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  • LUNAR AND PLANETARY EXPLORATION  (27)
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  • 1
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    Formation of planetesimals in the solar nebula (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The evolution of solid particles in the solar nebula (or other circumstellar disk) is described. Motions of bodies less than about 1 km in size were dominated by gas drag rather than gravity. An original population of microscopic grains had to produce greater than km-sized planetesimals before gravitational accretion of planets could begin. Planetesimals probably formed by coagulation of grain aggregates that collided due to differential settling, turbulence, and drag-induced orbital decay. Growth of such aggregates depended on sticking mechanisms and their mechanical properties, which are poorly understood. Their growth was aided by concentration of larger bodies toward the central plane of the disk. The nebula could remain optically thick during this process. It is unlikely that a particle layer formed by settling would undergo gravitational instability, as a small amount of turbulence would keep the particle layer from reaching the critical density. This conclusion is independent of the particle size, as even large bodies do not effectively decouple from the gas. Even in a laminar disk, shear in the particle layer would generate enough turbulence to keep it stirred up.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: Protostars and planets III (A93-42937 17-90); p. 1031-1060.
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  • 2
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    Capture of Planetesimals into a Circumterrestrial Swarm (1985)
    In:  CASI
    Details
    Publication Date: 2014-10-08
    Description: The lunar origin model considered in this report involves processing of protolunar material through a circumterrestrial swarm of particles. Once such a swarm has formed, it can gain mass by capturing infalling planetesimals and ejecta from giant impacts on the Earth, although the angular momentum supply from these sources remains a problem. The first stage of formation of a geocentric swarm by capture of planetesimals from initially heliocentric orbits is examined. The only plausible capture mechanism that is not dependent on very low approach velocities is the mutual collision of planetesimals passing within Earth's sphere of influence. The dissipation of energy in inelastic collisions or accretion events changes the value of the Jacobi parameter, allowing capture into bound geocentric orbits. This capture scenario was tested directly by many body numerical integration of planetesimal orbits in near Earth space.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 132-133
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  • 3
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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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  • 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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    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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  • 6
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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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  • 7
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    Evolution of Grains in a Turbulent Solar Nebula (1985)
    In:  CASI
    Details
    Publication Date: 2014-10-08
    Description: Planetesimal formation by gravitational instability of a dust layer requires a nonturbulent solar nebula. The assumption that the solar nebula is a turbulent, convective accretion disk is explored. Self consistent models of a convective disk depend on high opacity of the disk material, which must be provided by grains. Grain coagulation is implied by the requirement of forming planetesimals, as the turbulent velocities (approx. 1/3 sound speed) in the accretion disk would disrupt any dust layer. Collisional coagulation could form large planetesimals, simultaneously leaving a sufficient fraction of matter in small grains to maintain the nebula's opacity. Or, coagulation of grains into small (approx. cm) aggregates could lower the opacity enough for turbulence to decay. The evolution of a population of grains in a turbulent solar nebula is calculated numerically.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 140-142
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  • 8
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    Capture of planetesimals into a circumterrestrial swarm (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-01-25
    Description: The lunar origin model considered involves processing of protolunar material through a circumterrestrial swarm of particles. Once such a swarm has formed, it can gain mass by capturing infalling planetesimals and ejecta from giant impacts on the Earth, although the angular momentum supply from these sources remains a problem. Examined is the first stage of formation of a geocentric swarm by capture of planetesimals from initialy heliocentric orbits. The only plausible capture mechanism that is not dependent on very low approach velocities is the mutual collision of planetesimals passing within Earth's sphere of influence. This capture scenario was tested directly by many body numerical integration of planetesimal orbits in near Earth space. Results agree that the systematic contribution of angular momentum is insufficient to maintain an orbiting swarm under heavy bombardment. Thus, a circumterrestrial swarm can be formed rather easily, but is hard to sustain because the mean net angular momentum of a many body swarm is small.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Planetary Inst. Conf. on the Origin of the Moon; p 54
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  • 9
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    Late-stage planetesimals: How big? (1985)
    In:  Other Sources
    Details
    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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  • 10
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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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