Rosetta Probe Comet 67P's Weird Shape and Changing Features Get a Close Look

Recent studies parsing data from the Rosetta mission theorize how the comet's odd shape was formed and scrutinize various long-term changes in its structure, like collapsing cliffs.

Early in the Rosetta space probe mission, which the European Space Agency launched to conduct a close inspection of the Jupiter-family Comet 67P/Churyumov–Gerasimenko, investigators were intrigued by the comet's shape. Its bi-lobal structure led some people to joke that it was a rubber duckie in space.

Since then, many scientists have tried to figure out how that weird structure came to be. A new study investigates two possibilities: that the comet arose from two pieces coming together, or that it was carved out from a single parent body. Whatever the event, it would have happened early in the solar system's formation.

"We have evidence that both things [collision or carving] happened in the early solar system, but today, of course, the amount of material in the form of small bodies is so small and dilute that we don't see a lot of collisions actually happening," co-author Jonathan Lunine, a physical sciences professor at Cornell University, told Seeker.

The study, accepted for publication in Astrophysical Journal Letters and on preprint site Arxiv, was led by Olivier Mousis, an astrophysics professor and member of the University Institute of France.

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The paper's findings arrive on a wave of other 67P news. Mousis's study was published shortly before new results were released showing the changes in 67P as the comet drew closer to the sun. A study published in Nature Astronomy showed a cliff collapsing shortly after an outburst of dust and gas from the comet - the first such link that has been made definitively.

"Rosetta's images already suggested that cliff collapses are important in shaping cometary surfaces, but this particular event has provided the missing 'before–after' link between such a collapse, the debris seen at the foot of the cliff, and the associated dust plume, supporting a general mechanism where comet outbursts can indeed be generated by collapsing material," said Matt Taylor, the European Space Agency's Rosetta project scientist.

The collapse is thought to be brought on by long-term thermal changes in the comet and not a sudden temperature change, since the collapse happened at night. A complementary paper published in Science noted various long-term changes in the comet's structure, such as ripples appearing and disappearing, and cliffs changing.

"Monitoring the comet continuously as it traversed the inner Solar System gave us an unprecedented insight not only into how comets change when they travel close to the Sun, but also how fast these changes take place," said Ramy El-Maarry, leader of the second study.

Lunine and Mousis are long-time collaborators in studying the solar nebula, which was the primordial environment of gas and dust present in the early solar system while the sun was still growing up. To focus the question of when Comet 67P formed, the authors were interested in looking at what the comet is made of, and then trying to extrapolate the elements' formation to a time in the early solar system.

In particular, the authors focused on the isotopes of aluminum-26 and iron-60; they noted that the smaller the object is, the easier it is to get rid of that heat. They were attempting to model a body that still retained its volatiles in a reasonably thick layer near the surface.

What they found is that accretion would have occurred earlier than the formation of a single large body that then was carved out by a collisional event. That's because the larger body forms less quickly, and would have a higher content of radioisotopes per unit of surface area.

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The paper doesn't specify which scenario is more likely, but Lunine has a personal bias. The data shows that two smaller objects could have formed as early as a million years after the solar system's formation. A larger parent body would require a wait time of at least 4.5 million years, and up to seven million years.

He cited work from Julie Castillo-Rogez, a scientist at NASA's Jet Propulsion Laboratory who has performed work on when Iapetus, a moon of Saturn, was formed. Based on studies of radionucleides, she suggests that the moon would have been formed three million to five million years after the first solids formed in the solar system.

"If these [moons] formed in the Saturn system, three million to five million years after the birth of the solar system, it's hard to see the formation of a comet in six million or seven million years," Lunine said.

Another reason, he noted, is that the gas would dissipate in the solar nebula after a period of time, making it harder to justify a wait period, so to speak.

Lunine may do more Rosetta work in the future, but in the meantime he is working on a New Frontiers mission proposal to send a probe out to Enceladus. This icy moon of Saturn is known for its erupting geysers, and is considered a spot with a high probability of microbial life.

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