Planned Rosetta impact site

Rosetta will land near a pit containing 3-meter-sized "goosebumps." Some think they are the comet's building blocks; others say they are too big.

ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0

Rosetta spacecraft prepares to land on comet, solve lingering mysteries

All good things must come to an end, and so it will be on 30 September when the Rosetta spacecraft makes its planned soft landing onto the surface of comet 67P/Churyumov-Gerasimenko, the culmination of 2 years of close-up studies. Solar power has waned as 67P's orbit takes it and the orbiting Rosetta farther from the sun, and so the mission team decided to go on a last data-gathering descent before the lights go out. Contact with Rosetta will almost certainly be lost after it lands, as its communication dish is unlikely to be pointed toward Earth.

This last data grab—including detailed imaging of the comet's surface—is a bonus after a mission that is already changing theorists' views about how comets and planets arose early in the solar system. Several Rosetta observations suggest that comets form not from jolting mergers of larger cometesimals, meters to kilometers across, but rather from the gentle coalescence of clouds of pebbles. And the detection of a single, featherlight, millimeter-sized particle, announced on 29 September at a European Space Agency (ESA) press briefing, should further the view of a quiet birth. It appears to be a fragile dust mote that has survived, cradled in the comet, since the birth of the solar system. "This really is the seed at the starting point of everything," says Marco Fulle, a mission scientist at the Trieste Astronomical Observatory in Italy.

As the mission winds down, Rosetta scientists are finally in a position to answer one of their earliest questions about 67P: the origin of its two-lobed "rubber duck" shape. Were two small comets glommed together at some point in the past, or is one body being eroded by the sun into a dumbbell shape? A consensus is beginning to emerge. Rosetta's camera found onionlike layers created during formation but exposed on 67P's surface by erosion. The layers are oriented differently in the two lobes, suggesting separate formation and a later meld. But the union must have been gentle, because the comet lacks areas of high-density material that would have been created by a heavyweight crunch.

Rosetta scientists are also killing off the age-old notion that comets are "dirty snowballs," mostly ice with some dust mixed in. As earlier studies of other comets had suggested, the comet "is more like an icy dirtball," says Anders Johansen, an astronomer at Lund University in Sweden who is not involved in the mission—an idea backed up by Rosetta's observation of a high ratio of dust to water vapor in the material lofted into space by the sun's heat. This "dirt" is unlike anything found on Earth, Fulle says: a low-density mineral sponge, like pumice but extremely fragile. Some 70% of its volume is made up of voids, some of which are filled with organic molecules.

On its surface, 67P looks like a miniplanet. "It has a wide variety of terrains, almost like Earth," says Eberhard Gruen, a mission scientist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany: apparently smooth plains, cliffs, and mountains. But unlike Earth, its surface is being eroded as the sun's heat turns on jets of sublimating ices, kicking up dust. Rosetta analyzed many of these particles as they launched off the surface, and most were made of compact, dense minerals that require the heat of the inner solar system to form. They probably drifted outward to cooler parts of the solar system, where they were swept up into the comet—suggesting they are not so primordial, says Alessandra Rotundi, principal investigator of Rosetta's dust-analyzing instrument at the Parthenope University of Naples in Italy.

What we thought about the outer solar system may be wrong. Now, we have to fight to persuade people the comet formed in this way.

Marco Fulle, mission scientist at the Trieste Astronomical Observatory in Italy

But Rotundi's team also detected particles that entered the instrument while registering no mass. "We thought something was wrong with the instrument," she says. These "fluffy particles," also picked up by other sensors, turned out to have a density less than air on Earth. Researchers now estimate that 15% of 67P's volume is made up of fluffy particles, which are 99.95% empty space.

One of these delicate beasts found its way into an instrument that contains an atomic force microscope, capable of studying structures as small as a few nanometers. It showed that the fluffy particle has a fractal structure, like the branches of a tree or an indented coastline, with the same level of intricacy at scales both large and small, says Thurid Mannel, a scientist on the microscope instrument team at the Institute of Space Research in Graz, Austria, who presented preliminary results at the ESA briefing. The pattern is just what theorists would predict if it had been built up gently by nanometer-sized particles sticking together in the nebula that formed the solar system, gradually growing in size as more and more particles were added. Not everyone is convinced about fluffy particles, Gruen says. "The final word is not out yet."

But if they are as ancient and delicate as they appear, their survival on 67P suggests that the comet was not exposed to high temperatures or violent impacts. This runs counter to the traditional theory of comet (and planet) formation: that particles cling together into pebbles, which form boulders, which coalesce into kilometer-scale "cometesimals"—with collisions getting more violent at each stage. That scenario already faced challenges: Whereas small particles can stick electrostatically, it's harder to explain the formation of meter-sized objects, which don't exert much gravity.

A rival theory holds that pebbles tend to gather in bunches because of aerodynamic drag in the primordial gas cloud; eventually, giant flocks of pebbles collapse under their own collective gravity to form comet-sized objects directly. Several strands of evidence suggest 67P was formed from pebbles: The size distribution of material blown off the surface shows much is centimeter-sized; radio scans of 67P's internal structure found few of the large voids expected to form around boulders; and before the short-lived Philae lander ran out of battery power it sent close-up images revealing a wealth of pebbles. "What we thought about the outer solar system may be wrong," Fulle says. "Now, we have to fight to persuade people the comet formed in this way."

Not all evidence supports this view. Images of pits in the comet's surface show numerous lumps 3 meters across on the pits' sides—"like oranges in a box," says Holger Sierks of the Max Planck Institute for Solar System Research in Göttingen, Germany, chief of Rosetta's main camera. Although these lumps may have formed as ices sublimated out of the pit, some team scientists believe that these outsized "goosebumps" are 67P's primordial building blocks. They are a prime target for imaging during today's descent to the surface.

A slew of instruments will keep gathering data as Rosetta approaches the surface at the speed of a gentle stroll. For team members whose instruments have already been turned off to conserve power, the ending is bittersweet—but their work is far from over. Most instrument teams have only examined their own data, and are just now thinking about combining data sets. "We've just started collaborating with other teams," Sierks says. "This is the beginning of the story, not the end."

Read more of our coverage on Rosetta.