In a biggest surprise, oxygen was discovered on Comet Churyumov-Gerasimenko’s atmosphere. While such molecules are common in the earth’s atmosphere, their presence on comets had originally been ruled out.
Early on in the mission of the ROSINA mass spectrometer, in September of last year, researchers from the Center for Space and Habitability (CSH) at the University of Bern made an unexpected discovery when analyzing the comet’s gases: Between the expected peak values of sulfur and methanol, clear traces of oxygen (O2) molecules were detected.
Oxygen is in fact the fourth most common gas in the comet’s atmosphere, after water (H2O), carbon monoxide (CO) and carbon dioxide (CO2).
Molecular oxygen is very difficult to detect with spectroscopic measurements from telescopes, which explains why this molecule had not already been observed on other comets.
An in situ measurement by the ROSINA mass spectrometer on the Rosetta space probe was needed to make this discovery.
In contrast to comets, it is known that oxygen molecules occur on the moons of Jupiter and Saturn. That is explained by their being struck by high-energy particles from their respective mother planets, which does not exist in the case of Comet 67P/Churyumov-Gerasimenko.
The comet has been bombarded for 4.6 billion years, though, by high-energy cosmic radiation particles. These particles can split water, resulting in the formation of oxygen, hydrogen and ozone, among other substances.
Since its last meeting with Jupiter in 1959, which set the comet on its current orbit, it has consequently lost more than 100 metres of its material.
The most likely explanation is that the oxygen originated very early, before the formation of the solar system. Specifically, high-energy particles struck grains of ice in the cold and dense birthplaces of stars, the so-called dark nebulae, and split water into oxygen and hydrogen.
The oxygen was then not further “processed” in the early solar system. The oxygen measurements show that at least a significant part of the comet’s material is older than our solar system and has a composition typical of dark nebulae, from which solar nebulae and later planetary systems originate.
Rosetta is a space probe built by the European Space Agency launched on 2 March 2004. Along with Philae, its lander module, Rosetta is performing a detailed study of comet 67P/Churyumov–Gerasimenko (67P).
On 6 August 2014, the spacecraft reached the comet and performed a series of manoeuvres to be captured in its orbit.
On 12 November, the lander module performed the first successful landing on a comet. As of 2015, the mission continues to return data from the spacecraft in orbit and from the lander in the comet’s surface. During its journey to the comet, the spacecraft flew by Mars and the asteroids 21 Lutetia and 2867 Steins.
