Engineering topics

Veverka, J.

Publications and source records attributed to Veverka, J..

At least 37 records · Page 2

NASA NTRS1992

Galileo encounter with 951 Gaspra - First pictures of an asteroid

Galileo images of Gaspra reveal it to be an irregularly shaped object (19 by 12 by 11 kilometers) that appears to have been created by a catastrophic collisional disruption of a precursor parent body. The cratering age of the surface is about 200 million years. Subtle albedo and color variations appear to correlate with morphological features: brighter materials are associated with craters especially along the crests of ridges, have a stronger 1-micrometer absorption, and may represent freshly excavated mafic materials; darker materials exhibiting a significantly weaker 1-micrometer absorption appear concentrated in interridge areas. One explanation of these patterns is that Gaspra is covered with a thin regolith and that some of this material has migrated downslope in some areas.

Belton, M. J. S.↗

NASA NTRS1992

Mars Observer camera

The Mars Observer camera (MOC) is a three-component system (one narrow-angle and two wide-angle cameras) designed to take high spatial resolution pictures of the surface of Mars and to obtain lower spatial resolution, synoptic coverage of the planet's surface and atmosphere. The cameras are based on the 'push broom' technique; that is, they do not take 'frames' but rather build pictures, one line at a time, as the spacecraft moves around the planet in its orbit. MOC is primarily a telescope for taking extremely high resolution pictures of selected locations on Mars. Using the narrow-angle camera, areas ranging from 2.8 km x 2.8 km to 2.8 km x 25.2 km (depending on available internal digital buffer memory) can be photographed at about 1.4 m/pixel. Additionally, lower-resolution pictures (to a lowest resolution of about 11 m/pixel) can be acquired by pixel averaging; these images can be much longer, ranging up to 2.8 x 500 km at 11 m/pixel. High-resolution data will be used to study sediments and sedimentary processes, polar processes and deposits, volcanism, and other geologic/geomorphic processes.

Malin, M. C.↗

NASA NTRS1992

Gaspra's shape and surface features: Comparison to small satellites

The several thousand asteroids constitute the second largest population of substantial solid objects in the solar system (there are probably more comets), and have been studied from the Earth for nearly two centuries. Asteroids are important members of the solar system for several reasons, among them: (1) they contain the materials that reflect the processes going on early in the time of planet formation; (2) their impacts have been the major external influences on the surface of some terrestrial planets; and (3) their impacts on Earth have probably had major effects on biologic evolution. This paper examines and describes the asteroid Gaspra, and compares it to small satellites.

Thomas, P. C.↗

NASA NTRS1992

Satellites of Mars - Geologic history

The small, irregularly shaped satellites of Mars, Phobos and Deimos, provide the most detailed view of the geomorphic forms and processes important on small solar system bodies. The satellites appear to be very similar in composition, strongly resembling carbonaceous asteroids; however, recent groundbased spectra suggest that their surfaces have little bound or interlayer water. Despite their similar compositions, sizes and environments, Phobos and Deimos have radically different surface features. Phobos is densely covered by craters that are nearly lunar in appearance; Deimos' craters are subdued and largely filled in by debris. Phobos shows only local downslope movement of regolith; Deimos has it on a global scale. Phobos is criss-crossed by linear depressions; Deimos has none. Crater ejecta appear to be retained near their sources on Phobos while the ejecta are widespread on Deimos. The reasons for the differences between the satellites are not known; imaging of asteroids should tell us which, if either, satellite is typical of the many small bodies that populate the asteroid belt.

Thomas, P.↗

NASA NTRS1991

Voyager photometry of Triton - Haze and surface photometric properties

The Voyager whole-disk observations of Triton at 0.41, 0.48, and 0.56 micron filter wavelengths are analyzed using a model which combines an improved version of Hapke's photometric equation with a thin atmospheric haze layer in the appropriate spherical geometry. The model is shown to describe accurately the phase curves over a range of phase angles and to agree with disk-resolved brightness scans along the photometric equator and mirror meridian. According to the model, the photometric parameters of Triton's regolith are reasonably typical of icy satellites, except for the extremely high (close to unity) single-scattering albedo.

