Mysterious circles appear, grow on comet

Rosetta spacecraft spotted five round depressions in June, early July

comet circles

SEE SPOT GROW  The first of five growing circles on comet 67P appears in a June 5 image from Rosetta (left). By June 23, two spots were spreading across the surface (center). They were joined by a third on July 2 (right).


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.

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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