Splat went the comets

Spacecraft encounters with comets, such as last year’s meeting of the European Space Agency’s Rosetta with comet 67P/Churyumov-Gerasimenko, have revealed that layered structures and dumbbell-like shapes may be common in cometary nuclei. Motivated to understand how such morphology came to be, Erik Asphaug of Arizona State University and Martin Jutzi of the University of Bern ran some 100 collisional simulations, each taking one to several weeks to complete. They wanted to see what happens when two snowball-like objects collide at velocities consistent with comet-sized bodies falling gravitationally toward each other. For certain combinations of impact velocity and impact angle (from 90° for a head–on collision to 0° for a glancing one), their simulations produced bi-lobed shapes; for example, see the time series in the figure in which the initial impact causes the smaller body to leave bits of itself on the bigger one and slow down enough to be pulled back for a second impact. In more direct hits, the smaller body collided with the bigger one with a splat and became a pancake-shaped layer. The results support the idea that cometary nuclei formed from collisional accretion of similar-sized bodies. However, the researchers can’t yet distinguish whether those bodies were primordial, or whether they were fragments left over from more recent events, such as the breakup of an original comet by tidal forces near Jupiter, as happened to comet Shoemaker-Levy 9 in 1992, or the catastrophic collision of larger objects. Next on the agenda is to simulate the different scenarios and find out whether they produce distinct morphologies that can be compared to real comets. (M. Jutzi, E. Asphaug, Science 348, 1355, 2015. )