Nitrous oxide found in space could mean life

UC Riverside scientists suggest something is missing from the typical list of chemicals astrobiologists use to look for life on planets around other stars — nitrous oxide.

Chemical compounds in a planet’s atmosphere that could indicate life, called biosignatures, typically include gases that are abundant in Earth’s atmosphere today.

“There has been a lot of thought about oxygen and methane as biosignatures. Fewer researchers have given nitrous oxide serious thought, but we think this may be a mistake,” said Eddie Schwieterman, an astrobiologist at UCR’s Department of Earth and Planetary Sciences.

This conclusion and the modeling work that led to it are detailed in a paper published today in The Astrophysical Journal.

To achieve it, Schwieterman led a team of researchers that determined how much nitrous oxide living things on an Earth-like planet could possibly produce. Then they created models simulating this planet around different types of stars and determined the amounts of N₂O that could be detected by an observatory like the James Webb Space Telescope.

“In a star system like TRAPPIST-1, the closest and best system for observing the atmospheres of rocky planets, one could potentially detect nitrous oxide in a concentration comparable to CO₂ or methane,” Schwieterman said.

There are several ways that living things can create nitrous oxide, or N₂O. Microorganisms constantly convert other nitrogen compounds into N₂O, a metabolic process that can provide useful cellular energy.

“Life produces nitrogen waste products, which some microorganisms convert to nitrates. In an aquarium, these nitrates accumulate, which is why you have to change the water,” Schwieterman said.

“But under the right conditions in the ocean, certain bacteria can convert these nitrates into N₂Oh,” explained Schwietermann. “The gas then escapes into the atmosphere.”

Under certain circumstances, N₂O could be detected in an atmosphere and still does not indicate life. Schwieterman’s team took this into account when modeling. A small amount of nitrous oxide is caused by lightning, for example. But next to N₂Oh, lightning also produces nitrogen dioxide, which would give astrobiologists a clue that inanimate weather or geological processes created the gas.

Others who viewed N₂O as a biosignature gas often comes to the conclusion that it would be difficult to detect from such a great distance. Schwieterman explained that this conclusion points to N₂O concentrations in today’s Earth’s atmosphere. Because there isn’t much of it on this life-teeming planet, some believe it would be hard to spot anywhere else.

“This conclusion does not account for periods in Earth’s history when ocean conditions would have allowed for a much larger biological release of N₂O. Conditions in those periods could reflect where an exoplanet is today,” Schwieterman said.

Schwieterman added that ordinary stars like K and M dwarfs produce a spectrum of light that is less effective at breaking up the N₂O molecule than our sun is. These two effects together could greatly increase the predicted amount of this biosignature gas on an inhabited world.

The research team included UCR astrobiologists Daria Pidhorodetska, Andy Ridgwell and Timothy Lyons, as well as scientists from Purdue University, Georgia Institute of Technology, American University and NASA Goddard Space Flight Center.

The research team believes now is the time for astrobiologists to explore alternative biosignature gases such as N₂O, because the James Webb telescope may soon be broadcasting information about the atmospheres of rocky, Earth-like planets in the TRAPPIST-1 system.

“We wanted to put forward this idea to show that if we look for it, it’s not impossible that we’ll find this biosignature gas,” Schwieterman said.