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August 3, 2007

U of T researchers discover clues to early solar system

Crystals on meteorite hold a key to understanding building blocks of planets

By Karen Kelly

A University of Toronto-led study has uncovered tiny zircon crystals in a
meteorite originating from Vesta (a large asteroid between Mars and
Jupiter), shedding light on the formation of planetesimals, small
astronomical objects that form the basis of planets.

To date, studying zircons in eucrites -- meteorites formed by volcanic
activity -- has been difficult due to impact-induced fracturing and their
small size, typically less than five microns. Most eucrites are formed
within the asteroid belt that orbits Mars and Jupiter, a heap of
astronomical debris from the earliest epoch of the solar system. In a study
published in the recent issue of Science, researchers collected samples from
eucrites found in Antarctica believed to have originated from Vesta. The
researchers used new technology to reveal that asteroid's boiling rock
turned solid and crystallized within less than 10 million years of solar
system formation.

"Until now we have not been able to determine this time frame
unambiguously," said lead author Gopalan Srinivasan, a professor in U of T's
Department of Geology. "By pinpointing the timeframe we're able to add one
more piece to the geological and historical map of our solar system."

Scientists believe that at some point Vesta was quickly heated and then
melted into a metallic and silicate core, similar to the process that
happened on Earth. The energy for this process was released from the
radioactive decay that was present in abundance in the early solar system.
What has been unclear is when this process occurred.

Equipped with the ion microprobe at the Swedish National Museum, Srinivasan
and colleagues from four institutions set to analyze the zircons in the
eucrites, which formed when a radioactive element -- hafnium-182 -- was
still alive. Radioactive hafnium-182 decays to another element --
tungsten-182 -- with a nearly nine-million year half-life span. By studying
zircons for their 182 tungsten abundance, the researchers were able to
determine the crystallization ages of eucrites occurred within that
timeframe.

"Zircons on Earth and in space have basically the same characteristics,"
Srinivasan says. "They occur when boiling rock crystallizes and turns into
solid form primary crystallization products or they could be secondary
products caused by heating from impacts. We know Vesta became inactive
within first 10 million years of solar system formation which is nearly 4.5
billion years ago. This provides a snapshot of the early solar system and
clues to the early evolution of Earth's mantle and core."