The sun and the solar system's rocky inner planets, including the Earth, may have formed differently than previously thought, according to UCLA scientists and colleagues analyzing samples returned by NASA's Genesis mission. The data from Genesis, which collected material from the solar wind blowing from the sun, reveal differences between the sun and planets with regard to oxygen and nitrogen, two of the most abundant elements in our solar system, the researchers report in two studies in the June 24 issue of the journal Science. And although the differences are slight, the research could help determine how our solar system evolved.
"We want to understand how rocky planets form, particularly our rocky planet," said Genesis co-investigator and UCLA professor of Earth and space sciences Kevin McKeegan, who was the lead author of the Science study on oxygen. "To understand that, we need to understand how the isotope composition of the most abundant element in the Earth came to be what it is."
It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets. After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.
On Earth, the air contains three kinds, or isotopes, of oxygen atoms, which differ in the number of neutrons they contain. All three have eight protons, and almost all have eight neutrons (O-16), but a small proportion of isotopes contain nine neutrons (O-17) or 10 neutrons (O-18). Although isotopes of an element behave similarly, there are subtle differences in reaction rates according to the isotopic mass, McKeegan said.
"We found that the Earth and moon, as well as Martian and other meteorites, which are samples of asteroids, have a lower concentration of the O-16 than does the sun," McKeegan said. "The implication is that we did not form out of the same solar nebula materials that created the sun. Just how and why remains to be discovered."
"It's the most abundant element in the Earth, and it is isotopically anomalous," he said, adding that something chemically unusual happened to the material that eventually formed the Earth and other rocky planets some 4.6 billion years ago, after the sun had already formed.
The data were obtained from an analysis of material ejected from the outer portion of the sun. That material can be thought of as a fossil of our nebula because scientific evidence suggests that the sun's outer layer has not changed measurably in billions of years.
"This is the first time the heavy elements in the sun have had their isotope composition determined with precision, directly from solar material. The Genesis mission was a success. The mission has achieved its highest priority objectives. We are learning how planets form." McKeegan said.
Analyses of meteorites from Mars indicate that oxygen on Mars is very similar to oxygen on Earth, but not identical, McKeegan said.
Genesis launched in August 2001. The spacecraft traveled to the L1 Lagrange Point, about 1 million miles from the Earth, where it remained for 886 days between 2001 and 2004, passively collecting solar wind samples.
"The sun houses more than 99 percent of the material currently in our solar system, so it's a good idea to get to know it better," Burnett said.
A second paper in Science by different researchers details differences between the sun and planets with regard to the element nitrogen. Like oxygen, nitrogen has one isotope (N-14) that makes up nearly 100 percent of the nitrogen atoms in the solar system, but there is also a tiny amount of N-15.
Researchers studying the same Genesis samples found that compared to the Earth's atmosphere, nitrogen in the sun and Jupiter had slightly more N-14 — but 40 percent less N-15. The sun and Jupiter appear to have the same nitrogen composition, but as with oxygen, the nitrogen composition of the Earth and the rest of the inner solar system is very different.
For further information: http://www.universityofcalifornia.edu/news/article/25866