Was the study of 67P expected to yield results on our own atmosphere?
BM: It was one of the objectives of the Rosetta mission. First, to know where comets come from, if they originated in the solar system or elsewhere. We already knew they contained a lot of water, carbon, amino acids… what we wanted to find out was if and how they played a role in our planet’s formation, and if they had a part in the emergence of life.
We believe that many volatile elements were brought by what we call carbonaceous chrondites, meteorites which carry gas and organic material. These meteorites come from asteroids located between the orbits of Mars and Jupiter, so mainly from the internal solar system.
Comets represent our planet’s other potential source for this type of materials; we had to know if they had played a part in the formation of Earth’s oceans. But it doesn’t seem to be the case; prior analysis showed a much too high deuterium (hydrogen’s heavy isotope) to hydrogen ratio in comets. Comets’ deuterium content is 2 to 3 times higher than that of our oceans.
However, when it comes to our atmosphere, it’s another story. Our study shows that the xenon found in comets carries a special signature that is found in our atmosphere as well. This discovery links comets to our atmosphere on quantitative and qualitative levels, showing comets are likely responsible for 20% of our atmosphere’s primordial xenon.
What makes xenon so relevant in identifying the origins of our atmosphere?
BM: Xenon is a noble gas, and its isotopes are manufactured in stars reaching the end of their life cycle. This process gives xenon specific properties. It can have different numbers of neutrons, which doesn’t change its chemical properties but does make it easy to identify. Lighter 124Xe and 126Xe isotopes are produced in supernova explosions, but aren’t found in sufficient quantities on the comet.
On the other hand, the probe managed to identify 128Xe and 132Xe isotopes. These are the result of a slow production process at the end small and middle-sized stars’ life cycle (0.6 to 10 solar masses). Heavy 134Xe and 136Xe isotopes, which were also found on 67P, are only created when two neutron stars brutally collide. And Comet 67P/C-G’s xenon blend contains less 134Xe and 136Xe isotopes than what is normally found in the Solar System.
Thus it seems that xenon from outside the Solar System was trapped in the comet’s ice, which suggests it formed somewhere else. This xenon isotope blend is unique and had only been encountered in our atmosphere, to a smaller extent. It shows that comets are responsible for 20% of our atmosphere’s composition, while the remaining 80% was delivered by asteroids.
How did the Rosetta probe manage to provide such results?
BM: Not without taking risks. Noble gases are scarce, even in comets. The signal was very weak and Rosetta had trouble detecting it. It had to get as close as it could to the comet, between 5 and 8 km from its surface, to detect a stronger signal. And then it had to stay long enough to obtain usable data.
Astrophysicist Kathrin Altwegg from the University of Bern, lead scientist on the Rosina analyser, managed to negotiate 3 weeks of observation. This was a very high-risk operation; the probe and the comet were so far away from us that any command sent from Earth would take 30 minutes to arrive. This meant that to manage instability and avoid a crash, the probe had to be completely autonomous and constantly correct its trajectory, using a star tracker to triangulate its position using distant stars. But particles ejected by the comet could be mistaken for stars by the star tracker and confuse the probe. And that is exactly what happened. During an approach manoeuvre in May 2016, we lost control of the probe for several hours. But luckily everything went back to normal shortly after. In addition to measuring and identifying the Xenon blend, Rosina also analysed the comet’s chemical composition and determined the isotopic ratios for hydrogen, silicon, and amino acids.
“When it comes to scientific results, it has been the most productive instrument in the entire mission” according to Francis Rocard, Solar System Programme Manager at CNES.
B. Marty et al., (2017), Xenon isotopes in 67P/Churyumov- Gerasimenko show that comets contributed to Earth's atmosphere, Sciences, Vol. 356, Issue 6342, pp. 1069-1072, DOI: 10.1126/science.aal3496
- Bernard Marty, researcher at the Centre de recherches pétrographiques et géochimiques (CRPG): bmarty at crpg.cnrs-nancy.fr
- Francis Rocard, Solar System Programme Manager at CNES: francis.rocard at cnes.fr
Find out more
Rosetta on the CNES website.