Wm. Hayden Smith

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

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 A) geometric albedo spectrum from 3500 to 10,000 A, an IUE geometric albedo spectrum from 2000 to 3200 A, and high-resolution (∼0.1 A) observations of H2 4-0 quadrupole and 6818.9 A CH4 lines, determines the abundances of spectrally active gas species, such as the deep-atmosphere methane molar fraction, fCH4, and the mean ortho/para hydrogen ratio in the visible atmosphere, as well as stratospheric and tropospheric aerosol properties. Limits include: 0.023 < fCH4 < 0.037, and 0.85 < feH2 < 1.00, where feH2 denotes the fraction of H2 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, ms, mean modal radius, rm, and mean-wavelength imaginary index of refraction, ni, of particulates above the 10-mbar level are 4.0 < ms < 8 μg cm−2, 0.09 < rm < 1.0 μm, and 0.004 < ni < 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 ni, 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 A is limited to 0.08 < τH(6190 A) < 0.23, consistent with that previously reported and less than half that derived for Uranus. The wavelength-dependence of τH 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 PCld, with 3.2 < PCld < 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 A, similar to Uranian aerosols.

Digital array scanned interferometer: sensors and results

Digital array scanned interferometers (DASI) blend characteristics of a grating spectrometer and a two-beam interferometer for acquisition of hyperspectra. DASIs posses field-widened capabilities that permit very high throughput. Aspects of DASI design, hyperspectra, and data processing methods are presented. In particular, we provide data showing that photon-noise-limited hyperspectra are achievable for DASI data.

A polarimetric spectral imager using acousto-optic tunable filters

We report laboratory and telescopic observations with a polarimetric spectral imager based on an acousto-optic tunable filter (AOTF) where we demonstrate simultaneous acquisition of orthogonally polarized images at a spectral resolving power on the order of 103 and at a Rayleigh criterion spatial resolution of 100 line pairs per mm. This matches the spatial resolution of most digital arrays. An AOTF is shown to be usable as a fast shutter with a contrast of over 104 on a sub-millisecond time scale while providing an high transmittance of about 75% (both polarizations summed) when illuminated by white light. The polarization contrast between the orthogonally polarized images exceeds 104 across the field of view, permitting accurate measurement of the polarization parameters of incident light. These characteristics are now being used in a program of high spatial resolution imaging in narrow spectral bands. Some peculiarities and limitations of AOTFs are indicated.

Absorption coefficients for the 6190-A CH4 band between 290 and 100 K with application to Uranus' atmosphere

Abstract Absorption coefficients for the CH4 6190-A band as a function of temperature from 290 to 100°K are reported. We have developed and used a laser intracavity photoacoustic spectroscopy method which permits accurate control of the sample temperature due to the small sample volume required. Over the temperature interval from 290 to 100°K, we found the peak absorption coefficient to increase from 0.6 to 1.0 cm−1 (km-am)−1. The peak absorption increase was accompanied by significant band shape changes. The low-temperature data were then used to constrain further the Baines and Bergstralh (1986, Icarus 65, 406–441) standard model atmosphere for Uranus and found to yield an excellent fit to the bandshape near the minimum of the 6190-A band. The new low-temperature coefficients reduce the pressure level needed to fit the Neff et al. (1984, Icarus 60, 221–235) photometric data, as well as the Voyager Uranus observations, to ∼3 bar, rather than the 7–10 bar required by the room temperature CH4 absorption coefficients, thus producing agreement between models derived from individual H2 quadrupole lines and CH4 lines. The limitations of the present data are discussed in the context of the observations.

Pressure broadening of methane lines in the 6190Åand 6825Åbands at room and low temperatures

Abstract Measurements of pressure broadened half-widths have been performed for one vibration-rotation line in the 6190Amethane band and one vibration-rotation line in the 6825Amethane band. Self-, nitrogen-, hydrogen- and helium-broadening have been measured over a temperature range of 77–295 K. Pressure broadening coefficients and temperature dependence indices have been obtained for each line and broadening gas.

Radiative Lifetimes for Selected Astrophysically Important Resonance Transitions of F I, Si II, S I, II, III, P II, and CO

Radiative lifetimes for selected upper levels for transitions of F I, Si II, S I, II, and III, P II, and the CO molecule, lying between 950 A and 1350 A have been measured with the phase shift method. These species have been observed or sought via these transitions with the Copernicus satellite. The measured laboratory lifetimes ranged from 0.5 to 26 ns. Comparison is made with previous measurements where possible and the results of the Copernicus satellite in determining interstellar abundances.

The D/H ratio for Jupiter

Observations of Jupiters spectrum near the R5(0) HD line at 6063.88 A are reported. A feature with an equivalent width of 0.065 + or - 0.021 mA is coincident with the expected line. This feature is compared with HD profiles computed for inhomogeneous scattering models for Jupiter to yield a range for the Jovian D/H ratio of 1.0-2.9 x 10 to the -5th. This D/H ratio is in the lower range of previously reported D/H values for Jupiter and corresponds to an essentially solar D/H ratio for Jupiter. The detection of HD features in the presence of probable blends with spectral features of minor atmospheric hydrocarbon molecules is discussed. Such blends may make unambiguous identification of HD features difficult. 26 references.

D/H for Uranus and Neptune

Searches for absorption features of HD near 6050 A are reported for Uranus and Neptune. The existence of blends of the HD features with weak features due to minor species in the atmosphere of Uranus is demonstrated. These blends make the unambiguous identification of the weak HD features exceedingly difficult. The data are analyzed with inhomogeneous scattering models to ascertain a D/H upper limit for Uranus and Neptune of 0.0001, a factor of 2 smaller than the upper limit reported from CH3D measurements. 21 references.

Pressure broadening of ammonia lines in the 6475åband at room and low temperatures

Abstract Measurements of pressure broadened half-widths have been performed for four ammonia vibration-rotation lines in the 6475Aband. Self-, hydrogen- and helium-broadening have been measured over a temperature range of 175–295 K. Pressure broadening coefficients and temperature dependence indices have been obtained for each line and broadening gas. A rotational quantum number dependence for the line width has been observed for all broadening gases studied.

Hydrogen broadening of vibrational-rotational transitions of ammonia lying near 6450 Å

The hydrogen broadened half-widths of four ammonia vibrational-rotational transitions near 6450 A have been measured. The average value of the hydrogen broadening coefficient is 0.101 ± 0.004 cm-1 atm with no apparent quantum number dependence.