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Apt, J.

Publications and source records attributed to Apt, J..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Bolometric albedos of Titan, Uranus, and Neptune

The energy budgets of Titan, Uranus, and Neptune are reevaluated using new observational data on energy input as well as unpublished data on energy output. The bolometric geometric albedo of each object was determined, and preliminary determinations of the phase functions were used to compute the Bond albedos and effective temperatures. The values for the latter are 83 + or - 2 K for Titan, 57 + or - 2 K for Uranus, and 47 + or - 2 K for Neptune. The effective temperature of Titan is greater than the observed brightness temperatures in the thermal infrared region of the spectrum, indicating that the emissivity is less than unity for this part of the spectrum. An internal luminosity of (3.9 + or 1.1) x 10 to the 15th W is found for Neptune, and an upper limit of (0.6 + or - 1.4) x 10 to the 15th W is found for Uranus.

Neff, J. S.↗
NASA NTRS1984

Methane on Triton - Physical state and distribution

Infrared spectrophotometric measurements of Neptune's satellite Triton obtained between 1980 and 1982 in the spectral range 0.8-2.5 microns 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 Triton's orbital position. The data thus suggest that methane in the ice phase is mostly responsible for the bands in Triton's spectrum, and that the ice is distributed nonuniformly around the satellite's surface.

Cruikshank, D. P.↗
NASA NTRS1983

Methane on Triton and Pluto - New CCD spectra

Spectra of Triton and of Pluto from 4553-9558 A at 25 A resolution are presented. Both spectra show the methane absorption near 8900 A; its equivalent width was 16 times greater on Pluto than on Triton at the time of the observations. This is the first published observation of the 8900 A feature in Triton's spectrum. The previously reported 27 m-amagat abundance of the Pluto atmosphere ignored contributions due to methane ice and should therefore be regarded as an upper limit. The observations of the Pluto spectrum discussed here show sharp structure in the 8900 and 8600 A bands; the case for an atmosphere on Pluto may turn on whether new laboratory measurements show that such structure is present in methane ice.

Apt, J.↗
NASA NTRS1983

Speckle imaging for planetary research

The present study of speckle imaging technique effectiveness encompasses image reconstruction by means of a division algorithm for Fourier amplitudes, and the Knox-Thompson (1974) algorithm for Fourier phases. Results which have been obtained for Io, Titan, Pallas, Jupiter and Uranus indicate that spatial resolutions lower than the seeing limit by a factor of four are obtainable for objects brighter than Uranus. The resolutions obtained are well above the diffraction limit, due to inadequacies of the video camera employed. A photon-counting camera has been developed to overcome these difficulties, making possible the diffraction-limited resolution of objects as faint as Charon.

Nisenson, P.↗
NASA NTRS1982

Cloud height differences on Saturn

Spectrophotometry from 0.6-2.5 micron at 1.5% spectral resolution of Saturn's 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 Appleby's 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.

Apt, J.↗
NASA NTRS1982

Thermal periodicities in the Venus atmosphere

Evidence of periodic fluctuations was sought in the Pioneer Venus orbiter's IR radiometer data by averaging it in 10 X 10 deg latitude-longitude bins for each of the 72 days during which the instrument was in operation. The thermal brightness temperature time series obtained was then analyzed to determine the amplitude of fluctuations at periods ranging from two to 64 days, for cloud-top, 70, 80 and 90 km levels in the atmosphere. It is found that fluctuation amplitudes, which are small at equatorial latitudes, increase to a maximum at 60-70 deg latitude and reach a maximum at the cloud-top level, decreasing to a minimum at the 80 km channel before increasing again toward the 90 km channel. The period of the highest amplitude fluctuation is found to be 5.3 + or - 0.4 days, at all altitudes except at 70-80 deg latitude, where a 2.9-day period which may correspond to the polar dipole dominates the cloud-top channel.

Apt, J.↗
NASA NTRS1982

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 discrete detector arrays allows this type of array to open new possibilities 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.

Apt, J.↗
NASA NTRS1981

Radius and limb darkening of Titan from speckle imaging

The radius R of Titan is determined from ground-based, visible light observations using a two-dimensional speckle imaging technique. The value of R is found to depend on the limb-darkening exponent alpha, which is used in modeling the observed disk, and it is determined that the probable range extends from R values of 2680 + or - 100 km for alpha of 0.6 to 2900 + or - 100 km for alpha of 1.3. This range is in agreement with Voyager 1 and Pioneer Saturn observations. It is suggested that spatial resolution may be doubled by using a grid of 256 x 256 pixels at the time the image is digitized, and that further improvements are possible with speckle-reduction techniques under development.

