Found in space, laughing gas could mean life - Here's why

Schwieterman led a team of researchers in the study that determined how much nitrous oxide living things on a planet similar to Earth could possibly produce. They then created models that simulated the planet's orbit around various types of stars and calculated the amount of N2O that could be detected by an observatory like the 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 CO2 or methane,” Schwieterman said.

Nitrous oxide, or N2O, is a gas that is produced in a variety of ways by living things. Other nitrogen molecules are continuously converted by microorganisms into N2O through a metabolic process that can produce useful cellular energy.

“Life generates nitrogen waste products that are converted by some microorganisms into nitrates. In a fish tank, these nitrates build-up, which is why you have to change the water,” Schwieterman said.

“However, under the right conditions in the ocean, certain bacteria can convert those nitrates into N2O,” Schwieterman said. “The gas then leaks into the atmosphere.”

It is difficult to detect elsewhere

N2O can be found in an atmosphere and still not be a sign of life. This was taken into account in the modeling of Schwieterman's group. For instance, lightning can produce a small amount of nitrous oxide. However, lightning also produces nitrogen dioxide, giving astrobiologists a hint that the gas was produced by non-living meteorological or geological processes.

Others who have thought about N2O as a biosignature gas frequently conclude that it would be impossible to detect at such a distance. This conclusion, according to Schwieterman, is based on current N2O concentrations in the Earth's atmosphere. Some think it would also be difficult to detect elsewhere because there isn't much of it on this planet, which is packed with life.

“This conclusion doesn’t account for periods in Earth’s history where ocean conditions would have allowed for much greater biological release of N2O. Conditions in those periods might mirror where an exoplanet is today,” Schwieterman said.

According to Schwieterman, common stars like K and M dwarfs emit a light spectrum that is less effective than our sun at dissolving the N2O molecule. When these two effects are combined, the estimated concentration of this biosignature gas in a populated world could be significantly raised.

Abstract:

Nitrous oxide (N2O)—a product of microbial nitrogen metabolism—is a compelling exoplanet biosignature gas with distinctive spectral features in the near- and mid-infrared, and only minor abiotic sources on Earth. Previous investigations of N2O as a biosignature have examined scenarios using Earthlike N2O mixing ratios or surface fluxes, or those inferred from Earth's geologic record. However, biological fluxes of N2O could be substantially higher, due to a lack of metal catalysts or if the last step of the denitrification metabolism that yields N2 from N2O had never evolved. Here, we use a global biogeochemical model coupled with photochemical and spectral models to systematically quantify the limits of plausible N2O abundances and spectral detectability for Earth analogs orbiting main-sequence (FGKM) stars. We examine N2O buildup over a range of oxygen conditions (1%–100% present atmospheric level) and N2O fluxes (0.01–100 teramole per year; Tmol = 1012 mole) that are compatible with Earth's history. We find that N2O fluxes of 10 [100] Tmol yr−1 would lead to maximum N2O abundances of ∼5 [50] ppm for Earth–Sun analogs, 90 [1600] ppm for Earths around late K dwarfs, and 30 [300] ppm for an Earthlike TRAPPIST-1e. We simulate emission and transmission spectra for intermediate and maximum N2O concentrations that are relevant to current and future space-based telescopes. We calculate the detectability of N2O spectral features for high-flux scenarios for TRAPPIST-1e with JWST. We review potential false positives, including chemodenitrification and abiotic production via stellar activity, and identify key spectral and contextual discriminants to confirm or refute the biogenicity of the observed N2O.