Observations of the limb darkening of jupiter at ultraviolet wavelengths and constraints on the properties and distribution of stratospheric aerosols

M. G. Tomasko, E. Karkoschka, S. Martinek

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Abstract

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-Å-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/cm2 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/cm2 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.

Original languageEnglish (US)
Pages (from-to)218-243
Number of pages26
JournalIcarus
Volume65
Issue number2-3
DOIs
StatePublished - Jan 1 1986

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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