Rosetta reveals a complex comet

It’s time to stop thinking of comets as dirty snowballs. The Rosetta spacecraft’s first look at comet 67P/Churyumov–Gerasimenko shows a diverse, complex world, shaped by eruptions and erosion, that may hint at what the solar system was like 4.6 billion years ago.

A panoply of textures and structures shows that comet 67P is not a loose collection of ice and dust, the prevailing image of comets for decades.

“Rosetta has blown the dirty snowball idea out of the water,” says Nicolas Thomas, a planetary scientist at the University of Bern in Switzerland and member of the Rosetta team.

Previous missions had already hinted that comets were more complex than that. But the Rosetta mission, the first to orbit a comet and follow it as it approaches the sun, shows a patchwork of terrains weathered by blowing dust and gas eruptions.

The data, presented in seven papers in the Jan. 23 Science, reveal a thin layer of organic compounds slathered over the comet’s surface. There’s no evidence of surface ice, except for the occasional shiny nugget tucked away in a nook. Smooth plains of dust blanket a fractured, brittle crust, while towering cliffs expose layered bedrock. The entire landscape is littered with circular pits up to 300 meters across — and nearly as deep — carved by gas jets.

Rosetta launched in 2004 and spent 10 years in space before arriving at comet 67P last August (SN: 9/6/14, p. 8). In November, Rosetta dropped its lander, Philae, onto the comet — which subsequently bounced twice before settling in the shadow of a cliff and shutting down for lack of sunlight to power it. Rosetta will monitor the comet at least through its closest approach to the sun in August.

These data, taken through September 2014, are just “our first impression of the comet,” says Matt Taylor, Rosetta’s project scientist at the European Space Agency’s Space Research and Technology Center in Noordwijk, the Netherlands. This first look at comet 67P provides a reference point that will help researchers understand the changes that occur as the comet moves closer to the sun. “We’ve got the photograph,” Thomas says. “Now we want the movie.”

One question Thomas wants to answer is whether the surface diversity says something about how the comet formed. Comets — as well as asteroids and planets — are thought to have formed from tiny dust particles that swirled around the infant sun and stuck together. If the comet was assembled from debris that originated in different parts of the solar system, that might explain the assortment of terrains. But heat from the sun may have reworked the surface as well. With just a few months of data, it’s too early to tell.

Thomas says he was very surprised to see what appear to be dunes rippling across some of the plains of dust. “It’s bizarre,” he says. Dunes are common on Earth and Mars. “But to see them on a comet where there’s no atmosphere … how the hell do you do that?” He wonders if the gas jets seen on the comet, driven by sublimating ice (ice heated by the sun and turned into a gas), create a wind that can shift dust around.

“You don’t get these kinds of landforms on a snowball,” says Jessica Sunshine, a planetary scientist at the University of Maryland in College Park. Unlike asteroids, comets appear to be shaped by sublimating ice and occasional gas eruptions, which can remove material entirely or just drop it somewhere else on the surface. “This is a much more complicated beast that we’re trying to understand,” Sunshine says.

It’s great to see all this diversity up close, says Peter Thomas, a planetary scientist at Cornell University. If comet 67P “were just a fuzzy tennis ball vaporizing, it would be less interesting.” The cliffs, for example, appear to cut into the interior of the comet, exposing bedrock that may have built up in layers. “This opens up a lot of ways to tease out the history,” he says. “That wasn’t something that was a sure thing to see.”

Figuring out how that history unfolded will have to wait. Rosetta hasn’t seen the entire comet yet; the south pole is still hidden from the sun and won’t see light for several more months. Over the coming year, Rosetta will also be able to watch the comet change as it heats up, which should let researchers better understand how various terrains formed.

“They picked a really good comet to go to,” says Don Brownlee, a planetary scientist at the University of Washington in Seattle. Following 67P as it approaches the sun “is going to show us how comets really work.”