Scientists say they have made an unexpected discovery of molecular oxygen in the coma – or atmosphere – of comet 67P/Churyumov-Gerasimenko. The discovery, made by ESA's orbiting Rosetta spacecraft, could have important implications for the history and formation of the Solar System.
“It sounds not that spectacular, but actually it is the most surprising discovery we have made so far in 67P, because oxygen was not among the molecules expected in the cometary coma,” study co-author Kathrin Altwegg of the University of Bern said in a media briefing. The paper is published in the journal Nature.
The discovery was made using Rosetta’s ROSINA-DFMS, a mass spectrometer, which recorded an abundance of oxygen relative to water of 3.8%, making it the fourth most common gas in the coma after water, carbon monoxide, and carbon dioxide. It is surprising because molecular oxygen is very reactive. During the formation of the Solar System 4.6 billion years ago, when there was a lot of hydrogen around, most models predict that almost all the molecular oxygen would react with this hydrogen to form water, and no longer be present.
But the researchers found that the ratio of oxygen to water in the coma of the comet remained constant over several months. This means that molecular oxygen must be present in the whole body of the comet, possibly stored for billions of years, and is being leaked into the coma as the Sun heats the surface.
“We now have evidence that significant parts of this comet have in fact survived the heating or formation of the Solar System,” lead author André Bieler of the University of Michigan added at the briefing.
Rosetta has been in orbit since August 6, 2014. ESA/Rosetta/NAVCAM.
This in itself raises a number of questions. For example, the researchers aren’t entirely sure how the molecular oxygen got there in the first place. It surely must have been present at the formation of the comet, and it may have been trapped in the water ice.
Even if the molecular oxygen was there at the start of the comet, though, it shouldn’t have survived until now based on current theories for how the Solar System evolved. It suggests that the building process of the Solar System may have been more gentle than thought, with many objects forming further from the Sun than expected and being subject to less heat.
"This means that the material from which the comet was built from never got very close to the protostar during its formation," Bieler told IFLScience. Most comets now reside in the Kuiper Belt and Oort Cloud at the edge of the Solar System, but they are thought to have originated nearer the Sun. If the orbit of comet 67P wasn't too close, as this study suggests, then it would allow the molecular O2 to remain frozen on it.
Molecular oxygen has never been found on a comet before, because it is hard to detect through telescopes, instead requiring direct measurements like this by Rosetta. It has been spotted on other icy bodies though, such as moons of Jupiter and Saturn, but those detections can be explained by impacts of high-energy particles from their parent planets. The same process could not explain the existence of molecular oxygen throughout the body of comet 67P.
Thus, the existence of molecular oxygen on this comet remains confusing. It must surely have originated in the early Solar System – but it may force a rethink on how some bodies are formed, hinting at a more sedate beginning for us all than expected.