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Meteorites can reveal a lot about the composition and formation of their parent bodies, but such postcards from beyond come with no return address, making their provenance difficult to establish. Often, astronomers must observe the object's inbound trajectory and then trace its orbit backward through time to nail down the region of space or the specific parent body that a meteorite sprang from. (In one unprecedented case announced in March, a group of researchers had the added advantage of having spotted the object in space before its atmospheric entry.)
In 2007 a sky-watching program known as the Desert Fireball Network tracked a streak of light over Australia that led researchers to meteorite fragments on the ground. Through an analysis of its composition and orbital characteristics, the meteorite, known as Bunburra Rockhole, has revealed itself to be out of the ordinary, one of a small family of its kind to not originate from a large asteroid known as Vesta. The research, led by meteoritic and planetary scientist Philip Bland of Imperial College London, appears in this week's Science.
Vesta resides in the solar system's main asteroid belt, a rocky ring between the orbits of Mars and Jupiter. It may have once harbored an ocean of magma, as Earth's moon is thought to have had in its early history. That would explain the basaltic nature of the meteorites that originate from Vesta—basalt forms by magmatic or volcanic processes.
But the basaltic Bunburra Rockhole meteorite, named by convention for a feature near where it was found in Australia's Nullarbor Desert, appears to have sprung from a different parent body in the inner asteroid belt. Neither its orbital path nor its chemical signature, as measured by the relative prevalences of oxygen isotopes, agree with a derivation from Vesta or one of its related bodies. (An isotope is a traceable version of an element whose atoms have a different number of neutrons.)
Along with the three other such known meteorites, Bunburra Rockhole provides evidence for the existence of more bodies in the asteroid belt that harbor basaltic rock. Although asteroids such as Vesta—large enough to melt and then differentiate into distinct rock layers—are rare today, the solar system may have once had more of them, whose leftover fragments in the asteroid belt would explain the source of the anomalous meteorites.
Image of a fireball over the Nullarbor Desert courtesy of the Desert Fireball Network; funding from STFC and the EU
