Engineering topics

Veverka, J.

Publications and source records attributed to Veverka, J..

At least 109 records · Page 6

NASA NTRS1979

Grooves on asteroids - A prediction

The characteristics of the grooves on Phobos indicate that these features were formed by a large impact, an observation which implies that grooves may occur on other small bodies. Published data on fragmentation experiments suggest that the energy of the impact which produced the grooves on Phobos was in a critical range, strong enough to produce fracturing at large distances from the crater, but too weak to fragment the object. Other small bodies, such as asteroids, which have experienced an impact flux sufficient to include one such critical cratering event should show grooves, unless they have been fragmented subsequently by a more severe impact. It is estimated that between 1/12th and 1/4th of asteroids smaller than 100 km in diameter may have surface features resembling grooves associated with large impact craters. Several morphologically distinct types of grooves can be expected, depending on the thickness and composition of the regolith of the parent body.

Thomas, P.↗

NASA NTRS1979

Photometric evidence on long-term stability of albedo and colour markings on Io

Photometric measurements of the color and albedo of Io during the last 50 years are examined in order to determine the stability of the Ionian surface. Brightness measurements of Io relative to Ganymede show a slight, however inconclusive, increase with time, while light curve amplitudes and shapes are found to agree well during this period. The extent of volcanic activity observed by Voyager 1, however, is shown to be undetectable with the precision attainable by earth-based albedo determinations, and just barely detectable as color changes, which explains the observed photometric stability. Possible mechanisms for the preservation of the observed longitudinal distribution of color on Io are also discussed.

Morrison, D.↗

NASA NTRS1979

Frost streaks in the south polar cap of Mars

Viking Orbiter images of the annual south polar cap on Mars exhibit elongated bright features that are associated with craters and resemble wind streaks observed elsewhere on Mars. The study focuses on the well-documented frost streaks. The discussion covers the morphology of frost streaks, occurrence, seasonal behavior, thickness of frost in streak deposits, wind patterns inferred from frost streaks and other eolian features in the south polar region, formation of frost streaks, and other locales of preferential frost accumulation. The form and seasonal behavior of the bright elongated albedo markings which extend from the rims of many craters in the south polar cap suggest that they are accumulations of CO2 frost in the lee of craters. The frost streaks appear in the fall, increasing in length but not changing in direction during fall and winter. The frost streaks indicate a prograde circulation pattern of near-surface winds around the pole. Other details are also presented.

Thomas, P.↗

NASA NTRS1979

Spiral clouds on Mars - A new atmospheric phenomenon

Viking Orbiter images obtained during the 1978 northern summer on Mars show unusual spiral cloud patterns at high northern latitudes. Well-developed spirals occurred only during early summer at a time when the sublimation of the annual CO2 frost cap in the north had ceased. The systems ranged in size from 200 to 500 km and all spirals were wound in a counter-clockwise sense. An explanation is proposed, based on the radiatively driven instability described by Gierasch, Ingersoll, and Williams. The model predicts disturbances with the correct scale and lifetime. However, the successful development of the spirals depends on the existence of weak mean winds, suggesting that these instabilities can only develop at special null points in the general circulation. It is argued that such special circumstances are most likely to occur during early northern summer and are less likely to obtain at a comparable season in the Southern Hemisphere.

Gierasch, P.↗

NASA NTRS1979

Marsshine on Phobos

The Viking Orbiters have obtained several images of Phobos at large phase angles in which the portion of the satellite not directly illuminated by the sun is faintly visible. A photometric analysis of one such image is presented to prove that the phenomenon is real and can be explained by Marsshine (i.e., the illumination of Phobos by sunlight reflected from Mars). Such images provide cross sections of Phobos and are useful in determining the true shape and size of the satellite. The cross section observed in Picture 111A03 agrees closely with that predicted by triaxial ellipsoid model of Phobos developed by Duxbury (1974).

Goguen, J.↗

NASA NTRS1979

On matching the spectrum of Io - Variations in the photometric properties of sulfur-containing mixtures

The problem of comparing laboratory spectra of sulfur-containing binary mixtures with the spectrum of Io is discussed. For the satellite, the observable is the geometric albedo as a function of wavelength, whereas in the laboratory one often measures some other type of albedo. A previous paper demonstrated that for pure sulfur the multiplicative factor which converts the laboratory albedos to geometric albedos can be strongly wavelength dependent. The present paper demonstrates that this is also true for binary sulfur-containing mixtures. Furthermore, there is no universal conversion factor applicable to all binary mixtures, nor can the factor be interpolated for a particular mixture from the conversion factors of the two end members. The conversion factor is a function not only of the specific composition of a binary mixture but also of the relative particle size distributions of the two components, and must be measured specifically for each individual sample if a quantitative comparison between a laboratory sample and Io's surface is desired.

