An image of Comet 67 captured by Rosetta on 22 August 2015 63.4km from the comet's centre. Credit: ESA/Rosetta/Navcam - CC BY-SA IGO 3.0


Rosetta has made the first ever local detection of oxygen molecules in a comet's atmosphere, in a discovery that suggests the oxygen was incorporated into comet 67P Churyumov-Gerasimenko during its formation.

This discovery throws into question some models theorising the formation of the Solar System.

Over the year that the ESA spacecraft has been studying the comet, it has detected lots of different gases spewing from its nucleus.

The most abundant are water vapour, carbon monoxide and carbon dioxide.

But oxygen had not been associated with comets until now.

It is the third most abundant element in the Universe, but because it is highly reactive and easily breaks down to bind with other atoms, oxygen is relatively difficult to find, even in its simplest form as O2.

It is also difficult to detect using spectroscopic measurements by telescopes, meaning an opportunity such as that provided by the Rosetta mission was required to make the detection.

“We weren’t really expecting to detect O2 at the comet – and in such high abundance – because it is so chemically reactive, so it was quite a surprise,” says Kathrin Altwegg of the University of Bern, principal investigator of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument, ROSINA.

“We had never thought that oxygen could ‘survive’ for billions of years without combining with other substances.

“It’s also unanticipated because there aren’t very many examples of the detection of interstellar O2.

And thus, even though it must have been incorporated into the comet during its formation, this is not so easily explained by current Solar System formation models.”

The O2 was identified spewing from the comet following analysis of over 3,000 samples collected around 67P Churyumov-Gerasimenko between September 2014 and March 2015.

The amount detected was strongly related to the amount of water measured about the comet, suggesting the two are linked in both their origin within the comet and their release into space. O2 was less correlated with carbon monoxide and molecular nitrogen, even though they have a similar volatility to O2.

In contrast to comets, it is known that oxygen occurs on Jupiter and Saturn’s moons because they are continuously struck by high-energy particles from their host planets.

This is not the case for comets like 67P.

Cosmic radiation particles have been hitting the comet for 4.6 billion years.

These particles can cause water to spilt into oxygen, hydrogen and ozone, among others, but the particles only penetrate a few metres into the comet's surface.

However, each time 67P orbits the Sun it loses material from its surface. Since its last meeting with Jupiter in 1959, it is believed to have lost some 100 metres, thus exposing the gases contained within gradually over time.


Researchers believe the oxygen from 67P is located deep in the comet’s nucleus and originated before the formation of the Solar System, throwing into question some theoretical models as to the Solar System’s beginnings and evolution.