Chondritic Fractionation Patterns from Coupled Thermal-Coagulation Models of the Solar Nebula

Details Chondritic Fractionation Patterns from Coupled Thermal-Coagulation Models of the Solar Nebula Chondritic Fractionation Patterns from Coupled Thermal-Coagulation Models of the Solar Nebula

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.495c&link_type=abstract

Meteoritics, vol. 30, no. 5, page 495

Astronomy and Astrophysics

Astronomy

Abundance, Volatile Elements, Chondrites, Carbonaceous, Fractionation, Solar Nebula

Scientific paper

The thermal evolution of the primitive solar nebula was modulated by both dynamical processes (responsible for the release of gravitational energy) and the coagulation of solids (responsible for opacity and the regulation of the radiative environment). We have constructed models of the coupled thermal and coagulation history of the nebula with the goal of evaluating key nebula parameters by comparing model predictions with meteoritic data. The relative abundances of the moderately volatile elements in chondritic meteorites are especially suited for such purposes because (1) they plausibly reflect global nebula evolution rather than local processes[1,2]; and (2) their patterns permit a relatively straightforward, if heretofore qualitative interpretation [3]. It also happens that the relevant temperature range (about 600 K - 1350 K) is that which is least affected by the complications associated with uncertainties in the external protosolar luminosity field. The essential features of the models are the parameterized description of dynamical history (e.g., accretion rate and the evolution of total surface density), a prescription for the coagulation rate of solids that incorporates the main results of detailed calculations [4,5] and permits only the accumulation of locally condensable species, and the self-consistent determination of thermal evolution regulated by grain opacity. The coagulation rate of a given element is taken to be proportional to the local orbital frequency, species column density, and an efficiency factor which is treated as a free parameter, but the same for all species. Results are obtained by solving the species-by-species mass conservation equations, coupled with evolving temperature profiles determined by the balance of accretional energy and radiative losses and modulated self-consistently by the local fine grain column density. Condensed fine grains and evaporated species are presumed to be transported with the gas in a manner determined by the nebular accretion; coagulated solids are presumed to be decoupled from the flow and to retain their depositional radial locations. Model input parameters such as accretion rate and initial nebular mass are constrained by astronomical observations of T Tauri stars. The results are tested by comparison with solar system data: the distribution of solid material in the planets, the above-mentioned relative abundances of moderately volatile elements in meteorites, and, potentially, the elemental abundances of the terrestrial planets [6] and the deposition patterns of interstellar grains. Our main results so far are that the distribution of (non-icy) solids in the solar system can be reproduced by a diverse (but bounded) set of input parameters, which mainly constrain global nebula parameters such as total angular momentum and coagulation efficiency. Because radial transport is important, and coagulation is delayed in the inner regions of hot nebulae, the depositional patterns of coagulated solids are not simple reflections of the total nebula surface density. The smooth fractionation of moderately volatile elements found in the carbonaceous meteorites [1], if caused by global thermal and depositional evolution, requires that accretion rates and/or optical depths were sufficiently high to vaporize the dominant metal-silicate minerals early in nebular history. Models that fulfill this condition and reproduce the distribution of rocky solids in the solar system also readily provide the moderately volatile patterns of CV and CM meteorites, apparently because of the strong coupling between coagulation, opacity reduction and cooling. References: [1] Palme H. and Boynton W. V. (1993) in Protostars and Planets III (E. H. Levy and J. Lunine, eds.), pp. 979-1004, Univ. of Arizona, Tucson. [2] Grossman J. N. (1994) Chondrules and the Protoplanetary Disk, LPI, Houston. [3] Wai C. M. and Wasson J. T. (1977) EPSL, 36, 1-13. [4] Weidenschilling S. J. (1984) Icarus, 60, 555-567. [5] Weidenschilling S. J. and Cuzzi J. N. (1993) in Protostars and Planets III (E. H. Levy and J. Lunine, eds.), pp. 1031-1060, Univ. of Arizona, Tucson. [6] Kargel J. S. and Lewis J. S. (1993) Icarus, 105, 1-25.

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