Oceans of molten rock, or magma, covered some asteroids in the early solar system, reveals a new study of meteorites. But researchers are still puzzled over why other asteroids apparently did not melt at all.
In the solar system's first few tens of millions of years, collisions between rocky objects and the decay of radioactive isotopes melted the interiors of large objects. Magma oceans - perhaps hundreds of kilometres deep - lapped over the Moon, the Earth, and other planets, allowing dense material to settle towards their centres in a process called differentiation. But the extent of asteroid melting had remained unclear.
Now, Richard Greenwood at the Open University in Milton Keynes, UK, and colleagues have analysed groups of meteorites thought to have come from the 530-kilometre-wide asteroid Vesta and from a second, still-unknown, asteroid.
They found all of the meteorites from each source shared the same ratios of oxygen isotopes, suggesting both asteroids must have melted almost completely. "It's an exquisite piece of work," says Michael Drake, a geochemist at the University of Arizona in Tucson, US.
But the research fails to explain why other asteroids do not show any evidence of melting. Ceres, the largest known asteroid - 930 kilometres wide - appears to be totally undifferentiated.
Drake thinks the difference may be down to timing. Previous research has suggested asteroids were heated by the decay of radioactive aluminium-26 in the dusty disc from which the solar system condensed. That isotope has a half-life of only 700,000 years. So if it was the main heat source for the first asteroids, too little may have remained to warm those that formed later, Drake says.
Journal reference: Nature (vol 435, p 916)
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