The initial thermal and chemical state of a planet is largely determined by how it accreted. Although large bodies rapidly lose their memory of those initial conditions, smaller bodies do not: the Martian mantle has different isotopic reservoirs that were established early in its history and not subsequently homogenized , while the Martian dynamo may have been driven by an initially superheated core . Accretion is also inefficient; impacts can modify planetary bulk compositions in subtle  or dramatic  ways.There are two main pathways for melting and differentiation of silicate bodies. Small rapidly-accreted bodies melt from the inside out due to 26Al decay, potentially leaving an unmelted carapace . Large bodies melt due to release of gravitational energy via giant impacts. Both situations likely result in magma oceans, which may crystallize to yield unstable density structures . The lifetime of magma oceans is highly uncertain and depends on whether a flotation crust develops, and whether a thick primordial atmosphere is present . The Hf-W  and Pd-Ag  isotopic systems provide constraints on the timing and style of core formation. For instance, the rapid growth of planets in the ``Grand Tack'' model  may not be consistent with these constraints. The key uncertainty is the extent to which impactor cores equilibrate with the surrounding mantle during impacts. For example, the inferred rapid accretion of Mars  depends on an assumption of perfect re-equilibration. The physics of re-equilibration is imperfectly understood , and hard to model numerically ; laboratory experiments may provide a better approach . Dynamical models suggest that the Earth's feeding zone moved outwards with time . Isotopic  and element partitioning  models are consistent with this picture, suggesting that accreted material changed from volatile-poor and reduced to volatile-rich and oxidized as time progressed.  Halliday et al., SSR 96, 197-230, 2001.  Williams & Nimmo, Geology 32, 97-100, 2004  O'Neill & Palme, PTRSL 366, 4205-4238, 2008.  Benz et al., SSR 132, 189-202, 2007.  Elkins-Tanton et al., EPSL 305, 1-10, 2011.  Elkins-Tanton et al., MAPS 38, 1753-1771, 2003.  Zahnle et al., SSR 129, 35-78, 2007.  Kleine et al., GCA 73, 5150-5188, 2009.  Schonbachler et al., Science 328, 884-887, 2010.  Walsh et al., Nature 475, 206-209, 2011.  Dauphas & Pourmand, Nature 473, 489, 2011.  Dahl & Stevenson, EPSL 295, 177-186, 2010.  Kendall & Melosh, LPSC 43, 2699, 2012.  Deguen et al., EPSL 310, 303-313, 2011.  O'Brien et al., Icarus 184, 39-58, 2006.  Rubie et al. EPSL 301, 31-42, 2011.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- 5410 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Composition;
- 5420 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Impact phenomena;
- 5455 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Origin and evolution;
- 6207 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS / Comparative planetology