Scientists have long pondered what lies beneath the surface of comet 67P, but a study out in Nature this week has the answer: dust. Lots and lots of dust. I was hoping for space gremlins, but to planetary scientists this result is almost as exciting.

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Comets, frozen leftovers from the formation of the solar system, are a mixture of water ice and rock. If fully compact, we’d expect them to be denser than water, but often, that isn’t the case. Rosetta’s comet and others have very low densities, leading scientists to ask whether these time capsules are riddled with caves and caverns.

Now, this mystery has been solved for one of the most well-studied space rocks in the cosmos. Comet 67P isn’t a cosmic crunch bar. It’s more of a malt ball, solid all the way through but filled with very light, powdery dust grains.

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Astronomers reached that conclusion in a clever way: by studying the effect of the comet’s gravity on radio signals received on the ground. If the comet’s interior was pitted, its gravitational tug on Rosetta would be stronger during some parts of the spacecraft’s orbit than others, causing a change in acceleration. That, in turn, would produce a Doppler shift in the frequency of Rosetta’s radio signals. Since no large acceleration changes were detected, we can conclude that 67P is homogenous—but very, very fluffy. Scientists estimate the interior is about 75 percent dust grains and 25 percent water ice.

There could, however, still be small caves eluding our measurements. As Rosetta spirals toward 67P this fall in a dramatic suicide run, we’ll refine our measurements of the comet’s gravity field. Maybe the space gremlins are lurking somewhere, after all.

[Nature via ESA]

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Top: Images of Comet 67P taken by Rosetta’s navigational camera between August and November 2014, via ESA/Rosetta/NavCam

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