Comet dust yields surprises about universe

University of Washington astronomer Don Brownlee expected surprises from the pinch of comet dust collected by the Stardust spacecraft.

But he didn't foresee such a shake-up.

The tiny specks have changed ideas about the birth of our solar system and offered hints about the origin of life on Earth.

"We're learning incredible things," said Brownlee, principal investigator for the $212 million NASA mission.

Since the capsule parachuted to the Utah desert in January, nearly 200 researchers have employed some of the world's most powerful scientific tools to probe the particles. Their findings are laid out in seven reports in today's issue of the journal Science.

Among them is the discovery of organic molecules, very similar to the amino acids that are the building blocks of proteins. The compounds contain a biologically useful form of nitrogen, which would have been important to early microbes.

"The fact that we see these suggests that the presence of amino acids is not an insane idea ... though we haven't detected them yet," said Scott Sandford of NASA's Ames Research Center in California.

The comet dust also contains bits of organic material similar to tar or soot, Sandford said.

The organic molecules bolster the theory that a rain of comet dust may have delivered the basic ingredients of life to the early Earth. The fact that comet dust contains several types of organic molecules is especially intriguing, Sandford said.

"It makes the story more powerful, because you would presume that getting life started would be easier if you have a wide variety of things around you."

During its seven-year, 2.9 billion-mile voyage, the Stardust spacecraft flew within 150 miles of Comet Wild 2 (pronounced Vilt). Dust surrounding the comet's nucleus was captured in a low-density material called aerogel, like "collecting BBs by shooting them into Styrofoam," Sandford said. The mission marks the first time extraterrestrial material has been brought to Earth since the Apollo moon landings.

The second particle Brownlee and his colleagues pried from the aerogel rocked existing theories of comet birth.

Less than one-fifth the diameter of a human hair, the particle was made up of unusual minerals that were created at blistering temperatures — higher than 2,000 degrees F. But that contradicts the standard view that comets formed on the fringes of the solar system, where temperatures average around minus 400 degrees.

"It was stunning," Brownlee said. "People's jaws dropped."

The finding means material from the hot, innermost reaches of the early solar system was somehow propelled to the frigid netherlands beyond Pluto.

"It's like the solar system partly turned itself inside out," Brownlee said.

Scientists dubbed the super-high-temperature particle Inti, after the Incan sun god. So far, it is the only one of its type they have found, though most of the 10,000 particles collected still have not been examined. Bits of Inti have been distributed to other labs. Brownlee keeps his portion of the precious speck in a sealed clean room, inside a Petri dish taped to the table so an earthquake won't knock it to the floor.

Though they weren't forged in such a fiery furnace, crystalline mineral grains in the comet dust also support the notion that there was a lot of mixing in the cloud of dust that coalesced into the solar system about 4.6 billion years ago.

Many experts had expected comets to be composed primarily of interstellar dust, minuscule bits of matter that float through space and originate from stars that have exploded and died. But instead, the comet dust is chock-full of crystals that came from the inner portion of the young solar system.

"It looks like about 10 percent of the material came from the inner disc," Brownlee said. "No one has ever suggested anything like that in the past."

Comets are fascinating to astronomers because they are believed to be remnants of the early solar system, virtually unchanged since their creation.

"They've pretty much been in a deep freeze since they were made," Sandford said.

Working with the dust has been challenging. The largest particles are one-hundredth of an inch across. The smallest are one micron — a millionth of a meter.

Brownlee equips his lab with anti-static devices, and won't handle the particles if the humidity drops below 40 percent. In a dry, electrically charged atmosphere, the dust grains can simply fly into space.

"It's an odd little world dealing with things this small," he said.

To extract embedded particles from the aerogel, scientists have devised ingenious methods that include using glass needles to chip out tiny "keystones" smaller than a gnat. Tiny, two-pronged "pickle-forks" are used to spear the keystones and extract them from bigger chunks of aerogel.

Electron microscopes that can see individual atoms and particle accelerators the size of football fields have been used to scrutinize the dust and figure out its makeup.

"People will continue to study these particles for decades to come," Sandford said. "Just like people are still studying the moon rocks."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com