Engineering topics
Publications and source records attributed to Veverka, J..
Of the 10 small Uranian satellites, 1985U1 is the largest and the only one for which a resolved image was obtained by Voyager 2. In terms of albedo, the other nine satellites seem to be similar. Thus the single image of 1985U1 is important in suggesting what these other objects may be like. Size, shape, surface features, and photometry are examined.
A camera designed to operate under the extreme constraints of the Mars Observer Mission was selected by NASA in April, 1986. Contingent upon final confirmation in mid-November, the Mars Observer Camera (MOC) will begin acquiring images of the surface and atmosphere of Mars in September-October 1991. The MOC incorporates both a wide angle system for low resolution global monitoring and intermediate resolution regional targeting, and a narrow angle system for high resolution selective surveys. Camera electronics provide control of image clocking and on-board, internal editing and buffering to match whatever spacecraft data system capabilities are allocated to the experiment. The objectives of the MOC experiment follow.
Ejecta block characteristics observed on Phobos, Deimos, and the moon are examined. The analyzed source craters on Deimos are 0.8-2.3 km in diameter, those on Phobos are 1.5-10 km, and the lunar craters are between 0.2-3.5 km in diameter. The size and radial distribution of the ejecta blocks for the three bodies are compared. It is observed that the size distribution of the ejecta blocks surrounding craters on the three objects are basically similar, and the radial distribution of the blocks for Phobos and the moon are the same (within 2 radii of the crater center); however, the ejecta on Deimos are more dispersed (greater than or equal to 2 radii from the crater center).
For conducting a comparison of Voyager photometric measurements of individual areas on the Jovian satellite Io to laboratory data, it is necessary to reduce the two sets of information to a comparable format. In this connection, more accurate normal reflectance spectra for regions on Io are derived by means of a two-step process. The near-opposition limb-darkening characteristics are obtained for areas on the surface of Io. The new limb-darkening results are employed to determine the near-opposition phase behavior of selected regions on the satellite, taking into account also the effects of the opposition surge. A combination of the phase and limb-darkening data makes it possible to obtain improved normal reflectance spectra for regions on Io. These spectra can be readily compared with laboratory spectra of candidate materials.
It is pointed out that spectral reflectance curves depend largely upon the optical constants of the surface materials. In the present study, the results of laboratory measurements are employed to investigate the wavelength dependence of disk-integrated phase coefficients of some materials, taking into account a basalt, the ordinary chondrite Bruderheim, the carbonaceous chondrite Allende, and sulfur. These materials have been selected as appropriate analogs to the surfaces of the moon, S- and C-class asteroids, and the Jovian satellite Io, respectively. It is shown that the observed wavelength dependence of phase coefficients can be predicted (at least qualitatively) once the spectrum of an object is known. Attention is given to phase variation of disk-integrated spectral reflectance curves, the wavelength dependence of phase coefficients, a comparison with observations, and the effects of surface roughness on phase coefficients.
Viking Orbiter images of Mars are analyzed to define relationships between the observed contrast reversals (CR) and specific surface features. The link between CR phenomena and surface composition was first detected in contrast comparisons between UV and visible wavelength Mariner 9 data. Viking data, taken through red and violet filters, showed that the CRs occurred only with crater splotches and splotch-related streaks and in bright depositional and dark erosional streaks, both being low-albedo markings presumably caused by eolian forces. The splotch phenomena is confined mainly to the Oxia Palus region, although there are other regions where splotches and streaks commingle. Laboratory tests to mimic the CR characteristics showed that CRs are a common phenomena of different size fractions of iron oxides, e.g., goethite, where particles under 5 microns have been removed. The splotches, including dune formations, are therefore believed to indicate the presence of particles in the 100-800 microns diam range. Finer particles ride on the tops of the dust storms, and are continually removed from the surface by saltation.
