Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p21b0530p&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P21B-0530
Physics
5410 Composition, 5455 Origin And Evolution
Scientific paper
The well-defined orbital gradation of the uncompressed mean densities /lineρunc of the terrestrial planets attest to the existence of similar heliocentric gradations of temperature and pressure in the nebular gas from which these bodies condensed (Lewis, Science 186 440-443 1974). This trend of /lineρunc, coupled with the very different chemical signatures of each planet, such as the almost zero state of Fe oxidisation in Mercury, the dryness of Venus' atmosphere, and the likely existence of liquid FeS in the Earth's core, suggest that each planet condensed within a narrow feeding zone, close to its present orbit. These features are explained by the modern Laplacian theory [MLT] of solar system origin (Earth, Moon & Planets 87 11-55 2001; Abstract # 8061 in Mercury 2001 Workshop - see URL below). According to the MLT, the planetary system condensed from a concentric family of orbiting gas rings which were shed by the contracting proto-solar cloud [PSC]. Discrete ring shedding occurs if there exists a steep density inversion at the cloud's photosurface, with the gas density ρ rising ~ 35-fold. Previously it has been suggested that such an inversion comes about solely through the action of a large turbulent stress pt arising from supersonic convective motions within a uniformly superadiabatic interior (BAAS 23 1232 1991; Eos Trans. AGU 76 F332 1995). For a non-rotating cloud pt = β ρ GM(r)/r, where M(r) is the mass inside radius r and β ~ 0.1 is a constant. This requires pt rising to ~ 35pgas, which seems unlikely. Here pgas = ρ ℜ T/μ , T is temperature and μ is molecular weight. I now report a new PSC model which incorporates the findings of a numerical simulation of supersonic thermal convection in a model atmospheric layer (BAAS 32 1102 2000). The new model has an adiabatic core of radius r1 in which β = β 1, a constant. This core is surrounded by a superadiabatic envelope of polytropic index n = -1 in which β falls to 0 at the surface [s] according as β = β 1(θ - θ s})/(θ {1 - θ s). Here θ = μ cT(r)/μ Tc, c means the centre, θ 1 = μ c}T(r{1})/μ_{1 Tc, etc. If the controlling parameters β 1, θ s, θ 1 stay constant, then the contracting cloud sheds gas rings whose mean orbital radii Rn (n=0,1,2, ...) form a closely geometric sequence. The choice β = 0.1253, θ s = 0.00232 and θ 1 = 7.6 θ s leads to the detachment of a family of gas rings whose evolved radii Rn match the observed mean planetary spacings and whose condensate bulk chemical compositions yield densities in accord with the values /lineρunc. The maximum value of pt}/p{gas in the PSC, occurring at radius r = r1, is now only 11.3. The initial mass of the PSC is 1.197M&sun; . The loss of cloud mass during contraction to present solar size results in the orbital expansion of all gas rings and condensate material after ring detachment. Earth's gas ring was shed at 0.917 AU. Details of the gas ring temperatures, mean orbit pressures and condensate compositions are given in the URL below. Notably, Mercury formed at 1632 K and consists mostly of Fe-Ni-Cr-Co-V alloy (mass fraction: 0.670) and gehlenite (0.254). For Venus (911 K), the condensate contains metal alloy (0.326) and MgO-SiO2 (0.575). (Fe-Ni)S (0.087) and tremolite (0.102) first condense at Earth's orbit (674 K). FeO, as fayalite (0.180), first forms at Mars' (459 K). I thank Mr. David Warren [Tasmania], Dr. John D. Anderson [NASA/JPL] and the ARC for support.
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