Of asteroids and onions

摘要

A refined analysis of a group of meteorites has resolved controversy over the likely structure of their parent asteroid. A layered structure is revealed, rather like an onion, whose outer regions cooled fastest. The first stages of planetary accretion in the early Solar System produced small bodies, or 'planetesimals', of rock and ice. Most of these first-born were swallowed up by the present-day planets as they grew, but some survived as asteroids, orbiting between Mars and Jupiter where no large planet formed to consume them. These asteroids suffer continuing collisions, and rocks from their interiors may find their way to the Earth as meteorites. From such meteorites the temperature history of their parent planetesimal can be traced ― this is known as thermochronometry. On page 502 of this issue, Trieloff and colleagues give a comprehensive account of the temperature history of one particular planetesimal, as read from the properties of meteorites that once resided at various depths inside it.