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 to mutually rotate 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.

The simulation is part of a study published in Science Express by University of Burn astrophysicist Martin Jutzi and his colleague Erik Asphaug from Arizona State University. With their three-dimensional computer models, the researchers reconstruct what happened in the early solar system.

"Comets or their precursors formed in the outer planets region, possibly millions of years before planet formation," said Jutzi, according to a press release. "Reconstructing the formation process of comets can provide crucial information about the initial phase of planet formation, for instance, the initial sizes of the building blocks of planets, the so-called planetesimals or cometesimals in the outer solar system." About 100 simulations were performed, each of them 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/Borrelly.

"How and when these features formed is much debated, with distinct implications for solar system formation, dynamics, and geology," Jutzi said.

M. Jutzi & E. Asphaug, "The Shape and Structure of Cometary Nuclei As a Result of Low Velocity Accretion," Science Express, May 28, 2015.

The work was supported by the Swiss National Science Foundation and in part carried out within the frame of the Swiss National Center for Competence in Research (NCCR) PlanetS. The NCCR PlanetS is a collaboration between the Universities of Bern, Geneva and Zurich, ETH Zurich and EPF Lausanne.