Random Walk
And the Earth Was Without Form . . .
Scientists have found that the sun is different from the earth—no, really! You see, the sun and the planets all condensed out of roiling clouds of dust and gas that were presumably well stirred—and should therefore have been the same throughout—so these differences contain important clues as to how the solar system came to be, says emeritus professor of nuclear geochemistry Don Burnett. Burnett is the principal investigator for NASA’s Genesis mission, which returned samples of the solar wind to Earth in 2004. The solar wind is a high-speed stream of charged particles “blown” from the sun’s outer layers, which are believed to be a fossil remnant of that primordial solar nebula.
The work was done by two teams, both of which included Burnett. One team, led by Kevin McKeegan of UCLA, analyzed the samples’ oxygen levels. The other team, led by Bernard Marty from the Centre de Recherches Pétrographiques et Géochimiques at Nancy, France, looked at nitrogen levels. The results appeared in companion papers in the June 24 issue of Science.
Atoms of oxygen and nitrogen come in various forms, called isotopes, which differ in the number of neutrons they contain; these isotopes are chemically identical but have different masses. In both cases, the researchers found that the solar samples had fewer atoms of the heavier isotopes than does Earth.
Oxygen, for example, is the third most common element in the sun, and the chief component—in the form of silicon and other oxides—of the rocky planets. Oxygen-16 is the usual isotope, but Earth rocks, moon rocks, and rocky meteorites all have significant traces of the heavier oxygen-17 and oxygen-18, and the ratios of the isotopes are all roughly consistent, regardless of the rock’s origin. The ratio of oxygen-18 to oxygen-16 in the solar-wind samples, on the other hand, is about 7 percent less.
Nitrogen is the fifth most common element in the sun, and makes up about 78 percent of Earth’s atmosphere. The ratio of nitrogen-15 to nitrogen-14 in the sun proved to be roughly 40 percent less than in our atmosphere. Giant gasbag Jupiter, however, appears to have the same nitrogen-isotope ratio as the sun.
As the solar system formed, the sun would have condensed first, from the thickest part of the cloud, leaving a thin disk of material around itself like the rings around Saturn. One popular explanation for the different isotope ratios holds that, as the sun began shining outward onto the disk, the ultraviolet light capable of breaking up molecules of carbon monoxide containing oxygen-16 got absorbed near the disk’s surface. Meanwhile, the wavelengths able to dissociate the heavier isotopes penetrated more deeply, releasing oxygen-17 and oxygen-18 into the materials that would form the inner solar system. Alternatively, the heavier isotopes might have already sought out the dustier regions that would become rocky bodies—which then, of course, raises the question of how that happened.
Genesis, launched in August 2001, spent 886 days collecting solar-wind samples at Earth’s L1 Lagrange point—a locale about a million miles to the sunward side of Earth, where the two bodies’ gravitational forces balance. On September 8, 2004, the spacecraft’s sample-return capsule came to rest in the Utah desert after executing what might euphemistically be described as a geobraking maneuver when the parachute failed to deploy. But, as Burnett told E&S in 2007, “We went out and picked up the pieces. You couldn’t destroy the atoms in the crash—the only thing you can do is contaminate them.” Almost a decade invested in salvaging, sorting, and decontaminating the samples is now starting to pay off. —DS
Many scientists, including Eileen Stansbery of the Johnson Space Center, spent weeks in a set of clean rooms extracting the shards of the shattered solar-wind collection plates from Genesis’s mangled remains.
