Comet Dust Presents Riddle: Scientists Forced to Rethink Solar System

There is something comforting about a theory. A good one, like Albert Einstein's theory of relativity, imposes order on a series of seemingly unrelated observations. A bad one, though, imposes order where there is none -- and that seems to be the situation now facing planetary scientists.

Material collected from the tail of the Wild 2 comet in 2004 by NASA's Stardust spacecraft has now been exhaustively examined. The results, published in Friday's issue of the journal Science, show that the formation of our solar system may not have been quite as orderly as researchers thought. The prevailing assumption had been that comets were made of space dust from outside the solar system. But scientists also found tiny bits of material forged in the ultra-hot inner solar system among the collected particles.

"It was extremely exciting," University of Washington astronomer Donald Brownlee, who headed up the research team which published in Science, told Reuters. "We expected the comet to be largely made out of interstellar grains, materials that formed before the solar system formed and were never really affected much by the solar system."

In other words, the primordial cloud of material, which 4.5 billion years ago became Mars, Earth, Jupiter and Neptune, was much less settled than first thought. Somehow, material created in the extreme temperatures near the sun was catapulted into the outer reaches of the solar system where the comets formed. Among the tiny granules collected by Stardust -- dust particles smaller than a grain of sand -- were bits of calcium aluminium, which is among the oldest matter in the solar system, and magnesium olivine, the greenish compound found in the sand of some beaches in Hawaii.

"You were transporting material over really big distances," Phil Bland, a scientist from Imperial College in London who was also involved in the research, told Reuters. "So that's kind of wacky for us planetary scientists."

The findings are the first results published from the Stardust project. The $200 million mission sent the small spacecraft through the tail of the comet to scoop up whatever it could out of its tail. After spending some seven years in space, Stardust passed close by Earth early this year and ejected a capsule which landed in the Utah desert on Jan. 15.

Scientists will now have to revise their ideas about what the early solar system was like. Much of the material collected by Stardust was indeed dust from outside the planetary orbits. But just how dust from the interior became part of Wild 2 is a mystery.

cgh/reuters/ap