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.”
Back to the Beginning
Comets are icy, space-traveling time capsules. Unlike planets, where internal ovens have more or less cooked and rearranged the planet’s ingredients, a comet’s original building blocks are preserved. So, scientists can use the icy dirtballs to peer back in time, all the way to the beginning of the solar system when small bits of frozen debris were colliding and forming comets. As the thinking goes, the molecules trapped in a comet reflect the composition of the dusty primordial nebula that swirled around the very young sun.
Out there, far beyond the orbit of Neptune, temperatures were obviously quite cold. But until now, no one thought it was cold enough or placid enough for two oxygen atoms to meet, link up, and stay together.
“All the models say it shouldn’t be there,” says study author Andre Bieler of the University of Michigan in Ann Arbor.
And while molecular oxygen only accounts for a small percentage of the total amount of stuff escaping from the comet – about 3.8 percent, relative to water – finding it at 67P is still enough to make scientists reconsider the composition and temperature of that primordial dust cloud. “This ice hasn’t been heated up enough to be reprocessed,” Bieler says.
Gassy molecular oxygen has only been observed around two other stars, suggesting that it’s a rare component of the interstellar medium. Perhaps, scientists now say, that result reflects the difficulty of detecting O2 remotely.
“When we find new molecules in comets, they’ve nearly always been found in the interstellar medium,”says Mike A’hearn of the University of Maryland, College Park. But, he adds, “the abundance [in 67P] is low enough that it’s unlikely we would have ever seen it in remote sensing.”
O2 and the Search for Little Green Microbes
Here’s what the problem could be for scientists hunting for the signatures of life on exoplanets: No one thinks there are exhaling microbes on 67P. Yet molecular oxygen, as Seager says, is at the top of the list of gases that could indicate the presence of extraterrestrial life. And if it is naturally common in the cosmos, then O2 might need to be reconsidered as a potential biosignature.
On the other hand, high levels of O2 in a planet’s atmosphere could still reflect the presence of life.
“The O2 lifetime is so short in atmospheres it won’t be present for long unless it is continually produced,” Seager says, noting that there are many ways to produce molecular oxygen that don’t necessarily involve life. “The comet shows us there are situations we hadn’t considered, and this will happen over and over again.”