Nitrous oxide (also known as laughing gas) — a product of microbial nitrogen metabolism — is a compelling exoplanet biosignature gas with distinctive spectral features in the near- and mid-infrared regions of the electromagnetic spectrum.
One of the most compelling drivers of exoplanet science is the search for inhabited planets like Earth, which may be identified through remote biosignatures — chemical compounds in a planet’s atmosphere that could indicate life.
For such inhabited worlds to be positively identified from atmospheric spectra, they must possess global biospheres with a robust exchange of gases between life and the atmosphere as well as generate biosignature features that can be remotely detectable with foreseeable technologies.
“There’s been a lot of thought put into oxygen and methane as biosignatures. Fewer researchers have seriously considered nitrous oxide (N2O), but we think that may be a mistake,” said Dr. Eddie Schwieterman, an astrobiologist with the University of California, Riverside, the Blue Marble Space Institute of Science, NASA’s Alternative Earths and NExSS Virtual Planetary Laboratory teams.
Dr. Schwieterman and colleagues determined how much nitrous oxide living things on a planet similar to Earth could possibly produce.
They then made models simulating that planet around different kinds of stars and determined amounts of nitrous oxide that could be detected by an observatory like the NASA/ESA/CSA James Webb Space Telescope.
“In a star system like TRAPPIST-1, the nearest and best system to observe the atmospheres of rocky planets, you could potentially detect nitrous oxide at levels comparable to carbon dioxide or methane,” Dr. Schwieterman said.
There are multiple ways that living things can create nitrous oxide.
Microorganisms are constantly transforming other nitrogen compounds into nitrous oxide, a metabolic process that can yield useful cellular energy.
Under certain circumstances, this gas could be detected in an atmosphere and still not indicate life.
A small amount of nitrous oxide is created by lightning, for example. But alongside nitrous oxide, lightning also creates nitrogen dioxide, which would offer astrobiologists a clue that non-living weather or geological processes created the gas.
Others who have considered nitrous oxide as a biosignature gas often conclude it would be difficult to detect from so far away.
“This conclusion is based on nitrous oxide concentrations in Earth’s atmosphere today,” Dr. Schwieterman said.
“Because there isn’t a lot of it on this planet, which is teeming with life, some believe it would also be hard to detect elsewhere.”
“This conclusion doesn’t account for periods in Earth’s history where ocean conditions would have allowed for much greater biological release of nitrous oxide. Conditions in those periods might mirror where an exoplanet is today.”
“Common stars like K and M dwarfs produce a light spectrum that is less effective at breaking up the nitrous oxide molecule than our Sun is. These two effects combined could greatly increase the predicted amount of this biosignature gas on an inhabited world.”
The authors believe now is the time for astrobiologists to consider alternative biosignature gases like nitrous oxide because Webb may soon be sending information about the atmospheres of rocky, Earth-like planets in the TRAPPIST-1 system.
“We wanted to put this idea forward to show it’s not out of the question we’d find this biosignature gas, if we look for it,” Dr. Schwieterman said.
The team’s paper was published in the Astrophysical Journal.
Edward W. Schwieterman et al. 2022. Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach. ApJ 937, 109; doi: 10.3847/1538-4357/ac8cfb
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