Why does Rosetta's comet look like a rubber ducky?
Ever since a Europe's Rosetta probe landed on the surface of a comet last year, scientists have been puzzled over its rubber duck shape.
Either the strange shape was due to localized erosion on the surface of Comet 67P/Churyumov-Gerasimenko or possibly the merger of two comets.
Now, a study in Nature on Monday concludes it was a collision that caused the comet's unique, double-lobed appearance. The European Space Agency's Rosetta probe landed on the comet in November, the first successful attempt to land on a frozen remnant of the solar system's birth.
By using high-resolution images taken between August 6, 2014 and March 17, 2015 to study the layers of material seen all over the nucleus, they have shown that the shape arose from a low-speed collision between two fully fledged, separately formed comets.
"How the comet got its curious shape has been a major question since we first saw it," said Holger Sierks, of the Max Planck Institute for Solar System Research in Göttingen and principal investigator with the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS), the main imaging system of the Rosetta mission. "Now, thanks to this detailed study, we can say with certainty that it is a contact binary."
To reach their conclusion, scientists first used images to identify over 100 terraces seen on the surface of the comet, and parallel layers of material clearly seen in exposed cliff walls and pits. A 3D shape model was then used to determine the directions in which they were sloping and to visualize how they extend into the subsurface.
"It is clear from the images that both lobes have an outer envelope of material organized in distinct layers, and we think these extend for several hundred meters below the surface," said Matteo Massironi, the lead author of the study from the University of Padova, Italy, and an associate scientist of the OSIRIS team.
"You can imagine the layering a bit like an onion, except in this case we are considering two separate onions of differing size that have grown independently before fusing together," he said.
It also became clear that the features were coherently oriented all around the comet's lobes and in some places extended to a depth of about 2,132 feet.
"This was the first clue that the two lobes are independent, reinforced by the observation that the layers are inclined in opposite directions close to the comet's neck," Matteo said.
In one case, they modeled the comet as a single body with a center of mass close to the neck. In the other, they worked with two separate comets, each with its own center of mass.
The team found that the orientation of a given layer and the direction of the local gravity are closer to perpendicular in the model with two separate objects, rather than with one.
"This points to the layered envelopes in the comet's head and body forming independently before the two objects merged later," Matteo said. "It must have been a low-speed collision in order to preserve such ordered strata to the depths our data imply."
Bjorn Davidsson, a co-author from Uppsala University, Sweden, said they also found "striking structural similarities between the two lobes" that implies they formed through "similar accretion process" despite their "initially independent origins."
"Layering has also been observed on the surface of other comets during previous flyby missions, suggesting that they also underwent a similar formation history," Davidsson said.
While the collision caused the shape, the team found that erosion did play a minor, but important role.
Local variations seen in the structure of the surface likely result from different rates of sublimation - when ice turns directly into a gas - of frozen gases embedded within the individual layers, which are not necessarily distributed evenly throughout the comet.
"This result adds to our growing knowledge of the comet -- how it formed and its evolution," Rosetta project scientist Matt Taylor said.
"Rosetta will continue to observe the comet for another year, to get the maximum amount of information on this celestial body and its place in the history of our solar system," he said.