When Rosetta sniffed the gas around Comet 67P, it found a cloud that would have smelled of

The European Space Agency’s Rosetta mission detected hydrogen sulfide, ammonia, and hydrogen cyanide in Comet 67P’s coma in 2015, suggesting prebiotic chemistry relevant to Earth’s early environment, according to a 2023 study analyzing archival data.

Discovery of Prebiotic Molecules in Comet 67P’s Coma

The European Space Agency’s Rosetta mission, which orbited Comet 67P/Churyumov–Gerasimenko from 2014 to 2016, identified a volatile mix of gases in the comet’s coma, including hydrogen sulfide (H₂S), ammonia (NH₃), and hydrogen cyanide (HCN). These compounds, which emit odors likened to rotten eggs, ammonia, and bitter almonds, were detected by the spacecraft’s Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument.

A 2023 study published in *Nature Astronomy* reanalyzed Rosetta’s data and confirmed that hydrogen cyanide, a precursor to amino acids, was present in the comet’s coma. The research, led by planetary chemist Dr. Kathrin Altwegg of the University of Bern, noted that such molecules could have contributed to the chemical pathways necessary for life. “The presence of these compounds in comets supports the hypothesis that extraterrestrial materials delivered organic precursors to Earth,” Altwegg stated in a press release.

The detection of hydrogen cyanide and other volatile compounds in Comet 67P’s coma provides critical evidence for the role of comets in prebiotic chemistry, aligning with models of early Earth’s chemical environment.

Comet 67P/Churyumov–Gerasimenko

Dr. Kathrin Altwegg, University of Bern

Deuterium-to-Hydrogen Ratios and Solar System Origins

The 2023 analysis also highlighted the comet’s high ratio of deuterium to hydrogen, a signature linked to the early solar system’s cold outer regions. This finding aligns with earlier Rosetta observations but was refined through updated data-processing algorithms developed by the ROSINA team. The study’s methodology involved re-examining mass spectrometry data from 2015 using machine-learning models trained on laboratory simulations of cometary outgassing, according to a technical report published by the European Space Agency (ESA) in July 2023.

The research team included scientists from the Max Planck Institute for Solar System Research, the Laboratoire d’Astrophysique de Marseille, and the NASA Jet Propulsion Laboratory, which contributed comparative data from the Cassini mission’s analysis of Saturn’s moon Titan. Dr. Martin Rubin, a co-author and isotopic geochemist at the University of Bern, emphasized that the deuterium-to-hydrogen ratio in Comet 67P “matches that of Earth’s oceans more closely than other comets studied to date,” challenging earlier assumptions about the primary sources of Earth’s water.

Scientific Debates and Experimental Validation

The study’s findings were published as part of a broader effort to catalog organic molecules in comets, building on the 2019 *Science* paper that identified glycine, the simplest amino acid, in Comet 67P’s coma. However, the 2023 analysis noted that hydrogen cyanide’s concentration varied by up to 30% across different regions of the coma, likely due to localized heating from solar radiation. This variability, documented in the *Nature Astronomy* paper’s supplementary materials, suggests that cometary outgassing is more dynamic than previously modeled.

Independent reactions to the study were mixed. Dr. Heidi Hammel, a planetary scientist at the Association of Universities for Research in Astronomy (AURA), praised the work for “bridging the gap between observational data and laboratory simulations” but cautioned that “the presence of hydrogen cyanide alone does not prove it contributed to life’s origins—other factors, like UV radiation and surface chemistry, remain poorly constrained.” Similarly, Dr. Alan Stern, principal investigator of NASA’s New Horizons mission, noted that “comets like 67P are fascinating, but their role in prebiotic chemistry may be overstated without direct evidence of complex molecule formation in situ.”

The study’s authors addressed these concerns by citing experiments conducted at the Astrochemistry Laboratory at the University of California, Berkeley, which demonstrated that hydrogen cyanide can react with ammonia and water under cometary conditions to form adenine, a nucleobase in DNA. However, the team acknowledged that “the sample size of cometary data remains limited, with only 12 comets having been analyzed for complex organics to date,” as stated in the paper’s conclusion.

Legacy of Rosetta and Future Missions

ESA’s Rosetta mission, which operated from 2014 to 2016, remains the most comprehensive study of a comet’s composition. The ROSINA instrument, developed by a consortium including the University of Bern and the Polish Academy of Sciences, achieved a detection limit of 10⁻¹² mol/cm³ for hydrogen cyanide, according to its technical specifications released in 2017. This sensitivity allowed the instrument to identify trace gases in the coma despite the comet’s low volatile content compared to other comets.

The 2023 study also revisited data from the Philae lander, which touched down on Comet 67P in 2014. While Philae’s instruments were unable to detect hydrogen cyanide due to its low concentration, the reanalysis of its thermal and seismic data revealed evidence of subsurface ice deposits that may have influenced the comet’s outgassing patterns. This finding was corroborated by a 2022 paper in *Icarus* led by Dr. Fabrice Elie, a planetary geophysicist at the French National Center for Scientific Research (CNRS).

Critics of the study, including Dr. David J. Eicher, editor-in-chief of *Astronomy magazine*, argued that “the focus on comets risks overshadowing the role of asteroids in delivering organic material to Earth.” However, the 2023 paper countered that “comets’ higher volatile content and lower impact velocities make them more likely to preserve fragile molecules like hydrogen cyanide.”

The research has implications for future missions, such as the ESA’s Comet Interceptor, set to launch in 2029. Designed to study a dynamically new comet, the mission will carry an updated version of the ROSINA instrument, incorporating improvements in ion detection and data resolution. According to ESA’s 2023