Engineering topics

Sinton, W. M.

Publications and source records attributed to Sinton, W. M..

At least 19 records

NASA NTRS1991

Ground-based near-infrared imaging observations of Venus during the Galileo encounter

Near-infrared images of Venus, obtained from a global network of ground-based observatories during January and February 1990, document the morphology and motions of the night-side near-infrared markings before, during, and after the Galileo Venus encounter. A dark cloud extended halfway around the planet at low latitudes and persisted throughout the observing program. It had a rotation period of 5.5 + or - 0.15 days. The remainder of this latitude band was characterized by small-scale (400 to 1000 km) dark and bright markings with rotation periods of 7.4 + or - 1 days. The different rotation periods for the large dark cloud and the smaller markings suggest that they are produced at different altitudes. Midlatitudes (+ or - 40 to 60 deg) were usually occupied by bright east-west bands. The highest observable latitudes (+ or - 60 deg to 70 deg) were always dark and featureless, indicating greater cloud opacity. Maps of the water vapor distribution show no evidence for large horizontal gradients in the lower atmosphere of Venus.

Crisp, D.↗

NASA NTRS1991

Spectroscopic observations of bright and dark emission features on the night side of Venus

Near-infrared spectra of a bright and a dark thermal emission feature on the night side of Venus have been obtained from 2.2 to 2.5 microns at a spectral resolution of 1200 to 1500. Both bright and dark features show numerous weak absorption bands produced by CO2, CO, water vapor, and other gases. The bright feature emits more radiation than the dark feature throughout this spectral region, but the largest contrasts occur between 2.21 s 2.32 microns, where H2SO4 clouds and a weak CO2 band provide the only known sources of extinction. The contrast decreases by 55 to 65 percent at wavelengths longer than 2.34 microns, where CO, clouds, and water vapor also absorb and scatter upwelling radiation. This contrast reduction may provide direct spectroscopic evidence for horizontal variations in the water vapor concentrations in the Venus atmosphere at levels below the cloud tops.

Bell, J. F., III↗

NASA NTRS1990

Discovery of hotspots on Io using disk-resolved infrared imaging

First results are presented using two new techniques for ground-based observation of Io's hotspots. An IR array camera was used to obtain direct IR images of Io with resolution better than 0.5 arcsec, so that more than one hotspot is seen on Io in Jupiter eclipse. The camera was also used to make the first observations of the Jupiter occultation of the hotspots. These new techniques have revealed and located at least three hotspots and will now permit routine ground-based monitoring of the locations, temperatures, and sizes of multiple hotspots on Io.

Spencer, J. R.↗

NASA NTRS1989

The nature of the near-infrared features on the Venus night side

Images of the Venus nightside recently acquired at wavelengths near 1.74 and 2.3 microns, in narrow atmospheric windows between the CO2 and H2O bands, allow the identification of mechanisms responsible for bright contrast figures moving from east to west in the direction of the cloud-top atmospheric superrotation. The 6.5-day east-west rotation period of the features indicates equatorial wind speeds of the order of 70 m/sec in this upper layer. The bright features are indicative of partial clearings in the cloud deck which could decrease the efficiency of the atmospheric greenhouse maintaining the high surface temperatures of Venus.

Crisp, D.↗

NASA NTRS1988

Io hot spots - Infrared photometry of satellite occultations

Io's active hot spots, which are presently mapped on the basis of IR photometry of this moon's occultation by other Gallilean satellites, are obtained with greatest spatial resolution near the sub-earth point. A model is developed for the occultation lightcurves, and its fitting to the data defines the apparent path of the occulting satellite relative to Io; the mean error in apparent relative position of occulting satellites is of the order of 178 km. A heretofore unknown, 20-km diameter hot spot is noted on Io's leading hemisphere.

Goguen, J. D.↗

NASA NTRS1986

Research in planetary astronomy and operation of the 2.2-meter telescope

The turn-around in the secular change in brightness of Pluto was detected, the planet is now beginning to brighten. The spectral signature was found of an ammonia bearing compound in the surface ices of Europa. Specific Io volcanic hotspots were detected from infrared photometry of the mutual events of the Jovian satellites. The polarized emission from the volcanic hotspots on Io was discovered and the locations of those hotspots were determined. The orbit of Charon was refined from speckle observations of the Pluto-Charon system. The spectral properties of liquid nitrogen-methane mixtures with reference to the infrared spectrum of Triton were studied. Major progress was made in modeling asteroid lightcurves from a modern theory of photometric properties of the asteroid surfaces. Several additional olivine-rich asteroids from near-infrared spectrophotometry were discovered. Important photometric, spectroscopic, narrow-band imaging data on Comet P/Halley was acquired, along with Schmidt photographs of Comet P/Halley and its tail.

