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Using the data from Rosetta, scientists have developed a detailed scenario for the birth process that created Comet 67P/C-G.
During its two-year sojourn at Comet 67P/Churyumov–Gerasimenko, Rosetta has revealed a picture of the comet as a low-density, high-porosity, double-lobed body with extensive layering, suggesting that the lobes accumulated material over time before they merged.
The unusually high porosity of the interior of the nucleus provides the first indication that this growth cannot have been via violent collisions, as these would have compacted the fragile material. Structures and features on different size scales observed by Rosetta’s cameras provide further information on how this growth may have taken place.
Earlier work showed that the head and body were originally separate objects, but the collision that merged them must have been at low speed in order not to destroy both of them. The fact that both parts have similar layering also tells us that they must have undergone similar evolutionary histories and that survival rates against catastrophic collision must have been high for a significant period of time.
In other words, the comet’s two lobes formed slowly as separate bodies but always in the same general region, and then moved closer and closer together until they gently merged. Based on this scenario, Comet 67P/C-G had to have formed very early in the solar system, and also was not in the inner solar system — as it is now — when the great early bombardment occurred there about a billion years ago.