How Comet 67P’s Face Changed During Its Trip Around the Sun

http://www.nytimes.com/2017/03/23/science/rosetta-comet-67p-landslides-cliff-collapse.html

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Cliffs collapsed, boulders moved, cracks opened up, and jets of dust and gas erupted.

Those are some of the transmogrifications that scientists discovered as they pored over two years of photographs of Comet 67P/Churyumov-Gerasimenko taken by Rosetta, the European Space Agency spacecraft whose comet-trailing mission came to a conclusion in September.

The unexpected shape of the comet — like a rubber duck instead of a simple sphere — was most likely the result of a merger of two comets in the distant past. As 67P approached the sun, jets of dust and gas sped up the spin of the comet, increasing stresses on the body.

A crack in the neck of the comet, initially about 550 yards long when Rosetta arrived in August 2014, lengthened by 30 to 45 yards A new crack, about 160 yards long, opened up nearby.

“This comet is probably eventually going to split up as it speeds up more and more,” M. Ramy El-Maarry, a planetary scientist at the University of Colorado, said during a news conference on Tuesday at the Lunar and Planetary Science Conference outside Houston. “When is that exactly going to happen? Of course, I have no idea.”

Dr. El-Maarry and his colleagues published their findings this week in the journal Science. A second paper, in the journal Nature Astronomy, described how the collapse of one particular cliff set off an eruption of dust and gas from the comet’s surface.

Until Rosetta, astronomers had only brief glimpses of comets. Rosetta’s journey around the solar system allowed the spacecraft to accompany Comet 67P, about 2.5 miles wide, for two years of its 6.5-year orbit as it approached the sun and then swung back out.

In two close-up images of the same spot on the comet, one taken in May 2015, before the comet made its closest approach to the sun, and the other about a year and half later as it was moving away, most features of the landscape were unchanged. But a boulder that was bigger than most houses had moved more than 160 yards, or more than the length of a football field.

In the minuscule gravity of the comet, that would have required the same amount of force for lifting a 550-pound boulder on Earth, Dr. El-Maarry said.

Calculations indicate that a powerful gas outburst could have shoved the boulder.

In some places, ripple patterns appeared, disappeared, then appeared again. In other places, the surface eroded quickly, at a pace of several yards a day, when the comet was closest to the sun.

Yet none of the events that Rosetta observed could have created the giant cliffs and huge depressions seen on the surface, and the duck-shaped comet, as it sped away from the sun, looked pretty much like the same duck as when Rosetta arrived.

That indicated that the comet was much more active earlier in its lifetime, Dr. El-Maarry said.

In the Nature Astronomy paper, Maurizio Pajola, a postdoctoral researcher at the NASA Ames Research Center in Mountain View, Calif., and his colleagues described a collapsed cliff that he had spotted in an image taken in December 2015. But an image of the same spot the previous year showed a rock overhang that had a 230-foot-long, three-foot-wide crack.

Dr. Pajola then searched through other images to figure out what had happened.

The cliff was intact on July 4, 2015. The next time Rosetta observed it, on July 15, it had collapsed, leaving new boulders piled at the foot of the 440-foot-high cliff, with bright, reflective material from the comet’s interior.

Dr. Pajola estimated that 780,000 cubic feet of cliff had collapsed — about the volume of nine Olympic-size swimming pools.

A lower-resolution camera observed a plume of dust rising off the same region of Comet 67P on July 10.

“We were pretty sure it was an outburst directly related to the cliff collapse,” Dr. Pajola said. “So we were lucky this cliff collapse happened right in front of our eyes.”