Hall, D. N. B.↗

NASA NTRS1985

Characterization of Io's volcanic activity by infrared polarimetry

The thermal emission from Io's volcanic hot spots is linearly polarized. Infrared measurements at 4.76 micrometers show disk-integrated polarization as large as 1.6 percent. The degree and position angle of linear polarization vary with Io's rotation in a manner characteristic of emission from a small number of hot spots. A model incorporating three hot spots best fits the data. The largest of these hot spots lies to the northeast of Loki Patera, as mapped from Voyager, and the other spot on the trailing hemisphere is near Ra Patera. The hot spot on the leading hemisphere corresponds to no named feature on the Voyager maps. The value determined for the index of refraction of the emitting surface is a lower bound; it is similar to that of terrestrial basalts and is somewhat less than that of sulfur.

Goguen, J. D.↗

NASA NTRS1985

Observational tests for sulfur allotropes on Io

It has been established that, if the Io surface is primarily composed of sulfur allotropes, a change in reflectivity at certain wavelengths should be observable shortly after eclipse reappearance (ER). Four ERs were observed during July and August 1983 with a 61-cm telescope on Mauna Kea, Hawaii, and no posteclipse brightening effects in the filter bands selected for sensitivity to color changes in sulfur were found. The proposed model of the brightness change of elemental sulfur (S8) implies that this material covers less than 50 percent of the Io surface. It is concluded that no significant condensation of optically thick SO2 occurred on the Io surface during the four eclipses.

Hammel, H. B.↗

NASA NTRS1984

Photometric standard stars for L' and M filter bands

L' (3.8 microns) and M (4.9 microns) photometry of 20 primary and 21 secondary standard stars that are sufficiently faint for large telescope observation is presented. The estimated uncertainties in the magnitudes of the primary standards are 0.01 at L', 0.02 at M, and 0.015 for the L'-M color. A color transformation between the present L' magnitudes and the L magnitudes of Elias et al. (1982) is obtained and found to be near agreement with that predicted for black-bodies.

Sinton, W. M.↗

NASA NTRS1983

Io - The near-infrared monitoring program, 1979-1981

The results of a program to monitor the brightness of Io in the near infrared are reported. While Io's 2.2 micron flux can be explained very well by conventional albedo analysis, at 3.8 and 4.8 microns Io has large intrinsic variations presumably associated with volcanism. At the latter two wavelengths, Io appears brighter and more active on its trailing hemisphere. These wavelengths also have possibly significant terms at the Jupiter corotational periods of 13 and 6.5 hr. These are particularly significant for observations of the trailing hemisphere and have a marked peak when Io is in the active sector. The data also show that the variability occurs in outbursts. Eight of these were observed and four were studied, deriving color temperatures near 700 K which decayed to about 300 K after several hours. It is argued that the apparent peak and the significance of the corotational terms may be fortuitous.

Lindwall, D.↗

NASA NTRS1983

Determination of the Io heat flow. 1: Eclipse observations

The thermal emission from Io during eclipse by Jupiter yields data from which the total thermal flux from the volcanoes on the satellite surface can be estimated. Thermal infrared observations in spectral bands between 3.5 and 30 microns of five Io eclipse reappearances and one eclipse disappearance are reported and discussed. The thermal emission of the volcanoes which occurs almost all of the time was determined from the Io heat flux data. The thermal observations of Io are discussed with respect to previous thermophysical theories.

Sinton, W. M.↗

NASA NTRS1983

On Io, all that flickers is not cold

The 4.8 micron flux from the Galilean satellite Io was shown to fluctuate with an amplitude of approximately 10 percent on time scales of every 28 seconds, 40 minutes, night-to-night, and perhaps year-to-year. Such behavior was found to be the result of random fluctuations for which the mean square fluctuation varies inversely with frequency for constant bandwidth measurement. The theory developed for thermionic emission from barium oxide cathodes in vacuum tubes might be applicable to this situation. If so, the fluctuations in the flux from Io's volcanoes may be caused by diffusion of hot convective cells onto the surface of Io. Long term fluctuations may furnish a means by which the Io volcanism can shut down and conserve energy. Thus the discrepancy that exists between measurements of the current heat flow from Io and calculations of tidal dissipation may be resolved. Tests for rapid flickering at 10 microns showed no fluctuations greater than one percent. This agrees with the prediction of the flow model theory in which the 10 microns volcanic thermal emission arises from cooling of old flows.

