Comet Sample Has Surprising Origin

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Comets have a reputation of being outer solar system natives made up of pristine remnants of the building blocks of the sun’s planets and their moons.

But a new analysis of a tiny piece of one comet suggests that some of its material actually formed in the inner solar system — just like that of meteorites — before it was kicked outward and incorporated into the comet. The study provides the first information on when comet material formed, in this case almost 2 million years after the first material in the solar system was forged, about 4.6 billion years ago along with the birth of the sun.

The particle under study, dubbed “Coki,” was picked up from Comet Wild 2 by NASA’s Stardust mission.

To figure out when and where the material in Coki formed, Jennifer Matzel, a postdoctoral researcher at Lawrence Livermore National Laboratory in California, and her colleagues looked to see if the particle contained remnants of a particular radioactive isotope of aluminum, which would have been long gone after the first few million years of the solar system’s history.

This isotope is a signature of the oldest solids thought to have formed in the solar system, called calcium-aluminum-rich inclusions (CAIs).

But Coki seemed to contain little, if any, of the magnesium isotope that the radioactive aluminum decays into. The lack of these isotopes suggests that the material in Coki did not form until at least 1.7 million years after the first material in the solar system formed. The material in Coki also seems to have undergone periods of melting, suggesting that it formed in the inner solar system, closer to the sun. Some meteorite particles also show these signs of heating and melting, which suggest that material for both of these solar system bodies formed in similar environments.

The research is detailed in the Feb. 26 issue of the journal Science.