The more astronomers learn about the clunkily-named Comet 67P/Churyumov-Gerasimenko, the stranger it seems. The icy wanderer is currently hosting the European Space Agency’s Rosetta probe, which entered the comet’s orbit roughly a year ago. Since then the spacecraft and its robotic lander Philae have revealed a trove of new insights about the comet—many of them contradictory. Now scientists analyzing the data are in a muddle, wondering how 67P ever acquired its bizarre combination of traits, in particular its seemingly incongruous chemical composition and shape. Unraveling the mystery may upend scientists’ understanding not just of this comet, but also of the history of the solar system at large.
The conventional wisdom about comets like 67P is that they are untouched relics from the formation of the solar system 4.5 billion years ago—pieces of leftover material not integrated into our solar system’s main planets. This picture is based in part on their apparent chemical composition, which tends to include significant amounts of volatile chemicals that sublimate easily into space when heated. The fact that they retain these materials today, the thinking goes, suggests that they have been relatively unchanged over time and have somehow escaped impacts with other space rocks that would have burned off these compounds. Rosetta’s measurements at 67P support this idea. The spacecraft observed carbon monoxide and molecular nitrogen on the comet—chemicals that should have been lost if 67P had been in any significant fender benders.
Yet 67P possesses a very peculiar shape that contradicts the notion of it representing a pristine relic from long ago. Its form calls to mind a duck, with two roundish lobes for the head and body connected by a thin neck. Such a fragile structure suggests it could not have been kicking around the debris-strewn solar system for millennia in this state because repeated impacts would have destroyed it. “You really don’t need much to break the neck of this thing,” Willy Benz, director of the Physics Institute at the University of Bern in Switzerland, said earlier this month at the general assembly of the International Astronomical Union in Honolulu. “It is really unlikely that it has been like that for 4.5 billion years.” He and others suspect 67P is actually a smaller fragment that broke off from a once-larger body in a collision with another rock more recently. That interpretation clearly conflicts with the assertion that 67P has avoided crashes. “I believe,” Benz says, “that resolving this paradox deeply probes our understanding of the origin of our solar system.”
This is not the first time that researchers have reconsidered the comets-as-relics theory. Mounting evidence indicates that the early solar system was a chaotic place crowded with planetary bits and pieces that were often smashing together. While this contradiction raises questions about the survival of comets in general, it is exacerbated in the case of 67P because of this object’s extreme shape, Benz says. Alessandro Morbidelli, an astronomer at Nice Observatory in France who studies the dynamics of bodies like 67P, says that his money is on the comet being a broken-off fragment of a larger body. “If we found convincing evidence that 67P is not a collisional fragment, then the current models of solar system formation and evolution have to be revised,” he says.
Alternatively, perhaps it is our understanding of comets, even under the conventional theory, that is in need of some tweaking. It’s possible, for example, that comet volatiles have more staying power than scientists tend to assume. “Maybe there is a way to disrupt a large ice ball without heating it too much—without losing all the volatiles that are trapped in the ice,” Benz suggests. “We need simulations or lab work to see how much of a shock an object needs in order to lose its volatiles.”
It is also conceivable that 67P’s peculiar shape did not result from a collision, but from some other process, such as erosion, which could have worn away some of its bulk to leave the thin neck exposed. Comparable forms on Earth, however, arise when easily eroded terrain, such as limestone, wears away next to a harder rock like granite. This type of rock layering is rare on comets, so it is difficult to explain how 67P might have eroded to create the duck shape.
Fortunately for celestial sleuths, Rosetta is still collecting new data that can help fill in the blanks. The mission is nominally planned to continue through December, and could go longer if the spacecraft continues to function well. The future measurements are valuable because so few comets have ever been subject to such close examination. The more scientists learn about 67P, the more they can hope to extrapolate to other cases and piece together a more coherent picture of solar system and comet formation. “This is the first mission that has allowed us to study the development of the comet in real time over a large fraction of the interesting part of its orbit,” Benz says. “It has achieved unprecedented resolution, and so far only the very first analysis has been carried out.”