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  • 1
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    Satellite-sized planetesimals and lunar origin (1975)
    In:  Other Sources
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    Publication Date: 2011-08-16
    Description: Exploratory calculations using accretionary theory are made to demonstrate plausible sizes of second-largest, third-largest, etc., bodies at the close of planet formation in heliocentric orbits near the planets, assuming asteroid-like size distributions at the start of the calculation. Many satellite-sized bodies are found to be available for capture, cratering, or collisional fragmentation. In the case of earth-sized planets, the models suggest second-largest bodies of 500 to 3000 km radius, and tens of bodies larger than 100 km radius. Many of these interact with the planet before suffering any fragmentation events with each other. Collision of a large body with earth could eject iron-deficient crust and upper mantle material, forming a cloud of refractory, volatile-poor dust that could form the moon. Other satellite systems may have been affected by major capture or collision events of chance character.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 2
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    Orbital Resonances and Planetary Accretion in the Solar Nebula (1985)
    In:  CASI
    Details
    Publication Date: 2014-10-08
    Description: Planetesimal orbital evolution in a resisting medium near an accreting protoplanet was studied to explore mechanisms for capture into Trojan and satellite orbits. Various mechanisms for capture into libration were proposed, e.g., increase in Jupiter/Sun mass ratio, change in Jovian orbital radius, and collisions of asteroids with interplanetary dust. Studies include effects of solar nebula gas drag on orbital evolution. In general, the gas deviates from Keplerian motion, causing secular decay of planetesimal orbits, as well as damping eccentricity. The motion of bodies near Jupiter under the effect of a resisting medium was numerically explored. The equations of motion were integrated using the formalism of the planar restricted three body problem, modified to include effects by gas drag and a growing Jupiter.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 134-136
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  • 3
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    Models of Angular Momentum Input to a Circumterrestrial Swarm from Encounters with Heliocentric Planetesimals (1985)
    In:  CASI
    Details
    Publication Date: 2014-10-08
    Description: 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.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 129-131
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  • 4
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    Asteroid Collisional Evolution Studies (1985)
    In:  CASI
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    Publication Date: 2014-09-30
    Description: 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.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 77-79
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  • 5
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    Catastrophic disruption of asteriods and satellites; Proceedings of the International Workshop, Pisa, Italy, July 30-August 2, 1985 (1986)
    In:  Other Sources
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    Publication Date: 2016-03-09
    Description: Theoretical, numerical, and experimental investigations of the violent disruption of asteroids or planetary satellites are discussed in reviews and reports. Topics examined include acceleration techniques and results of experiments simulating catastrophic fragmentation events; laboratory simulations of catastrophic impact; scaling laws for the catastrophic collisions of asteroids; asteroid collisional history, the origin of the Hirayama families, and disruption of small satellites; and the implications of the inferred compositions of a steroids for their collisional evolution. Diagrams, graphs, tables, and a summary of the discussion at the workshop are provided.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: (ISSN 0037-8720)
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  • 6
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    An integrated dynamical and geochemical approach to lunar origin modelling (1984)
    In:  Other Sources
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    Publication Date: 2019-01-25
    Description: 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.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Planetary Inst. Conf. on the Origin of the Moon; p 51
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  • 7
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    Progress on Experimental Impact Studies (1985)
    In:  CASI
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    Publication Date: 2014-10-08
    Description: Experimental impact studies were undertaken. Ejecta velocity distributions and related results from regolith targets were studied. The velocity distributions, energy partitioning, and related properties from 14 impacts at speeds 5 to 2321 m/s in vacuo into regolith-like fine powders were derived. Data are analyzed on: velocity and mass distributions as a function of azimuth around the crater during oblique impacts catastrophic disruption of spherical targets of variable strength, and catastrophic disruption of irregular-shaped natural rock targets and artificial granular aggregate targets.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: in NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 170
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  • 8
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    Late-stage planetesimals: How big? (1985)
    In:  Other Sources
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    Publication Date: 2019-01-25
    Description: 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.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst. Terrest. Planets: Comp. Planetology; p 4
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  • 9
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    Questions about Mercury's role in comparative planetary geophysics (1985)
    In:  Other Sources
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    Publication Date: 2019-01-25
    Description: 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.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst. Terrest Planets: Comp. Planetology; p 6
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  • 10
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    Origin of the moon from a circumterrestrial disk (1986)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: 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.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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