re: Moon formation

From: Amara Graps (amara@amara.com)
Date: Wed Feb 12 2003 - 01:37:59 MST

  • Next message: Max M: "http://www.futureport.dk/news - 2003-02-12 (21 articles)"

    EvMick@aol.com:
    >I'd like to know more about this...where might I look?

    Current work is by Canup and Ward and Agnor and Levison and Cameron and others.

    Here is one:

    http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999AJ....117..603C&db_key=AST&high=3e3fa1a9f903461r

      Title: Evolution of a Terrestrial Multiple-Moon System
      Authors: Canup, Robin M.; Levison, Harold F.; Stewart, Glen R.
      Affiliation: AA(Southwest Research Institute, 1050 Walnut Street,
                          Suite 426, Boulder, CO 80302), AB(Southwest Research
                          Institute, 1050 Walnut Street, Suite 426, Boulder, CO
                          80302), AC(Laboratory for Atmospheric and Space
                          Physics, University of Colorado, Campus Box 392,
                          Boulder, CO 80309-0392)
      Journal: The Astronomical Journal, Volume 117, Issue 1, pp.
                          603-620. (AJ Homepage)
      Publication Date: 01/1999
      Origin: AJ
      AJ Keywords: MOON, PLANETS AND SATELLITES: GENERAL, SOLAR SYSTEM:
                          FORMATION
      Abstract Copyright: (c) 1999: The American Astronomical Society
      Bibliographic Code: 1999AJ....117..603C

                                       Abstract

    The currently favored theory of lunar origin is the giant-impact hypothesis.
    Recent work that has modeled accretional growth in impact-generated disks
    has found that systems with one or two large moons and external debris are
    common outcomes. In this paper we investigate the evolution of terrestrial
    multiple-moon systems as they evolve due to mutual interactions (including
    mean motion resonances) and tidal interaction with Earth, using both
    analytical techniques and numerical integrations. We find that multiple-moon
    configurations that form from impact-generated disks are typically unstable:
    these systems will likely evolve into a single-moon state as the moons
    mutually collide or as the inner moonlet crashes into Earth.

    ===============

    another

    http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999Icar..142..219A&db_key=AST&high=3e3fa1a9f903874

      Title: On the Character and Consequences of Large Impacts in
                          the Late Stage of Terrestrial Planet Formation
      Authors: Agnor, Craig B.; Canup, Robin M.; Levison, Harold F.
      Affiliation: AA(Space Studies Department, Southwest Research
                          Institute, Boulder, Colorado), AB(Space Studies
                          Department, Southwest Research Institute, Boulder,
                          Colorado), AC(Space Studies Department, Southwest
                          Research Institute, Boulder, Colorado)
      Journal: Icarus, Volume 142, Issue Icarus, pp. 219-237. (Icarus
                          Homepage)
      Publication Date: 11/1999
      Origin: ICAR
      Abstract Copyright: (c) 1999: Academic Press
      Bibliographic Code: 1999Icar..142..219A

                                       Abstract

    We perform three-dimensional N-body integrations of the final stages of
    terrestrial planet formation. We report the results of 10 simulations
    beginning with 22-50 initial planetary embryos spanning the range 0.5-1.5
    AU, each with an initial mass of 0.04-0.13M⊕. Collisions are treated
    as inelastic mergers. We follow the evolution of each system for 2x108 years
    at which time a few terrestrial type planets remain. On average, our
    simulations produced two planets larger than 0.5M⊕ in the terrestrial
    region (1 simulation with one m>=0.5M⊕ planet, 8 simulations with two
    m>=0.5M⊕ planets, and 1 simulation with three m>=0.5M⊕ planets).
    These Earth-like planets have eccentricities and orbital spacing
    considerably larger than the terrestrial planets of comparable mass (e.g.,
    Earth and Venus). We also examine the angular momentum contributions of each
    collision to the final spin angular momentum of a planet, with an emphasis
    on the type of impact which is believed to have triggered the formation of
    the Earth's Moon. There was an average of two impacts per simulation that
    contributed more angular momentum to a planet than is currently present in
    the Earth/Moon system. We determine the spin angular momentum states of the
    growing planets by summing the contributions from each collisional
    encounter. Our results show that the spin angular momentum states of the
    final planets are generally the result of contributions made by the last few
    large impacts. Our results suggest that the current angular momentum of the
    Earth/Moon system may be the result of more than one large impact rather
    than a single impact. Further, upon suffering their first collision, the
    planetary embryos in our simulations are spinning rapidly throughout the
    final accretion of the planets, suggesting the proto-Earth may have been
    rotating rapidly prior to the Moon-forming impact event.

