"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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