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

Abstract

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.