default header
14 March 2006
NASA Stardust Findings May Change View of Comet Formation
Comet samples have been distributed to 150 scientists for study
Washington – Comet samples returned to Earth in January by NASA’s Stardust spacecraft have surprised scientists by indicating that the formation of at least some comets may have included materials ejected by the early sun to the far reaches of the solar system.
Scientists found minerals formed near the sun or other stars in the samples, suggesting that materials from the center of the solar system could have traveled to the outer reaches where comets formed.
This finding may change the way scientists view comet formation and composition.
"The interesting thing is,” said Stardust principal investigator Donald Brownlee during a briefing March 13 from Johnson Space Center (JSC) in Houston, “we are finding these high-temperature minerals in materials from the coldest place in the solar system."
Comets are leftover debris from the formation of the solar system 4.5 billion years ago and are thought to consist mostly of ice, dust and rock.
The Stardust mission launched in February 1999, carrying a set of instruments that included one provided by the University of Chicago to monitor the impact of cometary dust.
In January 2004, the spacecraft came within 241 kilometers of comet Wild 2 and collected thousands of tiny dust particles streaming from its nucleus.
The Stardust sample-return canister parachuted onto the desert salt flats of Utah January 15 after a journey of nearly 4.8 million kilometers. (See related article.)
The science canister with the Wild 2 sample arrived at JSC January 17, and samples have been distributed to about 150 scientists for study.
COMETS FROM THE SUN
Scientists long have thought of comets as cold, billowing clouds of ice, dust and gases formed on the edges of the solar system. But they may prove to be diverse bodies with complex histories.
Comet Wild 2 seems to have a more complex history than scientists thought.
"We have found very high-temperature minerals,” said Michael Zolensky, Stardust curator and co-investigator at JSC, “which supports a particular model where strong bipolar jets coming out of the early sun propelled material formed near to the sun outward to the outer reaches of the solar system."
Comets may not be composed entirely of volatile rich materials, he added, “but rather are a mixture of materials formed at all temperature ranges, at places very near the early sun and at places very remote from it."
One mineral found in the material that Stardust brought back is olivine, a main component of the green sand found on some Hawaiian beaches. It is among the most common minerals in the universe but scientists were surprised to find it in comet dust.
Olivine is a compound of iron, magnesium and other elements. The Stardust sample is mainly magnesium.
The comet traveled the frigid environs of deep space until 1974, when a close encounter with Jupiter brought it to the inner solar system.
Besides olivine, the dust from Wild 2 also contains exotic, high-temperature minerals rich in calcium, aluminum and titanium.
"I would say these materials came from the inner, warmest parts of the solar system or from hot regions around other stars," Brownlee said.
"The issue of the origin of these crystalline silicates still must be resolved,” he added. “With our advanced tools, we can examine the crystal structure, the trace element composition and the isotope composition, so I expect we will determine the origin and history of these materials that we recovered from Wild 2."
The grains are tiny, most smaller than a hair's width. Thousands of them seem to be embedded in a glass-like substance called aerogel.
A single grain of 10 microns – one-hundredth of a millimeter – can be sliced into hundreds of samples for scientists to study.
STARDUST@HOME
During its seven-year journey, Stardust also gathered interstellar dust samples. The team at JSC hopes to begin detailed scanning of the interstellar tray within a month.
They will initiate the “Stardust@Home” project that lets volunteers from the public help scientists locate particles.
After registering, Stardust@Home participants download a virtual microscope. The microscope will connect to a server and download "focus movies."
The movies are images of the Stardust Interstellar Dust Collector from an automated microscope at the Cosmic Dust Lab at JSC. Participants will search each field for interstellar dust impacts.
Meanwhile, the Stardust spacecraft someday may see more comets.
After dropping the space return canister to fall back to Earth, the spacecraft was placed in an orbit around the sun that could bring it to another comet in February 2011.
More information about the Stardust mission is available on the NASA Web site.
(The Washington File is a product of the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)
