Rosetta's comet may be made of pebbles entirely

Chuck Bednar for redOrbit.com – @BednarChuck

The comet currently being observed by the ESA’s Rosetta spacecraft may actually be made entirely of pebbles, scientists involved with the mission explained earlier this week at the 46th annual Lunar and Planetary Science Conference in Houston, Texas.

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According to NewScientist, François Poulet of the CIVA camera team presented an in-depth analysis of the first images of Rosetta’s Philae lander on the surface of Comet 67P/Churyumov-Gerasimenko (67P). (What a mouthful.) He reported that the pictures indicate that parts of the surface around that location appear to be made out of aggregated pebbles, while others tended to be smoother.

The CIVA instrument is a suite of six micro-cameras used to capture panoramic pictures of the surface, as well as a spectrometer studies the composition, texture and the albedo (or reflectivity) of samples collected from the comet’s surface. Poulet said that the discovery of the pebbles was a good sign because they match-up with one existing model of how comets form.

That model proposes that miniature particles in the early universe clumped together to form pebbles approximately one-centimeter in size, but that those pebbles did not necessarily grow to be increasingly larger objects. Rather, they come together but retain their original shape, which is what Poulet’s team discovered, indicating that Comet 67P “could be totally made of pebbles.”

Pebble accretion

The comet originated from the Kuiper belt, a region located beyond the dwarf planet Pluto that is filled with icy objects left over from the formation of our solar system over 4.5 billion years ago. If the data collected by Philae during the short period it was operational after it touched down on comet is true, it will force experts to re-examine how pebbles as large as those observed on 67P were able to form so far away from the sun, Discovery News explained on Thursday.

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“The pebble accretion is kind of a new idea,” Donald Brownlee, an astronomer at the University of Washington, told the website. He explained that the theory suggests that the motion of gas can trap and transport medium-sized pebbles, which in turn attract more gas “like a magnet.”

Rosetta’s comet could be the first entity to provide actual evidence of this hypothesis, he noted. However, he and his colleagues previously found hints that comets might be large collections of pebbles in 2004, when NASA’s Stardust spacecraft flew past a comet called Wild-2 and brought back samples back to Earth.

Brownlee told Discovery News that his team found “some blocks that were clearly stronger than their surrounding material, but… 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.”

[STORY: Is Rosetta's comet falling apart?]

That material may have been collected to form the comet, and the fact that they were able to survive intact indicates that they could not have been travelling at high speeds, he added. The Rosetta and Philae mission scientists have reportedly mapped hundreds of these objects.

“We can’t tell yet if these are really all throughout the interior of the comet,” Holger Sierks, an astronomer at the Max Planck Institute for Solar System Research in Germany, told Discovery News. However, he noted that the pebbles found thus far are too large to be in line with existing models of how objects in the outer solar system form.

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