Core formation in silicate bodies

Details Core formation in silicate bodies Core formation in silicate bodies

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmmr31b..04n&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #MR31B-04

Physics

5420 Impact Phenomena, Cratering (6022, 8136)

Scientific paper

Differentiation of a body into a metallic core and silicate mantle occurs most efficiently if temperatures are high enough to allow at least the metal to melt [1], and is enhanced if matrix deformation occurs [2]. Elevated temperatures may occur due to either decay of short-lived radio-isotopes, or gravitational energy release during accretion [3]. For bodies smaller than the Moon, core formation happens primarily due to radioactive decay. The Hf-W isotopic system may be used to date core formation; cores in some iron meteorites and the eucrite parent body (probably Vesta) formed within 1 My and 1-4~My of solar system formation, respectively [4]. These formation times are early enough to ensure widespread melting and differentiation by 26Al decay. Incorporation of Fe60 into the core, together with rapid early mantle solidification and cooling, may have driven early dynamo activity on some bodies [5]. Iron meteorites are typically depleted in sulphur relative to chondrites, for unknown reasons [6]. This depletion contrasts with the apparently higher sulphur contents of cores in larger planetary bodies, such as Mars [7], and also has a significant effect on the timing of core solidification. For bodies of Moon-size and larger, gravitational energy released during accretion is probably the primary cause of core formation [3]. The final stages of accretion involve large, stochastic collisions [8] between objects which are already differentiated. During each collision, the metallic cores of the colliding objects merge on timescales of a few hours [9]. Each collision will reset the Hf-W isotopic signature of both mantle and core, depending on the degree to which the impactor core re-equilibrates with the mantle of the target [10]. The re-equilibration efficiency depends mainly on the degree to which the impactor emulsifies [11], which is very uncertain. Results from N-body simulations [8,12] suggest that significant degrees of re- equilibration are required [4,10]. Re-equilibration is also suggested by mantle siderophile abundances [13], though simple partitioning models do not capture the likely complex P,T evolution during successive giant impacts. The timescale of Martian core formation is currently uncertain (0-10 My) [14], though it is clear that Martian core formation ended before that of the Earth. [1] Stevenson, in Origin of the Earth, 1990. [2] Groebner and Kohlstedt, EPSL 2006. [3] Rubie et al., Treatise Geophys. 2007. [4] Kleine et al., GCA submitted. [5] Weiss et al., LPSC 39, 2008. [6] Keil and Wilson, EPSL 1993 [7] Wanke and Dreibus, PTRSL, 1984. [8] Agnor et al. Icarus 1999 [9] Canup and Asphaug, Nature 2001 [10] Nimmo and Agnor, EPSL 2006. [11] Rubie et al., EPSL 2003 [12] O'Brien et al, Icarus 2006 [13] Righter, AREPS 2003. [14] Nimmo and Kleine, Icarus 2007.

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