Cometary Breakup and Reassembly

Yesterday’s look at organic compounds on Comet 67P/Churyumov-Gerasimenko needs to be augmented today by a just released study of the comet with implications for how all comets evolve. But first, a renewed pointer to the Kickstarter campaign for KIC 8462852, the unusual star whose light curves continue to baffle astronomers. Please consider contributing to the project, which would raise enough money ($100,000) to support a year of observations.

We’re about halfway through the campaign but not yet at the halfway point in funds. Have a look at the information provided on the Kickstarter page, or in my essay A Kickstarter Campaign for KIC 8462852, which also has the relevant links. We know the light curves of ‘Tabby’s Star’ are not periodic, so we need continuous monitoring to gain more data on what may be happening there. If we can raise the funds, the Las Cumbres Observatory Global Telescope Network, already supporting the project, can give us the multi-wavelength observations we need.

A Comet’s Evolution

The rubber-duck shape of Comet 67P/Churyumov-Gerasimenko has long been noted. The ‘neck’ of the comet is what connects the two larger lobes, as is obvious in the image below. As a new study led by Masatoshi Hirabayashi (Purdue) and Daniel Scheeres (University of Colorado) points out, two large cracks appear on the neck connecting the two larger lobes. The team simulated rotation rates for the twin-lobed assembly different from its actual 12-hour spin.

The result: Two cracks similar enough to those on 67P to show just how much stress is imparted. The rotation rate is variable in an object like this one because flybys of the Sun or of Jupiter can produce a gravitational torque. And as also appears in the photo, cometary outgassing is a factor, with compounds like carbon dioxide and ammonia sublimating from the surface. A fast enough spin produced by these factors can cause the two lobes to separate. Seven hours per rotation is what it takes for the head of the ‘duck’ to break off.

Image: Comet 67P’s distinctive shape tells us much about its history. Credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0.

The researchers used numerical models that examined 1000 instances of 67P ‘clones’ under varying conditions over a 5000 year period. What Hirabayashi and Scheeres have learned is that the breakup and reassembly is an ongoing process as comets respond to these stresses. It’s also one that could last the lifetime of the comet. Says Scheeres:

“The head and body aren’t going to be able to escape from each other. They will begin orbiting each other, and in weeks, days or even hours they will come together again during a slow collision, creating a new comet nucleus configuration.”

As strange as it looks, Comet 67P may not be all that unusual. So far we have imaged seven comets at high resolution, five of which are bi-lobed. The researchers have learned that all of the bi-lobed comets have similar volume ratios between each lobe, an indication that the same cycle of disassembly and reassembly is happening in them as well. In some, there are similarities to what we find in a certain kind of asteroid. From the paper:

…bilobate nuclei observed by spacecraft encounters or ground-based radar have component volume ratios consistent with their nuclei being trapped in a similar cycle to that of 67P’s nucleus. For bilobate nuclei with a volume ratio between their lobes larger than about 0.2, the total energy of these systems will be negative after fission. This means that they are bounded in a similar way to some rubble pile asteroids; however additional sublimation effects could further erode or spin up the individual lobes before re-impact.

The process may be a major factor in cometary evolution, giving us insights into how these objects change over time:

Taking material density to be constant, we computed the volume ratios of the imaged bilobate nuclei of comets 1P/Halley, 8P/Tuttle, 19P/Borrelly, 67P and 100P/Hartley 2; we found that all of these nuclei had a volume ratio higher than 0.2… Observed nuclei with a single component might either be primordial, or have been part of a multi-component object, from which smaller parts are more easily shed.

Window into the Late Heavy Bombardment?

67P/Churyumov-Gerasimenko is a Jupiter-family comet orbiting the Sun every 6.5 years; such periodic comets are thought to originate in the Kuiper Belt, far beyond Neptune’s orbit. We learn that chaotic spin rate changes and the subsequent breaking into parts and reassembling probably caused the breakup of many ancient periodic comets originating at similar distances from the Sun. Enough erosion would have been produced by the continuing reconfiguration of their nuclei to reduce their ability to survive migration into the inner Solar System.

This could explain why comets were not a strong factor in the late heavy bombardment some four billion years ago, when numerous asteroids collided with the early terrestrial planets — two recent papers have made this case. “The reconfiguration cycles of short-period cometary nuclei,” the paper adds, “constitute a new evolutionary process that could affect their ability to survive during migration into the inner solar system.”

ESA’s mission to Comet 67P may, in other words, be giving us insights into the primordial bombardment that reshaped terrestrial worlds. The paper is Scheeres et al., “Fission and reconfiguration of bilobate comets as revealed by 67P/Churyumov–Gerasimenko,” Nature 1 June 2016 (abstract).