Hillier, J.↗

NASA NTRS1991

The wavelength dependence of Triton's light curve

Using Voyager observations, it is demonstrated that Triton's orbital light curve is strongly wavelength-dependent, a characteristic which readily explains some of the apparent discrepancies among pre-Voyager telescopic measurements. Specifically, a light curve amplitude (peak to peak) is found that decreases systematically with increasing wavelength from about 0.08 magnitude (peak to peak) near 200 nm to less than 0.02 magnitude near 1000 nm. Peak brightness occurs near 90 deg orbital longitude (leading hemisphere). The brightness variation across this hemisphere is close to sinusoidal; the variation across the darker hemisphere is more complex. The decrease in light curve amplitude with increasing wavelength appears to be due to a decrease in contrast among surface markings, rather than to atmospheric obscuration. The model also explains the observed decrease in the amplitude of Triton's light curve at visible wavelengths over the past decade, a decrease related to the current migration of the subsolar latitude toward the south pole; it is predicted that this trend will continue into the 1990s.

Hillier, J.↗

NASA NTRS1991

The photometric roughness of Ariel is not unusual

Voyager 2 images of the Uranian satellite Ariel are reanalyzed, with a focus on the large-scale roughness parameter theta-bar. The techniques used to reduce the data and fit the Hapke parameters are described, and the results are presented in tables and graphs. Using the whole-disk phase curve, theta-bar of 27 + or - 1 deg is obtained, somewhat higher than but in good general agreement with the values for the other Uranian satellites. The anomalously high value (42 deg) found by Helfenstein et al. (1988) is attributed to miscalibration of the clear-filter image for phase angle 144.7 deg.

Skypeck, A.↗

NASA NTRS1991

Uranus satellites - Surface properties

The post-Voyager knowledge of the photometric, colorimetric, spectral, and thermal properties of the Uranian satellites is reviewed, focusing on such fundamental physical properties as albedo, color, and surface texture. While albedo variations of at least a factor of 2 exist, color differences are almost absent (Miranda) or subdued (Oberon). In the case of Titania, the strong opposition effect reported by ground-based observers was confirmed by Voyager. Voyager did not observe the opposition parts of the phase curves of the other satellites. Voyager thermal observations of Ariel and Miranda suggest that both have highly porous regoliths, thermophysically similar to those of Jupiter's icy satellites. At the time of the flyby (south pole facing the sun), maximum surface temperatures reached or exceeded 85 K, but nighttime polar temperatures are predicted to drop to 20 to 30 K because each pole spends about 40 yr in darkness. Ground-based spectroscopy identified water ice as an important surface constituent.

Veverka, J.↗

NASA NTRS1990

Voyager disk-integrated photometry of Triton

Hapke's (1981) photometric model has been combined with a plane-parallel thin atmospheric haze model to describe Voyager whole-disk observations of Triton, in the violet, blue, and green wavelength bands, in order to obtain estimates of Triton's geometric albedo, phase integral, and Bond albedo. Phase angle coverage in these filters ranging from about 12 to 159 deg was obtained by combining narrow- and wide-angle camera images. An upturn in the data at the highest phase angles observed can be explained by including scattering in a thin atmospheric haze layer with optical depths systematically decreasing with wavelength from about 0.06 in the violet to 0.03 for the green filter data.

Hillier, J.↗

NASA NTRS1989

Miranda - Color and albedo variations from Voyager photometry

Voyager 0.48-micron images of Miranda covering phase angles of 13-39 deg, together with 0.35-micron 0.41-micron, and 0.56-micron data at 16-deg phase angle, are the bases of the Minnaert parameters from which the present albedo/color maps are derived. Color ratios are found to be essentially constant over all terrain types and albedo ranges on the satellite; spectrally, Miranda is gray. The color data are noted to conform with a model in which varying amounts of materials spectrally resembling the asteroidal F-material are mixed with an icy component.

Hillier, J.↗

NASA NTRS1989

Minnaert photometric parameters for the satellites of Uranus

After deriving the Minnaert (1961) parameters for the five largest satellites of Uranus as functions of both wavelength and phase angle, the well-documented case of Titania is invoked to assess the usefulness of the Minnaert description vs the more rigorous approach of Hapke (1981). While the limb-darkening Minnaert parameter k possesses only a weak wavelength dependence, it exhibits a strong dependence on phase angle that varies to some extent from satellite to satellite; in the case of Titania, this dependence is similar to that for the earth's moon.

Veverka, J.↗

NASA NTRS1989

Physical characterization of asteroid surfaces from photometric analysis

The feasibility of using photometric models like Hapke's (1981, 1984, 1986) equation for deriving physical properties of asteroids from photometric observations is discussed. Using data for Ceres and Vesta, it is shown that the incomplete phase-angle coverage limits the reliable determination of Hapke's photometric parameters from asteroid disk-integrated phase curves (the second limitation is the nonsphericity of many asteroids). However, within this limitations, certain trends among asteroids can be compared. It is shown that there is a general similarity among the Hapke's parameters other than the h parameter, which characterizes the width of the opposition surge in terms of soil structure (porosity, particle-size distribution, and the rate of compaction with depth), derived for similar objects, e.g., average C asteroids and Ceres, and average S asteroids, 1982 Apollo, and Vesta.

