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Comet 67P/Churyumov–Gerasimenko became historic nearly a year ago when the European Space Agency successfully landed the Philae probe on its surface—marking humanity's first landing on a comet's nucleus. However, the mission faced challenges as the probe bounced upon landing, complicating the operation.
Using instruments on the Rosetta spacecraft, which orbits the comet and deployed Philae, scientists have identified molecular oxygen in the comet's coma, as detailed in a study published today in Nature. This oxygen was found in the gas cloud surrounding the comet's nucleus, marking the first detection of oxygen in a comet's coma.
Rosetta has observed a variety of gases escaping from the comet's nucleus, mainly water vapor, carbon monoxide, and carbon dioxide. Intriguingly, molecular oxygen was the fourth most abundant gas relative to water. “We didn’t just find oxygen—we found it in significant quantities,” said Kathrin Altwegg from the University of Bern, a co-author of the study, during a press briefing on Tuesday.
Analyzing over 3000 samples collected between September 2014 and March 2015 by the ROSINA mass spectrometer on Rosetta—which started orbiting the comet in May 2014 after a decade-long journey—Altwegg, a lead researcher on the instrument, and her team discovered oxygen trapped in icy particles. On average, oxygen makes up approximately 3.8 percent of the material, relative to water, in the comet’s coma. (The ESA noted in a statement that the detected oxygen levels correlated strongly with water levels, indicating a shared origin and release process.)
This discovery is unexpected because oxygen, the third most abundant element in the universe, is highly reactive and typically bonds with other elements. Scientists previously believed that in the early solar system, oxygen would have combined with hydrogen to form water. However, the presence of oxygen molecules in the comet suggests a different narrative. “We never imagined oxygen could persist for billions of years without reacting with other elements,” Altwegg stated.
The researchers believe this finding could shed light on the chemical processes during the formation of our solar system. Comets, which formed around 4.6 billion years ago in the outer regions of the solar system, are considered the most primitive bodies in our system. Typically, about 95 percent of the gas in a comet’s coma consists of hydrogen dioxide, carbon monoxide, and carbon dioxide. Sulfur compounds and complex hydrocarbons have also been found on comets, but molecular oxygen had never been detected before—only on icy moons like those of Jupiter and Saturn.
Another instrument on Rosetta, the ALICE far-ultraviolet spectrograph, may have also identified molecular oxygen in 67P through spectroscopic analysis, according to Paul Feldman, a co-investigator on ALICE.
“This research is a remarkable achievement in mass spectrometry and a highly significant discovery,” Feldman told mental_floss. “It corroborates our findings from far-ultraviolet spectroscopy, which suggested the presence of O2 as a key driver of cometary activity.” The ALICE results will soon be published in a special edition of the journal Astronomy and Astrophysics focused on the Rosetta mission.
Nicolas Biver, a co-investigator on Rosetta's MIRO instrument, which measures temperature and detects chemicals, recently published a study in Science Advances revealing that comet Lovejoy is releasing a mixture of alcohol and sugar into space. Although not part of the oxygen study, he was informed about it by his Rosetta colleagues.
“We didn’t anticipate finding significant amounts of O2 in cometary nuclei,” Biver told mental_floss. “To validate this discovery, we need to measure O2 levels in other comets—though this is challenging since O2 is hard to detect remotely (and impossible from Earth).”
As Altwegg pointed out, oxygen is difficult to detect through telescopic spectroscopy. However, she believes it might be prevalent in comets. The team is currently analyzing Halley's comet for comparative purposes, with research still underway.
This finding could challenge our understanding of life-detection in the universe. While oxygen and methane are seen as biosignatures on Earth, their presence in the comet suggests these indicators may not always signify life. “When studying exoplanets, our aim is to identify biosignatures to determine if life exists,” Altwegg explained. “Previously, the combination of methane and O2 was considered evidence of life. On the comet, we find both, yet there’s no life. This implies it might not be a reliable biosignature.”
