Violent collision could have made comet in just hours

New modelling challenges accepted theories on how well-known comet formed. Andrew Masterson reports.

The comet Chury, photographed by Rosetta.
The comet Chury, photographed by Rosetta.

The comet 67P/Churyumov–Gerasimenko, or “Chury” for short, may have been formed from a violent collision between two smaller objects, new modelling suggests.

Chury – which was visited by the Rosetta spacecraft in 2014 and 2015 – has a very distinctive double-lobed shape, a property it shares with a number of other comets and which has long intrigued astronomers.

The theory that the comet is the result of two smaller ones coming together is not new. However, previous studies have suggested that the impact must have been slow. “A very gentle merger must have occurred,” concluded a major paper in 2015 published in the journal Astronomy & Astrophysics.

The main reason for the slow-and-steady hypothesis is that a high-speed encounter would have resulted in both colliding comets being destroyed.

Now, however, a team led by Patrick Michel of the Universite de Nice-Sophia Antipolis in France has modelled a different scenario. The key question for the researchers revolved around Chury’s continued existence: if its component parts were fragile, they wondered, how come it is still around? Would not the frequency with which it continues to collide with smaller objects in space have led to its gradual destruction?

Using numerical simulations, Michel and his colleagues revealed that in a high speed destructive collision between two comets only a relatively small amount of material on the leading edges of the objects would be pulverised.

This video shows modelling of how a high-speed collision between two objects could have resulted in the comet 67p/Churyumov–Gerasimenko.
ESA/Rosetta/Navcam - CC BY-SA IGO 3.0

On the other sides of the objects, materials rich in volatile elements would be ejected, but not reduced to dust. The ejection would involve slow enough speeds that gravitational force would be sufficient to allow the loosened pieces to gradually draw together and adhere.

Initially, many small clumps would thus be created. These would then gradually amalgamate to make larger concentrations, eventually forming a large, lobed whole.

The scientists found that this re-clumping process could take place in just a matter of days, even hours, after the initial impact. The modelling predicts the process would occur even if the collision occurred at one kilometre per second – the typical speed of objects moving in the Kuiper Belt, where Chury originated.

Neither the collision, while sudden, nor the subsequent amalgamation, would generate enough force to heat or compress the foundation elements and cause them to change their chemical compositions. Thus, Michel says, Chury remains a trove of primordial material, even if it turns out that it was formed much more recently than thought.

The research is published in the journal Nature Astronomy.

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Andrew Masterson is news editor of Cosmos.
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