Scientists Discover Oxygen in a Comet

The comet 67P/Churyumov–Gerasimenko made history just under a year ago when the European Space Agency landed the Philae probe on its surface—the first time we’ve ever landed a probe on a comet nucleus. Unfortunately, it bounced when it landed, greatly complicating the mission. 

Now scientists using instruments aboard the Rosetta spacecraft, which orbits the comet and deployed Philae, have discovered molecular oxygen in the coma of the comet, as they describe in a study published today in Nature. The oxygen was detected in the coma, or cloud of gas, surrounding the nucleus of the comet. This the first time oxygen has been discovered in a cometary coma.

Rosetta has detected an abundance of different gases pouring from the comet's nucleus, primarily water vapor, carbon monoxide, and carbon dioxide. Surprisingly, the fourth-most abundant material was molecular oxygen, relative to water. “It’s not only that we have oxygen—we have a lot of oxygen,” the University of Bern’s Kathrin Altwegg, a co-author on the paper, said in a press teleconference on Tuesday.

Using more than 3000 samples gathered from September 2014 to March 2015 by the ROSINA mass spectrometer aboard Rosetta—which began circling the comet in May 2014 after a 10-year journey—Altwegg, a principal investigator on the instrument, and her colleagues detected oxygen embedded in icy grains. It accounts for on average about 3.8 percent of the material, relative to water, in the comet’s coma. (The amount of molecular oxygen detected showed a strong relationship to the amount of water measured at any given time, suggesting that their origin on the nucleus and release mechanism are linked, the ESA said in a statement.)

The finding is surprising because oxygen, the third-most abundant element in the universe, is highly chemically reactive; it likes to combine with other chemicals. It was previously thought that in the early solar system it must have combined with the abundant hydrogen then present to form water. The oxygen molecules in the comet perhaps tell a different story. “We had never thought that oxygen could ‘survive’ for billions of years without combining with other substances,” Altwegg said in a statement.

The researchers say the finding may help illuminate the chemistry of the formation of our solar system. Comets are the most primitive bodies in our solar system, forming in its outer reaches some 4.6 billion years ago, as the planets were still forming. Usually, about 95 percent of the total gas density in the comas of comets is composed of hydrogen dioxide, carbon monoxide, and carbon dioxide. Sulfuric compounds and complex hydrocarbons have been discovered on comets too. But molecular oxygen has never been detected on a comet before. It’s only been found on other icy bodies like the moons of Jupiter and Saturn.

Another instrument aboard Rosetta appears to have found oxygen too. The ALICE far-ultraviolet spectrograph may have also detected molecular oxygen spectroscopically in 67P, according to Paul Feldman, a co-investigator on ALICE.

“The work is a tour de force of mass spectrometry and a very welcome result,” Feldman told mental_floss. “It supports our inference from far-ultraviolet spectroscopy of the presence of O2 as one of the volatile drivers of cometary activity.” The ALICE findings will be published soon in a special issue of the journal Astronomy and Astrophysics devoted to the Rosetta mission.

Nicolas Biver is a co-investigator on Rosetta's MIRO, a microwave instrument that senses temperature and can identify chemicals. Last week he published a study in Science Advances detailing how the comet Lovejoy is spewing a cocktail-ready mix of alcohol and sugar into space. He wasn't involved in the oxygen study but alerted to it by his Rosetta colleagues. 

“We were not expecting to find much O2 in the cometary nuclei,” Biver told mental_floss. “We need to measure the abundance of O2 in other comets to confirm this discovery—and also because each technique can have its own bias, but this will not be easy as O2 is difficult to observe remotely (and impossible from the ground).”

As Altwegg explained, that’s because oxygen is difficult to observe from telescopes using spectroscopy. Nevertheless, she suspects it might be quite common in comets. The team is looking at Halley's comet right now for comparison. That research is ongoing.

This discovery could complicate our search for life in the universe. While oxygen and methane are considered biosignatures of life on Earth, their presence in the comet suggests we may need to rethink that idea. “If we look at exoplanets, our goal, of course, will be to detect biosignatures, to see if the planet contains life," Altwegg said. "And as far as I know, so far the combination of methane and O2 was a hint that you have life underneath it. On the comet, we have both methane and O2, but we don’t have life. So it’s probably not a very good biosignature.”