Nature http://doi.org/bbvn (2016)

Credit: NPG

Finding water on a comet might not come as a big surprise, after all it is one of the main components of the cometary nucleus and comas are made mostly of water vapour. But finding exposed water ice is news, because many comets observed so far — including 67P/Churyumov–Gerasimenko (67P; pictured), made famous by the Rosetta mission — are dark and dusty; true 'icy dirtballs'. Now, using the Rosetta mission cameras and Visible Infrared and Thermal Imaging Spectrometer (VIRTIS), Gianrico Filacchione and colleagues have identified two regions of exposed water ice.

Filacchione et al. spotted two bright patches in the Imhotep region of 67P. Taking a closer look at these features, they found that they were debris falls where ice has been exposed. The VIRTIS spectra confirm this is indeed pure water ice made of millimetre-sized grains — much larger than one would expect from vapour condensation. The large ice grains are likely the result of more complex processes such as growth by vapour diffusion or sintering, suggesting that the coating of the nucleus keeps evolving by the build-up of overlapping dirt and ice layers.