After a decade-long journey through space, the Rosetta spacecraft has spent the past year less than 100km (about 62 miles) from the nucleus of comet 67P Churyumov-Gerasimenko, capturing some stunningly detailed images. But despite this wealth of visual evidence for researchers there is a lot we still don’t know about the comet—including why it is covered in organic material rather than just ice and what causes its powerful jets of dust and ice.
One of the big surprises of the Rosetta mission has been discovering just how dark 67P is: completely unlike a “dirty snowball,” which was how astronomer Fred Whipple described comets in the 1950s. Although images from missions to other comets have shown surfaces that are more likely to be mixtures of ice and rock, findings from an instrument called the Visible and Infrared Thermal Imaging Spectrometer onboard Rosetta have shown that 67P is rocky and almost completely covered in a layer of organic compounds—which was not expected. So where is all the ice?
Now researchers using the same instrument have come up with an answer. They discovered a day-night cycle of ice sublimation (where solid turns to a gas without first turning to liquid) and re-condensation linked to the amount of sunlight. In short, when parts of the comet are in shadow, it is cold enough for ice to form; when the sun shines on the surface, the ice disappears.
