The strange, duck-shaped comet that ESA’s Rosetta spacecraft has been orbiting for more than a year just got a bit stranger: Like plants on Earth, the comet is blowing molecular oxygen, O2, into the space around it. Molecular oxygen is thought to be rare in the cosmos – or at least exceptionally tricky to detect.
“It is the most surprising discovery we have made so far,” says Rosetta team member Kathrin Altwegg of the University of Bern. The team first spotted the oxygen about a year ago and took its time ruling out sources other than the comet itself. “The first time we saw it,” Altwegg says, “I think we all went a little bit into denial because it is not expected to be found in a comet.”
Of course, molecular oxygen is common on Earth, having first been pumped out in enormous quantities by photosynthetic blue-green algae about 2.5 billion years ago. Until now, though, astronomers have only spotted gaseous O2 in a handful of other places, including two distant molecular clouds. The new observations, reported today in Nature, not only force a reconsideration of the very early solar system, they also throw a bit of a curveball at scientists hoping to identify the signatures of life on other worlds.
“The finding is definitely a wake up call for exoplanets and the search for life,” says Sara Seager of MIT. “O2 is the most prominent gas on our biosignature gas list.”
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