Jerome Apt

Methane on Triton: Physical state and distribution

Abstract Infrared spectrophotometric measurements of Neptunes satellite Triton obtained between 1980 and 1982 in the spectral range 0.8–2.5 μm show six individual absorption bands attributable to methane. An additional band in the Triton data is not methane. The Triton spectral data conform more closely to a laboratory spectrum of frozen methane than to a synthetic spectrum of methane gas computed for conditions of low temperature expected at the satellite. Additionally, the strength of the bands vary with Tritons orbital position. The data thus suggest that methane in the ice phase is mostly responsible for the bands in Tritons spectrum, and that the ice is distributed nonuniformly around the satellites surface.

Bolometric albedos of Titan, Uranus, and Neptune

Abstract The geometric albedos of Titan, Uranus, and Neptune have been measured from 2000 to 3175 A by J. Caldwell, T. Owen, A. R. Rivolo, V. Moore, G. E. Hunt, and P. S. Butterworth (1981, Astron. J. 86 , 298–305), from 3500 to 10,500 A by J.S. Neff, D. C. Humm, J. T. Bergstralh, A. L. Cochran, W. D. Cochran, E. S. Barker, and R. G. Tull (1984, Icarus 60 , 221–235), and from 6440 to 25,360 A by J. Apt, R. N. Singer, and R. N. Clark (private communication). The integrated solar flux in this spectral interval amounts to 97% of the solar constant. These data sets were combined and integrated to find the bolometric geometric albedo of each object. Preliminary determinations of the phase functions were used to compute the Bond albedos and effective temperatures. The effective temperatures are compared with bolometric temperatures determined from brightness temperatures in the 10-μm to 5-mm region of the spectrum. An improved value for the internal luminosity of Neptune is (3.9 ± 1.1) × 10 15 W and an upper limit to the internal luminosity of Uranus 15 W. Titan was found to have an effective temperature greater than the observed brightness temperatures in the thermal infrared indicating that the emissivity in this spectral region is less than unity.

Thermal periodicities in the Venus atmosphere

Abstract A search was made for periodic fluctuations in the thermal brightness temperatures recorded by the Pioneer Venus orbiters infrared radiometer. Data were averaged in 10 × 10° latitude-longitude bins for each of the 72 days the instrument was in operation. This time series of thermal brightness temperatures was then analyzed to determine the amplitude of fluctuations at periods from 2 to 64 days at four levels in the atmosphere (at the cloud tops and at approximately 70, 80, and 90 km). The amplitude of such fluctuations is small at equatorial latitudes and increases to a maximum at 60–70° latitude at most altitudes. The period of the highest amplitude fluctuation is 5.3±0.4 days (at all altitudes) except at 70–80°, where a 2.9-day period which appears to correspond to the polar dipole dominates the cloud-top channel. The amplitude of the periodic fluctuations is a maximum at the cloud tops, decreasing to a minimum at the 80-km channel, and increasing again at the 90-km channel.

Comparison of band model and integrated line-by-line synthetic spectra for methane in the 2.3 μm region

Abstract The 2.3 μm spectral region of methane can be used to retrieve cloud properties of planetary spectra, provided parameters for the methane spectrum are known. Two standard techniques for calculating absorption spectra in this region are compared here. A Voigt profile Mayer-Goody random band model is applied, using coefficients empirically fitted by Fink et al. to CH 4 spectra recorded with high absorping amounts at 10 cm −1 resolution. Calculation of the absorption is also done with a line-by-line direct integration method for the same gas conditions using molecular parameters obtained by combining an older unpublished list of observed positions and estimated line strengths (derived from 0.04 cm −1 resolution data) with quantum assignments from the literature. The molecular parameters have been evaluated for the 4180–4590 cm −1 region by comparing new laboratory spectra with 0.01 cm −1 resolution recorded at 296 and 153K with synthetic spectra calculated at the same conditions. The deficiencies of the molecular parameters and random band coefficients for this spectral region of CH 4 are then discussed qualitatively and demonstrated by comparing 10 cm −1 resolution synthetic spectra calculated by both methods for the same gas conditions at 296, 153, and 55 K. Curves of growth of the total equivalent width are calculated at 296 and 55K for a pathlength of 50 cm and pressures up to 10 atm. Changing the mean line spacing in the band model gives better agreement between the spectra calculated by the two techniques at low gas temperatures. The required multiplier has been determined for the mean line spacing for pressures from 10 −6 to 10 −1 atm at 55, 100, and 150 K.

Comparison of ground-based and spacecraft observations of the infrared emission from Venus: The nature of thermal contrasts☆

Abstract Comparisons are made between observations of spatial variations in the thermal emission from Venus obtained with ground-based telescopes and those from spacecraft. In particular, we concentrate on measurements of solar-related structure at low and mid-latitudes, limb-darkening, and on the high-contrast polar structure. We conclude that (1) the solar-related emission is predominantly wavenumber 2, although it contains a significant diurnal component; (2) the relative amplitudes of the semidiurnal and diurnal components vary with latitude; (3) thermally excited temperature waves or, alternatively, solar-driven vertical motions of the cloud top are better able to account for the magnitude of the solar-locked emission than brightness temperature contrasts resulting from variations in aerosol microphysical properties; (4) the equatorial limb-darkening shows the top of the main cloud to be diffuse and approximately uniformly mixed with the gas; (5) polar collars are persistent at least for several months but disappear on occasion; and (6) collars have been observed at both poles simultaneously, but whether simultaneous appearance is the exception or the rule is still in question.

Speckle imaging for planetary research

Abstract The use of speckle imaging techniques for planetary research is discussed. Data are gathered with a video camera system. Image reconstruction uses a division algorithm for Fourier amplitudes and the Knox-Thompson algorithm for Fouroer phases. Bias correction techniques for arbitrary “photon shapes” have been developed. Results have been obtained with a laboratory optical simulator and on Io, Titan, Pallas, Jupiter, and Uranus using the 2.24-m telescope on Mauna Kea. All evidence indicates that spatial resolutions less than the seing limit by a factor of about 4 can be obtained. This applies to objects brighter than Uranus. For fainter objectsbias correction is inadequate and images cannot be recovered. Resolutions obtained are well above the diffraction limit. The reasons for this performance are understood in terms of inadequacies in the video camera. A photon-counting camera has been developed which should eliminate these problems. If so, it should be possible to obtain diffraction-limited resolution on objects as faint as Charon.

Cloud height differences on Saturn

Abstract Spectrophotometry from 0.6–2.5 μm at 1.5% spectral resolution of Saturns equator and visibly dark South Equatorial Belt near a 1980 ring-plane crossing shows substantially less absorption by methane over the equator than over the SEB. Model fits using Applebys model atmosphere and a multiple scattering model lead to the conclusion that the pressure levels of the clouds in the two regions differ by 20%. The region of high clouds is coincident with the location of the equatorial fast jet observed by Voyager.

Near-Infrared Linear Array Spectrometer For Space Applications

A 128-element InSb linear detector array read out with a field effect transistor (FET) multiplexing scheme has been developed at the Jet Propulsion Laboratory: State-of-the-art NEP, uniform response, on-detector charge integration, and very low focal plane heat load compared to that of discreet detector arrays allows this type of array to open new possi-bilities for spaceborne near-infrared imaging spectrometers. Large number of cooled detector elements are feasible with the new technology, making it practical to design sensitive spectrometers even for spinning spacecraft where simultaneous wavelength coverage is necessary to avoid confusion of spatial with spectral variations.