Scientists have solved the mystery of why the comet being studied by Europe's Rosetta spacecraft is shaped like a rubber duck -- it started off as two separate comets, a new study shows.
Ever since Rosetta sent back pictures of its twin-lobed target more than a year ago, scientists have debated whether the comet, known as 67P/Churyumov-Garasimenko, could be the result of two comets that merged together during the solar system's early years.
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The other option is that the so-called "neck region" between 67P's two lobes experienced some particularly active and still unexplained outgassing over the eons, eroding its more spherical shape into a body that resembles a rubber duck.
"Our study rules out the possibility that the comet shape is the outcome of erosion," planetary scientist Matteo Massironi, with the University of Padova in Italy, wrote in an email to Discovery News.
Rather, the neck region is where two independent bodies collided, analysis of high-resolution images taken by the orbiting Rosetta spacecraft shows.
"The heating and partial melting at the impact location and the subsequent cooling and gluing of the two bodies explain the shape of the neck region," Massironi wrote.
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In a paper published in this week's Nature Geoscience, scientists say that 67P's lobes are made up of similar, but independent stratified, "onion-like" layers.
"Geological sections through the comet show that the larger lobe is made up of strata up to 650 meters (2,133 feet) thick, which are independent of analogous stratified layers on the smaller lobe," the scientists wrote.
"Our analysis ... clearly shows that the layers of the body and the head of the comet are not related," Massironi said.
Combining the geological data with measurements of the comet's gravity, and Rosetta scientists conclude that 67P is the result of two independent bodies that gently and repeatedly impacted before merging together soon after the formation of the solar system some 4.6 billion years ago.
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Scientists don't know if comet twins are common, or if 67P is an oddity, but the results may provide an important clue about how comets -- and the planets -- formed.
Because the comet's lobes are similar in structure and composition, scientists suspect they formed from similar pebble-sized comet bits into bodies about 1 kilometer (0.62 miles) in diameter before merging.
"Theoretical models of the solar system formation have to include gentle collisions and merging process," Massironi said. "But to better understand how much 67P is representative of the comet-formation process, the exploration of a larger sample of comets is definitively needed."