THE HISTORY OF THE ASTEROID BELT INFERRED FROM METEORITE PHYSICAL PROPERTIES

The connection between meteorites and asteroids is well established. Every known meteoriod orbit can be traced back to the asteroid belt, and the mineral composition inferred by asteroid spectra is similar to that of meteorites. However, density studies of meteorites and asteroids show that the physical state of asteroids is very different from that of meteorites. Most meteorite densities are 20% to 50% (or more) higher than the densities of those asteroids that share similar spectral properties. Meanwhile, studies of meteorite thin sections show that most meteorites are very well compacted, with porosities (typically around 10%) due primarily to shock microcracks, not incomplete compaction.

From this one can infer a likely impact history of asteroids. First, impact events (producing shocks on the order of 1 - 10 GPa) lithified the meteorites very early in solar system history. After lithification, more severe catastrophic impacts rubblized the meteorite parent bodies, probably within the first half billion years of the solar system's history. Since then, occasional catastrophic impacts have created a relatively small number of asteroid families and, as evidenced in the cosmic ray exposure data, distinct (rather than continuous) events sending material into Earth-crossing orbits.

This density and porosity evidence is open to other interpretations, however. It is possible that most asteroids are made of material not sampled in our meteorite collection. Perhaps many low-density asteroids are not “piles” of rock-like rubble but rather large, poorly lithified accumulations of dust, or devolatilized comet nuclei, which would not survive impacts with Earth's atmosphere. But recent studies suggest that the proportion of material hitting the upper atmosphere of Earth is within a factor of two of the impact rate of meteorites inferred from the statistics of meteorite finds in dry deserts. And observations of rubble on Eros and Itokawa, the asteroids best studied by spacecraft to date, suggest that the fragments that make up these asteroids are made of well-lithified material similar to those in our meteorite collections.