Comet sampler: specimens show that inner and outer solar system mixed.

Just as the solar system was forming some 4.6 billion years ago, it turned itself inside out. Some of the hottest material, residing so close to the sun that it almost vaporized, sped out to the chilliest reaches of deep space. These bits of formerly high-temperature dust ultimately became parts of the icy balls known as comets.

That startling scenario--in stark contrast to a widely held view that outlying regions grew up isolated from the inner solar system--is revealed by the first analyses of cometary-dust grains brought back to Earth by a spacecraft. NASA's Stardust craft passed through the dusty shroud of a comet called Wild 2 in 2004 and last January dropped to Earth a canister of the material that the mission had collected (SN: 3/25/0G, p. 182).

Researchers filled seven papers in the Dec. 15 Science with descriptions of just a tiny fraction of the roughly 10,000 dust grains captured.

Some planetary scientists, including members of the Stardust team, had conjectured that many microscopic comet grains had formed near stars other than the sun and then entered the solar system during its youth. If so, the grains would contain a wide variety of isotopes from elements heavier than lithium.

Instead, Stardust investigator Don Brownlee of the University of Washington in Seattle and his colleagues found that the isotopic composition of nearly every grain that they analyzed matched that of the inner solar system, demonstrating that the rocky material in Wild 2 indeed originated in the solar system.

One specific grain, the second particle that the researchers analyzed, grabbed their attention. The grain consists of calcium-aluminum minerals that could have formed only at the high temperatures extremely close to the sun. That rare material, sometimes seen in meteorites that have roamed the inner solar system, had never before been detected in a comet.

The particle also contained magnesium olivine, a component of Hawaii's green sand. Like the calcium-aluminum mineral, magnesium olivine was one of the first materials that formed around the young sun.

About 10 percent of the matter in comets, which typically reside beyond the orbit of Neptune in a region called the Kuiper belt, comes from the sun's fiery vicinity, Brownlee and his colleagues calculate. "It is amazing that more than a trace amount of comets comes from the inner, hotter regions of the solar nebula where Earth formed," says Brownlee.

"It appears inescapable that during the formation of the solar system, materials formed near the sun were mixed as far out as the Kuiper belt," notes Don Burnett of the California Institute of Technology in Pasadena in a commentary accompanying the Science papers.

Before Stardust, Brownlee notes, planetary scientists recognized that solids from the innermost part of the solar system had reached the asteroid belt, which lies between the orbits of Mars and Jupiter. "But now we know they also reached the region [far] beyond where Pluto is," says Brownlee.

For decades, theorists have proposed that a planet-making disk of gas, dust, and ice surrounded the newborn sun. Frank Shu of the University of California, San Diego has conjectured that jets of gas or a strong wind generated by the newborn sun hurled into space molten dust particles from the inner disk.