    ===================

    another
    http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1996Icar..119..427C&db_key=AST&high=3e3fa1a9f904013

      Title: Accretion of the Moon from an Impact-Generated Disk
      Authors: Canup, Robin M.; Esposito, Larry W.
      Affiliation: AA(Laboratory for Atmospheric and Space Physics,
                          University of Colorado, Boulder, Colorado),
                          AB(Laboratory for Atmospheric and Space Physics,
                          University of Colorado, Boulder, Colorado)
      Journal: Icarus, Volume 119, Issue 2, pp. 427-446. (Icarus
                          Homepage)
      Publication Date: 02/1996
      Origin: ICAR
      Abstract Copyright: (c) 1996: Academic Press
      Bibliographic Code: 1996Icar..119..427C

                                       Abstract

    We present the first published numerical calculations of accretion of an
    impact-generated protolunar disk into a single large Moon. Our calculations
    are based on the model developed by R. M. Canup and L. W. Esposito (Icarus
    113, 331-352, 1995) to describe accretion in the Roche zones around the
    giant planets. Previous numerical simulations of a large impact event
    predict the formation of a disk of material centered near or within the
    Roche limit (~2.9R_⊕). A natural expectation based on our previous
    results and comparison with the satellite systems of the outer planets would
    be for multiple small moons to arise from such a protolunar disk. Multiple
    moonlets could accrete to form a single Moon if they evolved into crossing
    orbits due to tidal interaction with the Earth. This would occur if the
    innermost moonlet in the disk were also the most massive, so that it evolved
    outward at the relatively fastest rate and swept up all exterior material.
    Our calculations, which include both moonlet accretion and orbital
    evolution, demonstrate that forming massive moonlets in the inner disk near
    the Roche limit is extremely difficult. We conclude that an Earth system
    with multiple moons is the final result unless some particularly severe
    constraints on initial conditions in the disk are met. A disk with a lunar
    mass of material exterior to a ~ 3.5-4R_⊕ or an extremely steep radial
    surface density profile at the onset of collisional growth is required for a
    single, lunar-sized body to result from accretion of silicate density
    material in a protolunar disk. The former corresponds most closely to disks
    produced by impactors with nearly twice the mass of Mars and about twice the
    angular momentum of the current Earth/Moon system. Other processes, such as
    gravitational instability or primary accretion of an iron core in the inner
    disk, might be able to ``seed'' accretional growth and allow for the
    formation of a single Moon if disk temperature and compositional
    requirements are met. Our analysis demonstrates the need for more detailed,
    higher resolution impact simulations.

    ===================

    another

    http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001M%26PS...36....9C&link_type=ARTICLE&db_key=AST

      Title: From interstellar gas to the Earth-Moon system
      Authors: Cameron, A. G. W.
      Affiliation: Lunar and Planetary Laboratory, University of Arizona,
                          Tucson, Arizona, 85721 USA; acameron@lpl.arizona.edu
      Journal: Meteoritics & Planetary Science, vol. 36, no. 1, p.
                          9-22 (2001).
      Publication Date: 01/2001
      Origin: M&PS
      Bibliographic Code: 2001M&PS...36....9C

                                       Abstract

    This paper reports the current status of my SPH simulations of the formation
    of the Moon. Since the Moon has recently been found to have been formed
    approximately 50 million years after the solar nebula itself was formed, I
    have placed the lunar formation problem in the entire context of the
    formation and early evolution of the solar nebula. This set of processes
    remains controversial, and I have outlined what I believe to be the
    essential physical processes involved. These start with the formation of
    short-lived (now extinct) radioactive nuclides in a massive supernova. Then
    follows the probable role of the supernova ejecta in triggering the collapse
    of a core in a molecular cloud to form the solar nebula, and the injection
    of the radioactivities into the collapsing cloud core. Most of the solar
    nebula dissipates to form the Sun, and what remains becomes relatively
    quiescent. Gas drag acting on interstellar grains and the dustballs formed
    from them, due both to vertical descent to midplane and inward spiralling in
    midplane, quickly causes growth of the solid materials to form
    planetesimals. When these bodies reach the kilometer size range and beyond,
    gravitational forces dominate the accumulation process. The accumulation of
    the Earth requires of the order of 10(8) years. About half-way through that
    process the giant impact occurs with the next largest accumulating body near
    the protoearth. I have been simulating the giant impact using smoothed
    particle hydrodynamics (SPH) with 100,000 particles. The simulations of
    three of these runs are depicted in detail with a series of color images. It
    is shown that conventional accumulation simulations that assume Keplerian
    orbits and that merge bodies upon collision are misleading because they
    cannot take account of tidal stripping nor of loss and gain of particles
    during the accumulation. In addition, the large rotational flattening of the
    protoearth renders the orbital motions nonkeplerian. The simulations that
    are shown in detail have been followed for just over a week of real time,
    and in that time the largest accumulating clump has reached about half or
    more of the mass of the Moon and additional clumps have accumulated into
    bodies in the range of 1 to 20 percent of a lunar mass. It is important to
    note that although these runs have given very promising results, the
    parameter space that could plausibly be associated with the giant impact is
    not yet adequately explored.

