An article just published in the journal “Nature” describes the discovery of oxygen molecules in the coma of the comet 67P/Churyumov-Gerasimenko. It’s a surprising discovery, and in fact is the first time that this presence is detected in a comet because molecular oxygen – O₂ – is very reactive so it tends to combine for example with hydrogen to form water. This oxygen is a “survivor” from the time of formation of the solar system.

Over a year ago, ESA’s Rosetta space probe started analyzing the coma of the comet 67P/Churyumov-Gerasimenko using its ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument. It detected substances predicted by scientists such as sulfur and methane but the surprise came from the discovery of molecular oxygen.

Kathrin Altwegg of the University of Bern, the principal investigator of the ROSINA instrument, stated that she and her colleagues didn’t expect this discovery because molecular oxygen is chemically very reactive. It was a surprise also because there aren’t many examples of detection of interstellar molecular oxygen. Although it must have been incorporated into the comet 67P/Churyumov-Gerasimenko during its formation, this is not easily explained by current models of solar system formation.

It’s possible that molecular oxygen is present in other comets but it’s very difficult to detect it with the spectroscopic analyzes carried out by ground-based telescopes. It’s no coincidence that it was discovered by the ROSINA instrument installed on the Rosetta space probe sent on site.

The researchers analyzed more than 3,000 samples taken around the comet 67P/Churyumov-Gerasimenko between September 2014 and March 2015 to identify the O₂. They determined that it’s the fourth most common gas in the coma after the water gas, carbon monoxide and carbon dioxide.

Another surprise came from the discovery of a stable correlation between water and oxygen. This means that the ratio between the two molecules doesn’t change in different areas of the comet or at different times. This suggests that their origin in the nucleus and the mechanism of their release are linked.

According to the researchers, the most likely explanation for the presence of O₂ is that it’s molecules dating back to before the formation of the solar system. Water existing at the time was disgregated by high energy particles that broke it up into oxygen and hydrogen molecular.

Somehow, the O₂ remained protected during the growth phase of the comet 67P/Churyumov-Gerasimenko. This means that this process has occurred gently to prevent molecular oxygen from reacting with other substances. This discovery has implications on the models of the solar system formation and provides new information to better understand this process.

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