We know that exoplanets are common in our galaxy, but it is a matter of guessing questions about alien life and extraterrestrial civilization. Given that we have not yet found evidence for any of them, astronomers are currently interested in how we can find them. New research suggests a strange approach: look at pollution. The work has been accept for publication in the Astrophysical Journal and is available online as a pre-print. In it, the team discusses how nitrogen dioxide (NO2) in gas can used to detect alien civilizations. On Earth, the gas is a by-product of combustion and can see in observations of distant planetary atmospheres with next-generation telescopes.
“On Earth, most of the nitrogen dioxide is emitted from human activities – vehicle emissions and combustion processes such as fossil fuel power plants,” said Ravi Kopparapu, lead author of NASA’s Goddard Space Flight Center, in a statement. “In the lower atmosphere (about 10 to 15 kilometers or about 6.2 to 9.3 miles), NO2 from human activities predominates over non-human sources. Thus, observing NO2 on habitable planets may indicate the presence of an industrial civilization.”
Astronomers are looking for some clues to life in the atmosphere of these exoplanets. These are biosignatures. Now they are also discovering “techno-natures” – molecules that were present in the air of that distant world in the presence of industrial civilization. Blue Marble co-author Jacob Huckett-Mishra added, “Other studies have tested chlorofluorocarbons (CFCs) as possible technologists, including industrial products that were widely used as refrigerators in ozone depletion until their phases,” added Jacob Huck-Misra.
“CFCs are a powerful greenhouse gas that can be used to transform a planet like Mars by providing additional warmth from the atmosphere. As far as we know, CFCs not produced by biology at all, so these are more obvious technical signatures than NO2. However, CFCs are very specific manufactured chemicals that may not be prevalent elsewhere; NO2, by comparison, is a common by-product of any combustion process.” Discovering nitrogen dioxide would not be a “gotcha”, we got alien life. Gas also produced naturally, so astronomers have to guess whether the signal they are seeing is enough to produce by a technological society.
“On Earth, about 76 percent of NO2 emissions are due to industrial activity,” explained Giada Arney, co-author of the paper NASA Goddard. “If we observe NO2 on another planet, we will have to run a model to estimate the maximum possible NO2 emissions from non-industrial sources alone. If we observe more NO2 than our models suggest is acceptable from non-industrial sources, the remainder of NO2 may be responsible for industrial activity. Yet there is always a false positive possibility in the pursuit of life beyond the earth, and certain future actions needed to have confidence in distinguishing the positive from the positive.”
Clouds and other aerosols can also create false positives. For the next step in this project, the team wants to create a better model for the planet’s atmosphere so that it can have a more realistic view of what the nitrogen dioxide signal will look like.
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