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C. Agnor, E. Asphaug (University of California, Santa Cruz)
Estimates of Earth's accretion timescale based on modeling (e.g. Wetherill 1990) and isotopic evidence (Halliday and Porcelli 2000) indicate that the Earth formed in 25-100 Myr. At least a portion of this accretion took place in the presence of the solar nebula. While the problem of nailing down the nebular lifetime remains open, observations of dust disks surrounding young stars and meteoritic evidence suggest that the gas disk existed and was involved in making planetary material for ~10 Myr (e.g. Podosek & Cassen 1994, Trilling et al. 2001). The persistence of a remnant of the nebula's original gas disk during terrestrial planet accretion is certainly plausible. The existence of this remnant nebula has dynamical (Agnor & Ward 2002, Kominami & Ida 2002) and geochemical (Porcelli & Pepin 2000) implications for terrestrial planet formation.
Nakazawa et al. (1985) explored the structure of Earth's primordial atmosphere as the solar nebula was dissipating. They found that even for low surface densities of nebular gas ( \sigmagas~1 g cm-2 or ~0.1% of the minimum mass nebula), Earth can capture a significant primordial atmosphere directly from the nebula (i.e. total mass up to a few lunar masses, or ~105 times the current atmosphere). Such a massive primordial atmosphere may have played a dynamical role in the formation of the Moon (e.g. models of lunar capture have employed aerodynamic drag in Earth's atmosphere as the primary mechanism for reducing the Moon's orbital energy, Nakazawa et al. 1983). Conversely, the formation of the Moon may have played a role in removing Earth's primordial atmosphere. Giant impacts have been suggested as one possible mechanism that could accomplish global atmospheric removal (Ahrens 1993).
We are using smooth particle hydrodynamics (SPH) to model the removal of Earth's primordial atmosphere via giant impact. We employ initial conditions similar to recent works on lunar formation (e.g. Canup & Asphaug 2001) but also include ideal gas atmospheres on the colliding bodies. In addition to exploring the hydrodynamics and efficiency of atmospheric removal via giant impact, we also examine the influence of Earth's protoatmosphere on the ejecta velocity distribution and formation of the proto-lunar disk.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.