PerspectivePlanetary Science

A Vitrage of Asteroid Magnetism

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Science  16 Nov 2012:
Vol. 338, Issue 6109, pp. 897-898
DOI: 10.1126/science.1230532

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Summary

In the early solar system, some protoplanets experienced large-scale melting, leading to the formation of a metallic core overlain by a rocky mantle. This differentiated structure has persisted to the present day in large bodies such as Earth. In Earth's core, vigorous churning of molten metallic liquid generates the geomagnetic field in a process known as the dynamo (1). Although we have no samples of Earth's core, many smaller protoplanets were catastrophically shattered over the intervening eons, producing the present-day asteroid belt and providing us with meteorite samples from their deep interiors. These samples provide a natural cross section of asteroid interiors, with stony meteorites thought to have formed at shallow depths and iron meteorites in the core. Intermediate in composition between these two types are the pallasites, spectacular mixtures of translucent, gem-quality olivine crystals and iron-nickel metal that, when backlit, take on the quality of a medieval stained-glass window (vitrage) (see the first figure). The origin of pallasites and the nature of their parent body (2) have perplexed meteoriticists since the first, eponymous meteorite Pallas was described in 1794 (3). On page 939 of this issue, Tarduno et al. (4) suggest that the pallasite parent body was the product of a near-catastrophic impact of a molten body onto a differentiated protoplanet with an active core dynamo.