Particle Gives New Clues to Comets' Origins

For many years, based on solid scientific evidence, researchers have believed that comets were formed in the outer fringes of the solar system, out of material that also created the planets and the moons billions of years ago. This theory seemed to be very sound, and therefore gained wide acceptance among astronomers. However, a new study, conducted on small particles recovered from such a cosmic wanderer, shows that this theory may not be entirely accurate. The small clump of matter points to an inner solar system origin for some of the cometary components, which means that some of the materials they contained formed closer to the Sun, before being ejected away from the star.

In this sense, experts say, these cometary components resemble meteorites, which are formed in the same manner. The new investigation was conducted on a piece of comet that was collected directly from its source, so to speak. The NASA's Stardust mission snatched it from the Comet Wild 2, and returned it to our planet for analysis. Experiments have revealed that the particle, called “Coki”, was formed about 2 million years after the first materials appeared in the solar system. In other words, it is about 4.6 billion years old, and may have formed at the same time the Sun appeared.

Investigators analyzing the sample focused their work on determining whether the particles contained a specific type of radioactive aluminum isotope. The study was conducted by experts at the US Department of Energy's (DOE) Lawrence Livermore National Laboratory (LLNL), in California. Generally, geologists and chemists look at these isotopes as clear indicators that a material formed in the earliest stages of the solar system, when the first solids, known as Calcium-Aluminum-rich inclusions (CAI), appeared. Details of the new work appear in the February 26 issue of the esteemed journal Science, Space reports.

Isotope data collected from Coki seems to suggest that this particular material experienced repeated melting, which in turn would suggest that it formed nearer to the Sun. These records would have looked a lot different had the comet formed in the outer fringes of the solar system, the expert say. The same traces have been discovered in meteorite fragments as well, and so scientists believe that part of the materials in both classes of space objects may have formed at the same location. “I think we're still learning a lot about the diversity of particles in the Stardust collection,” LLNL postdoctoral researcher Jennifer Matzel says.