Photometry of Saturn at large phase angles

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Abstract
Limb‐darkening curves are derived from Pioneer 11's imaging photometry of a bright zone (−11° ≤latitude ≤−6°) and a dark belt (−17° ≤latitude ≤−15°) on Saturn in red and blue light at 12 phase angles between 13° and 151°. The atmosphere of Saturn scatters light only 1/3 as efficiently in the forward direction as does the atmosphere of Jupiter, and the belt on Saturn is only ∼5% fainter than the zone instead of the factor of 2 contrast seen on Jupiter in blue light. Vertically inhomogeneous models of the scattering of light in the atmosphere of Saturn were computed to fit these photometric data by using the cloud structure derived by Gehrels et al. (1980) from polarimetry in blue light at 88° phase (aerosol optical depth =1 at ∼300 mbar, aerosol scale height ≈ 1/4 gas scale height). These computations show that the absorbers of blue light in the atmosphere must be mixed with the aerosols responsible for scattering, not concentrated above them. The single‐scattering phase function of the aerosols is well represented by a combination of two Henyey‐Greenstein functions. Moderately forward scattering functions fit well in both red and blue light, although a definite difference between the two colors is found. If the particles are approximated by spheres, the small asymmetry parameter (≈ 0.32) of their phase function in red light constrains their average radius to be smaller than about 0.13 µm, less than 1/4 the size of the cloud particles on Jupiter. The aerosols on Saturn are similar in size to those in the thin, high‐altitude layer of haze reported by Stoll (1980) on Jupiter. The phase integral of the scattering models computed here is about 1.46 in red light and 1.36 in blue light. If Saturn scatters similarly at other wavelengths, its bolometric geometric albedo together with the effective temperature of 96.5 K±2.5 K reported by Orton and Ingersoll (this issue) imply that Saturn radiates between 2.3 and 4.2 times as much energy as it receives from the sun.