A new analysis of comet ice and dust collected by Rosetta, a spacecraft launched by the European Space Agency, finds that comets made a significant contribution to Earth’s atmosphere. Bernard Marty (Centre de Recherches Pétrographiques et Géochimiques, France), a member of the DCO Reservoirs and Fluxes Community and Scientific Steering Committee, in collaboration with 29 other researchers from six countries, made a detailed analysis of the trace gas xenon in the cloud surrounding the 67P/Churyumov-Gerasimenko comet, sampled by Rosetta. In a new paper in the journal Science , the group reports that based on the characteristics detected in 67P’s xenon, they estimate that about 22% of xenon in Earth’s atmosphere came from comets. Considering that comets are also rich in organic carbon, these findings suggest that they may have carried substantial amounts of carbon and other volatile compounds to early Earth.
Rosetta was the first spacecraft to orbit a comet, giving researchers the opportunity to monitor trace gases and volatile components like carbon, nitrogen, and water. The volatiles are trapped in icy regions of the comet but, when heated by the sun’s energy, they change directly from a solid into a vapor in a process called sublimation. These gases form a cloud called the coma, which the orbiting spacecraft sampled. The researchers could then identify the collected particles, based on their size, using an onboard high-resolution mass spectrometer called ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis).
The researchers took a special interest in the different isotopes of the rare noble gas xenon. An isotope is an atom for which the number of neutrons is known. Different isotopes of an element have different masses but identical chemical properties. Xenon in Earth’s atmosphere contains smaller proportions of “heavy” isotopes with additional neutrons than xenon in the solar wind or meteorites, which scientists have struggled to explain. “We were interested in xenon isotopes because it has nine isotopes so it’s potentially very rich in terms of information,” said Marty, “but it’s the least abundant noble gas.”
The low levels of xenon in the coma required close and extended sampling from the comet. The researchers worked with the flight engineers to establish a flight path that placed the spacecraft within five to 10 kilometers of the four-kilometer-wide comet for three weeks. “That’s always a little bit risky,” said Marty. “The flight engineers were not very happy.”
The researchers then compared the isotope profile of the comet to Earth’s atmosphere, the solar wind, and to meteorites. They saw that the heavy xenon in Earth’s atmosphere can be explained by the addition of cometary xenon, which makes up more than one fifth of the xenon in the atmosphere.
“The comet contains an isotope signal that fits quite well with atmospheric xenon,” said Marty. “We could establish a genetic link between atmospheric xenon and cometary xenon.”
As described in an earlier paper  by the group, the comet also contained the amino acid glycine, and the precursor molecules methylamine and ethylamine, which can serve as building blocks for life. Based on the new paper’s estimates of cometary xenon in the atmosphere, Marty roughly calculates that comets may have contributed organic carbon in amounts comparable the current organic carbon present in the biosphere.
In future work, Marty plans to build a comet in the lab by simulating cometary ice, and irradiating it to release the volatiles trapped inside. He wants to investigate processes of cometary formation and to observe how sublimation affects the isotope profiles of the released gases.
The Rosetta spacecraft ended its mission in 2016 by landing on 67P/Churyumov-Gerasimenko, but Marty hopes that in the future, a spacecraft will return comet samples back to Earth. Though this type of mission will be complicated and expensive, the samples will be invaluable for illuminating the prebiotic materials from comets that may have led to life on Earth. Both NASA and the Japan Aerospace Exploration Agency (JAXA) are considering such a mission, said Marty, but none are currently scheduled.
The Rosetta mission poster showing the spacecraft and its deployment of the Philae lander to comet 67P/Churyumov–Gerasimenko. Credit: ESA/ATG medialab; Comet image: ESA/Rosetta/Navcam