Growth and Evolution of Asteroids

Asteroids are what is left of the precursors to the terrestrial planets. They are stunning in their diversity, ranging from charcoal-black worlds the size of a hilltop, spinning like a carnival ride, to dog-bone-shaped metallic remnants of some cataclysmically disrupted planetary core, to worlds as stately as Ceres and Vesta (and fragments thereof ), to garden-variety fractured and blocky nuggets that dominate near-Earth space. Asteroid belts are common around Sun-like stars. When properly seen as unaccreted residues, as scraps on the floor of the planetary bakery, the diversity of asteroids can be fully appreciated, for to paraphrase Tolstoy, accreted planets are all alike; every unaccreted planet is unaccreted in its own way. 413 A nn u. R ev . E ar th P la ne t. Sc i. 20 09 .3 7: 41 344 8. D ow nl oa de d fro m a rjo ur na ls. an nu al re vi ew s.o rg by A nn u Re v on 0 5/ 18 /0 9. F or p er so na l u se o nl y. ANRV374-EA37-18 ARI 1 April 2009 12:33 Meteoroid: a geologic mass in space that is smaller than an asteroid (D < 50 m) 1. ASTEROID BASICS Asteroids are small rocky planetary bodies. Like many definitions in planetary science, this one has outliers and exceptions. It is also vague: small is not well defined, and rocky takes on a variety of geochemical and rheological meanings. But these concepts serve as an outline for the study of asteroids, because understanding small is to characterize the transition from one realm of geophysics to another—asteroids straddling the two—while describing rocky is to address some of the fundamental aspects of meteoritics, itself a venerable discipline in search of geologic context. Sometimes asteroids are equated with planetesimals, which are the building blocks of terrestrial planets. But this definition applies only insofar as asteroids are primordial. Nearly every asteroid we see today, whether of primitive or evolved composition, is the product of a complex history involving accretion followed by one or more episodes of catastrophic disruption. Energetic collisions among asteroids, in the aftermath of planet formation, have resulted in dynamical families of smaller asteroids, most of them now mixed into the background population, but all with distinct and indicative petrogenic relationships. Dozens, perhaps hundreds, of early asteroids grew large enough to thermally differentiate, leaving scattered pieces of their metal-rich cores and, more rarely, their mantles and crusts. Asteroids represent stages on the rocky road to planet formation. Most of the matter that survives from the original protoplanetary disk is bound into planets. But the unaccreted small bodies of the terrestrial planet forming region—the asteroids—are fated to wander the solar system at the whims of the larger dynamical bodies. They have stories to tell. 1.1. Primitive Bodies The study of asteroids has emerged as one of the most active research areas in planetary science. One reason for this is the novel context of near-Earth space, where many asteroids reside, transported from much larger populations elsewhere in the solar system. Another is the utility of asteroids as tracers of the past history of planetary evolution: Having borne witness to the solar system’s origin, they are like fossils to a paleontologist. Every day, hundreds of tons of asteroidal fragments rain down on Earth, mostly in the form of micrometeorites and sandto pea-sized grains (shooting stars), few of which reach the ground. Those that do mostly go undiscovered except when they land on rock-free deserts or ice sheets, or are witnessed falling. Space missions are under way, and others planned, to bring back samples from asteroids in order to make a rigorous connection between solar system origins, planetesimals, planets, and the hundreds of tons of meteorites that are found on Earth. The largest meteorites are too massive to transport. The 60-ton Hoba meteorite is a∼1 × 3 × 3 m tabloid of iron (a IVB nickel-rich ataxite) that sits where it landed 80,000 years ago in Namibia. It is larger than the smallest asteroids observed in modern observational campaigns. This has rendered the distinction between asteroids and meteoroids arbitrary; 50 m is often applied as the cutoff. By number, the population of asteroids is dominated at the smallest sizes, approximated by a power law