The photometry data indicate little change in the brightness of planetary features in the year between the two Pioneer encounters. Photometric models that fit the Pioneer 10 data fit the Pioneer 11 data remarkably well with essentially the same phase functions. Using a two-cloud model, we find that our models best fit the limb darkening at 12° phase when the belt absorbers are evenly distributed in both the clouds. There is no evidence for rainbow-like bumps on the single-scattering phase functions in the range of scattering angles from 120° to 140° as might result from scattering by spherical particles.

For 2 weeks continuous imaging, photometry, and polarimetry observations were made of Jupiter and the Galilean satellites in red and blue light from Pioneer 11. Measurements of Jupiter's north and south polar regions were possible because the spacecraft trajectory was highly inclined to the planet's equatorial plane. One of the highest resolution images obtained is presented here along with a comparison of a sample of our photometric and polarimetric data with a simple model. The data seem consistent with increased molecular scattering at high latitudes.

Attention is given to the solar and thermal radiation fields of Venus. Direct measurements and the results of numerical models based on direct measurements are presented. Radiation outside the atmosphere is considered with emphasis placed on global energy budget parameters, spectral and angular dependences, spatial distribution, and temporal variations of solar and thermal radiation. Radiation fluxes inside the atmosphere below 90 km are also considered with attention given to the solar flux at the surface, solar and thermal radiation fluxes from 100 km to the surface, and radiative heating and cooling below 100 km.

Photopolarimetric observations of a prominent bright zone and a dark belt of Jupiter in red and blue light are analyzed which were performed by Pioneer 10 at phase angles of 12, 23, 34, 109, 120, 127, and 150 deg. Geometric and photometric reductions of the imaging data are described, the instrument sensitivity at various times is evaluated, and the data are referred to an absolute scale. The observations are analyzed in detail by comparing the data with results of radiative-transfer calculations for specific scattering models of Jupiter's atmosphere. These models include those with a vertical structure consisting of a layer of Rayleigh-scattering gas above a semiinfinite mixture of cloud particles and gas, those having a small quantity of aerosols in the gas above either the diffuse cloud in a reflecting-scattering model or the top cloud of a two-cloud-layer model, those in which a forward-scattering haze is mixed uniformly with gas, and those containing dust layers. It is found that in both the belt and the zone in red as well as blue light, cloud phase functions are required which provide both strong forward scattering and some backscattering.

Several photometric measurements of Venus made from the Pioneer Venus orbiter and probes indicate that solar near-ultraviolet radiation is being absorbed throughout much of the main cloud region, but little above the clouds or within the first one or two optical depths. Radiative transfer calculations were carried out to simulate both Pioneer Venus and ground-based data for a number of proposed cloud compositions. This comparison rules out models invoking nitrogen dioxide, meteoritic material, and volatile metals as the source of the ultraviolet absorption. Models involving either small ( approximately 1 micrometer) or large ( approximately 10 micrometers) sulfur particles have some serious difficulties, while ones invoking sulfur dioxide gas appear to be promising.

The current state of knowledge of the Venusian clouds is reviewed. The visible clouds of Venus are shown to be quite similar to low level terrestrial hazes of strong anthropogenic influence. Possible nucleation and particle growth mechanisms are presented. The Pioneer Venus experiments that emphasize cloud measurements are described and their expected findings are discussed in detail. The results of these experiments should define the cloud particle composition, microphysics, thermal and radiative heat budget, rough dynamical features and horizontal and vertical variations in these and other parameters. This information should be sufficient to initialize cloud models which can be used to explain the cloud formation, decay, and particle life cycle.

An assessment is made of the development status of concepts for cloud and aerosol compositions, vertical and horizontal distributions, and microphysical properties, in the Jovian upper troposphere and stratosphere. Attention is given to several key photochemical species' relationships to aerosol formation as well as their transport process implications, treating photochemistry in the context of comparative planetology and noting differences and similarities among the outer planet atmospheres; since this approach emphasizes observational data, a variegated assortment of ground-based and spacecraft observations is assembled. Current views on the tropospheric distribution of clouds are challenged, and a rationale is presented for alternative accounts.

