An Analysis of Neptune ' s Stratospheric Haze Using High-Phase-Angle Voyager Images

Abstract We have inverted high-phase-angle Voyager images of Neptune to determine the atmospheric extinction coefficient as a function of altitude and the scattering phase function at a reference altitude. Comparisons between theoretical models and observations help separate the contributions from molecular Rayleigh and aerosol scattering and help determine the variation of the aerosol size, concentration, and scattering properties with altitude. Further comparisons between models and data allow us to place constraints on the location and composition of the hazes, the concentration and downward flux of certain condensible hydrocarbon gases, the eddy diffusion coefficient in the lower stratosphere, and the thermal profile in parts of Neptune's stratosphere. We find that a distinct stratospheric haze layer exists near 12 +1 -1 mbar in Neptune's lower stratosphere, most probably due to condensed ethane. The derived stratospheric haze production rate of 1.0 +0.2 -0.3 × 10 -15 g cm -2 sec -1 is substantially lower than photochemical model predictions. Evidence for hazes at higher altitudes also exists. Unlike the situation on Uranus, large particles (0.08-0.11 μm) may be present at high altitudes on Neptune (e.g., near 0.5 mbar), well above the region in which we expect the major hydrocarbon species to condense. Near 28 mbar, the mean particle size is about 0.13 +0.02 -0.02 μm with a concentration of 5 +3 -3 particles cm -3 . The cumulative haze extinction optical depth above 15 mbar in the clear filter is ∼3 × 10 -3 , and much of this extinction is due to scattering rather than absorption; thus, if our limb-scan sites are typical, the hazes cannot account for the stratospheric temperature inversion on Neptune and may not contribute significantly to atmospheric heating. We compare the imaging results with the results from other observations, including those of the Voyager Photopolarimeter Subsystem, and discuss differences between Neptune and Uranus.

[1]  M. Allen,et al.  Hydrocarbon photochemistry in the upper atmosphere of Jupiter. , 1991, Icarus.

[2]  H. Hammel,et al.  Clouds and hazes in the atmosphere of Neptune. , 1995 .

[3]  H. Hammel,et al.  Clouds, hazes, and the stratospheric methane abundance in Neptune. , 1994, Icarus.

[4]  Paul N. Romani,et al.  Methane photochemistry on Neptune : ethane and acetylene mixing ratios and haze production , 1993 .

[5]  R. Vervack,et al.  The Distribution Hydrocarbons in Neptune's Upper Atmosphere , 1993 .

[6]  M. Allen,et al.  Solar System Hydrocarbon Photochemistry: Impact of New CH4 Lyα Photodissociation Quantum Yields , 1993 .

[7]  Yongge Wang,et al.  Modeling the Thermal Structure of Neptune's Stratosphere , 1993 .

[8]  M. Ashfold,et al.  Primary product channels in the photodissociation of methane at 121.6 nm , 1993 .

[9]  William E. Blass,et al.  Thermal spectroscopy of Neptune: the stratospheric temperature, hydrocarbon abundances, and isotopic ratios , 1992 .

[10]  M. Allen,et al.  The relative abundance of ethane to acetylene in the Jovian stratosphere. , 1992, Icarus.

[11]  M. Allen,et al.  Hydrocarbon nucleation and aerosol formation in Neptune's atmosphere. , 1992, Icarus.

[12]  B. Bézard,et al.  Stratospheric ethane on Neptune: Comparison of groundbased and Voyager IRIS retrievals , 1992 .

[13]  J. Moses.  Meteoroid ablation in Neptune's atmosphere , 1992 .

[14]  S. Atreya,et al.  Voyager 2 ultraviolet spectrometer solar occultations at Neptune - Constraints on the abundance of methane in the stratosphere , 1992 .

[15]  G. F. Lindal,et al.  The atmosphere of Neptune : an analysis of radio occultation data acquired with Voyager 2 , 1992 .

[16]  P. Zarka,et al.  Dust distribution around Neptune: Grain impacts near the ring plane measured by the Voyager Planetary Radio Astronomy Experiment , 1991 .

[17]  S. Limaye,et al.  Winds of Neptune - Voyager observations of cloud motions , 1991 .

[18]  F. M. Flasar,et al.  Thermal structure and dynamics of Neptune's atmosphere from Voyager measurements , 1991 .

[19]  B. Bézard,et al.  Hydrocarbons in Neptune's stratosphere from Voyager infrared observations , 1991 .

