Stardust's memories of solar system

Comet dust scooped up by NASA's Stardust spacecraft in 2004, and returned to Earth 11 months ago, is telling scientists that the early solar system was a far more turbulent place than they expected.

Tiny grains of silicate crystals they've found among the comet dust could only have formed in high temperatures -- heat unknown in the icy regions beyond Neptune's orbit where the comets condensed 4.6 billion years ago.


That means the grains -- the building blocks of rock -- must have been forged near the sun, only to be hurled far out into the outer solar system, where they mixed with the other dust, frozen water and gases of which comets are made.

"This mixing of material around the solar system dramatically complicates our ability to say, 'Here's a comet, with this composition and form, and it tells us what conditions were at this place in the protoplanetary disk,'" said University of Maryland astronomer Michael F. A'Hearn. "We would have liked this to have been easy."


On the other hand, he added, "This makes it more interesting."

The $212 million Stardust mission was launched from Cape Canaveral in February 1999. In January 2004 it flew within 147 miles of the hamburger-shaped comet Wild 2, passing through its tail of dust and gases.

As it passed, Stardust extended a paddle-shaped dust collector that captured more than 10,000 dust grains in an aerogel medium, much like flies caught on flypaper.

The spacecraft flew on around the solar system, then re-entered Earth's atmosphere Jan. 15 at 29,000 mph -- the fastest re-entry of any spacecraft in history. It slowed and parachuted to the desert near Dugway, Utah.

Since then, about 200 scientists have been busy analyzing the particles. Their initial findings -- revealed with electron microscopes, mass spectrometers and other high-tech machines -- were published today in a series of articles in the journal Science, including one by A'Hearn.

They didn't have much comet dust to work with. The entire collection from Wild 2 amounts to less than one-tenth of an ounce.

But it is precious cargo -- the first extraterrestrial material returned to Earth since the last Apollo astronauts returned from the moon in 1972. It's also the first retrieved from anyplace beyond the moon.

Comets are often described as dirty snowballs -- lumps of ice, frozen gases and dust.


Short-period comets such as Wild 2, with orbital periods of less than 20 years, are believed to have formed in the icy Kuiper Belt, beyond Neptune.

Scientists are intrigued by comets, regarding them as timeless, frozen remnants of the early solar system, bearing clues to the conditions that prevailed when and where they formed. Hoping to learn more about the formation of the Earth and other planets, they analyzed the comets' reflected light and inventoried the chemical composition of their dust and gases.

In 1985, they began sending an international fleet of spacecraft on comet flybys, gathering close-up pictures and more detailed observations of their chemistry and physics.

Last year NASA's Deep Impact mission, led by A'Hearn, crashed an 815-pound projectile into comet Tempel 1. It blasted out 10,000 tons of material for remote study by Deep Impact's instruments, and by astronomers on Earth.

But until Stardust returned last January, scientists never had any pristine comet dust to study directly.

Wild 2 (pronounced "Vilt 2") is about 2.8 miles long. It's believed to have circled the sun in the Kuiper Belt for billions of years.


Over time, the gravitational tug of the outer planets drew it closer. And in 1974, Jupiter's gravity sent the comet into its current orbit, passing between the orbits of Mars and Jupiter, and within reach of a spacecraft from Earth.

Wild 2 was expected to provide scientists with insights into conditions in the early solar system beyond Neptune's orbit. Its dust should have included bits of rock and dust created under the very cold conditions of the outer solar system, or elsewhere in the galaxy by earlier generations of stars.

Instead, a "major portion" of the largest particles were crystalline silicates, minerals that form in temperatures above 1,300 degrees Fahrenheit -- so high they must have been created very close to a star.

Just which star became clear when studies by a team led by University of California, Los Angeles, astrophysicist Kevin D. McKeegan found that the relative abundance of various elemental isotopes in the silicates (varieties of carbon, hydrogen, nitrogen and oxygen atoms) looked like those found in our inner solar system. It was a kind of DNA match with our sun.

"At this early stage, it appears that a major fraction of the ... larger silicates in Wild 2 were produced in our solar system," concluded one team led by astronomer Don Brownlee of the University of Washington, Seattle.

Somehow, those silicates, forged near the sun, found their way out beyond Neptune and got themselves embedded in Wild 2's ices. But how?


"Most of us would have preferred they be interstellar grains from other stars," A'Hearn said. "It would have been easier to understand."

In 2002, French comet specialist Dominique Bockelee-Morvan theorized that "radial mixing" could quickly sweep up the crystalline silicates near the sun and spew them across the solar system. Her theory was never widely embraced, even though it seemed to be happening elsewhere.

Astronomers looking at very young stars have observed that matter in the "circumstellar disk" of dust and gas that surrounds the star gradually spirals in around the star's equator and falls in.

Often there's also an outflow that sweeps up nearby material and hurls it up from the poles and far out into space, like smoke rising from a forest fire and blowing far downwind.

"We see this in lots of places in the universe, around other very young stars," A'Hearn said.

Frank H. Shu, an astrophysicist at the University of California, Berkeley, predicted that this "X-wind" would be strong enough to launch some fairly heavy, calcium- and aluminum-rich particles, called CAIs, from the hottest regions of the early solar nebula, far out to the icy zones where Wild 2 formed.


Stardust scientists found one CAI grain among the thousands returned from Wild 2.

A'Hearn said such surprising results add spice to science. Studies comparing the scientific questions astronomers set out to answer with what they actually discovered on a mission have found "only a small overlap."

More often, he said, "you find out new and unexpected things, and you have to rethink your previous ideas of how things work."