Mysterious circles appear, grow on comet

A comet is growing its own version of crop circles. Over the course of a month, five expanding disklike depressions appeared on the surface of 67P/Churyumov-Gerasimenko in images taken by the Rosetta spacecraft, which has been orbiting 67P since August 2014 (SN: 9/6/14, p. 8).

The first roundish feature showed up on June 3 and was joined by a second 10 days later. Within a month, the first spot had grown to 220 meters across and 5 meters deep, and it had run into its neighbor. Three more spots appeared in early July, flanking the original two, researchers report online September 15 in Astronomy & Astrophysics.

The spots form around weak points in the surface, where sunlight can easily turn buried ice to vapor. That vapor in turn erodes the surrounding smooth plain — though the pits are growing too fast for sublimation alone to be their cause. Loosely bound dust or heat released by ice as it transforms from a disarray of water molecules into orderly crystals might help the spots along. Unlike with crop circles, bored pranksters are probably off the hook.