Veverka, J.↗

NASA NTRS1979

Phobos and Deimos - A preview of what asteroids are like

The surfaces of Phobos and Deimos are discussed, as the best available examples of what asteroid surfaces may be like. Attention is given to shape, regolith properties, crater densities, albedo markings and surface gravities. It is found that although the surfaces of these two similarly-sized asteroid-like bodies are nearly identical in terms of many disk-integrated properties, they are strikingly different in surface morphology.

Veverka, J.↗

NASA NTRS1978

Phobos - Photometry and origin of dark markings on crater floors

Viking Orbiter 1 close encounter pictures of Phobos reveal unusual dark patches on the floors of many craters. Photometry of these features indicates that they have a similar normal reflectance but a phase coefficient 30% larger than the average surface. These facts suggest that the 'dark' material has a similar composition but a much rougher texture than the average surface of Phobos. By analogy with terrestrial impact, explosion and laboratory craters, the dark patches are interpreted as impact generated melt pools which remain visible on Phobos due to the low value of g and consequent small amount of fallback ejecta and slumping. Since blanketing by ejecta from subsequent impacts and micrometeoroid erosion should eventually obliterate such features, they should be more conspicuous in the fresher craters. Similar features should be visible on small asteroids and other low g bodies. However, they are not prominent on the surface of Deimos - possibly because most of the craters imaged at high resolution on the outer satellite appear to have been blanketed by several meters of fine-grained material.

Goguen, J.↗

NASA NTRS1978

On the nature and visibility of crater-associated streaks on Mars

The paper considers Mariner 9 and Viking data that contradict Kuzmin's (1975) hypothesis that all crater-associated wind streaks on Mars are depositional and consist of unresolved barchan-like dunes. According to Kuzmin's hypothesis, any streak can appear either bright or dark relative to its surroundings depending on the sun's position. The spacecraft images, however, show examples of dark and light streaks visible at the same azimuth angle of the sun. Evidence that bright and dark streaks differ both in morphology and in character is considered. It is suggested that the common ragged dark streaks are probably erosion scars while most bright streaks probably represent accumulations of bright dust-storm fallout.

Veverka, J.↗

NASA NTRS1978

The puzzling moons of Mars

Viking Orbiter observations of Phobos and Deimos are presented with attention to physical dimension and surface features. Both satellites are approximately 1.4 times as long as they are wide; Phobos having a length of 27 km, while Deimos is half that size. In addition, both satellites are tidally locked, as is earth's moon, and are held together by cohesive forces as well as by gravity. Phobos and Deimos are both heavily cratered, and Phobos has deep (30 m) grooves in the vicinity of its largest crater, Stickney; indicating surface fracturing under meteorite impact. The craters on Deimos are largely filled with a fine-grained substance, probably crater ejecta.

Veverka, J.↗

NASA NTRS1978

Deimos encounter by Viking - Preliminary imaging results

Recent close flybys of Deimos by Viking revealed a smooth-appearing surface void of grooves. Higher-resolution pictures showed that the surface was actually covered with craters but that a regolith filled the smaller craters, giving the smooth appearance. The surface was also covered with boulders and bright streak-like markings analogous to base-surge or ejecta cloud deposits.

Duxbury, T. C.↗

NASA NTRS1978

Lunar occultation of Saturn. II - The normal reflectance of Rhea, Titan, and Iapetus

An inversion procedure to obtain the reflectance of the central region of a satellite's disk from lunar occultation data is presented. The scheme assumes that the limb darkening of the satellite depends only on the radial distance from the center of the disk. Given this assumption, normal reflectances can be derived that are essentially independent of the limb darkening and the diameter of the satellite. The procedure has been applied to our observations of the March 1974 lunar occultation of Tethys, Dione, Rhea, Titan, and Iapetus. In the V passband we derive the following normal reflectances: Rhea (0.97 plus or minus 0.20), Titan (0.24 plus or minus 0.03), Iapetus, bright face (0.60 plus or minus 0.14). For Tethys and Dione the values derived have large uncertainties, but are consistent with our result for Rhea.

