Engineering topics

Greenberg, R.

Publications and source records attributed to Greenberg, R..

At least 37 records · Page 2

NASA NTRS1986

Planetary astronomy

Comets were studied during their relatively quiescent phases at large distances from the sun. This program, has resulted in a variety of CCD images of comets in several visible and near-IR bands selected to sample the continuum, rather than emission features. The most extensive dataset was obtained for P/Halley during January 1985 (plus scattered earlier data), near the time the comet turned on. The data were reduced in a preliminary fashion. Other comets predicted to be relatively inactive (including Neujmin 1 and Arend-Rigaux) were most active than expected. P/Gehrels 3 was also observed.

Chapman, C. R.↗

NASA NTRS1986

The formation and origin of the IRAS zodiacal dust bands as a consequence of single collisions between asteroids

Debris fom the single collisions between asteroids that are presently suggested as an explanation for zodiacal dust bands discovered by IRAS is distributed about the plane of the ecliptic, as particles undergo differential precession of their ascending nodes because of their semimajor axes' dispersion. On time scales of 100,000 to 1,000,000 yr, two bands are formed on each side of the ecliptic for each collision; the IRAS band pairs are probably due to collisions of aproximately 15-km diameter asteroids which occurred within the last several million yr. The model presented suggests that asteroid collisions suffice as a basis for most of the observed zodiacal emission.

Sykes, M. V.↗

NASA NTRS1986

Origin of the moon from a circumterrestrial disk

The paper reveals the possibility that the moon could have formed from the long-term evolution of a circumterrestrial disk emplaced during the earth's final stages of formation. A model is presented which emphasizes silicate enrichment (or iron depletion) of lunar material within the disk. This model depends on the late-stage planetesimal population being dominated by small bodies.

Weidenschilling, S. J.↗

NASA NTRS1985

Asteroid Collisional Evolution Studies

Understanding asteroid collisional evolution is important for characterizing the physical state of asteroids today and for learning about the processes that acted in this region of the solar system early in its history. The collisional outcome algorithm in the numerical simulation of asteroid evolution was revised to reflect pressure-strengthening. Asteroid collisions are now treated as a distribution of oblique impacts rather than as only head-on collisions. The initial and evolved size distribution of a plausible asteroid population is compared with the observed size distribution. Asteroid accretion times and reconstruction of the primordial solar nebula suggest that there was significantly more mass in this part of the solar system when the asteroids were accreting.

Davis, D. R.↗

NASA NTRS1985

A Circum-terrestrial Compositional Filter

A major question about the moon is its under abundance of iron. It is the purpose of this research to understand whether a metal-silicate fractionation of heliocentrically orbiting bodies can be achieved through collisional interactions with a circum-terrestrial swarm. Rates of diffusion are investigated and the mutual collisional destruction within the population is examined. The interactions of these differentiated planetesimals and their collisional products (both silicate mantle fragments and iron cores) with a swarm of Earth orbiting lunesimals (perhaps ejecta from the Earth) of km scale, totaling a mass of order 0.1 lunar mass, extending out 10 or 20 Earth radii are considered. It is found that such a small near Earth population of lunesimals can filter out silicate rich material, while passing iron cores, and form a moon composed partly of terrestrial material, but more substantially of the captured silicate rich portions of the planetesimals.

Chapman, C. R.↗

NASA NTRS1985

Models of Angular Momentum Input to a Circumterrestrial Swarm from Encounters with Heliocentric Planetesimals

Models of lunar origin in which the Moon accretes in orbit about the Earth from material approaching the Earth from heliocentric orbits must overcome a fundamental problem: the approach orbits of such material would be, in the simplest approximation, equally likely to be prograde or retrograde about the Earth, with the result that accretion of such material adds mass but not angular momentum to circumterrestrial satellites. Satellite orbits would then decay due to the resulting drag, ultimately impacting onto the Earth. One possibility for adding both material and angular momentum to Earth orbit is investigated: imbalance in the delivered angular momentum between pro and retrograde Earth passing orbits which arises from the three body dynamics of planetesimals approaching the Earth from heliocentric space. In order to study angular momentum delivery to circumterrestrial satellites, the near Earth velocities were numerically computed as a function of distance from the Earth for a large array of orbits systematically spanning heliocentric phase space.

