The collapse of cliffs on comets can create plumes of gas and dust, which contribute to comets' characteristic tails.

Credit: ESA/ROSETTA/NAVCAM Such outbursts are frequent, but their cause has been unclear. Maurizio Pajola at the NASA Ames Research Center in Moffett Field, California, and his colleagues analysed images taken by high-resolution cameras on the European Space Agency's Rosetta spacecraft as it orbited the comet 67P/Churyumov–Gerasimenko. By comparing images taken before and after an outburst on 10 July 2015 (pictured), the team traced the origin of the event to the collapse of an already fractured cliff, and a subsequent landslide.

In a separate study, Mohamed Ramy El-Maarry at the University of Colorado, Boulder, and his colleagues used Rosetta's instruments to map 67P's surface over an 18-month period. They concluded that most such surface changes are caused by increased exposure to sunlight — which evaporates ice and weakens land structures — when the comet's orbit takes it close to the Sun every 6.5 years. Given that these landscape shifts are relatively cosmetic, the authors suggest that the landforms on comet 67P have not changed significantly since at least 1959, when it entered its current orbit.

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