A speck of Stardust, a window on the young solar system

Bob Pepin reads the history of the solar system in grains of comet dust

See larger image
An aggregate of dust particles from Comet Wild-2 left this track in a piece of aerogel, a space-age sponge used by NASA's Stardust mission. University physicist Bob Pepin and his colleagues studied some of it and found hints to the dust's origin.

Photo courtesy Christopher Snead, UC-Berkeley

By Deane Morrison

January 4, 2008

One of the joys of star watching is seeing a speck of dust shed by a comet streak across the sky as a meteor. Old song lyrics to the contrary, no one ever catches a "falling star."

But four years ago, NASA's Stardust spacecraft did the next best thing when it chased down a comet and collected grains of dust blowing off its nucleus. Samples of the comet dust were shipped to scientists all over the world, including University physics professor Bob Pepin.

By studying gases trapped in the dust, he and his colleagues found evidence that after being generated near--but not in--the infant sun, the gases blasted their way into nearby dust particles. Previous work by other researchers suggests that the dust was later flung out past Neptune, where it helped form comets. The work by Pepin's team appears in the January 4 issue of Science.

A Wild ride

Comet Wild-2 used to be a garden-variety comet, its orbit stretching as far out as Uranus and only as close to the sun as Jupiter. But in September 1974 the comet passed too close to Jupiter and had its orbit rearranged by the giant planet's gravity. Now, Wild-2 comes about as close to the sun as Mars. It was in that neighborhood when the Stardust spacecraft rendezvoused with the comet, which was making only its fifth appearance in its new orbit. Because it has spent so little time this close to the sun, most of its material should be pristine.

The studies of cometary dust are part of a larger effort to trace the history of our solar system. Planetary scientists regard comets as the most pristine remnants of the ancient cloud of gas and dust that condensed into the sun and the myriad bodies that orbit it.

"We want to establish what the solar system looked like in the very early stages," says Pepin. "If we establish the starting conditions, we can tell what happened in between then and now." One early event was the birth of Earth's moon, about 50 million years after the solar system formed, he says.

Also, the gases he studies have relevance even closer to home.

"Because some scientists have proposed that comets have contributed these gases to the atmospheres of Earth, Venus, and Mars, learning about them in comets would be fascinating," he says.

A space-age comet cleanser

Comet Wild-2 (pronounced Vilt-two) is thought to have originated in the Kuiper Belt, a comet-rich region stretching from just inside the orbit of Neptune to well beyond Pluto. As it grew in this roughly -360 F region, it incorporated grains of dust and ambient gas.

"The particles probably came from the first million years, or even less, of the solar system's existence," Pepin says.

In early January 2004, the Stardust spacecraft visited the comet, veering as close as 149 miles to its nucleus and flying through the stream of particles blowing off it. At the moment of encounter, the spacecraft sponged up some of the dust with an ultralight glass-fiber material called aerogel, held in a supporting framework.

"It looked like a tennis racket," says Pepin of the apparatus. "It was exposed for approximately 20 minutes."

Basalt and battery

About 30 years ago, colorful University astrophysicist Ed Ney donned what appeared to be long johns and gave a physics colloquium on his studies (in cold weather) of Comet Kohoutek, a once-promising comet that turned out to be a visual dud. When Ney asked what the audience thought of the comet, physics professor Hans Courant heaved an egg in Ney's general direction. Ney went on to report that he and his students had detected basalt, an igneous rock material, in the comet. "Basaltic rock?" scoffed an audience member. "This comet has volcanoes?"

The aerogel trapped aggregates of fine particles that hit at nearly six kilometers per second and split on impact. The collisions left drumstick-shaped trails pointing inward from the surface of the aerogel. Yet so thin was the dust that the entire mission returned less than half a milligram of cometary material.

Noble prizes

The spacecraft returned and parachuted its payload safely back to Earth in January 2006. A few months later, Pepin received three sub-samples of particles while colleagues at Nancy University, France, received two others, all from the same particle "hit."

Their task was to analyze gases locked in tiny dust grains about a quarter of a billionth of a gram in weight. As a first step, the researchers heated the grains to about 1,400 degrees C., liberating gases imprisoned for eons.

"The particles probably came from the first million years, or even less, of the solar system's existence," Pepin says.

That would be close to 4.6 billion years ago. If our middle-aged sun were 50 years old, then the particles were born in the first four days of its life.

The gases of interest were helium and neon--two of the "noble gases" of the periodic chart, so named for their inability to combine chemically with other elements. Because of their chemical aloofness, they stay the same as they were when the comet dust formed and so reflect the conditions that existed at that time and place.

One thing that persists through the millennia is the ratio of different forms, or isotopes, of helium and neon. For example, the sun has much more of the lighter helium-3 isotope and less of the heavier helium-4 isotope than do Earth's atmosphere or meteorites. Thus, the ratio of isotopes holds a clue to the origin of the noble gases.

Looking at the isotope ratios and overall amounts of helium and neon in the comet dust, Pepin and his colleagues concluded that those gases didn't come from the sun. Instead, they have much more in common with gases seen in "primitive" meteorites, which have never been exposed to very high temperatures. The gases most likely came from a hot environment pierced by huge magnetic flares that must have been close to the young, evolving sun.

In support of this hypothesis, many tiny igneous "rocks"--which form only in very hot environments like volcanoes--have been found in the comet's dust. Also, University astronomy professor Ed Ney and his students detected similar material 30 years ago in Comet Kohoutek.

So what are nice grains of igneous rock doing in a place like the Kuiper Belt? Such rock couldn't possibly have formed in that cold expanse; therefore, how it got so far from its source is the number one question, Pepin says.

"Somehow these little high-temperature particles were transported out very early in the life of the solar system," he explains. An idea circulating among physicists is that the particles were hurled outwards ballistically from somewhere close to the young sun. If this really happened, it could have played a major role in shaping the solar system.

Of course, no one knows for sure. But then figuring it out is what's fun.

"And there's lots of fun still ahead," says Pepin. "We've looked at one track carved by a comet particle. There are at least 200 more in Stardust's aerogel, and we already have our eye on some of them."