Time scales of small body differentiation

TL;DR

This paper investigates the time scales of internal differentiation in small planetary bodies, emphasizing how variations in physical properties affect melt migration and the likelihood of magma ocean formation, challenging previous models.

Contribution

It introduces a model showing how temperature-dependent properties influence melt drainage times, impacting theories of small body differentiation and thermal evolution.

Findings

Drainage time depends on viscosity and size parameters.

Variations in properties can alter melt migration efficiency.

Global magma oceans likely formed in early accreted bodies.

Abstract

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: $\mu_m/R^2$, the ratio of the matrix viscosity to the square of the body radius and $\mu_f/a^2$, 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 $^{26}$Al 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.