Davis, D. R.↗

NASA NTRS1985

Collisional history of asteroids - Evidence from Vesta and the Hirayama families

Numerical simulations of the collisional evolution of hypothetical initial asteroid populations have been run which are subject to three constraints: they must evolve to the current asteroid size distribution, preserve Vesta's basaltic crust, and produce at least the observed number of major Hirayama families. A 'runaway growth' initial asteroid population distribution is found to best satisfy these constraints, and a model is developed for the calculation of fragment size distribution in the disruption of large, gravitationally bound bodies in which the material strength is enhanced by hydrostatic self-compression. This model predicts that large asteroids behave as intrinsically strong bodies despite histories of collisional fracture.

Davis, D. R.↗

NASA NTRS1985

Late-stage planetesimals: How big?

Numerical simulation of the early stages of planet growth show that a few bodies nearly 1000 km in diameter may have formed within approx. 100,000 yr after solid material grew into km scale planetesimals by gravitational instability. Even after such large bodies formed, the bulk of the mass of the future terrestrial planet zone resided in small bodies. Subsequent evolution is difficult to model because it requires simultaneous consideration of continuum (multitudinous small bodies) and discrete (a few large bodies) evolution. Some relevant issues include definition of accretional feeding zones, evaluation of the range of gravitational influence, viscous transport and diffusion, orbital commensurabilities, role of gas, etc. The first large bodies may have been (1) the embryos of the final planets, which grew by accreting tiny planetesimals, or (2) merely the first of many 1000+ km bodies, which grew independently and later collided to form the planets. Models of late stage accretion that assume all bodies to be initially nearly Moon sized provide insight into relevant collisional and dynamical processes. The chief point in this research is that the correct size distribution during the later stages of planet growth remains unknown.

Greenberg, R.↗

NASA NTRS1985

Questions about Mercury's role in comparative planetary geophysics

Problems which have arisen in formulating a mutually consistent picture of Mercury's evolution are outlined. It appears that one or more of the following widely adopted assumptions are wrong about Mercury: (1) its original composition at least approximately resulted from equilibrium condensation; (2) its magnetic field arises from a still-active dynamo; (3) its thermal evolution should have yielded early core formation followed by cooling and a global contraction approaching 20 km in the planet's radius; (4) Mercury's surface is basaltic and the intercrater plains are of volcanic origin. It is suggested that Mercury's role in comparative planetology be reevaluated in the context of an alternative timescale based on the possibility that Mercury was subjected to a continuing source of cratering projectiles over recent aeons, which have not impacted the other terrestrial planets. Although such vulcanoids have not yet been discovered, the evolution of Mercury's orbit due to secular perturbations could well have led to a prolonged period of sweeping out any intra-Mercurian planetesimals that were originally present. Mercury's surface could be younger than previously believed, which explains why Mercury's core is still molten.

Chapman, C. R.↗

NASA NTRS1984

Satellite Masses in the Uranus and Neptune Systems

Satellite masses are derivation with emphasis on implications for bulk densities and albedos is reviewed. In the Uranian system the inner satellites have lower densities and/or higher albedos than the outer ones. However, uncertainties are great enough that all five satellites may have nearly equal densities. In such a case the albedo would decrease with semimajor axis. A more severe constraint is placed on Miranda's mass, and hence on its density and albedo. The recent radiometric value for Triton's diameter, combined with mass determinations, yields a density greater than 4 gm/cu cm.

Greenberg, R.↗

NASA NTRS1984

From icy planetesimals to outer planets and comets

The growth of Neptune and Uranus and the origin of the Oort comet cloud are simulated numerically, applying the planetesimal-growth model developed by Greenberg et al. (1978) for the terrestrial planets. The results of 12 experiments are presented in graphs and tables and discussed in detail. In the model which best fits observations, the planets grow relatively quickly from icy planetesimals of diameter 10 km or less which form by gravitational clumping of grains as they settle nonhomologously to the midplane of the solar system; the comets then represent unaltered remnants of the original planetesimal populations in the zone.