The paper presents cloud structure models for Jupiter's Great Red Spot, Equatorial and North Tropical Zones, North and South Temperate Zones, and North and South Polar Regions. The models are based on images of Jupiter in three methane bands and nearby continuum; radiative transfer calculations include multiple scattering and absorption from three aerosol layers. The model results include the transition in the upper-cloud altitude to 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 temperature zones, and the presence of aerosols up to about 0.3 bar in the Great Red Spot and Equatorial Zone. It is concluded that polarization data are sensitive to aerosols in and above the upper cloud layer but insensitive to deeper cloud structure.

Three series of spectra of Jupiter covering the spectral range from 0.22 to 0.33 μm were obtained with the International Ultraviolet Explorer (IUE) satellite in November 1979. The absolute reflectivity of Jupiter was obtained in 50-A-wide regions centered at 0.221, 0.233, 0.252, and 0.330 μm from these observations. One of the three spectral series includes 7 spectra at various latitudes along Jupiter's central meridian. These data show a strong decrease in reflectivity for latitudes greater than about 30°, in agreement with measurements made by Voyager ( C. W. Hord, R. A. West, K. E. Simmons, D. L. Coffeen, M. Sato, A. L. Lane, and J. T. Bergstrahl, 1979 , Science ( Washington, D.C. ) 206, 956–959). A total of 24 spectra were also obtained in a west-east series along the equator and another near 40°N latitude. Both west-east series of spectra were obtained by using the motion of a Galilean satellite to pull the 3-arcsec-diameter IUE aperture across the disk of Jupiter. Spectra which straddled the edge of the disk were used to determine the locations of all the spectra in both west-east series to high accuracy. The west-east series show limb darkening at high latitudes and brightening toward the illuminated limb at low latitudes. Comparisons of model calculations with the data obtained near 40°N indicate a significant absorption optical depth (increasing from ∼0.3 at 0.25 μm to nearly 0.6 at 0.22 μm) centered near pressure levels of 20 to 30 mbar. Models in which the haze particles have effective radii within a factor of about 2 of 0.2 μm are favored. Smaller particles have difficulty fitting the variation with wavelength in our data (even with rapidly varying amounts of absorption with wavelength) and larger particles rapidly fall out of the high atmospheric layers. The aerosol mass loading of the atmosphere at high latitudes is estimated at 20 μm/cm 2 above the 50-mbar level. The required variation of the imaginary index of refraction of the aerosol material with wavelength is derived for several possible aerosol distributions. The variation measured by M. Podolak, N. Noy, and A. Bar-Nun (1979 , Icarus 40 , 193–198) for polyacetylene photochemical products is in reasonable agreement with the IUE observations for one of the vertical haze distributions presented, although mixtures of materials produced by irradiating various combinations of methane, hydrogen, and some nitrogen-bearing compounds with energetic particles may also be able to reproduce the observations. Near the equator, the haze aerosols produce much less absorption than near 40°N, and the derived aerosol distributions and optical properties are more dependent on the assumed location and reflectivity of the top of the tropospheric cloud. The equatorial haze aerosols can be as optically thick as the high-latitude aerosols only if they are concentrated much deeper in the atmosphere (near 150 mbar). However, if the haze aerosols extend up to pressures as low as 50 mbar or less at low latitudes as suggested by the eclipse studies of D.W. Smith (1980 , Icarus 44 , 116–133), then they have 5 to 10 times less absorption optical depth near the equator than at 40°N. Comparisons with the satellite eclipse studies and analyses of polarimetry near the limb at large phase ( P.H. Smith and M.G. Tomasko, 1984 , Icarus 58 , 35–73) indicate that the haze aerosols at low latitudes can have sizes in the same range as found near 40°N. A radius estimate of 0.2 μm yields a mass loading of some 3 μm/cm 2 for the haze aerosols near the equator above the 150-mbar pressure level. Assuming that the haze aerosols have the same composition at high and low latitudes implies that the single-scattering albedo of the tropospheric cloud particles at low latitudes decreases strongly from 0.33 to 0.22 μm.

