By far the most interesting and defining quality of comet 67P, when the Rosetta Probe visited it earlier this year, was its double-lobed shape. While not being completely new, this was the first example of such a process happening with a comet.
Contact binaries are assumed to have been two asteroids (or comets) that joined together through a collision slow enough that they wouldn't fly past each other, or destroy each other. A defining quality for these is the notable double-lobe nature of them when a telescope points itself at them. Naturally, based on the images, that's what I would have thought if I didn't know better. However I have a couple of reasons to believe otherwise.
First, long exposure images of the comet have found that both lobes of the comet are erupting. And, based on a JPL small-body database search, asteroids outnumber comets just a bit over 200:1. The chances of two comets merging together is less than 0.1%. To further this, every picture taken shows most of the outgassing coming from the center of the comet, along with the shape of the comet indicating that the area was dug away rather than gravitationally connected- a contact binary owuld have a distinct divider between each asteroid, with gravitational attraction flattening only the closest parts to one another, giving it more of a bowling pin look than an apple-core look.
Second, the density of the comet is 0.4 g/cm<sup>3</sup>, which is typical of a comet, but based on such a size and a density, a contact binary alone would not be sufficient to mold the two lobes of the comet together to such a high degree. Plus, you would expect the density to be higher had two asteroids collided with each other, pushing each other together and giving a density of at least a few more decimals than the found density.
Lastly, notice that, contrary to the rest of the comet, the 'bridge' is relatively smooth, without any craters. This shows that the surface is young in comparison, and that any craters that form are quickly wiped away as the dust that they are imprinted upon flies off the comet.
Based on the given evidence, I can conclude that 67P is a single comet, but through an unknown force, the center of it is being melted away quicker than the rest of the comet, and that we are perhaps observing live exactly how comets fragment. It may be quite a while before this one fragments, but I think the mechanism behind this quicker core sublimation would be a good target for more in-depth study, and that it will help us better understand exactly how comets work.
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