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Rosetta's Comet Revealed: It's Dry on the Outside, Fluffy on the Inside

Reports from the European Space Agency's Rosetta mission reveal that the comet it has been following has a dried-out surface and a fluffy interior.
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The comet that's being shadowed by the European Space Agency's Rosetta spacecraft is getting its most detailed once-over yet in a series of scientific papers — revealing a candy-bar structure with a dry, crunchy surface and a soft, fluffy interior.

However, the cosmic candy bar known as Comet 67P/Churyumov-Gerasimenko is being unevenly cooked as it approaches the sun, and the initial reports published in the journal Science suggest it will undergo dramatic changes in the months ahead. That's exactly what the scientists on Rosetta's team are hoping for.

"We are trying to see how a comet evolves over time, and also through the course of its orbit," University of Maryland astronomer Michael A'Hearn, a co-author of three of the seven papers published this week, said in a news release. "Gaining this detailed time series is what distinguishes Rosetta from other missions."

Gallery: Scientists Show Off Comet Close-Ups

Other space missions — ranging from ESA's Giotto flyby in 1986 to NASA's Deep Impact smashup in 2005 — have characterized comets before, but only on a one-shot basis. Rosetta, which made its rendezvous with Churyumov-Gerasimenko last August after a 10-year, 4 billion-mile trip, is designed to monitor the comet as it swings around the sun this August.

Last November, Rosetta sent down a lander dubbed Philae for a historic touchdown on the surface of the comet — and although Philae's batteries were drained down to hibernation levels due to some equipment snags, scientists still hope that the lander will recharge itself as the comet nears the sun.

Churyumov-Gerasimenko is a little more than two and a half miles (4.1 kilometers) long in its widest dimension, with a mass of about 22 trillion pounds (10 trillion kilograms). It's shaped like a rubber ducky, with a head, a body and a slender neck. The comet is big enough and active enough to have its own magnetosphere — apparently created by the ionized gas surrounding Churyumov-Gerasimenko's nucleus.

Readings from Rosetta's instruments indicate that the nucleus is less than half as dense as water, suggesting that the interior is extremely porous. However, scientists found that the surface was dehydrated, with a high concentration of dust and simple organic compounds. The high-resolution camera on Rosetta, known as the Optical, Spectroscopic and Infrared Remote Imaging System or OSIRIS, spotted ripple-like structures on the surface, wind tails and smooth depressions filled with dust.

Image: Comet geography
The Rosetta team defined 19 regions on Comet 67P/Churyumov-Gerasimenko's surface, based on large-scale unit boundaries. The names for the regions were inspired by figures in ancient Egyptian history and mythology.N. Thomas et. al / Science

The Rosetta mission's scientists said the look of the terrain "suggests that surface dust transport is of major importance in defining the uppermost surface layer in many regions." They also saw fractures in the surface, probably due to the release of pressurized gas from beneath the surface as the comet warms up.

The researchers said the evidence provides further support for the view that "localized gas-driven transport" moves material around on the comet's surface.

They also saw fractures in the surface, probably due to the release of pressurized gas from beneath the surface as the comet warms up unevenly. "The surface and subsurface temperatures vary quite a lot as the comet rotates and changes its orientation toward the sun during its orbit," Peter Schloerb, an astronomer at the University of Massachusetts at Amherst, said in a news release.

For months, Rosetta's team has been tracking the emissions of dust and gas into the coma that surrounds the comet's nucleus. Most of the emissions are coming from the comet's illuminated side, and from its "neck." Those emissions are expected to accelerate with potentially explosive effects as Churyumov-Gerasimenko approaches the sun.

"We are already seeing more activity," the University of Maryland's Dennis Bodewits said. "Jets are sprouting up everywhere. We've been surprised to see how active it is. It already has more jets than many other comets do at perihelion."

The journal Science has created an online portal page for this week's reports on Rosetta research.