The paper presents a summary of the data currently available (June 1984) describing the planet-enshrouding particulate matter in the Venus atmosphere. A description and discussion of the state of knowledge of the Venus clouds and hazes precedes the tables and plots. The tabular material includes a precis of upper haze and cloud-top properties, parameters for model-size distributions for particles and particulate layers, and columnar masses and mass loadings.

The Solar Flux Radiometer (LSFR) experiment on the large probe of the Pioneer Venus (PV) mission made detailed measurements of the vertical profile of the upward and downward broadband flux of sunlight at a solar zenith angle of 65.7°. These data have been combined with cloud particle size distribution measurements on the PV mission to produce a forward-scattering model of the Venus clouds. The distribution of clouds at high altitudes is constrained by measurements from the PV orbiter. Below the clouds the visible spectrum and flux levels are consistent with Venera measurements at other solar zenith angles. The variations in the optical parameters with height and with wavelength are summarized in several figures. The model is used to evaluate the solar heating rate at cloud levels as a function of altitude, solar longitude, and latitude for use in dynamical studies.

Spatially resolved measurements of Saturn's absolute reflectivity in methane bands at 6190, 7250, and 8900 A and in nearby continuum regions are presented. Images were obtained through narrow-band interference filters with a 500 x 500-pixel charge-coupled device. Band/continuum ratios were measured to high accuracy by referencing to the ring brightness in each image. Several data processing techniques enhanced the quality of the observations. These are the use of the ring symmetry to find center position and orientation, accurate subtraction of ring light, and constrained image deconvolution. Uncertainty in the continuum absolute reflectivity is within 10%. Uncertainties in band/continuum ratios are from one to several percent. The Equatorial Zone was much brighter than any other latitude in the strong 8900 band image. Northern midlatitudes were brighter than southern midlatitudes. The latter observation indicates fewer high-altitude aerosols in the south, a possible result of atmospheric dynamics or seasonal sublimation of NH3 crystals. The data are tabulated and presented in a form suitable for quantitative scattering model analyses.

The suggestion by Elliot et al., (1989) that a haze layer near the surface of Pluto may be photochemical in origin and similar to the aerosol hazes in the atmospheres of other outer solar system bodies is evaluated. The nature of hazes which may be produced in the Hubbard et al., (1989) atmosphere is explored as well. It is concluded that the very low pressure in Pluto's atmosphere requires an aerosol production rate equal to the total maximum methane photolysis rate expected at Pluto.

The radiative heat flux from 0.1 to 10 bars is estimated on the basis of a “two-cloud” scattering model that fits available spectral data and Pioneer photometry. Deeper than a few bars, the flux is 4.5 W m −2 , compared with the 18.8 W m −2 used in an earlier study by Trafton and Stone. A temperature profile is computed, with the H 2 pressure-induced opacity; the temperature at 1 bar is found to be 156°K, rather than the commonly accepted 170°K. An additional optical depth of unity at the 0.67-bar level could restore the conventional value; otherwise a considerably cooler atmosphere is a serious possibility.

