to the typical and necessary sequence of events required for the fragmentation of an initially spherical metallic impactor into stable droplets.

![A conceptual view of the fragmentation and equilibration of a metal volume falling into a magma ocean. Typical temperature/concentration profiles are represented for the sheet and the ligament.[]{data-label="fig:sed"}](sed.pdf){width="\linewidth"}

In our conceptual fragmentation model (figure \[fig:sed\]) and in laboratory experiments on which it is based [@deguen_2014; @landeau_2014], the liquid metal phase is vigorously stirred and stretched by the turbulent flow following the impact, before fragmentation actually happens. This suggests that mass and heat transfer between metal and silicates may be aided, or even controlled, by *stretching enhanced diffusion*, a mechanism identified as crucial in the context of the mixing of a stirred diffusive heterogeneity [*e.g.* @coltice_2006; @duplat_2008; @kellogg_1987; @olson_1984; @ottino_1989; @ranz_1979; @ricard_2015; @venaille_2008; @villermaux_2004]. Stretching enhanced diffusion works as follows: take an initially compact patch of a diffusing tracer (temperature or concentration of solute) advected by a flow. Unless the flow is uniform, the patch will be deformed according to the local strain tensor. In 2D, it will be stretched in one of the principal strain directions, and compressed in the other one (thus leading to the formation of 2D 