Greenberg, R.↗

NASA NTRS1984

Saturn ring particles as dynamic ephemeral bodies

Although Saturn's rings are within the Roche zone, the accretion of centimeter-sized particles into large aggregates many meters in diameter occurs readily, on a time scale of weeks. These aggregates are disrupted when tidal stresses exceed their very low strengths; thus most of the mass of the ring system is continually processed through a population of large 'dynamic ephemeral bodies', which are continually forming and disintegrating. These large aggregates are not at all like the idealized ice spheres often used in modeling Saturn's ring dynamics. Their coefficient of restitution is low, hence they form a monolayer in the ring plane. The optically observable characteristics of the rings are dominated by the swarm of centimeter-sized particles.

Davis, D. R.↗

NASA NTRS1984

How fast do Galilean satellites spin?

Each of the Galilean satellites, as well as most other satellites whose initial rotations have been substantialy altered by tidal dissipation, has been widely assumed to rotate synchronously with its orbital mean motion. Such rotation would require a small permanent asymmetry in the mass distribution in order to overcome the small mean tidal torque. Since Io and Europa may be substantially fluid, they may not have the strength to support the required permanent asymmetry. Thus, each may rotate at the unknown but slightly nonsynchronous rate that corresponds to zero mean tidal torque. This behavior may be observable by Galileo spacecraft imaging. It may help explain the longitudinal variation of volcanism on Io and the cracking of Europa's crust.

Greenberg, R.↗

NASA NTRS1984

Asteroids and meteorites - Origin of stony-iron meteorites at mantle-core boundaries

Stony-iron meteorites formed at the core/mantle interfaces of small asteroidal parents. The mesosiderites formed when the thick crust of a largely molten parent body (100-200 km in diameter) foundered and sank through the mantle to the core. Pallasites formed in smaller parent bodies (50-100 km) in which olivine crystals from the partially molten mantle sank to the core/mantle interface and rafted there. Subsequent collisions stripped away the rocky mantles of both kinds of parent bodies, exposing the stony-iron surfaces of their cores to direct impacts, which continue to knock off meteorite fragments.

Greenberg, R.↗

NASA NTRS1984

An integrated dynamical and geochemical approach to lunar origin modelling

The three major categories of models of lunar origin which explain the Moon's properties are complete and a more general scenario is presented. The model presented is as the Earth grew by planetesimal bombardment, a circumterrestrial cloud of particles was created from a combination of impact ejected mantle material and planetismals captured directly into orbit around the Earth. The compositional properties are explained two ways: (1) a few big late planetismals of diverse composition are captured in orbit and/or hit the Earth; and (2) the circumterrestrial swarm acts as a filter, preferentially capturing small weak silicate bodies, while passing large iron planetismals.

Greenberg, R.↗

NASA NTRS1984

A circumterrestrial compositional filter

A major question about the Moon is its under abundance of iron. The proposal of whether a metal-silicate fractionation of heliocentric bodies could be achieved through collisional interactions with a circumterrestrial swarm is examined. The whole scenario works only if there is a way to maintain the hypothesized circumterrestrial swarm, which otherwise would collisionally diffuse on a time scale of 100 yr (much of it collapsing on the Earth); the source of angular momentum to maintain the swarm remains a mystery.

Chapman, C. R.↗

NASA NTRS1984

Orbital resonances among Saturn's satellites

When orbital properties constituted most of our knowledge of Saturn's satellites, resonances provided important clues to physical properties and history. With the discoveries of the 1980s, including close-up direct observations of physical and geological properties, resonances are more important than ever. This chapter reviews the kinetics of the various classes of resonances represented among the classical and newly discovered satellites and describes how resonances have played a fundamental role in shaping the Saturn system as it is seen today.

Greenberg, R.↗

NASA NTRS1984

Planetary rings - A dynamic, evolving subject

Attention is drawn to current attempts to present the results of theoretical dynamical studies of the planetary rings, and to propagate observational results on ring structure and physical properties that are relevant to dynamical interpretations. Like earlier, optical models, dynamical models tend to assume simplified, single sized, smooth, and round, ring-constituting particles; a tendency is noted which confuses particles dominating optical properties with those that dominate dynamics.

Greenberg, R.↗