Rosetta's Comet May be Made of Pebbles

The comet being studied by Europe’s Rosetta spacecraft may be made entirely of pebbles, challenging currently held theories of how bodies form in the far reaches of the solar system.

The comet, called 67P/Churyumov–Gerasimenko, or 67P for short, is a transplant from the Kuiper belt, a region beyond Pluto littered with icy bodies left over from the formation of the solar system some 4.6 billion years ago.

Rosetta arrived at the comet in August 2014 and three months later released a companion lander named Philea for additional studies. Philea touched down, but then bounced several times before coming to rest sideways wedged in a hole or jammed against a cliff wall.

The lander was able to run through a preprogrammed series of initial experiments before losing power. Scientists hope to regain contact with the probe in the next few months so it can resume its studies, the first to be conducted on the surface of a comet.

During its brief mission, Philae collected some key data, including images that show the entire comet may be made of pebbles, scientists said at the Lunar and Planetary Science Conference in Houston this week.

If confirmed, scientists will have to rethink how pebbles as big as some seen on 67P could form so far from the sun.

“The pebble accretion is kind of a new idea,” University of Washington astronomer Donald Brownlee told Discovery News.

The theory is that motions of gas can effectively transport and trap medium-sized pebbles, which in turn attract more gas. “It’s like a magnet effect,” Brownlee said.

“The cool thing is then accretion speeds are basically zilch. Instead of (objects) whamming in at a kilometer-per-second, or a fraction of a kilometer-per-second, they can just drift around at meters-per-second. They’re still orbiting the sun, but relative to each other, the speeds can be very low,” he said.

Rosetta might be the first body to provide proof of the idea, added Brownlee, who is not involved with the mission.

Scientists have had previous hints that comets may be piles of pebbles. NASA’s Stardust spacecraft flew past a comet called Wild-2 (pronounced “Vilt-2”) in 2004 and later returned samples back to Earth.

“On Stardust, we did see some blocks that were clearly stronger than their surrounding material, but our resolution was 12 meters,” said Brownlee, who was lead scientist on the mission.

“We thought the surrounding material was just ablating away and leaving their original blocks. Rosetta is seeing huge numbers of these things, so my guess is those are original accreted materials ... debris that was out there in the Kuiper belt that went in to form the comet. The fact that they survived means they couldn't have been zipping in at kilometers-per-second speeds,” he said.

Scientists on the Rosetta and Philea missions have mapped several hundred of these objects.

“We can’t tell yet if these are really all throughout the interior of the comet,” astronomer Holger Sierks, with the Max Planck Institute for Solar System Research in Germany, told Discovery News.

Read more at Discovery News