Methylated gases are made when organisms add a carbon and three hydrogen atoms to an undesirable chemical element. Called methylation, this process can turn potential toxins into gases that float safely away into the atmosphere. If these gases were to be detected in the atmosphere of an exoplanet using telescopes, they would be suggestive of extraterrestrial life.
“Methylation is so widespread on Earth, we expect life anywhere else to perform it. Most cells have mechanisms for expelling harmful substances,” said Michaela Leung, a Ph.D. student at the University of California, Riverside, NASA’s Alternative Earths and NExSS Virtual Planetary Laboratory teams.
One methylated gas, methyl bromide (CH3Br) has several advantages over other gases traditionally targeted in the search for life outside our Solar System.
In a new study, Leung and her colleagues explored and quantified these advantages.
For one, methyl bromide remains in the atmosphere for a shorter time than traditional biosignature gases.
“If you find it, the odds are good it was made not so long ago — and that whatever made it is still producing it,” Leung said.
Methyl bromide is more likely to have been made by something living than a gas like methane, which can be made by microbes. But it could also be a product of a volcano or other geologic process.
In addition, this gas absorbs light near a cousin biosignature, methyl chloride, which makes both of them, and the presence of life, easier to find.
Though methyl bromide is extremely common on Earth, it is not easily detectable in our atmosphere because of the intensity of our Sun’s UV light.
Ultraviolet radiation starts chemical reactions that break up water molecules in the atmosphere, splitting them into products that destroy the gas.
However, the authors determined methyl bromide would be more easily detectable around an M dwarf star than it is in our Solar System or ones like it.
M dwarfs produce less of the type of UV radiation that leads to the breakup of water.
“An M dwarf host star increases the concentration and detectability of methyl bromide by four orders of magnitude compared to the Sun,” Leung said.
Leung and co-authors are set to investigate the potential for other methylated gases to serve as targets in the search for extraterrestrial life, since this group of gases is especially closely associated with life, and only life.
“We believe methyl bromide is one of many gases commonly made by organisms on Earth that may provide compelling evidence of life from afar. This one is just the tip of the iceberg,” said Dr. Eddie Schwieterman, an astrobiologist at the University of California, Riverside, the Blue Marble Space Institute of Science, NASA’s Alternative Earths and NExSS Virtual Planetary Laboratory teams.
The team’s work appears in the Astrophysical Journal.
Michaela Leung et al. 2022. Alternative Methylated Biosignatures. I. Methyl Bromide, a Capstone Biosignature. ApJ 938, 6; doi: 10.3847/1538-4357/ac8799
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