Earth’s Xenon May Have Been Brought Here by Comets
published during a full moon.

The presence of xenon, the heaviest of the stable noble gases, in Earth’s atmosphere has always baffled scientists. It’s been clear for decades that xenon, which has eight stable isotopes and over 40 unstable ones, has cosmic origins, but researchers’ models haven’t been able to account for a major source of the element—until now. New research by a group of scientists from France, Switzerland, Israel, and the U.S. suggests that nearly a quarter of the xenon in Earth’s atmosphere comes from comets.


Pie chart showing the main components of Earth’s atmosphere. Credit: Life_of_Riley

The breakthrough in the ongoing mystery of our atmosphere’s xenon was made possible by the Rosetta spacecraft, which gathered data on comet 67P/Churyumov-Gerasimenko from 2014-2016 (in addition to depositing the Philae lander on the comet’s surface). When researchers examined the spectrometry data gathered from 67P, they found xenon frozen inside the comet’s ice. It appears to have been trapped there for a long, long time—since before the formation of our solar system.


Selfie taken by the Rosetta spacecraft. Credit: NASA/JPL

Comets likely delivered about 22% of Earth’s atmospheric xenon.

Upon further analysis, scientists were able to match the isotopic signature of the comet’s xenon to the xenon of undetermined origin in the Earth’s atmosphere. In a study published in Science Magazine, scientists walk through other possibilities for the presence of xenon in our atmosphere but ultimately reject those hypotheses in favor of the evidence obtained from Rosetta. The researchers conclude that comets likely delivered about 22% of Earth’s atmospheric xenon.


Xenon (54 Xe). Credit: Hi-Res Images of Chemical Elements

One of the objectives of the Rosetta mission was to look for organic materials to help determine whether comets might have brought life to Earth, in addition to delivering water to our planet.

Comet 67P contains organics such as sulfur and methane—some of the basic ingredients for life. The findings not only shed light on xenon but also raise the possibility that other elements of unknown origin may have come from comets. Ultimately, such studies also help scientists understand what the early solar system looked like, as well as the conditions and ingredients that made life possible on Earth—and may make life possible elsewhere.