Both exogenic and endogenic effects have been proposed to explain the major observed characteristics of satellite surfaces. The current view is that the basic properties of most surfaces result from the intrinsic composition of a body and its geologic history. Exogenic effects have, however, played a role in modifying the appearance of nearly all surfaces. The most important exogenic effect is impact cratering, one manifestation of which is the production of micrometeoroid gardened regoliths on airless bodies. On large, silicate bodies the micrometeoroid bombardment can produce an optically mature, dark agglutinate-rich soil; the nature of regoliths on predominantly icy satellites remains uncertain. Direct accumulation of infalling material does not appear to play a major role in modifying most surfaces. Solar wind radiation effects have not altered greatly the optical properties of solar system objects; magnetospheric charged particles may have modified the optical properties of some outer planet satellites (e.g., sulfur ion bombardment in the case of some of the satellites of Jupiter). Other effects, such as aeolian and liquid/solid chemical weathering, may be important on satellites with atmospheres like Titan and Triton.
Satellites smaller than Mimas (r = 195 km) are distinguished by irregular overall shapes and by rough limb topography. Material properties and impact cratering dominate the shaping of these objects. Long fragmentation histories can produce a variety of internal structures, but so far there is no direct evidence that any small satellite is an equilibrium ellipsoid made up of noncohesive gravitationally bound rubble. One many bodies that orbit close to their primary the tidal and rotational components of surface gravity strongly affect the directions of local g and thereby affect the redistribution of regolith by mass wasting. Downslope movement of regolith is extensive on Deimos, and is probably effective on many other small satellites. It is shown that in some cases observed patterns of downslope mass wasting cold produce useful constraints on the satellite's mean density. The diversity of features seen in the few high-resolution images of small satellites currently available suggests that these objects have undergone complex histories of cratering, fragmentation, and regolith evolution.
Voyager imges of Hyperion are analyzed with respect to spin state, dimensions and shape, surface features, photometry, and colors. It is suggested that the two most important observations concerning Hyperion are the demonstration of the satellite's unusual spin state and the discovery of the apparent low density of large craters on the satellite's surface. A possible implication of such a reduced density of craters is that the last important fragmentation of Hyperion might have occurred near the end or after the period of initial heavy bombardment.
Topographic features affect the scattering properties of planetary surfaces by casting shadows and altering the local incidence and emission angles. Measurements of this phenomenon were obtained on the Cornell goniometer for both high and low albedo surfaces. For the low albedo surface, the decrease in reflected radiation due to topography increases sharply with increasing phase angle, whereas for the high albedo sample the effects are approximately constant between phase angles of 30 and 70 deg. The observations are in good agreement with a theoretical model in the case of the dark surface. However, for the high albedo surface the model overestimates the effects by a factor of 2, since it does not include the partial illumination of shadows by multiple scattering. For both high and low albedo surfaces, the effects of topography do not become significant until a phase angle of 30-40 deg.
The extent to which the unusual spectral properties of Io and Alamathea can be accounted for by sulfur glass or at least by sulfur rich glasses was investigated. Whether or not the temperature dependence of the characteristic spectrum of pure sulfur can be used to set limits on the abundance of sulfur as a surface constituent of Io was also explored. Data from Voyager imaging observations of Saturn's small satellites, of the unusual retrograde satellite Phoebe, and of Hyperion were also reduced, analyzed, and interpreted.
The brightest, coldest areas on Io, the white regions, may act as cold traps for SO2 gas, and thus have an important role in governing the pressure, diurnal variation, and flow of the satellite's tenuous SO2 atmosphere. Therefore, it is essential to derive accurate albedos for the brightest regions, where the necessary albedos are those in the energy balance equation of the surface used to compute temperatures. Forty-one of the brightest of the white areas, each 60 to 120 km on a side were studied. The simplest way to estimate the required energy balance albedo for each region is to determine the Bond slbedo of a planet covered with that type of material. This process is outlined and resulting albedos are given. with the exception of several darker regions on the poorly-resolved post eclipse face of Io, typical albedos are 0.6 to 0.7. The brightest areas studied are located in the cluster of white regions east of Prometheus (longitudes 90 to 40 deg W). It is possible using Voyager data and fits to Hapke's equation to derive albedos for the bright regions without making any assumptions about the phase integrals.