    ===================

    another

    http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001E%26PSL.192..545H&db_key=AST&high=3e3fa1a9f904013

      Title: In search of lost planets - the paleocosmochemistry of
                          the inner solar system
      Authors: Halliday, A. N.; Porcelli, D.
      Affiliation: AA(Department of Earth Sciences, ETH Zentrum,
                          Sonneggstrasse 5, CH-8092, Zurich, Switzerland),
                          AB(Department of Earth Sciences, ETH Zentrum,
                          Sonneggstrasse 5, CH-8092, Zurich, Switzerland)
      Journal: Earth and Planetary Science Letters, Volume 192, Issue
                          4, p. 545-559. (E&PSL Homepage)
      Publication Date: 11/2001
      Origin: ELSEVIER
      Abstract Copyright: (c) 2001 Elsevier Science B.V.
      Bibliographic Code: 2001E&PSL.192..545H

                                       Abstract

    The depletion of moderately volatile elements in planetesimals and planets
    is generally considered to be a result of removal of hot nebula gases. This
    theory can be tested with Sr isotopes. The calculated initial 87Sr/86Sr of
    the angrite parent body (APB), eucrite parent body (EPB), the Moon and the
    Earth are significantly higher than the initial Sr isotopic composition of
    the solar system despite the volatile-depleted nature of all of these
    objects. Calculated time-scales required to accomplish these increases in
    87Sr/86Sr with a solar Rb/Sr in a nebula environment are >2 Myr for the APB,
    >3 Myr for the EPB and >10 Myr for the Moon. These times are more than an
    order of magnitude longer than that expected for cooling the nebula in the
    terrestrial planet-forming region and correspond to the period during which
    most of the mass already should have been accreted into sizeable
    planetesimals and even planets. Therefore, incomplete condensation of the
    nebula does not provide an adequate explanation for the depletion in
    moderately volatile elements. The data are better explained by a protracted
    history of depletion via more than one mechanism, including processes
    completely divorced from the earliest cooling of the circumstellar disk. The
    Sr model ages are maximum formation ages of the APB and EPB and indicate
    that these are most probably secondary objects. With independent estimates
    of their minimum age, a time-integrated Rb/Sr can be calculated for the
    precursor materials from which they formed. These are consistent with
    accretion of the APB and EPB from objects that at one stage may have
    resembled carbonaceous chondrite parent bodies in terms of volatile budgets.
    At some late stage there were large losses of volatiles, the most likely
    mechanism for which is very energetic collisions between planetesimals and
    proto-planets that, in the case of the Asteroid Belt, have since been lost.
    The same applies to the Moon, which presently has Rb/Sr=0.006 even though
    the material from which it formed had a time-integrated Rb/Sr ratio of
    ~0.07, consistent with a precursor planet (Theia) that was even less
    volatile element-depleted than the present Earth (Rb/Sr=0.03). The
    time-integrated Rb/Sr of Theia is similar to the present Rb/Sr of Mars
    (0.07). There is suggestive evidence of a similar time-integrated value for
    the proto-Earth (~0.09). Therefore, prior to the later stages of planet
    formation involving giant impacts between large objects, the inner solar
    system may have had relatively uniform concentrations of moderately volatile
    elements broadly similar to those found in volatile-depleted chondrites.
    Correlations of the present Rb/Sr ratios in planets and planetesimals with
    ratios of other volatile elements to Sr can be used to infer the
    time-integrated composition of precursor materials. The time-integrated
    inferred K/U ratios of the proto-Earth, as well as Theia, were ~20000, so
    that early radioactive heat production may have been ~40% greater than that
    calculated by extrapolating back from the Earth's present K/U. Higher C and
    S bulk concentrations may have led to concentrations in proto-cores of
    0.6-1.5% C and 4-10% S. These are significantly higher than those
    anticipated from the degree of volatile depletion of the present silicate
    Earth (~0.12% C, ~1.3% S). If the late history of accretion did not involve
    large-scale re-equilibration of silicates and metal, the present core may
    have inherited such high C and S concentrations. In this case, S would be
    the dominant light element in the present core.

    ===================

    and more

    http://adsabs.harvard.edu/cgi-bin/nph-abs_connect?return_req=no_params&db_key=AST&text=We%20perform%20three-dimensional%20N-body%20integrations%20of%20the%20final%20stages%20of%20terrestrial%20planet%20formation%20We%20report%20the%20results%20of%2010%20simulatiginning%20with%2022-50%20initial%20planetary%20embr

    NASA ADS is your friend.

    -- 
    Amara Graps, PhD
    Istituto di Fisica delle Spazio Interplanetario (IFSI)
    Consiglio Nazionale delle Ricerche (CNR), Roma, ITALIA
    Amara.Graps@ifsi.rm.cnr.it
    


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