The solar flux radiometer aboard the Pioneer Venus large probe operated successfully during its descent through the atmosphere of Venus. Upward, downward, and net fluxes from 0.4 to 1.0 micrometers were obtained at more than 390 levels between 185 millibars (at an altitude of approximately 61 kilometers) and the surface. Fluxes from 0.4 to 1.8 micrometers were also obtained between 185 millibars and about the level at which the pressure was 2 atmospheres. Data from 80 to 185 millibars should be available after additional decoding by the Deep Space Network. Upward and downward intensities in a narrower band from 0.59 to 0.66 micrometers were also obtained throughout the descent in order to constrain cloud properties. The measurements indicate three cloud regions above the 1.3-atmosphere level (at an altitude of approximately 49 kilometers) and a clear atmosphere beneath that level. At the 67 degrees solar zenith of the probe entry site, some 15 watts per square meter are absorbed at the surface by a dark ground, which implies that about 2 percent of the solar energy incident on the planet is absorbed at the ground.

A 2.5-centimeter telescope aboard Pioneer 10 is capable of making two-dimensional spin-scan maps of intensity and polarization in red and blue light at high spatial resolution. During the recent flyby of Jupiter, a large quantity of imaging and polarimetric data was obtained on Jupiter and the Galilean satellites over a wide range of phase angles.

If a radiometer having a narrow field of view is used to measure the radiance of a source such as a quasi-isotropic atmosphere, a knowledge of the out-of-field responsivity is critical. For example, if a radiometer with a field of view of 5 deg (full-angle) has a relative responsivity of 0.0001 for the out-of-field radiation, the contribution of the out-of-field radiation (assuming an isotropic source subtending 2 steradians) is 10.5% of the total signal. Either the stray light suppression of the radiometer must be extremely high or methods of determining the out-of-field response must be developed. A description of one method of determining the effect of out-of-field response and its application to a planetary atmospheric radiometer is presented.

During the scientific commissioning phase of IUE several spectra were acquired from objects residing in the Solar System. The activities focused on testing numerous parameters which would indicate the usefulness of IUE for planetary science. It seems that IUE can successfully tackle many important questions and the data analysis and interpretation of the initial set of observations has begun.

A spectrally selective optical-radiation detector can be normalized to an equivalent detector having unique values for its average responsivity R¯ and its cut-on and cutoff wavelengths λ1 and λ2 It is shown that the radiant power of any source that is described by a second-degree polynomial can be determined exactly between limits λ1 and λ2 Calculations were made for several real detectors and Planckian radiators at several temperatures to demonstrate the low errors inherent in this scheme.

The imaging photopolarimeter (IPP) aboard the Pioneer 11 spacecraft measured the linear polarization of the integrated disk of Titan in red and blue light at a variety of phase angles from 15 to 97 deg. The large polarization (54%) measured in blue light at 90 deg phase constrains the size of the aerosols near the top of Titan's atmosphere to have radii smaller than about 0.09 microns if they have a refractive index of 2.0. The polarization at 90 deg phase in red light is smaller (41%) and implies that the optical thickness of the layer of small aerosols is about 0.6 above an effectively depolarizing surface. The shape of the polarization versus phase curve in blue light suggests increasing particle size with increasing depth into the atmosphere. The limb darkening of Titan was measured at 28 deg phase by using the imaging mode of the IPP. The limb darkening observed in both colors is reasonably consistent with that given by the scattering models derived from the polarization observations.

