Scientists involved with NASA’s Stardust mission are both surprised and delighted by the first-ever comet samples the spacecraft brought to Earth in January. The first results, presented in March at the Lunar and Planetary Science Conference in League City, Texas, showed that the samples contain fairly large pieces of high-temperature minerals.
“Oh, it was absolutely incredible,” says University of Washington astronomer Don Brownlee, Stardust’s principal investigator. “It was really exciting.”
Why? Because astronomers believe comets formed in the outermost fringes of the solar nebula, the cloud of gas and dust that accompanied the Sun as it formed 4.6 billion years ago. The classic model of comets, proposed in 1986 by J. Mayo Greenberg, pictures the objects as forming solely out of submicron-size interstellar dust grains that existed before the Sun took shape. “Against that,” Brownlee says, “we’re finding rocks up to tens of microns in size, at least some of which formed clearly in the inner part of the solar system.”
What is this stuff? “These are things related to calcium-aluminum intrusions [CAIs] that are believed to form in the hottest regions of the solar nebula disk inside the orbit of Mercury,” Brownlee explains. CAIs are the oldest things in the solar system – older than the other components of meteorites by a few million years – and formed in the hottest regions of the solar nebula.
Stardust encountered Comet Wild 2 (pronounced “Vildt 2”) in 2004 and passed through its tail, ensnaring particles blown off the comet in a glass-air substance called aerogel. Astronomers believe the comet formed beyond Neptune, at the outer edge of the solar nebula, where we today find the icy bodies that make up the Kuiper Belt. Only in the last few million years has Wild 2 ventured into the inner solar system, so astronomers thought it would contain pristine material preserved since the solar system formed. “We basically found solids that formed in the hottest possible regions of the solar nebula, and we found them in the coldest possible place,” Brownlee says.
Six research teams, comprising more than 200 people around the world, are scrutinizing Stardust’s samples. “This is ongoing science,” Brownlee cautions, “but my view of it is that we’re seeing the Kuiper Belt is a collector of the entire solar-nebula disk.”
At first glance, the result seems to validate the so-called X-wind theory, first proposed in 1988 by Frank Shu, now at Taiwan’s National Tsing Hua University. The theory suggests that fast rotation and intense magnetic fields at the innermost edge of a forming star’s nebula create an extraordinary version of outflows like the Sun’s present-day solar wind.
“He actually predicted that when the Sun formed, about a third of a solar mass of material was ejected outward,” Brownlee notes. Some of it then would fall back on the disk, and some of it was ejected from the solar system.
There are other possibilities. Some argue that turbulent mixing in the solar nebula can transport materials across the disk.
“Just finding a single piece of this stuff out there shows that, however it was done, there was a mechanism to transport materials from the innermost regions to the outermost regions,” Brownlee says. “You can speculate on that forever, but actually having proof of it is something different.”