An international team of scientists has proposed that Comet 67P Churyumov-Gerasimenko, which the Rosetta spacecraft is orbiting, was formed by two separate comets merging into one. Their new study, published in the journal Nature, suggests a low-velocity impact caused the single body to form.

Comet 67P has two separate lobes connected by a “neck,” and scientists had been unsure whether this was the result of a larger original comet being worn down by the Sun over time, or two separate objects fusing slowly together. This latest research points heavily to the latter.

Using data from the OSIRIS camera on the Rosetta spacecraft, the study led by Matteo Massironi of the University of Padova in Italy found that the lobes of the comet were made of "onion-like" layers of rock, known as strata. The strata on the larger, main lobe were fairly extensive, about 650 meters (2,130 feet) thick. We often see strata on Earth as layers of sediment and, owing to Earth’s gravity, the horizontal layers are stacked on top of each other – rising up vertically, perpendicular to the ground.

If the comet had always been a single body, then the strata on both lobes would be layered towards that single body’s centre of gravity, but this is not apparent. “In 67P’s case [the strata are] perpendicular to gravity vectors calculated for separate objects, and they are not perpendicular to gravity vectors calculated for a complete body,” Massironi told IFLScience. “This means that stratification formed before the junction of the two lobes,” meaning the two must have a separate origin.

The strata of the body (the main lobe) and the head (the minor lobe) also do not match across the neck, lending further evidence to the idea that the comet is formed of two bodies. It’s likely that those two bodies are of a fairly similar origin though, as the comet itself looks almost homogeneous in its appearance. “Even if they were formed at different positions, they have gone through the same process of evolution,” said Massironi.

This would be the first-ever confirmed “contact binary” comet, which is essentially one object formed from two. The impact must have been of a low velocity, as the comet was able to retain its structure, leading co-author Björn Davidsson of Uppsala University in Sweden to suggest that it “should have formed very early in the Solar System.” Had the original two comets merged later, they would have had larger velocities and likely would have destroyed each other in the impact.

Other theories for the comet’s formation had suggested its neck was eroded over time by the Sun, but Massironi is confident this theory is not correct. “I am very confident, because we have two strong lines of evidence: the one related to the inner stratification which show that the head and the body have two distinct onion-like inner structures, and the one related to gravity, proving that strata formed before the junction of the two lobes,” he said.

Using more data from the Rosetta spacecraft and Philae lander, it’s hoped that more information on the comet’s stratification can be gleaned, to lend further credence to what is now looking a very plausible theory.

^{Image in text: The stratified morphology of the comet is seen in teh background. Credit:  ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.}