Scientists at UC Riverside believe something is missing from the normal list of compounds used by astrobiologists to look for life on planets orbiting other stars. Biosignatures are chemical components in a planet's atmosphere that potentially suggest life. They often contain gases that are abundant in the Earth's atmosphere today.
According to Eddie Schwieterman, an astrobiologist in UCR's Department of Earth and Planetary Sciences, "a lot of thought has gone into oxygen and methane as biosignatures." Fewer researchers have given nitrous oxide substantial consideration, but we believe this is a mistake.
This finding, as well as the modeling work that led to it, is described in full in a paper published today in The Astrophysical Journal. Schwieterman headed a team of researchers that calculated how much nitrous oxide living beings on a planet like Earth might conceivably create. They then created models that simulated the planet's orbit around several types of stars and calculated the quantity of N2O that could be discovered by an observatory like the James Webb Space Telescope.
According to Schwieterman, nitrous oxide might be detected at amounts equivalent to CO2 or methane in a star system like TRAPPIST-1, the nearest and best system for studying the atmospheres of rocky planets.
Nitrous oxide, or N2O, may be produced by living beings in a variety of ways. Other nitrogen molecules are continually transformed into N2O by microorganisms, a metabolic process that can generate important cellular energy.
According to Schwieterman, life creates nitrogen waste products that are transformed into nitrates by some microbes. These nitrates accumulate in a fish tank, which is why the water must be changed. Certain microorganisms in the water, however, may turn those nitrates into N2O under the correct circumstances. The gas subsequently seeps into the environment.
Under some conditions, N2O in the atmosphere might be detected but not imply life. This was taken into account by Schwieterman's team in their modeling. Lightning, for example, produces a little quantity of nitrous oxide. However, in addition to N2O, lightning produces nitrogen dioxide, which might provide astrobiologists with evidence that the gas was generated by non-living meteorological or geological processes. Others who have investigated N2O as a biosignature gas frequently conclude that it would be impossible to detect from such a long distance. This conclusion, according to Schwieterman, is based on current N2O concentrations in the Earth's atmosphere. Some feel it would be difficult to detect elsewhere since there isn't much of it on our planet, which is packed with life.
According to Schwieterman, this conclusion ignores times in Earth's history when ocean conditions would have allowed for substantially more biological output of N2O. Conditions throughout those times may have been similar to where an exoplanet is now.
Schwieterman went on to say that common stars, such as K and M dwarfs, have a light spectrum that is less efficient than our sun at breaking up the N2O molecule. The combination of these two factors might significantly boost the estimated quantity of this biosignature gas on a populated planet.
Daria Pidhorodetska, Andy Ridgwell, and Timothy Lyons of UCR were part of the study team, as were scientists from Purdue University, the Georgia Institute of Technology, American University, and the NASA Goddard Space Flight Center. The study team feels that now is the moment for astrobiologists to examine alternate biosignature gases such as N2O because the James Webb telescope will soon be relaying data on the atmospheres of TRAPPIST-1's rocky, Earth-like planets.
We wanted to bring this notion forth to illustrate that it's not impossible to locate this biosignature gas if we seek it, according to Schwieterman.
Journal Information: Edward W. Schwieterman et al, Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac8cfb