Engineering topics
Publications and source records attributed to Chapman, C. R..
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Attention is given to the properties of, and dynamical processes affecting individual particles of Saturn's rings. Because particles tend to be gravitationally bound when located on the surfaces of larger particles, and since net tidal stresses within the particles are small, particle collisions should produce accretion in Saturn's rings. Rapid accretionary processes within the rings are counterbalanced by tidal disruption of the larger accreted aggregates, which are presently designated 'dynamic ephemeral bodies'. The coefficient of restitution is probably very low, implying that the large particles containing most of the rings' mass are in a monolayer, although the small particles responsible for most of the rings' visible cross section form a layer many particles thick. Kinematic viscosity and interparticle erosive process models should incorporate these properties.
Asteroid physical strengths, which critically affect the quantity of ejecta that can be placed in orbits evolving so as to cross that of the earth, vary widely due to initial composition and size, and subsequent geophysical and collisional evolutions. The meteorite yield on earth additionally depends on meteorite strength, which affects longevity in space and survival through the atmosphere. It is presently shown that meteorites may be primarily derived by cratering rather than disruptive fragmentation, and from large, main-belt asteroids rather than from small, earth-approaching bodies. The model presented combines a variety of evidence from various disciplines to yield results that are consistent with meteorite statistics, although no claim is made for the uniqueness of the model and many of its elements remain uncertain.
The small solar system bodies broadly defined as asteroids have been considered the parent bodies for most or all meteorites. This association requires that the compositional differences between the various classes of meteorites be convincingly related to the observed compositions of asteroids. An investigation has been conducted regarding the compositional relationship between S-type asteroids and the common types of differentiated and undifferentiated meteorites. It is found that spectroscopic data for S-type asteroids are consistent with a simple, undifferentiated model and with certain variations of a differentiated model. However, the differentiated models for S-type asteroids are not supported by meteoritic evidence. It appears, therefore, that most S-type asteroids are undifferentiated bodies, similar to ordinary chondrites in composition.
The fundamental geological and geophysical properties of asteroids were studied by theoretical and simulation studies of their collisional evolution. Numerical simulations incorporating realistic physical models were developed to study the collisional evolution of hypothetical asteroid populations over the age of the solar system. Ideas and models are constrained by the observed distributions of sizes, shapes, and spin rates in the asteroid belt, by properties of Hirayama families, and by experimental studies of cratering and collisional phenomena. It is suggested that many asteroids are gravitationally-bound "rubble piles.' Those that rotate rapidly may have nonspherical quasi-equilibrium shapes, such as ellipsoids or binaries. Through comparison of models with astronomical data, physical properties of these asteroids (including bulk density) are determined, and physical processes that have operated in the solar system in primordial and subsequent epochs are studied.
Knowledge of the compositions of the asteroids is obtained by the remote sensing of reflected and emitted radiation from what are essentially star-like points of light. Since asteroids are a remnant population of planetesimals that were never accreted into the larger planets, their compositions and properties can provide insight into the nature of planetary matter in early epochs, before most of it was physically and chemically modified by geological processes within the planets. The progress made during the past decade in learning about asteroids through remote sensing is reviewed.
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The question of whether the crater population on Phobos represents a production population or an equilibrium population is considered. The absolute ages of cratered surfaces are interpreted and analyzed. A computer program was developed to study the dynamics of material ejected from Martian satellites and to investigate the hypothesis that at least some of the extensive set of linear features discovered on the surface of Phobos could be the result of secondary cratering from the Stickney impact. The possibility that Deimos was catastrophically disrupted by a large impact but subsequently reaccreted is considered as well as the probability the Phobos had an impact nearly large enough to disrupt it are also discussed.