Helfenstein, P.↗

NASA NTRS1989

Spacecraft exploration of asteroids - The 1988 perspective

Planned and proposed missions to study asteroids are examined. The history of asteroid studies in the 1980s is reviewed, including the proposed Mainbelt Asteroid Orbiter/Flyby mission. Main-belt asteroid flyby missions are discussed, including the Galileo, Cassini, and Comet Rendezvous Asteroid Flyby mission. Also, consideration is given to proposed missions to near-earth asteroids, the Vesta mission to perform in situ studies on two large asteroids, the Piazzi mission to flyby an Apollo-Amor-Aten asteroid using a Giotto-derived spacecraft, and the possibility of a Japanese asteroid flyby mission.

Veverka, J.↗

NASA NTRS1988

Brighter material on Deimos - A particle size effect in a carbonaceous material?

The values obtained for brightness ratios between contiguous bright and dark areas on Deimos from Viking Orbiter images, together with the lack of a significant wavelength dependence of these ratios in the 0.4-0.6-micron range, are presently noted to be consistent with particle size fraction measurements of the Murchison CM meteorite. These data, and a near-coincidence of Deimos absolute reflectances with those of laboratory samples, render the present data consistent with both brighter and darker materials on Deimos being akin to carbonaceous chondrites; the material with smaller average particle size is associated with the brighter patches.

French, L. M.↗

NASA NTRS1988

Photometry and polarimetry of Mercury

Information available on the photometric and polarimetric properties of Mercury's surface is discussed, with special consideration given to data concerning the nature of the Mercurial regolith and the comparison of the Mercury's photometric properties with those of the moon. The results from an analysis performed in terms of Hapke's (1981, 1984, 1986) photometric function confirm the remarkable similarity of photometric properties of the two celestial bodies. However, their highland/mare alabedo ratios differs considerably; while the ratio is about 2 on the moon, it is only about 1.4 on Mercury. Mercury also shows slightly lower values of the maximum positive polarization observed, a difference that is consistent with either a slightly higher average reflectance for Mercury, or with a difference in regolith texture, or both.

Veverka, J.↗

NASA NTRS1987

Satellites of Uranus - Disk-integrated photometry from Voyager imaging observations

Voyager 2 imaging observations over a wide range of phase angles are used to determine the fundamental photometric parameters for the five largest satellites of Uranus. Over the spectral range covered by Voyager cameras (approximately 350-600 nm) the disk-averaged colors are moderately gray (no redder than the spectrum of Saturn's satellite Phoebe). Geometric albedos range from 0.19 for Umbriel to 0.40 for Ariel. Phase coefficients determined generally between phase angles of 10 and 60 deg vary from 0.021 mag/deg for Ariel to 0.028 mag/deg for Miranda. Phase integrals lie in the range of 0.5-0.65. The Bond albedos are about 0.1 for Umbriel and about 0.2 for the other satellites.

Veverka, J.↗

NASA NTRS1987

Titania's opposition effect - Analysis of Voyager observations

Voyager 2 obtained images of Titania over phase angles ranging from 0.8 to 150 deg and at sufficient resolution to investigate the photometric behavior of different surface units. The large, relatively narrow opposition surge detected from earth was confirmed and can be successfully modeled with Hapke's (1986) photometric theory. Opposition effects do not vary greatly between bright areas (craters and ejecta) and dark areas, but the brightness of the brighter areas decreases more slowly with increasing phase angle than that of the dark areas. Thus the fresher craters and their ejecta become less prominent in relation to the background as opposition is approached. This effect is best explained by a modest difference in single-scattering albedo.

Thomas, P. C.↗

NASA NTRS1987

Voyager observations of 1985U1

During its discovery of the 75-km average diameter Uranian satellite 1985U1, between the epsilon ring and Miranda, Voyager was able to resolve this irregularly shaped satellite in sufficiently great detail to allow the determination of its size, shape, and photometric properties. It is presently noted that 1985U1's albedo, at between 0.07 and 0.09, is both substantially lower than those obtained for the large Uranian satellites and slightly higher than that of the Saturn moon, Phoebe. A V-system opposition magnitude of about +20.5 is conjectured.

Thomas, P.↗