U.S. scientists say they are surprised by the composition of stardust samples that an American spacecraft returned from a comet in January. They contain mineral grains forged in searing heat that they never expected to be inside a comet formed in ultra-frigid conditions beyond Neptune.
The tens of thousands of cosmic grains are from the comet Wild-2, collected in a wispy glass material called aerogel on the U.S. Stardust probe as it plowed through the gas and dust surrounding it two years ago. A smaller number of particles, perhaps only 100, are interstellar grains captured on the way to Wild-2.
Now, two months after their return to Earth, the mission's principal investigator describes the surprising finding after inspecting some of the particles in a special clean room in Houston operated by the U.S. space agency NASA.
"Remarkably enough, we have found fire and ice," said University of Washington astronomer Donald Brownlee. "In the coldest part of the solar system, we found samples that formed at extremely high temperatures. It's thought that the ices in comets formed at temperatures about 30 degrees above absolute zero, and yet we're finding minerals that formed at over 1,500 degrees."
The minerals in question include magnesium compounds with iron or aluminum and a titanium. They make up only a fraction of the particles brought back, but researchers are perplexed about their origin. Brownlee says they could have formed in a distant star before being cast into our developing solar system 4.5 billion years ago, or they could have formed near the sun and been hurled outward.
The NASA curator of the samples, Michael Zolensky, talks of this latter possibility.
"If these are really from our own sun, they've been ejected out all the way across the entire solar system and landed out there," he said. "That means these materials were basically on a big conveyor belt being shot out and then gradually drifting in and being shot out again."
The scientists say they will be able to determine the origin of the minerals with modern laboratory techniques that can look at their atomic structure. Donald Brownlee says the atoms of minerals from outside our solar system have a slightly different structure than those inside.
"So it's not a matter of conjecture," added Mr. Brownlee. "We have very, very, very strong clues at the atomic level that will enable us to untangle this. We can go grain by grain and say, 'This is from the solar system and this is not from the solar system.' At least that's our expectation."
In April, scientists will begin examining the interstellar grains not from the comet. In a program called "Stardust at Home," they plan to load millions of pictures of the particles onto an Internet site so that volunteers at their computers can help identify them.