Numerical simulations of the differentiation of accreting planetesimals with ²⁶Al and ⁶⁰Fe as the heat sources

Abstract

Abstract-Numerical simulations have been performed for the differentiation of planetesimals undergoing linear accretion growth with 26 Al and 60 Fe as the heat sources. Planetesimal accretion was started at chosen times up to 3 Ma after Ca-Al-rich inclusions (CAIs) were formed, and was continued for periods of 0.001-1 Ma. The planetesimals were initially porous, unconsolidated bodies at 250 K, but became sintered at around 700 K, ending up as compact bodies whose final radii were 20, 50, 100, or 270 km. With further heating, the planetesimals underwent melting and igneous differentiation. Two approaches to core segregation were tried. In the first, labelled A, the core grew gradually before silicate began to melt, and in the second, labelled B, the core segregated once the silicate had become 40% molten. In A, when the silicate had become 20% molten, the basaltic melt fraction began migrating upward to the surface, carrying 26 Al with it. The 60 Fe partitioned between core and mantle. The results show that the rate and timing of core and crust formation depend mainly on the time after CAIs when planetesimal accretion started. They imply significant melting where accretion was complete before 2 Ma, and a little melting in the deep interiors of planetesimals that accreted as late as 3 Ma. The latest melting would have occurred at <10 Ma. The effect on core and crust formation of the planetesimal's final size, the duration of accretion, and the choice of ( 60 Fe/ 56 Fe) initial were also found to be important, particularly where accretion was late. The results are consistent with the isotopic ages of differentiated meteorites, and they suggest that the accretion of chondritic parent bodies began more than 2 or 3 Ma after CAIs.

Differentiation and core formation in accreting planetesimals

“…Hence, the buoyant force due to the density contrast would act downwards. In our model, we treat the process of iron melt migration in the same manner as the migration of the silicate melt (Sahijpal et al 2007;Stevenson 1990) and assume the migration of molten metal through a mixed iron-silicate matrix by buoyant flow.…”

“…Miyamoto et al 1981;Grimm & McSween Jr. 1993;Sahijpal et al 1995;Bennett & McSween Jr. 1996;Ghosh & McSween Jr. 1998Merk et al 2002;Ghosh et al 2003;Yoshino et al 2003;Sahijpal & Soni 2005;Bizzarro et al 2005;Hevey & Sanders 2006;Sahijpal et al 2007;Moskovitz & Gaidos 2011). The aim of these works is the study of the thermal metamorphism and/or melting of asteroids and planetesimals.…”

“…Therefore, segregation of iron is found to occur only after the crust formed when the decay of the remaining 60 Fe generates melt fractions in excess of 50% (Taylor 1992). In the model by Moskovitz & Gaidos (2011) -as before in the model by Sahijpal et al (2007) -it is assumed that silicate melt can rise toward the surface by porous flow and form a basaltic crust. An assumption that differs from another recent model by Elkins-Tanton et al (2011), who argue that silicate melt would not be able to reach the surface because of its higher density in comparison to that of the primordial crust.…”

“…Similarly, accretion is not modelled in Ghosh & McSween Jr. (1998), Hevey & Sanders (2006) and Moskovitz & Gaidos (2011). Velocities of melt migration are not calculated self-consistently but are either assumed to be constant (Sahijpal et al 2007) or do not depend on radius, i.e. the decrease of gravity towards the centre of the planetesimal is neglected (Moskovitz & Gaidos 2011).…”

“…Molten planetesimals, it was further proposed, may be a source of the chondrule melt droplets. Sahijpal et al (2007) performed numerical simulations of the differentiation of planetesimals undergoing a linear accretion growth with both 26 Al and 60 Fe as the heat sources. For one set of models (labelled A in their study) they calculated the gradual growth of an iron core due to the flow of Fe,Ni-FeS melt towards the centre of the planetesimal and of a basaltic crust for specific melt percolation velocities.…”