Fire in the Ice

This 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 the gemstone peridot. It is surrounded by a thin rim of melted aerogel, the substance used to collect the comet dust samples. The particle is about 2 micrometers across. Click on image for a larger view.
Credit: NASA

Material recovered from ‘the Siberia of our solar system’ was forged in fire, said Donald Brownlee, principal investigator for the Stardust mission. Brownlee and his colleagues presented initial results from the mission Monday at the NASA-sponsored 37th Lunar and Planetary Science Conference being held near the NASA Johnson Space Center.

Stardust traveled 3.2 billion kilometers (2 billion miles) to collect particles shed by the comet Wild 2 (pronounced ‘Vilt 2’), and then made the return journey to Earth, where it landed in the Utah desert on January 15 of this year.

Wild-2, like other comets, formed some 4.5 billion years ago out near Pluto, at the same time the sun and planets were forming. Scientists were interested in collecting material from a comet because it was believed to be primordial material that has not been altered in the billions of years since the solar system took shape. By studying this material, they hoped to learn what the outer solar system was like in its infancy.

In the two months since Stardust landed, researchers in laboratories around the world have been busy analyzing the tiny particles it brought back. And they do, indeed, include material from the outer solar system, as expected. But they also contain minerals that require extreme heat to form. Such minerals – olivine, which gives the green-sand beaches of Hawaii their distinctive hue – are common on Earth. But no one expected them to be common out by Pluto.

"In the coldest part of the solar system, we’ve found samples that have formed at extremely high temperatures," Brownlee said. "When these minerals formed, they were either red-hot or white-hot grains."

This aerogel array, which was mounted atop the Stardust spacecraft, was used to collect interstellar dust particles as well as dust from the tail of comet Wild 2.
Credit: Berkeley

That means they had to form close to a star. The most likely star, of course, is our own sun. The sun and planets condensed out of a thin rotating disk of gas and dust. Scientists speculate that as the sun formed (and these high-temperature minerals were forged), a fountain of material was spewed up above the plane of the disk and transported billions of miles to the outer solar system, where it was incorporated into comets like Wild 2. Astronomers have seen jets of material being ejected from newly forming stars in our galaxy, but they do not have a complete picture of a transport mechanism that could carry this material to the outer reaches of a presolar disk.

But scientists are still not certain whether the high-temperature materials captured from Wild 2 come from within our own solar system. They may have come from another star. Additional tests will be able to resolve this question in time. Materials that come from our own solar system have characteristic signatures. If the Wild 2 particles match up, they’re locals. If not, they must be interlopers from another star system.

"We can tell the difference, absolutely, for something that formed in our own solar system, and something that formed around another star," said Brownlee.

Stardust used impressive technology to capture its cargo. The spacecraft contained a grid of 132 aerogel cells, each about the size of an ice cube. Aerogel contains the same material as glass, silicon dioxide, but it is 1,000 times less dense; it is 99.8 percent air. Yet it was able to slow down and capture particles shed by Wild 2 without damaging or significantly altering them. Due to the spacecraft’s speed, comet particles slammed into the aerogel at 6.1 kilometers per second (more than 13,000 miles per hour), and were slowed to a halt within the space of a few centimeters (an inch or two).

Researchers did not necessarily expect to be able to see any of the captured particles without a microscope. Before Stardust returned its cargo to Earth, said Peter Tsou, who designed the aerogel system, "we thought if we had one of those particles that we could see with the naked eye, we’d be happy." Tsou and his colleagues were pleasantly surprised. More than 45 particles were visible, although even the largest of these is only a few hundred microns across, smaller than a grain of sand. Associated with each of the main particles is a complex debris trail, resembling a carrot or a ginseng root, composed of thousands of smaller particles that crumbled off (or in some cases, exploded outward) as the original impacting particle burrowed through the aerogel. Many of these secondary particles are large enough for study, as well.

Comet Wild 2 imaged by Stardust just after flyby. The image highlights the remarkably rugged surface of the comet, which in close-up stereo views shows hardened impact craters, cliffs, and mesas in the landscape.
Credit: NASA/JPL

To date, researchers have examined particles from only 6 of the 132 aerogel cells. Intensive investigation by the principal research team will continue for at least another 6 months.

In April, a second component of the investigation will begin. In addition to capturing comet particles, Stardust also captured interstellar dust particles (IDPs), on a second aerogel grid. These were captured in ’empty’ space, not near the comet. IDPs are microscopic, and the Stardust team expects to have recovered only about 45 of them. But they have no easy way to determine where in the aerogel they are located. To find them, they have established a massive Stardust at Home program that will enlist people all over the world, working on their home computers, in the search.

Related Web Pages

Stardust Site
Image of Particle Tracks
Image of Comet
Live Webcam of Stardust Mission
Early Wild Success for Stardust
Telescopes for Stardust
Harpooning a Comet
Two-Way Asteroid Trip Takes Off
Tale of a Comet
We Are All Made of Stars
Stardust’s Return