Comets born of cosmic splats

Spacecraft carrying imaging equipment have visited a total of six comets. Four of them have binary nuclei (1P/Halley, 19P/Borrelly, 103P/Hartley, and 67P/Churyumov–Gerasimenko) and four appear to have layered structures (67P/Churyumov–Gerasimenko, 19P/Borrelly, 81P/Wild, and 9P/Tempel). How such morphologies came to be has been something of a mystery for astronomers. Erik Asphaug of Arizona State University and Martin Jutzi of the University of Bern now think they’ve solved at least a piece of that puzzle. They ran some 100 collision simulations, each taking one to several weeks to complete, to see what happens when two loosely packed icy spheres—something like giant snowballs—collide at velocities consistent with comet-sized bodies falling gravitationally toward each other. ¹

The two astronomers’ efforts were motivated by the “talps” model (splat spelled backward) proposed in 2007 by Michael Belton, an emeritus astronomer at Kitt Peak National Observatory in Arizona, and his collaborators, based on images of comet 9P/Tempel taken by NASA’s Deep Impact spacecraft. (For more on Deep Impact and other missions to comets, see the article by Don Brownlee, Physics Today, June 2008, page 30 .) In the talps model, Jupiter-family comets—those with periods of less than 20 years and orbits that extend out to near Jupiter—have nuclei that consist of layered piles of material formed from cometesimals smashing into each other. ²

Asphaug and Jutzi had been investigating how the Moon came to be lopsided. (See Physics Today, January 2014, page 14 .) The near side is low and flat with a thin crust, whereas the far side is dominated by heavily cratered and rugged highland terrain. To test the possibility that the highlands on the far side formed when the Moon accreted a smaller hypothetical companion, they ran simulations of low-velocity (2–3 km/s) collisions of the two bodies. ³ When Asphaug ran across the work by Belton and company, he recognized that their two models were strikingly similar. “In both cases it is easier for the projectile to become deformed than it is for the projectile to excavate and displace the target material. So it splats.”

Meanwhile, last year’s rendezvous of the European Space Agency’s Rosetta with 67P/Churyumov–Gerasimenko, shown in figure 1, spectacularly revealed a comet with a bi-lobed shape and signs of layering—smooth regions whose edges look stratified and exposed rocky-looking faces with linear features. ⁴ “When Rosetta arrived, it was clear that the comet’s structure required that the problem be tackled head on,” says Asphaug.

In the new simulations for cometary accretion, the researchers parameterized collisions in terms of a projectile-to-target mass ratio, impact velocity, and impact angle (from 90° for a head-on collision to 0° for a glancing one). Such simulations aren’t new, but the two went one step further by using algorithms developed by Jutzi to precisely model the behavior of low-strength material under microgravity conditions. Those algorithms allowed them to include realistic materials properties such as density, porosity, tensile strength, compression strength, and friction.

For colliding bodies with low density and little tensile strength, as one would expect for cometesimals, the simulations produced three distinct types of outcomes. The combination of high impact velocity and low impact angle produced hit-and-runs in which the two bodies failed to accrete. In direct hits, the smaller body collided with the bigger one with a splat and became a pancake-shaped layer of rubble. Between the two extremes, the simulations produced bi-lobed shapes as shown in figure 2.

The results appear to confirm 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 remnants of a young solar system or fragments left over from more recent events. For example, Asphaug and Jutzi’s colliding bodies could just as well have come from the breakup of an original comet by tidal forces near Jupiter, as happened to comet Shoemaker–Levy 9 in 1992, or from the catastrophic collision of larger objects.

To distinguish between the different scenarios is crucial because the compositional makeup of comets, which generally includes volatile gases that appear to have survived intact from when the Sun formed, doesn’t square up with modern dynamical models of solar- system formation. According to those models, especially ones involving giant-planet migration, Jupiter-family comets would have experienced catastrophic disruptions multiple times. Comets that still possess primordial ingredients seem out of place in that picture.

“I think future models and observations will be able to discriminate among the different options,” predicts Asphaug, “but right now we have to throw up our hands and say take your pick.” Next on the pair’s agenda is to test that prediction by simulating events similar to the breakup of Shoemaker–Levy 9 to find out whether they produce distinct morphologies that can be compared with real comets. Then would come the even more difficult task of simulating collisions between bodies as large as the 963-km- diameter dwarf planet Ceres.