Rosetta's up-close views of comet: stunning, but puzzling

Nearly 11 years after launch and five months into its cosmic road trip with a comet, the European Space Agency's Rosetta orbiter is providing stunning views and raising puzzling questions about its traveling companion: comet 67P/Churyumov-Gerasimenko.

The comet, 67P for short, sports a rubber-duck-shaped nucleus with goosebumps. It's a Baby Huey, sporting a mass of 10 billion metric tons. It displays more variety in the amount and relative abundance of the gases it sheds than researchers expected. And it sports an array of dust and chunks of debris up to six feet across that orbit the nucleus, like bees unwilling to leave the hive's neighborhood.

These are among the observations the Rosetta science team is unveiling in Friday's issue of the journal Science. Rosetta arrived at the comet last August. The observations researchers have described were gathered during the first two months Rosetta and 67P became co-travelers.

The results so far are "tremendous; they are completely changing what we know about comets," says Dennis Bodewits, a researcher at the University of Maryland, College Park, who focuses on behavior and evolution of comets. "Being able to orbit a comet while it is flying close to the sun, we can see things and really figure out how comets work" at an unprecedented level of detail.

Comets, along with asteroids, represent the construction rubble left over from the solar system's planet-building stage some 4.5 billion years ago. Comets in particular are thought to carry some of the most pristine ingredients the young sun and its extended disk of dust and gas had to offer as raw material for planets. Comets also are known to carry organic compounds and are thought to be one type of vehicle that delivered water and organic chemicals to Earth – chemicals that could serve as building blocks for more-complex molecules underpinning organic life.

Comet 67P is providing the most rigorous test yet for ideas about how comets form and evolve as they make their periodic pilgrimages toward the sun and back.

Using Rosetta's OSIRIS cameras – one for detailed close-ups of the surface and one for wider views – the mission's science team has uncovered an amazing variety of surface features.

On large scales, some portions of the surface appear brittle, with sections hundreds of feet across looking as though they'd collapsed after being undermined. Other regions of the surface appear to be vast rubble piles, while others appear as smooth plains. One region hosts a cliff nearly 3,000 feet tall that rises from the adjoining plain.

Virtually the entire surface is covered in a layer of dark dust. The craft's VIRTIS spectrometer has uncovered an array of molecules with high carbon content in the surface material, but precious little ice. This is unlike other comets similar to 67P – members of a class known as Jupiter-family comets with return periods of about 20 years or less. They get their name from the influence Jupiter's gravity has in shaping their trajectories.

"67P represents a different species in the cometary zoo," writes the VIRTIS team in its contribution to the package of Rosetta results in Science.

The virtually uniform composition of the surface material, despite multiple trips around the sun that could have altered it, suggests that the material is fairly pristine, and so likely signals that the material deeper in the nucleus is pristine as well. Given the nature of the organics in the dust, the team says that its measurements point to a comet that formed in a chilly portion of the early disk of dust and gas that surrounded the sun – likely out beyond Neptune's current orbit.

On smaller scales, the surface also is dotted with circular pits, with many ranging in size from 164 to nearly 1,000 feet across and 30 to 660 feet deep. These represent the "nozzles" for the jets of dust and gas the comet releases as the ices beneath its insulating layer of dust heat and shift directly from ice to gas. The surface also hosts goose bumps, closely packed chunks of debris 10 feet across that cover football-field-size patches of surface. [Editor's note: The original has been corrected to more accurately describe the "goose bump" features.]

Two outstanding puzzles: How and where did 67P form? Its two lobes connected by a neck of material could represent one object with a highly erodible waist. Or the lobes could represent two smaller objects that gently merged in an accretion process representative of the process by which the planets formed.

And if the composition of the two lobes differ, it could imply that one of 67P's lobes formed in one part of the solar system, then picked up a second lobe as it was migrating toward its current orbit.

Part of the answer may rest with the shifting nature of the gases in the halo of dust and gas surrounding the nucleus.

As Rosetta approached 67P, researchers expected that the relative abundance of carbon monoxide (CO), carbon dioxide (CO2), and water vapor in the halo of dust and gas that surround the nucleus known as the coma, would remain constant even as the comet warmed and emissions increased. That's what one might expect if the comet assembled itself in one region of origin. Instead, they've found that the relative abundances vary, as well as the overall amount, with changes in the rotating comet's seasons and in its day-night cycle.

The varying abundances could strictly be an artifact of the day-night or seasonal cycles. The three ices sublimate at different temperatures, meaning that on the dark portion of the comet, temperatures would be cold enough to freeze water while still being warm enough to allow CO or CO2 to sublimate.

But it's also possible that these variations reflect variations in the composition of the nucleus itself, says Myrtha Hässig, a scientist at the Southwest Research Institute in San Antonio and a member of the team using ROSINA, a package designed to analyze the chemical composition of gases 67P releases as well as collective pressure of their flows.

If that's the case, and given the comet's shape, it could be that 67P was built from two elements that merged. This would likely have occurred during a period in the early solar system when the giant planets were migrating to their current orbits and their gravitational influences, particularly that of Jupiter, the solar system's largest planet, were disrupting the orbits of comets, asteroids, and even larger planet-wannabes.

If the two lobes are different, that "might actually point toward early migration in the solar system," says Dr. Hässig, who is the lead author of the Science paper describing the ROSNIA measurements. Evidence for that migration currently comes mainly in the form of results from computer simulations of the solar system's formation.

A key test will come with the changing of the seasons on 67P, when the second of its two lobes picks up more sunlight. So far, it has remained largely in the dark, with the release of gas coming mainly from the lit neck of the duck, as well as from the currently lit lobe.

[Correction: This article has been updated to clarify that members of a class known as Jupiter-family comets have return periods of about 20 years or less.]