You'd never know from all the high-contrast photographs of Comet 67P/Churyumov-Gerasimenko that the comet is actually quite dark. This recently released composite image reveals how little light there is emanating from 67P/C-G, relative to other objects in our solar system.


The composite image was included in a recent post about albedo (the term astronomers use to describe the reflectivity of an object) at ESA's Rosetta Blog. It's not none-more black, but 67P/C-G is pretty damn dark:

one way of giving at least a suggestion of just how dark comets are is to show 67P/C-G against a number of other Solar System objects exhibiting a wide range of albedos.

The montage below compares 67P/C-G with the Moon, the Earth, and Enceladus, a moon of Saturn. The brightness of each object in the montage has been scaled according to its mean albedo: for 67P/C-G, we have taken an albedo of 5%; for the Moon, 12%; for Earth, 31% (with deserts having an albedo of roughly 40%, thin clouds 30–50%, thick clouds 60–90%, and oceans 7–10%). Finally, for simplicity, an albedo of 100% has been taken for the brightest parts of the ice-covered surface of Enceladus, the most reflective body in the Solar System.

It's hard to do this scientifically accurately, partly because the actual albedo in a given image of an object depends on a whole host of factors and because the human eye and brain don't respond linearly to different light levels. But hopefully this comparison gives at least an impression of quite how dark 67P/C-G is and how diverse the Solar System's bodies can be.


More here.