Scientists on the Rosetta mission have found oxygen in the gas cloud around comet 67P in what they described as the most surprising discovery about the comet to date.

It is the first time that molecular oxygen - the form of the gas we breathe - has been detected on a comet, and points to a gentle birth for comet 67P some time before the formation of the solar system.



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Measurements from the European Space Agency’s orbiting Rosetta probe show that oxygen is the fourth most abundant gas in the tenuous atmosphere of comet 67P / Churyumov-Gerasimenko, to use its full name, after water vapour, carbon monoxide and carbon dioxide.



“It’s actually the most surprising discovery we have made so far on 67P because oxygen was not among the molecules expected in a cometary coma,” said Kathrin Altwegg at the University of Bern.



The finding is puzzling because oxygen is highly reactive and scientists do not expect it to hang around for long in space. “We had never thought that oxygen could ‘survive’ for billions of years without combining with other substances,” said Altwegg.



Working with André Bieler at the University of Michigan, Altwegg showed that oxygen levels around the comet remained high over seven months of observations from September 2014 to March 2015. Because the surface of the comet is constantly being shed, the finding suggests that oxygen is present all through the body.



The scientists are unsure precisely how the oxygen got to be in the comet, but suspect that the answer lies in the cold and dense birthplace of the solar system called the dark nebula. One possibility they describe in the journal Nature, is that high energy particles hurtling through space slammed into ice grains in the dark nebula and broke the bonds in water molecules, creating oxygen that became locked in voids in the ice.



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Over billions of years, on the edge of the fledgling solar system, these ice grains became incorporated into the comet. The comet must have formed far from the newborn sun, because the heat would have freed the oxygen into space.



If the researchers are right, it bolsters the theory that comets are primordial and pristine remnants of the early solar system. But the existence of oxygen in the comet contradicts some long-held ideas about how the solar system formed. One is that the solar system’s oxygen originally formed beyond the solar system in what is called the interstellar medium. “This evidence of oxygen as an ancient substance will likely discredit some theoretical models of the formation of the solar system,” said Altwegg.



The discovery of oxygen on the comet has not led scientists to speculate about the presence of life on the 10 billion-tonne, Mont Blanc-sized body.

In November last year, the Rosetta mission made history by landing a small probe on the surface of the comet. The Philae lander bounced on 67P’s surface before coming to rest at the foot of a cliff. The tiny lander (which returned a wealth of data before its batteries died), is still on the surface of the comet, which swung past the sun in August and is now steadily making its way back out into the farther reaches of the solar system.

