Asteroids Formed Early on in Solar History

Article excerpt

Bits of dust and gas gathered into clumps, and boulder-size bodies careened into each other, forming asteroids and ultimately planets. A lot went on in the infant solar system, and new research suggests that some of it happened in a hurry.

A study published this week supports theoretical models in which some planetesimals--asteroids and other building blocks of planets--had already formed, heated up, and partially cooled a mere 5 million years after the birth of the solar system. The analysis identifies the radioactive isotope aluminum-26 as the heat source that melted these primitive rocks.

A highly accurate method for radioactive dating of meteorites underlies the findings. Gopalan Srinivasan and his colleagues at the Physical Research Laboratory in Ahmedabad, India, describe their work in the May 21 SCIENCE.

The researchers focused on a meteorite called Piplia Kalan, named for the village in western India where it crashed 3 years ago. The rock belongs to a group of meteorites known as eucrites and considered to be fragments chipped from the giant asteroid 4 Vesta.

To study processes in the early solar system, Srinivasan and his colleagues needed to trace a radioactive isotope with a half-life shorter than 1 million years, so they chose aluminum-26. Theorists have long suspected that the heat emitted by this isotope could have melted some of the solar system's first solid bodies.

Srinivasan and his colleagues wanted to determine when in the solar system's history a fragment of Piplia Kalan had cooled enough to solidify. To do so, they compared the rock's aluminum-26 abundance with that in the solar system's oldest known solids, primitive grains found on some meteorites. Although the aluminum-26 present in the early solar system has by now disappeared, researchers can infer its presence by measuring its decay product, magnesium-26. …