Scientists have discovered dimethyl sulfide (DMS), a molecule thought to have only living sources, on a cold, lifeless comet. The finding calls into question the molecule’s usefulness as a biosignature and the significance of an earlier hint of it in the atmosphere of an alien planet.
“This is the first sign of an abiotic source,” says Nora Hänni, a chemist at the University of Bern who presented the discovery last week at the General Assembly of the European Geosciences Union.
Tim Lichtenberg, a planetary astronomer at the University of Groningen who wasn’t involved in the study, says the work highlights how astronomers should be cautious about presumed biosignatures. “You need to look into all sorts of abiotic sources for interesting chemical compounds.”
DMS is the smell of sea air. The simple molecule is produced abundantly by tiny algae that live in the oceans. Because DMS on Earth is known to be produced only by life, astronomers studying atmospheric gases on exoplanets have considered it a strong biosignature. “The astrobiology community is trying to find the perfect biosignature to look for,” says Lena Noack, a geodynamicist of the Free University of Berlin. “DMS is now the latest.”
Last year, a team of astronomers drew headlines when it claimed a possible detection of DMS in the atmosphere of K2-18b, an exoplanet 8.6 times as massive as Earth orbiting within the habitable zone of its star. Based on the cocktail of other gases detected in its atmosphere, the team believed K2-18b might have a surface ocean and that the DMS, if confirmed, would be a sign of marine life. The paper sparked breathless headlines, despite the thin evidence for the DMS. “In the media, everybody was like, ‘Oh, maybe we have detected life!’” Hänni says.
Hänni wasn’t so sure. She studies the surprisingly complex organic molecules found in comets, including nucleobases, the building blocks of DNA. After hearing about the K2-18b claim, Hänni wondered whether she could also find the molecule on an unquestionably lifeless body, the comet 67P/Churyumov-Gerasimenko, which the European Space Agency’s Rosetta spacecraft chased for 2 years, directly sampling the cloud of dust and gas shed from the icy space rock.
Just 1 day of data from Rosetta’s mass spectrometer, an instrument that can identify molecules by their specific weights, was enough for Hänni and her colleagues to find DMS. She says lab experiments will now be needed to pin down exactly how DMS forms in space, where ultraviolet light and cosmic rays can power the synthesis of complex organic molecules. Another important question is whether comets could deliver significant amounts of DMS to a planet—and perhaps account for detections like the K2-18b claim. “If it impacted the atmosphere, it could contaminate the atmosphere of the planet,” Noack says, potentially complicating searches for alien life.
In the meantime, Hänni says, researchers will need to be more careful about interpreting DMS biosignatures.
“If we want to rely on such simple molecules as biomarkers,” she says, “then you need all the information you can possibly get.”
More: https://www.science.org/content/article/what-presumed-sign-life-doing-dead-comet