The solar flux radiometer (LSFR) aboard the Pioneer Venus (PV) sounder probe measured the intensity of sunlight in five directions to the vertical using narrow angular fields of view. The measurements in a narrow spectral channel (0.59–0.67 µm) and two broad channels (0.4–1.0 and 0.4–1.8 µm) were reduced to yield the profiles of upward and downward solar flux with a vertical resolution of 100–500 m. All the flux profiles show three distinct cloud layers with bottoms at altitudes of 57.5, 49.7, and 47.9 km. The azimuthal structure of the intensity samples at the highest altitudes implies a cloud optical depth between 3.5 and 4.0 above 64.3 km. The narrow-band data were interpreted to give the optical depths and single scattering albedos of the clouds at a wavelength of 0.63 µm. The results for the upper cloud are generally consistent with those reported by the cloud particle size spectrometer (LCPS) experiment on PV. In the middle and lower cloud the LSFR data imply that the largest particle size mode has about one third the optical depth deduced from the LCPS data under the assumption that these particles are spherical. The total optical depth of the clouds was found to be about 25. Models of the broad-band fluxes confirm these conclusions and indicate that about half the sunlight absorbed by Venus is absorbed above our first measurements. In addition, absorption of solar radiation occurs in the upper cloud and below 35 km; the middle and lower cloud layers absorb remarkably little sunlight. Models consistent with our measured flux profiles and the spherical albedo of Venus were used to scale our measured net fluxes to a bolometric globally averaged solar energy deposition profile, assuming that the sounder site is typical of planetwide conditions. Averaged over the planet, about 17 W/m² are absorbed at the ground (some 2.5% of the total solar energy incident on the planet). The solar net flux profile in the lower atmosphere is not very sensitive to changes in the thickness of the nearly conservative lower cloud.

In the present paper, the single-scattering phase function of Saturn's aerosols is determined from the vertical structure of Saturn's atmosphere, derived in a previous analysis, and Pioneer's imaging photometry of bright and dark zones on Saturn in red and blue light. The single-scattering phase function is well represented by a combination of two Henyey-Greenstein functions. Moderately forward scattering functions fit well both in red and blue light, although a definite difference between the two colors is found.

Instruments flown on the Pioneer Venus orbiter and probes measured many of the properties of the atmosphere of Venus which control its thermal balance and support its high surface temperature. Estimates based on orbiter measurements place the effective radiating temperature of Venus at 228±5 K, corresponding to an emission of 153±13 W/m², and the bolometric Bond albedo at 0.80±0.02, corresponding to a solar energy absorption of 132±13 W/m². Uncertainties in these preliminary values are too large to interpret the flux difference as a true energy imbalance. A mode of submicron particles is suggested as an important source of thermal opacity near the cloud tops to explain the orbiter and probe thermal flux measurements. Comparison of the measured solar flux profile with thermal fluxes computed from the measured temperature structure and composition shows that the greenhouse mechanism explains essentially all of the 500 K difference between the surface and radiating temperatures of Venus. Precise comparison of the observed and computed value of this difference is hindered by uncertainties in the local variability of H_(2)O and in the thermal opacity of CO_2 and H_(2)O at high temperature and pressure. The directly measured thermal flux profiles at the small probe sites are surprisingly large and variable in the lower atmosphere. Observed zonal and meridional circulation are qualitatively as required to produce the observed uniformity of temperature structure. However, the present lack of quantitative estimates of the horizontal and vertical dynamical heat transports implied by these measurements is a significant gap in the understanding of the thermal balance of the atmosphere of Venus.

The results presented represent a synthesis of data from those Pioneer Venus experiments directed toward studying cloud problems. These orbiter and multiprobe experiments show the cloud system to consist of three altitude regions populated by cloud particles and smaller haze particles which extend above and below as well as coexist with the cloud particles. The optical properties derived are only consistent with the largest particles, having platelike morphology. The smallest particles are shown to require changes in chemical composition to explain observed behavior. The medium-sized H2SO4 droplets of 2 micrometers diameter appear to be the least volatile and are the best understood. The role of the cloud particles in precipitation dynamical processes, lightning, and radiation are all discussed.

The model predictions were compared with the Pioneer Venus probes and orbiter to determine the composition of the UV absorbing materials. The simulations were carried out with radiative transfer codes which included spacecraft constraints on the aerosol and gas characteristics in the Venus atmosphere; gaseous SO2 (a source of opacity at the wavelengths below 0.32 microns), and a second absorber (which dominates above 0.32 microns) were required. The UV contrast variations are due to the optical depth changes in the upper haze layer producing brightness variations between equatorial and polar areas, and to differences in the depth over which the second UV absorber is depleted in the highest portion of the main clouds.