Elliot, J. L.↗

NASA NTRS1978

Phobos: Photometry and origin of dark markings on crater floors

High resolution photographs of Phobos taken during close flybys of Viking Orbiter 1 reveal many dark patches on the floors of several craters. The apparently dark material is only prominent at large phase angles. Analysis of the photometric properties indicates that the dark patches represent areas of unusually rough textures whose reflectance near zero phase is similar to that of the mean surface (approximately 6 percent in the visible), but whose phase curve is much steeper. The contrast of such areas is less than 10 percent zero phase but approaches 100 percent near phase angles of 90 degrees. It is proposed that these intricately textured deposits represent patches of vesticular impact melt.

Goguen, J.↗

NASA NTRS1978

Origin of the grooves on Phobos

The age, surface density, and association with the 10-km crater Stickney of the grooves (long linear depression) on Phobos are investigated, and the results support the cratering hypothesis of groove formation and seem to rule out a tidal mechanism. The appearance of the grooves is described. The age of the grooves is estimated from the density of superimposed impact craters; the density of impact craters within the grooves is compared with the average value for all of Phobos. The old age of the grooves as well as their evident association with the crater Stickney are considered to be inconsistent with the tidal hypothesis.

Thomas, P.↗

NASA NTRS1978

Scattering of light from particulate surfaces. I - A laboratory assessment of multiple-scattering effects

In planetary applications it is sometimes important to know the degree to which multiple scattering is influencing the photometric properties of a surface. An investigation is conducted concerning the point at which the single-scattering approximation used in the theory developed by Hapke (1963) and Irvine (1966) breaks down. It is shown that the Hapke-Irvine photometric function provides an adequate representation of the scattering properties of particulate surfaces whose geometric albedos (or normal reflectances) do not exceed 0.3. An experiment is conducted to estimate the normal reflectance below which the Hapke-Irvine equation considered provides an adequate representation of the scattering properties of a particulate surface. The investigation makes use of a set of samples which were prepared by mixing magnesium oxide and charcoal powders. The samples have a range in normal reflectance from 0.04 to 1.04.

Veverka, J.↗

NASA NTRS1978

How to compare the surface of Io to laboratory samples

Since one does not know the photometric functions of various parts of Io, one cannot convert the observed geometric albedo of the satellite to a parameter more directly measurable in the laboratory. One must therefore convert laboratory reflectances to geometric albedos before quantitative comparisons between Io's surface and a laboratory sample are made. This procedure involves determining the wavelength dependence of the sample's photometric function. For substances such as sulfur, whose reflectance varies strongly with wavelength, it is incorrect to assume that the photometric function, and hence the ratio (laboratory reflectance/geometric albedo) is independent of wavelength. To illustrate this point, measurements of the color dependence of this ratio for sulfur are presented for the specific case in which the measured laboratory reflectance is the sample's normal reflectance. In general, unless the laboratory reflectance is precisely the geometric albedo, a wavelength-dependent correction factor must be determined before the laboratory sample can be compared quantitatively with Io's surface.

Veverka, J.↗

NASA NTRS1978

A statistical study of crater-associated wind streaks in the north equatorial zone of Mars

The physical characteristics of all crater-related streaks within the north equatorial zone of Mars (between 0 and 30 deg N) are studied. The survey includes 25% of the Martian surface area and covers the latitude band of major interest to the Viking missions. Nine characteristic parameters are measured for each of 2325 streaks, of which 82.3% are classified as light tone, 8.1% as dark tone, and 9.6% as mixed tone. The location of streaks in the investigated zone is considered together with the indicated wind regimes, and the characteristics of the streaks and of their associated craters are discussed. The results show that: (1) the streaks are not uniformly distributed throughout the north equatorial zone but are strongly concentrated in certain areas; (2) the density of streaks displays a significant latitude variation not apparently related to terrain type between 0 and 30 deg N; (3) the percentage of craters with streaks reaches a maximum between 10 and 15 deg N and falls off significantly toward both the equator and higher latitudes; and (4) the density of streaks correlates well with geological terrain type.

Veverka, J.↗

NASA NTRS1978

The Martian satellites - 100 years on

Studies of Phobos and Deimos, the two satellites of Mars, may provide the best observations yet of the material populating the asteroid belt. Viking observations of Phobos showing crater chains and striations are analyzed, and Mariner-9 photometry exhibiting the albedo of the satellite is discussed. A mass determination for Phobos results in a value of 1.1 times 10 to the 19th power g; the mean density of the satellite is set at about 2 g per cu cm, a value suggesting formation of the satellite from material in the asteroid belt.

Hunt, G. E.↗