Meteorite gives clues to solar system's early radioactivity
UCLA NEWS RELEASE
Posted: September 4, 2000
Clues about our early solar system are revealed in an isotope analysis of a primitive meteorite, reported by a UCLA scientist and French colleagues in the cover story of the Aug. 25 issue of the journal Science.
Analyzing ancient calcium-aluminum-rich inclusions, known as CAIs, in samples of a meteorite that is more than four-and-a-half billion years old, Kevin McKeegan, a UCLA research geochemist, reports that these inclusions contain isotopes of the elements boron and beryllium in a ratio that indicates that when the CAIs crystallized, they contained a radioactive isotope, beryllium-10. This isotope has a half-life of approximately 1.5 million years and is therefore long extinct, having completely decayed to boron-10 long ago, McKeegan said.
"Four-and-a-half billion years ago, these calcium-aluminum-rich inclusions melted and crystallized in the solar nebula before the Earth and other planets were formed," McKeegan said. "These CAIs are the oldest known solar system rocks."
McKeegan believes that while the Allende meteorite -- which landed in Mexico in a shower of stones in 1969 -- came from the asteroid belt, the CAIs inside of it probably did not originate there. Instead, McKeegan surmises that these inclusions most likely formed much closer to the young sun, and were carried by a wind to the asteroid belt, where they conglomerated into an asteroid, a piece of which eventually became a meteorite that fell to Earth. He believes that the CAIs, the largest of which are about the size of a fingernail, ar older than the meteorite in which they are included.
How far did these CAIs travel before they were accreted into the meteorite that eventually landed in Mexico? Nearly four times the distance between the Earth and sun, McKeegan said.
This hypothesis is based on the work of University of California, Berkeley, astrophysicist Frank Shu and his colleagues. According to McKeegan, if this controversial idea is correct, then the sun went through an "energetic phase where it spewed out high-energy nuclear particles before the planets formed." Rocks, such as those inclusions found within the Allende meteorite, witnessed this violent process, he said. Very high temperature minerals coexist in the Allende meteorite with low temperature materials, which supports the theory that the high temperature materials came from a different region.
The CAIs contain a form of aluminum (aluminum-26) and calcium (calcium-41) that were originally radioactive, but remained so for less than a few million years, McKeegan said. Where did these radioactive "extinct isotopes" within the CAIs come from?
Most scientists believe the aluminum-26 and calcium-41, like other isotopes, were made in stars, and that little time elapsed from their synthesis to when rocks were formed in the solar system. Some astrophysicists have argued that to form the solar system in such a short time (less than one million years), a nearby supernova must have exploded, and that materials from this massive dying star were incorporated into the sun, the Earth, and the other planets. According to this theory, the exploding supernova would have provided the radioactivity in the CAIs. However, the finding of beryllium-10 casts doubt on this theory.
McKeegan and his colleagues consider it more likely that at least some of the radioactivity came from nuclear reactions induced by the collisions of energetic particles with dust or gas in the early solar system.
"We have compelling evidence in these CAIs that at the very beginning of the solar system there was a high-radiation environment that caused nuclear reactions to turn some of the rock radioactive," McKeegan said.
McKeegan and French research scientists Marc Chaussidon and Francois Robert measured the isotope composition and abundances of lithium, beryllium and boron from samples of CAIs known to contain aluminum-26. They analyzed the small samples in a high-resolution ion microscope, a powerful type of mass spectrometer, at Nancy, France. There are only seven of these instruments in the world, including the first one at UCLA.
Unlike almost all other elements, lithium, beryllium and boron are made not in stars, but primarily by high-energy nuclear reactions in interstellar space, McKeegan said.
McKeegan's research was funded by a cooperative French-American program through the National Science Foundation, and by NASA's Cosmochemistry and Astrobiology programs.