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A well-travelled meteorite
Posted: 03 March 2011

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Analysis of a pea-sized chunk of meteorite dating back to the dawn of the Solar System provides the first evidence that dust particles bound up in these ancient rocks are well-travelled and have experienced a range of environments.

The 4.57 billion year old meteorite displays evidence that its constituent dust grains travelled vast distances as the protoplanetary nebula condensed into planets, likely encountering the hot solar environment as well as the asteroid belt.

The journey of a dust grain through the protosolar disc. Image: Justin Simon/NASA

“This has implications for how our Solar System and possibly other solar systems formed and how they evolved,” says Justin I. Simon, a former University of California, Berkeley, post-doctoral fellow who led the research. “There are a number of astrophysical models that attempt to explain the dynamics of planet formation in a protoplanetary disc, but they all have to explain the signature we find in this meteorite.”

Primitive meteorites, in particular carbonaceous chondrites, contain millimetre to centimetre-sized calcium-aluminum-rich inclusions (CAIs) and chondrules, molten or partially molten droplets that become bound up in the asteroid. Since these were the first solids to condense out of the gaseous nebula surrounding our protosun, they therefore contain clues to the history of the Solar System before any planets formed.

The team used an ion microprobe called NanoSIMS (secondary ion mass spectrometer) to sample a pea-sized CAI from the Allende meteorite, a carbonaceous chrondrite that fell to Earth in 1969. “I chose the Allende CAI because most of what we know about CAIs has come from the Allende meteorite, and therefore any record that I found would likely reflect the histories of CAIs in general,” says co-author and deputy director of Lawrence Livermore National Laboratory (LLNL)’s Glenn T. Seaborg Institute Ian D. Hutcheon.

The researchers sampled the oxygen isotope composition of the CAI because the relative abundance of oxygen isotopes varied in the protoplanetary disc, so it is possible to pinpoint where a mineral formed based on the relative abundances of the isotopes oxygen-16 (16O) and oxygen-17 (17O). The team were able to probe its core and four distinct mineral layers to determine the abundance of the isotopes in each layer.

“If you were this grain, you formed near the protosun, then likely moved outward to a planet-forming environment, and then back toward the inner Solar System or perhaps out of the plane of the disc,” says Simon. “Of course, you ended up as part of a meteorite, presumably in the asteroid belt, before you broke up and hit the Earth.”

How the dust grains experienced these extremes of environments could, in part, be explained by a popular theory known as the X-wind model. This describes the Sun’s intense magnetic fields churning around the contents of the protoplanetary disc and tossing dust grains formed close to the Sun into the outer Solar System, often forcing the grains outside the plane of the Solar System. “It may have followed a trajectory similar to that suggested in the X-wind model,” says Hutcheon. “Though after the dust grain went out to the asteroid belt or beyond, it had to find its way back in. That’s something the X-wind model doesn’t talk about at all.”

The team’s next step is to probe other inclusions to determine whether this particular CAI is unique or typical of all CAIs.