December 12, 2014

Comet 67P Only Looks Gray, It’s Actually Black

The European Space Agency’s Rosetta mission has now released the first narrow-angle color composite image of Comet 67P – taken through a set of red, green, and blue filters.

By Caleb A. Scharf

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This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

The European Space Agency's Rosetta mission has now released the first narrow-angle color composite image of Comet 67P - taken through a set of red, green, and blue filters. And here it is, in all its glory from a mere 120 kilometers away, with a roughly 3.9 meter per pixel resolution.

Comet 67P in all its colourful glory (Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

You might be forgiven a guffaw, the cometary nucleus is remarkably, and uniformly, gray looking. No hint of surface water ice deposits (which might look bluish), just lots of grime and rock. Too subtle for the human eye, there is actually a slight enhancement in the red reflectivity of the nucleus compared to other colors, this is likely due to the optical properties of tiny dust grains.

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But even this image is misleading. The actual reflectivity of 67P is closer to that of coal or charcoal. So why doesn't it appear black? Well, it does, you just have to look in the shadows. Even out here, some 2.8 times further from the Sun than the Earth is, the sunlight is still brilliant. Where it strikes a surface directly, the few percent that's reflected is plenty for sensitive cameras and eyes to register (just like our own Moon). A little stretching of the image brightness doesn't hurt either. Yet that reflected light doesn't do much to illuminate the shaded zones. The stark differences between light and dark is one clue to the blackness of the nucleus - it's also because of the lack of any atmosphere diffusing the sunlight.

Of course this dark aspect, and the dust that makes it so, is absolutely fascinating. Future data from Rosetta will make use of more than just 3 color filters, the OSIRIS imaging experiment actually has 25 - allowing it to search for very specific minerals in the surface. And as 67P warms up in its path around the Sun it will reveal more of its compositional secrets by ejecting gas into the cometary coma where Rosetta can study it.

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