In order to derive more accurate normal reflectances of regions on Io, the near-opposition (alpha 10 deg) photometry of a set of regions on the 20 W face of the satellite, the face with the most complete near-opposition Voyager imaging coverage was analyzed. Sixty-eight regions were chosen for study, each approximately 120 km on a side, and divided into three coherent color classes: white, orange, and polar brown regions. Derived Minneart parameters, while generally consistent with previous limb darkening determinations, show that: (1) for the white regions, k rises linearly with increasing B(0), which is the trend to be expected for normal materials; (2) the white regions are slightly limb brightened (k 0.5) at low wavelengths (low values of B(0)); and (3) k appears to drop with rising B(0) for the orange regions, contrary to the trend expected for most materials. Despite similar reflectances, there is one group of orange regions with large opposition surges and a second group showing significantly lower surges, possibly indicating differences in surface texture (porosity and/or particle size).
Global and regional patterns on Mars were inferred from surface aeolian features, such as wind streaks and dune deposits, which were visible in Viking Orbiter images. Precise measurements of the dimensions of topographic obstacles, i.e., craters, hills, ridges, on Mars as well as their associated wind streaks were used to determine the aerodynamic shape of an obstacle affects near surface airflow. A classification of Martian wind streaks was developed on the basis of albedo contrast and the presence or absence of either topographic obstacles or sediment deposits at the point of origin of the wind streaks. It was concluded that local meteorological conditions, such as the stability of the atmospheric boundary layer, play a major role in determining why some Martian craters produce depositional wind streaks while others produce erosional ones.
Global mapping and photometry of selected areas on Mars are used to investigate the nature of bright and dark wind streaks that extend from topographic obstacles. Occurrence of both bright and dark streaks is strongly latitude dependent and is only weakly correlated with surface properties such as albedo and thermal inertia. Data on the colors, albedos, and phase behavior of streaks are consistent with models of bright streaks as mosaics of plains materials and brighter, redder dust. Less than 20 percent of the ground need be covered by the optically thick dust in the brightest parts of the streaks; the amount of dust in optically thick layers could be as little as 0.001 g/sq cm. Dark streaks can be interpreted as erosional windows in a patchy dust cover. The model of dust deposition in optically thick patches is sedimentologically different from scenarios involving the deposition of ubiquitous, optically thin layers. It has the advantage that large amounts of dust can be deposited without affecting regional albedos.
The results of a comprehensive analysis of disk-integrated photometry of Io derived from Voyager images are presented. Phase curves, rotation curves, and geometric albedos consistent with earth-based observations of Io are obtained, except for a previously noted tendency for the Voyager data to yield somewhat redder colors. It is demonstrated that the phase coefficient decreases with increasing wavelength. The phase integral ranges from about 0.7 in the ultraviolet to about 0.8 in the orange. The Bond albedo is estimated at 0.50 + or -0.10, a value consistent with, though slightly below, previous earth-based estimates.
Voyager images of Saturn's satellite Iapetus ranging in phase angle from 8 to 90 deg have been used to define the satellite's photometric properties and construct an albedo map of its surface. Iapetus shows variations in reflectance across its surface of a factor of 10 to 20, the greatest albedo range known for a solar system object. It is darkest at the apex of orbital motion, becomes brighter away from the apex, and is brightest near the poles. The 'boundary' between light and dark material is gradual rather than sharp. The photometric properties of the surface are adequately described by a lunarlike photometric function, but the surface phase function varies with albedo. The dark material on Iapetus is reddish, the bright material somewhat less so.
A laboratory apparatus was used to simulate the uniform fallout and deposition of particles 1 to 5 microns in diameter in an experimental study on how the spectral and photometric properties of representative Martian areas are affected by fallout of atmospheric dust (smaller than or equalling 60 microns) suspended during dust storms. In this study, measurements are made in the changes in reflectance at optical and near-infrared wavelengths (0.4 to 1.2 micron) caused by deposition of varying amounts of a Mars-analog dust on bright and dark substrates before and after deposition of 6 x 10 to the -5th to 1.5 x 10 to the -3rd g/sq cm of simulated fallout. It is believed that only small amounts of dust particles (approximately 3 x 10 to the -4th g/sq cm) are needed to make significant albedo changes in dark areas of Mars, and that this would rule out uniform dust deposition on the surface of the planet. Data also indicate that other high albedo features like bright crater-related wind streaks may not be areas of significant sediment deposits. Laboratory simulations have permitted estimates of how much the reflectance of an area on Mars would change given a certain amount of dust fallout (g/sq cm) or reflectance data. These simulations may also be useful in tracking the transport and deposition of the dust.