Nisenson, P.↗
NASA NTRS1980

The character of the thermal emission from Venus

Images of the thermal infrared emission from 10.6 to 12.6 microns from the earth-facing hemisphere of Venus were made from earth on each available opportunity over a 26-day period in 1977 and a 106-day period in 1978-1979, using a 1.5 m telescope. Compact, variable thermal features exist at latitudes higher than about 50 deg, some of which repeat at 5-day intervals. No such repeatability is observed for features near the equator. Day-to-day variations in limb darkening and the contrast of high latitude features appear to differ in the conjunction and post-conjunction images. A strong, solar-fixed component of the emission exists and exhibits wave number 1, 2, and 4 components. The thermal tides provide constraints upon the atmosphere structure. The solar-fixed structure shows significant asymmetries with respect to the equator.

Apt, J.↗
NASA NTRS1980

Structure and circulation of the Venus atmosphere

The Pioneer Venus data relevant to the dynamics and thermodynamics of the atmosphere is summarized and interpreted. On the day side there is a thermosphere in which temperatures increase with height to an exospheric temperature of about 300 K. On the night side there is a cryosphere in which temperatures decrease with height to an exospheric temperature of about 100 K. The atmosphere is stratified stably from the highest altitudes down to about 28 km except for a layer in the clouds between about 50 and 55 km which is nearly adiabatic. Horizontal thermal contrasts are approximately 1 to 2% in the deep atmosphere and 100% in the upper atmosphere. The temperatures generally decrease with latitude at and below the clouds on constant pressure surfaces. Above the clouds there is a reversed zonally averaged latitudinal temperature gradient. The dominant circulation of the atmosphere above the lowest one or two scale heights is a zonal retrograde motion with 100 m/s winds at 60 km altitude. There is also a superrotation at altitudes of 150 km and above.

Schubert, G.↗
NASA NTRS1980

Simple Cassegrain scanning system for infrared astronomy

To meet the need for a reliable, fast imaging system capable of being taken rapidly on and off the telescope, a simple, inexpensive, and compact Cassegrain reimaging system for scanning IR images was constructed. Using commercially available components without requiring close mechanical tolerances, the design solves the problem of beam stability pointed out by Koornneef and van Overbeeke (1976). For the moving-iron galvanometer scanner, it is noted that at the imaging frequency of 0.5 Hz, hysteresis in image plane motion was found to be less than 0.2 arc sec for a 64-arc sec scan, and the deviation from linearity with a triangular wave input was found to be less than 0.3 arc sec. This system and a scanning secondary were used to image Venus at 11.5 microns, and compared with the scanning secondary, the reimaging system did not appear to contribute any additional noise, considerably improved mechanical reliability, and eliminated cross-scan motion

Apt, J.↗
NASA NTRS1979

Anomalous features in thermal radiance maps of Venus

Seventeen thermal radiance maps of Venus were recorded in a 26-day interval near the 1977 conjunction. An average symmetrical two-dimensional function was removed from the data. Features remain near the east and west limbs which correspond in part to previously reported solar-related effects but there are important differences, and reasons are given for questioning the interpretation of some of the earlier data. It is concluded that these features cannot be solely under solar control. Features are also found at both poles, just inside the region covered by the ultraviolet polar hood. These are not inconsistent with disturbances rotating rapidly around the planet. There is weak evidence for a 5-day repetition in some features.

Apt, J.↗
NASA NTRS1979

Infrared image of Venus at the time of Pioneer Venus probe encounter

An image of the infrared emission from the Earth-facing hemisphere of Venus was obtained at the time the Pioneer Venus probes penetrated the atmosphere. The thermal structure of the atmosphere at the 85-millibar level included regions of rapidly varying polar features, a solar-related postdawn warm area, and a nonsolar-fixed nighttime warm area. The probes succeeded in entering each of these three thermal regions.

Apt, J.↗
NASA NTRS1977

Observations of the sodium emission from Jupiter, region C

Observations of the sodium emission near Jupiter made while testing an interferometric imaging device are discussed. The results obtained suggest that the sodium emission detected in the region (designated C2) between 3 and 7.5 arcmin (9 and 23 Jupiter radii) from the planet may be no more than terrestrial airglow. It is concluded that region C2 is implausible on theoretical grounds and would need very solid observational support for its existence to be accepted.

Goody, R.↗