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Buratti, Bonnie J.

Publications and source records attributed to Buratti, Bonnie J..

30 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Ganymede and Callisto - Surface textural dichotomies and photometric analysis

Complete solar phase curves of the Ganymede and Callisto leading and trailing hemispheres, which have been obtained by reducing Voyager imaging observations and combining them with ground-based telescopic data, are presently fit to scattering models in order to derive hemispherical values of the single scattering albedo, the single particle phase function (SPPF), the compaction state (CS) of the optically active portion of the regolith, and the mean slope angle of macroscopic features. While Callisto's leading side is composed of particles that are more strongly backscattering than the trailing side, no hemispheric differences are found in the CS, surface roughness, or SPPF.

Buratti, Bonnie J.↗
NASA NTRS1991

Photometry of the comet 2060 Chiron

The comet 2060 Chiron has proven to be an interesting and enigmatic object. Situated between the orbits of Saturn and Uranus, it was originally classified as the most distant asteroid. It began to show cometary behavior in 1987 by increasing a full magnitude in brightness and developing a coma; there is evidence also for similar earlier outbursts. A thorough study of Chiron is important for two reasons: (1) it is a transition object defining the relationship between comets, asteroids, and meteorites; and (2) a full description of its changes in brightness - particularly on time scale of hours - will provide an empirical foundation for understanding the physical mechanisms (including outgassing, sublimation of volatiles, and even significant mass ejections) driving the evolution of comets. Short term outbursts were observed in early 1989, and a rapid decrease in brightness of Chiron's coma was observed in 1990 in the V and R filters. Also, a rotational lightcurve was detected of the nucleus with an amplitude only 1/4 that observed in its quiescent state: this fact indicates the increased importance of the optically thin coma to the observed brightness.

Buratti, Bonnie J.↗
NASA NTRS1991

Comparative global albedo and color maps of the Uranian satellites

The surfaces of the Uranian satellites Ariel, Miranda, Oberon, Titania, and Umbriel are characterized on the basis of Voyager observations. Tables of spectrophotometric data and maps of normal reflectances, green/violet ratios, and possible geological formations are presented and discussed in detail. Variations in albedo are found to be associated with impact features, and it is inferred from color differences that the upper surface of Ariel contains a higher proportion of redder material (tentatively identified as accreted low-albedo meteoritic dust) than those of the other moons.

Buratti, Bonnie J.↗
NASA NTRS1991

Observation of a rapid decrease in the brightness of the coma of 2060 Chiron in 1990 January

Photometric observations of 2060 Chiron in the V and R filters were obtained with the 1.5-m telescope on Palomar Mountain during a 7-hr period on January 20, 1990 (UT). A general decrease of about 10 percent in integrated brightness occurred in both filters. No color dependence to the decrease was observed. A small (about 0.02 mag) rotational light curve, far smaller than the 0.09 mag (peak-to-peak) one observed by Bus et al. (1989) is superposed on the general decrease. On January 29, 1990, Luu and Jewitt (1990) observed an impulsive brightening of Chiron of approximately the same magnitude and time scale as the presently observed decrease in brightness. The combined results provide evidence that Chiron is currently exhibiting short-term fluctuations in the brightness of its coma, in addition to its well-established general decrease in brightness.

Buratti, Bonnie J.↗
NASA NTRS1990

Albedo and color maps of the Saturnian satellites

The paper discusses the production of maps of the albedos and colors of Mimas, Enceladus, Tethys, Dione, and Rhea over the full range of their imaged surfaces. Voyager images were used to prepare maps of the normal reflectances and color ratios (0.58/0.41 micron) of these satelites.

Buratti, Bonnie J.↗
NASA NTRS1989

Photometry from Voyager 2 - Initial results from the Neptunian atmosphere, satellites, and rings

Voyager 2 has performed photopolarimetric measurements for the planet, several of its satellites, and its ring system. The Great Dark Spot is noted to appear as a low-albedo region, while the bright companion appear to lie at a higher altitude than the Great Dark Spot. Triton's linear phase coefficients are consistent with a solid-surface object having high reflectivity; its rotational phase curve indicates two major compositional units at its surface. The deficiency of material in Neptune's ring system by comparison with that of Uranus may indicate the absence of a recent moon-shattering event.

