When European Space Agency (ESA) scientists landed a spacecraft on a comet last year, they weren't expecting to find oxygen.

Rosetta mission researcher Kathrin Altwegg told the Los Angeles Times, "the first time we saw it, we all went a little bit into denial because molecular oxygen was really not expected to be found on a comet,"


That's because, as ESA explained in a statement, oxygen is extremely reactive; it's easy to find a compound containing oxygen in space, but rare to find the molecule on its own. Per ESA:

Oxygen is the third most abundant element in the Universe, but the simplest molecular version of the gas, O2, has proven surprisingly hard to track down, even in star-forming clouds, because it is highly reactive and readily breaks apart to bind with other atoms and molecules… Despite its detection on the icy moons of Jupiter and Saturn, O2 had been missing in the inventory of volatile species associated with comets until now.

The scientists' findings were published in Nature this week. Using observations made by the Rosetta spacecraft's ROSINA spectrometer over the course of six months, the team was able to determine that the oxygen they found in the gaseous coma that surrounds comet 7P/Churyumov-Gerasimenko is primordial. Or, as lead author Andre Bieler told, "older than our Solar System."

Rosetta scientist Matt Taylor said in a statement that, "This is an intriguing result for studies both within and beyond the comet community, with possible implications for our models of Solar System evolution." explains:

Prevailing theories of the Solar System's birth posit a chaotic, collision-strewn mixing of matter flowing toward and away from the newly-formed Sun. Pristine, icy grains containing oxygen would not have made it through such violence intact, the scientists said, leading them to speculate that the process was, in fact, "gentler".

Altwegg told that "this evidence of oxygen as an ancient substance will likely discredit some theoretical models of the formation of our Solar System," adding, "we never thought that oxygen could 'survive' for billions of years."

Danielle Wiener-Bronner is a news reporter.