Differentiation time scales of small rocky bodiesOPEN ACCESS
Marc Monnereau, Jérémy Guignard, Adrien Néri, Michael J. Toplis, Ghylaine Quitté
Submitted as ‘Time scales of small body differentiation’ to Icarus on August 9th 2022
Update (1 October 2022): Differentiation time scales of small rocky bodies
“Highlights
• Differentiation of asteroids relies on the interplay of various time scales: heating due to 26Al radionuclide, cooling and drainage of silicate liquids.
• The drainage of silicate liquid has a major impact as it controls the drainage of 26Al heat sources.
• The drainage time of silicate liquids is proportional to two independent parameters: μm/R^2 , the ratio of the matrix viscosity to the square of the body radius and μf/a^2, the ratio of the liquid viscosity to the square of the matrix grain size.
• The classic idea that liquid migration was efficient enough to remove 26Al heat sources from the interior of bodies and dampen their melting relies on percolation rates of silicate liquids overestimated by six to eight orders of magnitude.
• In bodies accreted during the first few million years of solar-system history, drainage cannot prevent the occurrence of a global magma ocean.”
“The petrologic and geochemical diversity of meteorites is a function of the bulk composition of their parent bodies, but also the result of how and when internal differentiation took place. Here we focus on this second aspect considering the two principal parameters involved: size and accretion time of the body. We discuss the interplay of the various time scales related to heating, cooling and drainage of silicate liquids. Based on two phase flow modelling in 1-D spherical geometry, we show that drainage time is proportional to two independent parameters: μm/R2, the ratio of the matrix viscosity to the square of the body radius and μf/a2, the ratio of the liquid viscosity to the square of the matrix grain size. We review the dependence of these properties on temperature, thermal history and degree of melting, demonstrating that they vary by several orders of magnitude during thermal evolution. These variations call into question the results of two phase flow modelling of small body differentiation that assume constant properties.For example, the idea that liquid migration was efficient enough to remove 26Al heat sources from the interior of bodies and dampen their melting (e.g. Moskovitz and Gaidos, 2011; Neumann et al., 2012) relies on percolation rates of silicate liquids overestimated by six to eight orders of magnitude. In bodies accreted during the first few million years of solar-system history, we conclude that drainage cannot prevent the occurrence of a global magma ocean. These conditions seem ideal to explain the generation of the parent-bodies of iron meteorites. A map of the different evolutionary scenarios of small bodies as a function of size and accretion time is proposed.”
