On the Ways That Planetesimals Were Analogs to Terrestrial Planets

Abstract:

Terrestrial planets are now understood to accrete from smaller bodies – both planetesimals and planetary embryos – that are, at least in many cases, already internally differentiated into a metallic core and a silicate mantle. The final terrestrial planet retains few or no unmodified vestiges of the first bodies from which it formed; each planet is the result of multiple heating, melting, and differentiation events as it grew and also on the bodies from which it grew. Planetesimals and embryos, however, are imperfect miniature physical analogs to today’s Earth. The small gravity fields produce different physical processes, as would the potentially higher temperatures, the dominance of internal radiogenic heating, and the possibility of far higher volatile fractions.

The meteorite collection contains samples of undifferentiated primitive material from planetesimals along with both silicates and iron-nickel metal from differentiated planetesimals. Conspicuously lacking is olivine-dominated material similar to the Earth’s mantle or resulting from magmatic fractionation. Perhaps our collection simply lacks these samples but they did originally exist. Perhaps, however, in the small gravity field of a planetesimal magmas cannot efficiently fractionate, and no olivine-dominated material can be produced; the mantles and crusts of planetesimals may have been significantly different than our Earth, Venus, and Mars examples.

Even in the low-gravity environment, however, metal can segregate into a core, and this core can produce a magnetic dynamo. And because each planetary core is the result of combining many smaller, pre-differentiated cores from other bodies, each core (except the very first differentiation event) is a combination of many others that went through multiple melting, solidification, collision, and disruption events. Thus core materials and processes on planetesimals may have been higher-fidelity analogs to the cores of terrestrial planets than are their mantles and crusts.