Hatched in a gentle smash-up (Image: ESA)

Video: 67P formed when two baby comets got together

The rubber-duck comet formed not with a bang, but a whimper. Everyone’s favourite comet chaser, the European Space Agency’s Rosetta spacecraft, is yielding new insights about how its target, comet 67P, may have formed – and it wasn’t nearly as traumatic a birth as it could have been.

How comet 67P got its distinctive rubber ducky shape, with two lobes joined by a neck, is a mystery. There are two main theories: either it started as one body that eroded, or it was once two bodies that merged.


Hans Rickman of Uppsala University in Sweden and colleagues saw no reason why the neck would have eroded faster than the rest of the comet, so they focused on the merger idea.

They saw two possibilities. One is that two chunks of ice and dirt, the building blocks of comets, merged early in the solar system’s history. At this time, there would have been more gas around to exert drag on moving objects, and the large planets would not yet have formed, so their gravity would not have acted as a slingshot for debris. Impact speeds would thus have been much lower than now, allowing colliding objects to fuse rather than break up.

The alternative scenario is that the two lobes were part of a single larger body that shattered in a violent collision, after which the two chunks found each other again and stuck together.

Gentle union

Looking at data from Rosetta’s OSIRIS camera, Rickman and his colleagues find evidence for a gentle union. Previous observations showed that 67P is highly porous. If it formed violently, the material should be more compacted, says team member Simone Marchi of the Southwest Research Institute in Boulder, Colorado.

The team also noted that the comet’s nucleus contains volatile gases such as carbon monoxide and dioxide, which would escape if the comet was significantly heated, such as in a high-speed collision.

Separately, computer simulations by Martin Jutzi of the University of Bern in Switzerland and Erik Asphaug of Arizona State University in Tempe support the gentle merger theory as well. They ran around 100 3D simulations of spheres made of ice, 1 kilometre in diameter, approaching each other at low speeds.

Mating ritual

Jutzi and Asphaug found that following a collision between two bodies, one can be slowed by their mutual gravity and return to re-impact and fuse with its partner (see video). The resulting two-lobed shape is similar to what has been observed in about half of the comets seen by spacecraft, including 67P. “You get this dance or mating ritual, where things hit, splash off each other, and re-accrete,” Asphaug says.

But how likely is it that a comet formed in the early solar system would survive to this day? The early solar system was a violent place, and models suggest that the chances of a baby comet surviving until now without a major collision are fairly low, around 5 per cent.

“The one problem is, if you’re going to make [67P] primordially, then you have to somehow shelter it from all the bombardment predicted by the dynamical modelling and bring it into the inner solar system relatively unscathed,” Asphaug says.

This lends weight to the idea that the comet had shattered and re-formed, but neither scenario can be fully excluded, says Marchi. “We still don’t know which one of the two scenarios is the more probable.”

What does seem clear, argue Rickman and colleagues, is that whether the two lobes were the result of an early merger between baby comets, or whether they are reunited bits of a larger smashed body, their union took place at low speed.

Rickman says that understanding 67P’s origin will give a better picture of how dangerous and chaotic the early solar system was. “If our estimates of these parameters need revision, it would be important for the broad picture of solar system evolution.”

Journal reference: Study by Rickman et al.: arxiv.org/abs/1505.07021; Science Express, DOI: 10.1126/science.aaa4747