How comets were assembled

Source: PlanetS

Rosetta’s target «Chury» and other comets observed by space missions show common evidence of layered structures and bi-lobed shapes. With 3D computer simulations Martin Jutzi of PlanetS at the University of Bern was able to reconstruct the formation of these features as a result of gentle collisions and mergers. The study has now been published online in the journal «Science Express».

In a video sequence based on a computer simulation two icy spheres with a diameter of about one kilometer are moving towards each other. They collide at bicycle speed, start mutually rotating and separate again after the smaller body has left traces of material on the larger one. The time sequence shows that the smaller object is slowed down by mutual gravity. After about 14 hours it returns back to reimpact a day after the first collision. The two bodies finally merge to form one body that somehow looks familiar: The bi-lobed frame resembles the shape of comet 67P/Churyumov-Gerasimenko imaged by ESA’s Rosetta mission.

About 100 simulations were performed, each one taking one to several weeks to complete, depending on the collision type.

67P/Churyumov-Gerasimenko isn’t the only comet showing a bi-lobed shape and evidence for a layered structure. Crashing on 9P/Tempel 1 in 2005, NASA’s Deep Impact showed similar layers, a feature that is also presumed on two other comets visited by NASA missions. Half of the comet nuclei that spacecraft have observed so far have bi-loped shapes among them comets 103P/Hartley 2 and 19P/Borelly. 

As the three-dimensional computer simulations indicate, the major structural features observed on cometary nuclei can be explained by the pairwise low velocity accretion of weak cometesimals. The model is also compatible with the observed low bulk densities of comets as the collisions result in only minor compaction.