The asteroid belt that swirls around the sun between the orbits of Mars and Jupiter was once a constellation of about 70 small planets. How do we know? Meteorites say so.
"They provide us a snapshot of all of the processes that occurred in the first 100 million years of the solar system," said David Kring, a planetary scientist and meteorite specialist at the University of Arizona's Lunar and Planetary Laboratory.
That's because the bulk of the objects that have fallen from space spent most their histories in the deep freeze, essentially sitting out of the game as the solar system's principle planets and moons evolved.
Chemically, virtually nothing happened to these meteorites for billions of years as they tumbled through space as asteroids. While their chemistry was stable for all that time, the objects were experiencing tortuous physical changes - colliding with each other and gradually breaking down from 70 primitive mini-planets into the jumble of objects that make up the asteroid belt. Some rocks were thrown into erratic trajectories that eventually brought the pieces into collision with Earth.
Chemically unchanged since the solar system's prelude, they arrive on Earth as veritable time capsules, bearing evidence about the primitive solar system and carrying a record of their journeys through space.
"The same type of material accreted to form planets like the Earth," Kring said. "But the Earth, because it's so big, and was able to get so hot. It melted, and basically destroyed all of that chemical evidence. So the fact that these have been so well preserved out in that region of space is why we've been able to develop the story that we have."
That story is surprising in the level of detail that scientists have been able to piece together simply from studying rocks. One of the facilities that supports much of this research is NASA's Johnson Space Center, which curates the collections of lunar samples, meteorites found in Antarctica, and stratospheric dust. Everett Gibson, a geochemist at Johnson has spent his career analyzing lunar samples and meteorites.
"We are able to see in meteorites markers that reveal most of the major processes in the life of the parent bodies -- from the formation of the crust all the way down to the interior. And iron meteorites, we think, are samples from the core from the interior of these bodies."
Until the Apollo astronauts brought back rocks from the moon, meteorites were the only samples on Earth of extraterrestrial material. These simple special-deliveries from outer space prompted meteorite expert Carleton Moore to call them "poor-man's space probes."
That comparison is credited to Moore by Gibson, who studied under Moore at Arizona State University in the 1960s. Moore is curator of one of the world's largest meteorite collections at Arizona State in Tempe.
"Meteorites are the samples of our solar system and the cosmos which are delivered to us at no cost, which we then have samples of that we can study. And we do not have to launch a major exploration program to get them," Gibson explained.
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