Lane, Arthur L.↗
NASA NTRS1988

Enceladus - Implications of its unusual photometric properties

Voyager 1 and 2 images are used to ascertain the photometric properties of Enceladus up to a 43-deg phase angle. The lack of correlation noted between spectrophotometric properties and terrain type indicates that optical characteristics result from a recently-deposited, ubiquitous surface layer that may have originated on the E-ring of Saturn. Enceladus' distinctive phase and photometric functions can be accounted for by the high degree of surface multiple scattering; observations to date indicate that the Enceladus regolith's textural characteristics are similar to those of other icy satellites of Saturn.

Buratti, Bonnie J.↗
NASA NTRS1988

Surficial textures of the Galilean satellites

The observations of the Galilean satellites of Jupiter obtained with the IUE have been analyzed as a function of solar phase angle. By fitting the measurements to a shadowing model, comparative descriptions of the microtextures of the optically active portion of the surfaces of the satellites are derived. Important differences among the satellites and between leading and trailing hemispheres of individual satellites result from the different processes of meteoritic bombardment, magnetospheric interaction, and geological resurfacing that operate in the Jovian system. Io and Callisto have the most tenuous upper regoliths, whereas the surface of the leading side of Europa is the most compact.

Buratti, Bonnie J.↗
NASA NTRS1988

Spectrogoniometer for measuring planetary surface materials at small phase angles

This paper describes an instrument, termed spectrogoniometer, which was designed to measure the bidirectional reflectance properties of planetary surface analogs at very small phase angles. By using a pellicle beam splitter, a highly collimated laser light source, and a carefully aligned apparatus it was possible to measure the reflectance properties of geologic samples under any viewing geometry: the angles of incidence and emission ranging from 0 to 90 deg, and the azimuthal angle ranging from 0 to 360 deg. The results on the effect of porosity are demonstrated by comparing measurements on a compact sample (with a void space of 25 percent) and a fluffy sample (with a void space of 90 percent). The fluffy sample exhibited a 30-percent increase in intensity between 10 and 0 deg, whereas the compact sample exhibited only a 20-percent increase.

Buratti, Bonnie J.↗
NASA NTRS1987

Voyager 2 photopolarimeter observations of the Uranian satellites

The surfaces of the principal Uranian satellites are characterized on the basis of UV and IR geometric albedos, phase curves, and phase coefficients obtained in full-disk photopolarimetric observations during the Voyager 2 encounter with Uranus in January 1986. The data are presented in tables and graphs and found to be consistent with a heavily cratered terrain and loosely packed regolith. The Bond albedos are calculated as 0.22 + or - 0.1 for Ariel, 0.07 + or - 0.05 for Umbriel, 0.16 + or - 0.12 for Titania, and 0.19 + or - 0.22 for Oberon. The characteristics of the Uranian satellites indicate compositions (and probably formation conditions and surface-modification mechanisms) distinct from those of the Saturnian and Jovian satellites.

Nelson, Robert M.↗
NASA NTRS1987

Spectral geometric albedos of the Galilean satellites from 0.24 to 0.34 micrometers - Observations with the International Ultraviolet Explorer

An 8-year IUE satellite spectrophotometric observation campaign's results for the Galilean satellites of Jupiter indicate low albedos for all four, consistent with the distribution of sulfurous materials escaping from the surface of Io to the surfaces of the three other bodies by means of magnetospheric processes. A previously reported UV spectral asymmetry between Europa's leading and trailing hemispheres is confirmed. The greater trailing than leading side opposition surge observed in Europa implies that the trailing side is the less compact of the two. Ganymede exhibits UV spectral asymmetry also, but only at wavelengths greater than 0.28 microns.

Nelson, Robert M.↗