We’ve come a long way in our understanding of comet 67p/Churyumov-Gerasimenko. In March 2014, the ESA’s Rosetta probe took its first image of comet 67p after a deep-space hibernation.

comet 67p first image

By August 2014, Rosetta was just 12,000 km away from comet 67p and capturing stunning images.

comet 67p neck

One feature immediately caught scientists and the general public’s eye. What’s going on with the ‘neck’ that appears to separate two parts of the comet?

Today, scientists have an answer. Rosetta scientists looked at two possibilities. Either localised erosion created the ‘neck,’ or comet 67p formed after two comets merged.

Comet 67p’s shape is the result of a low-speed collision between two comets. Scientists poured over high-res images taken between August 6, 2014 and March 17, 2015. An in-depth study of the layers of material seen all over the nucleus point to a collision between the two comets as the reason behind the comet’s unique shape.

The image below (click for full-size) highlights the patterns that support this finding.

comet 67p layers

“It is clear from the images that both lobes have an outer envelope of material organised in distinct layers, and we think these extend for several hundred metres below the surface,” says Matteo Massironi, lead author of the study published in the journal Nature.

Massironi compares the layering to an onion. “Except in this case we are considering two separate onions of differing size that have grown independently before fusing together.”

A 3D shape model was created to figure out which directions the material and more than 100 identified terraces were sloping. “It soon became clear that the features were coherently oriented all around the comet’s lobes and in some places extended to a depth of about 650 m,” reads the press release.

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comet 67p two comets

Massironi called these “the first clue that the two lobes are independent.” To confirm their findings, the scientists “looked at the relationship between the local gravity and the orientations of the individual features all around the reconstructed comet surface.”

According to the ESA, layers of material should typically form at right angles to the gravity of an object. To test this, the scientists constructed two models. One with the comet as a single body with a center of mass near the ‘neck.’ The other one with two separate comets, with each having its own center of mass.

The model with two separate comets showed the layer and the direction of local gravity are closer to perpendicular than the one comet model.


Haven’t been keeping up on all things Rosetta? Check out my post last week about comet 67p’s water-ice cycle.

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