Once the Rosetta spacecraft arrived at Comet 67P/Churyumov-Gerasimenko last August, European scientists used an array of instruments to assess every nook and cranny of the remarkable two-lobed nucleus.

It took the comet-chasing Rosetta spacecraft 10½ years to reach its objective. That's a long time to wait to find out if all the systems and instruments are going to work as planned.

Fortunately, vindication for the European Space Agency and for scores of mission scientists came soon after the spacecraft reached Comet 67P/Churyumov-Gerasimenko last August 6th. In fact, even before the hitchhiking Philae lander made the first-ever soft landing on a comet on November 12th, the mother ship's 11 experiments had scrutinized the stark, two-lobed nucleus from tip to tip — occasionally from as close as 6 miles (10 km). The first two months' observations are summarized in a set of seven articles published last week in Science.

In my view, the most far-reaching discovery by Rosetta to date was actually announced some weeks ago, ahead of the magazine's print edition: Comet 67P's water molecules have a deuterium-to-hydrogen ratio far higher than that in Earth's oceans. Particularly because this is a Jupiter-family comet (JFC), the type that are most likely to have collided with Earth over the eons, this argues that comets contributed very little of our planet's water.

Comet 67P's terrain types
Rosetta scientists have identified 19 distinct regions on Comet 67P/Churymov-Gerasimenko, all named for ancient Egyptian deities.

Meanwhile, the spacecraft's basic measurements of this object are interesting in themselves. Far from being an idealized orb of ice and rock, the comet's nucleus has a two-lobed shape that invites wild speculation as to its origin.

In an article describing results from the OSIRIS camera, Holger Sierks (Max Planck Institute) and his team note that the larger lobe is 2.5 miles (4.1 km) long, while the smaller one spans 1.6 miles (2.6 km). They're joined by a narrow waist, which intriguingly has been the source of most of the comet's escaping gas and dust to date. It's not clear whether the comet assembled as two large masses or that its midsection was once much plumper but has gradually eroded away.

How dark is a comet's nucleus?
The Moon (on average) reflects 12% of the sunlight that strikes its surface. But the nucleus of Comet 67P is much darker still, with an albedo (reflectivity) of only 6%. These images are not to scale.
Moon image: R. Vanderbei, Princeton Univ.; comet: ESA / MPS for OSIRIS team

Four other details about the nucleus are intriguing. First, it's very dark overall, with an average reflectivity of just 6% — nearly black, much like charcoal. This jibes with the albedos found at Comet 9P/Tempel 1 nearly a decade ago at Comet 103P/Hartley 2 in 2010 by NASA's Deep Impact spacecraft. Ditto for the nucleus of 1P/Halley. There's an unmistakable trend here: when you think "comet," think black.

What has likely happened is that rapidly escaping gas has carried off bits of dust that are not moving as fast, so they eventually settle back onto the surface. The comet's low gravity (less than 1100,000 that on Earth), combined with its 12.4-hour spin, distribute these particles all around to create an even veneer.

An active pit on Comet 67P?
Rosetta captured this closeup of a large pit (at left) while 60 km from the comet's nucleus. Computer enhancement (at right) reveals detail in the shadow that appears to be a jet from the pit's interior.
ESA / Rosetta / MPS for OSIRIS team

Second, based on how strongly it attracts Rosetta, the nucleus must have a mass of about 10 billion metric tons. That's certainly hefty — it's about 25 million times the mass of the International Space Station, for example. But Sierks and his team report that the comet's overall density is just 0.47 g/cm3 — similar to wood. It's not made of wood, of course, but the ice-and-rock interior must be very "fluffy," with a porosity of 70% to 80%.

Third, scans by Rosetta's VIRTIS instrument (short for Visible, Infrared and Thermal Imaging Spectrometer) hasn't found any ice on the surface of 67P. Instead, a team led by Fabrizio Capaccioni (INAF, Italy) reports that the dark exterior is covered with complex, carbon-rich organic molecules. "VIRTIS clearly observed a comet that is different from the other JFCs encountered so far," the groups notes, because other such comets do have exposures of water ice on their surfaces. (The original leader of the VIRTIS team, Angioletta Coradini, never got to see these results; she died of cancer in 2011.)

The VIRTIS data suggest the organic molecules have lots of carbon-hydrogen bonds but few involving nitrogen. The team doesn't speculate as to what these are — but I will! It's possible that compounds are polycyclic aromatic hydrocarbons (PAHs) — much like the stuff coating the dark half of Saturn's moon Iapetus (not to mention what results when you grill a steak too long).

A crack in Comet 67P
This image of Comet 67P/Churyumov-Gerasimenko reveals a large fracture, roughly 500 m long, running through the nucleus.
ESA / Rosetta / MPS for OSIRIS team<./i>

Finally, the surface is Comet 67P/Churyumov-Gerasimenko is a wonderland of weird landforms. The OSIRIS team has subdivided Comet 67P into 19 regions, all named for ancient Egyptian deities, that correspond to five distinct terrain types: smooth, brittle with pits and circular structures, large depressions, dust covered, and consolidated ("rock-like"). For example, Hapi (the god who makes the Nile River flood each year) is the smooth terrain in the neck that, thus far, has dominated the comet's outgassing. Looming above Hapi is a striking cliff face. Gas-spewing pits dot the region called Seth (a violent god associated with storms and disorder), and elsewhere the camera recorded enigmatic mounds, about 10 feet (3 m ) across, that the OSIRIS team calls "goosebumps."

Not a bad scientific haul for two months' work, and many other results (such as water-escape rates and solar-wind interactions) are detailed in the Science papers. And yet the most exciting events are still months away. Comet 67P/Churyumov-Gerasimenko is moving inward toward its August 13th pass through perihelion, by which time the nucleus should be jetting much more vigorously.

Read all about the Rosetta mission in Sky & Telescope's August 2014 issue.


Image of Nzubenel


January 28, 2015 at 10:42 am

Great article, J. Kelly! You know, it is said that "any landing you can walk away from is a good one" applies well here. Even though Philae's harpoon/piton system failed and it "bounced," the instuments still work and have sent have sent us an incredible amount of good data! Also, if that "neck" joining the two lobes continues to sublime, we may have two comets sometime in the future!

Doug Z

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