NASA's Stardust Comet Samples Contain Minerals Born in Fire
his image shows a comet particle collected by the Stardust spacecraft. The particle is made up of the silicate mineral forsterite, which can found on Earth in gemstones called peridot.
Credit: NASA/JPL

Pieces of a comet returned to Earth by NASA's Stardust spacecraft apparently formed near the Sun or around another star altogether before being flung to the outer edges of the Solar System, mission scientists said Monday.

Researchers studying samples of Comet Wild 2 (pronounced "Vilt 2") embedded in Stardust's gel-filled collector found that the minerals formed under extremely high temperatures - such as those near a star - and not in the frigid cold expected at the Solar System's edge, where most short-term comets originate.

"In the coldest part of the solar system we've found samples that formed at extremely high temperatures," said Donald Brownlee, Stardust's principal investigator at the University of Washington in Seattle, during a Monday press conference. "When these minerals formed they were either red hot or white hot grains, and yet they were collected in a comet, the Siberia of the Solar System."

The finding - announced on the 20th anniversary of the European probe Giotto's rendezvous with Comet Halley in 1986 - perplexed Stardust researchers and added a new wrinkle in astronomers' understanding of how comets, and possibly the Solar System, formed.

Particle science

NASA's Stardust sample return capsule landed in the Utah desert on Jan. 15, 2006 after a nearly seven-year mission to retrieve comet and interstellar dust particles in an aerogel collector. The probe swung past the 4.5 billion-year-old Comet Wild 2 on Jan. 2, 2004.

Astronomers aren't sure whether the minerals found in Stardust's comet samples formed near the Sun or around another star, though isotope scans are expected determine that for sure in upcoming tests, Brownlee said. Olivine, a mix of iron and magnesium that appears green on some Earth beaches, is one of the several surprising compounds found in the Wild 2 samples, he added.

Michael Zolensky, Stardust curator and a mission co-investigator at NASA's Johnson Space Center (JSC), said astronomers believed that a sort of material "zoning" occurred during the Solar System's formation.

According to the model, substances formed under hotter temperatures closer to the Sun, while colder materials - such as the gases that make up the giant planets - took root further out, he added.

But if Stardust's comet samples are found to be local to the Solar System, and not from some distant start, they'd suggest a sort of transportation system to fling particles formed near the Sun out past the orbit of Pluto and into the comet realm, researchers said.

"If this mixing is occurring, as suggested by these results, than how do you preserve any kind of zoning in the solar system," Zolenksy said. "It raises more mysteries."

Awash in samples

Thanks to the Stardust mission, astronomers have a wealth of comet and dust samples at their disposal, at least 45 of which large enough to be seen with the naked eye. But the number of actual samples could range in the millions, researchers said.

"We're just learning how to handle the samples, how make the best use of them," Zolensky said. "Once we understand that then we'll open them up for widespread analysis. And that will go on for the rest of our lives."

Stardust researchers said that all of the probe's 132 sample cells contain comet particles ranging in size between 15 micrometers - less than the width of a human hair - down to about one micrometer, though the interstellar dust samples are much smaller. The largest of those dust samples can reach one micrometer, they added.

"With every day that goes by, we're able to handle smaller particles, so hopefully a year from now we'll be able to handle things down to a half a micrometer," Zolensky said.

Mission scientists expect to scan Stardust's sample cells and release them to the Stardust@home project, an effort that allows the public to search for comet and dust samples using their personal computers, in a few weeks.

Researchers also hope to pin down the origin of Comet Wild 2's high-temperature forming minerals in the next few weeks as well.

"It's a really exciting mystery story," Brownlee said of the new Stardust findings. "So stay tuned."