67P reveals recipe for a comet

To make one oddly shaped comet, take two smaller comets and squish them together. That probably explains why comet 67P/Churyumov-Gerasimenko looks like a rubber duck, a new study reports.

Since the Rosetta spacecraft’s arrival last August (SN: 9/6/14, p. 8), researchers have debated whether 67P was a comet that lost some weight around its waistline or two comets that got a little too attached to one another. Layers and terraces on cliffs gave away 67P’s coupling. Mismatched layers between the head and body imply that the two lobes formed independently and later fused together, Matteo Massironi, a geologist at the University of Padua in Italy, and colleagues report online September 28 in Nature.

The cliffs provide a peek at what passes for bedrock on a comet, revealing a stack of ice and dust coatings similar to strata seen in sedimentary rock formations on Earth. The strata in the head are slightly askew to those in the body. The researchers “do lot of very clever work to trace these layers,” says planetary scientist Michael Belton, president of Belton Space Exploration Initiatives in Tucson, Ariz.

“67p was big surprise to us,” Belton says. “Thinking back, it shouldn’t have been such a surprise.” Comet 103P/Hartley 2, visited by the Deep Impact spacecraft in 2010, has a similar dumbbell shape. Radar that bounced off comet 8P/Tuttle when it passed Earth in 2008 also showed two bulbous ends. Of the handful of comets that researchers have seen up close, roughly half show hints of being forged from two smaller bodies, notes Belton. This suggests that comets often bumped into each other over 4 billion years ago, when the solar system was more crowded than it is today.