Vol. 597
In section 10. Planets and planetary systems

How primordial is the structure of comet 67P/C-G? Combined collisional and dynamical models suggest a late formation

Formation of bi-lobed shapes by sub-catastrophic collisions. A late origin of comet 67P’s structure

Comets are generally thought of as being primordial objects. With its high porosity and low internal strength, comet 67P/Churmyov-Gerasimenko seemed to confirm this widely held opinion: how could such a fragile object have suffered collisions? In two papers, M. Jutzi and collaborators combine models of the dynamical evolution of the solar system and SPH collision simulations to address this problem. They show that objects such as 67P may have experienced many shape-changing collisions over the lifetime of the Solar System. If this is true, these comets may not be primordial. While the collisions lead to compaction of part of the material, some material is ejected and reaccumulated, leading to additional macroporosity. They also show that the formation of bi-lobed structures such as 67P/C-G is a natural outcome of low energy, sub-catastrophic collisions involving elongated, rotating bodies. This study sheds new light on comets and on the interpretation of the measurements and results of the Rosetta mission.