[20]  D. Gautier,et al.  The helium abundance of Neptune from Voyager measurements , 1991 .

[21]  Donald A. Gurnett,et al.  Micron‐sized particles detected near Neptune by the Voyager 2 plasma wave instrument , 1991 .

[22]  M. Tomasko,et al.  Properties of scatterers in the troposphere and lower stratosphere of Uranus based on Voyager imaging data , 1991 .

[23]  D. Hunten,et al.  Effects of size‐dependent emissivity on maximum temperatures during micrometeorite entry , 1991 .

[24]  R. West,et al.  Clouds and aerosols in the Uranian atmosphere , 1991 .

[25]  Julianne Ives Moses.  I. Phase Transformations and the Spectral Reflectance of Solid Sulfur: Possible Metastable Sulfur Allotropes on Io's Surface. II. Photochemistry and Aerosol Formation in Neptune's Atmosphere. , 1991 .

[26]  S. Atreya,et al.  Reanalysis of Voyager 2 UVS Occultations at Uranus: Hydrocarbon Mixing Ratios in the Equatorial Stratosphere , 1990 .

[27]  G. F. Lindal,et al.  The atmosphere of Neptune: Results of radio occultation measurements with the Voyager 2 spacecraft , 1990 .

[28]  R. West,et al.  Calibration of the 7- to 14-μm brightness spectra of Uranus and Neptune , 1990 .

[29]  S. K. Croft,et al.  Voyager 2 at Neptune: Imaging Science Results , 1989, Science.

[30]  J. Blamont,et al.  Ultraviolet Spectrometer Observations of Neptune and Triton , 1989, Science.

[31]  A. Ingersoll,et al.  Neptune's Wind Speeds Obtained by Tracking Clouds in Voyager Images , 1989, Science.

[32]  S. Atreya,et al.  Stratospheric aerosols from CH4 photochemistry on Neptune , 1989 .

[33]  H. Hammel.  Neptune Cloud Structure at Visible Wavelengths , 1989, Science.

[34]  L. Horn,et al.  Photometry from Voyager 2: Initial Results from the Neptunian Atmosphere, Satellites, and Rings , 1986, Science.

[35]  S. Atreya,et al.  Methane photochemistry and haze production on Neptune , 1988 .

[36]  J. Caldwell,et al.  Observations of Neptune and Uranus below 2000 A with the IUE , 1988 .

[37]  M. Tomasko,et al.  Nature of the stratospheric haze on Uranus: Evidence for condensed hydrocarbons , 1987 .

[38]  Paul N. Romani,et al.  The upper atmosphere of Uranus: EUV occultations observed by Voyager 2 , 1987 .

[39]  W. R. Thompson,et al.  Solid hydrocarbon aerosols produced in simulated Uranian and Neptunian stratospheres. , 1987, Journal of geophysical research.

[40]  W. Hubbard,et al.  Oblateness, radius, and mean stratospheric temperature of Neptune from the 1985 August 20 occultation , 1987 .

[41]  D. Strobel,et al.  Photochemistry of the atmosphere of Uranus , 1987 .

[42]  G. F. Lindal,et al.  The atmosphere of Saturn - an analysis of the Voyager radio occultation measurements , 1985 .

[43]  S. Atreya,et al.  Photochemistry and clouds of Jupiter, Saturn and Uranus , 1985 .

[44]  S. Atreya,et al.  Photolysis of methane and the ionosphere of Uranus , 1983 .

[45]  J. Pollack,et al.  Vertical distribution of scattering hazes in Titan's upper atmosphere , 1983 .

[46]  T. Ackerman,et al.  Algorithms for the calculation of scattering by stratified spheres. , 1981, Applied optics.

[47]  L. Soderblom,et al.  Radiometric performance of the Voyager cameras , 1981 .

[48]  Yuk L. Yung,et al.  Vertical transport and photochemistry in the terrestrial mesosphere and lower thermosphere (50–120 km) , 1981 .

[49]  R. Turco,et al.  A physical model of Titan's clouds , 1980 .

[50]  R. Turco,et al.  The NASA-AMES Research Center Stratospheric Aerosol Model. 1. Physical Processes and Computational Analogs , 1979 .

[51]  P. Stone.  The meteorology of the Jovian atmosphere , 1976 .

[52]  R. Lindzen.  TIDES AND GRAVITY WAVES IN THE UPPER ATMOSPHERE , 1971 .

[53]  H. V. Hulst.  Light Scattering by Small Particles , 1957 .