Sinton, W. M.↗

NASA NTRS1982

Hot spots of Io

The size and temperature, morphology and distribution, variability, possible absorption features, and processes of hot spots on Io are discussed, and an estimate of the global heat flux is made. Size and temperature information is deconvolved to obtain equivalent radius and temperature of hot spots, and simultaneously obtained Voyager thermal and imaging data is used to match hot sources with specific geologic features. In addition to their thermal output, it is possible that hot spots are also characterized by production of various gases and particulate materials; the spectral signature of SO2 has been seen. Origins for relatively stable, low temperature sources, transient high temperature sources, and relatively stable, high-tmperature sources are discussed.

Pearl, J. C.↗

NASA NTRS1981

The thermal emission spectrum of Io and a determination of the heat flux from its hot spots

Observations of thermal emission from Io in the near infrared made during an eclipse were combined with unpublished 8- to 13-micron intermediate band photometry and a 16- to 22-micron spectrum to specify Io's emission spectrum from 2.2 to 22 microns. Models were calculated having 'hot spots' at several different temperatures superposed on a surface, the major part of which is assumed to be at the solar equilibrium temperature. It was possible to fit the entire composite spectrum with this model. It is argued that the total emission from the hot spots can be equated to the nonsolar energy input into Io. The disk-averaged heat radiated by the hot spots is found to be 180 + or 60 microwatts/sq cm = 43 + or - 14 microcalories/sq cm-sec. A possible bimodal temperature distribution of the hot spots is discussed.

Sinton, W. M.↗

NASA NTRS1980

Io - Ground-based observations of hot spots

Observations of Io in eclipse demonstrate conclusively that Io emits substantial amounts of radiation at 4.8 and 3.8 micrometers and a measurable amount at 2.2 micrometers. Color temperatures derived from the observations fit blackbody emission at 560 K. The required source area to yield the observed 4.8-micrometer flux is approximately 5 x 10 to the -5th of the disk of Io and is most likely comprised of small hot spots in the vicinity of the volcanoes.

Sinton, W. M.↗

NASA NTRS1980

Io - Are vapor explosions responsible for the 5-micron outbursts

It is proposed that a vapor explosion of a submerged pool of liquid sulfur will remove the crust overlying an area of about 50-km diam. Thermal radiation from the exposed liquid sulfur pool with a surface temperature of 600 K is then presumed to be responsible for the 5-micron outbursts that have been observed. The explosive volcanoes are expected to leave black sulfur calderas, which are, indeed, found on the surface. The 5-micron outburst observed by Sinton (1980), on June 11, 1979 (UT), is identified with a new caldera found on Voyager 2 photographs but which had not been present on Voyager 1 pictures.

Sinton, W. M.↗

NASA NTRS1980

Infrared scans of Saturn

Scans are presented at five wavelengths between 7.8 and 25 microns north-south along Saturn's central meridian and east-west parallel to the equator through the subearth point. The brightening of Saturn's South Pole at 12.7 microns was more enhanced in 1977 than in 1978 due to the 5 deg greater declination of the polar axis in 1977. There is a plateau in the Southern Hemisphere between -30 and -60 deg latitude in the 7.8 and 12.7 micron scans. The apparent temperature of the rings decreased as Saturn approached the equinox. It is found generally that the strongest ring emission arises from the C ring.

Sinton, W. M.↗

NASA NTRS1980

Five-micrometer measurements of Uranus and Neptune

Five-micron brightness temperatures and brightness temperature upper limits for Uranus and Neptune have been obtained which are substantially lower than those of Jupiter and Saturn and which correspond to a geometric albedo of approximately 0.01, in agreement with results reported by Gillett and Rieke (1977). Phosphine and CH3D, which are observed at 5 microns on Jupiter and Saturn, are discussed as possible sources of opacity at 5 microns in the atmospheres of Uranus and Neptune.

Macy, W. W., Jr.↗