Crash landing ends up mission accomplished: wrecked probe yields data about the early solar system.
After painstakingly gathering and analyzing the shards, researchers say that Earth's chemistry is not like the sun's. Compared with the sun, the planet is enriched in two types of oxygen and one type of nitrogen, two teams report June 24 in Science.
"The big thing is that the planets around us are so different from the sun," says Donald Burnett, a cosmochemist at Caltech and the Genesis project leader. "We have uncovered something very fundamental about how the Earth as a planet evolved."
Nearly 4.6 billion years ago, the sun and then the planets formed out of a swirling disk of gas and dust. Because everything started from the same raw materials, scientists have assumed objects in the inner solar system would share the same chemistry. But in the 1970s, researchers discovered evidence that something must have altered the solar system's chemical composition during that early era. Analyses of certain meteorites revealed that they had different proportions of oxygen isotopes compared with Earth rocks.
Isotopes are variations of chemical elements that have differing numbers of neutrons in the atomic nucleus. Oxygen-16, for instance, has eight protons and eight neutrons, whereas oxygen-17 has eight protons and nine neutrons.
"We need to know what was the average starting composition of the oxygen in the solar system," says Kevin McKeegan, a cosmochemist at UCLA and leader of the team that analyzed the Genesis oxygen data. "If we knew where we started from, then we could better understand what happened."
Because scientists can't send a sample-return mission smack into the sun, the next best thing is to study the solar wind, a flood of charged particles that streams off the sun and presumably shares its chemistry. So after its 2001 launch, Genesis flew to a point upstream of Earth and gathered solar wind particles in collectors situated like a car windshield in a cloud of bugs.
After more than two years collecting samples, the mission was supposed to parachute gently home to the U.S. Army's Dugway Proving Ground in Utah. But the chute failed to open and the capsule containing the samples plunged to Earth, crumpling on impact. The science canister inside burst open, and particles collected pristinely in outer space were exposed to all the grime on Earth.
"If you must crash a spacecraft, the best planet to crash it on is the Earth," Burnett says. "Here you can go pick up the materials."
And pick them up scientists did. At first it wasn't clear whether they would be able to remove all the dirt contaminating the Genesis pieces, but lab technicians invented new ways to scrub the shards clean.
Researchers began publishing a slow but steady stream of papers on solar wind chemistry, including some showing that the sun and the Earth contain different amounts of the noble gases neon and argon. But the mission's main targets, oxygen and nitrogen, took more time because of the possibility of contamination from Earth, where oxygen and nitrogen are far more common than the noble gases.
To remove the polluted bits from one set of samples, McKeegan's team at UCLA stripped off the top layer of an unbroken collector crystal. Careful chemical analysis showed that Earth is about 7 percent richer in oxygen-17 and oxygen-18, relative to oxygen-16, compared with the sun.
At Nancy-Universite in France, a team led by Bernard Marty compared nitrogen-14 and nitrogen-15 in the solar wind particles with Earth's atmosphere. Even more than oxygen, the amounts of different nitrogen isotopes vary dramatically across the solar system. Marty's team reports in Science that solar nitrogen looks a lot like that in Jupiter's outer atmosphere and not much like Earth's nitrogen at all. In fact, the sun is about 40 percent poorer in nitrogen-15 compared with Earth's atmosphere.
Scientists don't know exactly how the sun got a different chemistry from Earth (and, presumably, the rest of the inner planets). But one leading theory fingers the strong ultraviolet radiation that streamed off the newborn sun. That radiation could have caused molecules containing oxygen and molecules containing nitrogen to split apart and then form other molecules in the coalescing planetary disk, McKeegan says. Some of those newly formed molecules would have been taken up in the rocky material starting to form the Earth, and thus would carry more of a particular isotope of oxygen or nitrogen with them into the newborn planet.
Both the oxygen and nitrogen work yielded "excellent and important papers," says Andrew Davis, a cosmochemist at the University of Chicago.
The Genesis team isn't through analyzing the shards of the spacecraft. Burnett says the team is now focusing on magnesium isotopes, which could clarify whether the solar wind changes its chemistry between when it leaves the sun and when it reaches Earth.
"In my terms, we had a list of 18 measurements of things we wanted to do, and we've done about five of them," Burnett says. "My bar is pretty high, and I'm not going to rest."
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|Title Annotation:||STORY ONE; NASA's Genesis mission|
|Date:||Jul 16, 2011|
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