How did Comet 67P Get Its Duckie Shape?

Comet 67P/Churyumov-Gerasimenko on March 25, 2015. Notice the surface features and gas jets emanating from beneath the comet's crust. The two lobes came from two different pieces of ice that collided to form the "duckie" shape of the comet's nucleus. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

 The Comet with the Odd Shape

Ever since the Rosetta mission studied the nucleus of Comet 67P/Churyumov-Gerasimenko, astronomers speculated about how it got its weird "duckie-looking" shape. There were two schools of thought about it: the first was that the comet was once a larger chunk of ice and dust that somehow eroded through frequent melting as it got near the Sun. The other idea is that there were two cometary ice chunks that collided and made one big nucleus.
After nearly two years of observations of the comet using high-resolution cameras aboard the Rosetta spacecraft, the answer became very clear: the nucleus of the comet is made of two smaller chunks that rammed together in a collision long ago. 

Each piece of the comet — called a lobe — has an outer layer of material on its surface that exists in distinct layers.  Those layers actually seem to extend down below the surface quite a long way — perhaps as much as a few hundred meters, almost like an onion. Each of the lobes is like a separate onion and each was a different size before the collision that fused them together. 

How did Scientists Figure Out the Comet's History?

To determine how the comet got its shape, Rosetta mission scientists studied images very closely and identified a number of features called "terraces". They also studied layers of material seen in cliff walls and pits on the comet, and created a 3D shape model with all the surface units to understand how the layers might fit into the nucleus. This is not terribly different from looking at layers of rock in a canyon wall here on Earth and analyzing how far they extend into a mountainside. 

In the case of Comet 67P, the astronomers found that the features in each lobe were oriented as if each lobe was a separate chunk. The layers in each lobe seemed to point in opposite directions away from the comet's "neck" region, where the two lobes seem to join together. 

Additional Tests

Simply finding the layers was only the beginning for the scientists, who wanted to make sure they could definitely prove the lobes were once separate ice chunks. They also studied the local gravity of the comet in various areas and the orientations of surface features. If the comet had been one big chunk that simply eroded, all the layers would be oriented at right angles to the gravitational pull. The comet's actual gravity pointed to the fact that the nucleus came from two separate bodies. 

What this means is that the "head" of the duckie and its "body" formed independently long ago. Eventually they "met" in a low-speed collision that joined the two pieces together. The comet has been one big chunk ever since. 

The Future of Comet 67P

Comet 67P/Churyumov-Gerasimenko will continue to orbit the Sun until its path is changed by gravitational interactions with other planets. Those changes could send it more directly near the Sun. Or, it might break apart if the comet loses enough to material to weaken its structure. This could happen on a future orbit as sunlight warms the comet, and causing its ices to sublimate (similar to what dry ice does if you leave it out). The Rosetta mission, which arrived at the comet in 2014 and landed a small probe on its surface, was designed to follow the comet through its current orbit, taking images, sniffing its atmosphere, measuring the comet's outgassing, and observing how it changes over time. It finished its mission by making a "soft crash landing" on the nucleus on September 30, 2016. The data it collected will be analyzed by scientists for years to come. 

Among its other findings, the spacecraft showed the highest resolution images of a comet nucleus ever collected. Chemical analysis of the ices showed that the comet's water ice is slightly different from Earth water, meaning that comets identical to Comet 67P probably did not contribute to the creation of Earth's oceans.