Comet 67P/Churyumov-Gerasimenko is probably the result of a 'gentle' merger between baby comets, or reunited bits of a larger smashed body, says a new study combining data from ESA's Rosetta spacecraft with computer simulations.
These are the most probable explanations of how 'Chury' got its distinctive rubber duck shape, with two lobes joined by a neck.
A gentle collision rather than a violent one accompanied the merger, the scientists suggest based on available evidence on the comet's composition.
Hans Rickman of Uppsala University in Sweden and colleagues looked at two possible mergers.
One goes back to the primordial solar system where two drifting chunks of ice and dirt fused together in the absence of planetary gravity. The abundance of gas provided the drag on moving objects to slow impact speeds.
The other scenario considers the two lobes were part of a single larger body shattered in a violent collision but merged in a subsequent meeting.
Rickman and his colleagues find evidence for a gentle union based on data from Rosetta's OSIRIS camera, reports New Scientist.
This data shows the comet is highly porous, which would not be the case under a high-impact collision.
Team member Simone Marchi of the Southwest Research Institute in Boulder, Colorado
notes that the comet's nucleus contains volatile gases such as carbon monoxide and dioxide, which would have escaped under the heat generated in a high-speed collision.
Supporting the gentle merger are computer simulations by Martin Jutzi of the University of Bern in Switzerland and Erik Asphaug of Arizona State University in Tempe.
The 100 3D simulations of collision between 1km wide objects made of dust and ice and approaching each other at low speeds suggests what Asphaug refers to as a 'mating ritual' where the two bodies slow down after collision by their mutual gravity and return to re-impact and fuse with each other.
The resulting two-lobed shape is similar to 67P and other comets observed so far.
The problem is in explaining how the primordial merged comet survived bombardment happening in the solar system. Chances are as low as 5%.
It makes the other scenario more probable where a comet shattered and re-formed.
Either way, a gentle merger is what resulted in 67P, agree the teams.
The 4km wide comet 67P with the orbiting Rosetta will reach its closest point to the sun by August 13 after which it will veer away into deep space.
Being the leftovers from the early solar system, comets are believed to have brought water and other molecules necessary for life on Earth. There is increasing conflict on their role as water bearers.