Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology

Abstract

Abstract Nitrogen dioxide (NO2) on Earth today has biogenic and anthropogenic sources. During the Covid-19 pandemic, observations of global NO2 emissions have shown a significant decrease in urban areas. Drawing upon this example of NO2 as an industrial byproduct, we use a one-dimensional photochemical model and synthetic spectral generator to assess the detectability of NO2 as an atmospheric technosignature on exoplanets. We consider cases of an Earth-like planet around Sun-like, K-dwarf, and M-dwarf stars. We find that NO2 concentrations increase on planets around cooler stars because there are fewer short-wavelength photons that can photolyze NO2. In cloud-free results, present Earth-level NO2 on an Earth-like planet around a Sun-like star at 10 pc can be detected with signal-to-noise ratio ∼5 within ∼400 hr with a 15 m LUVOIR-like telescope when observed in the 0.2–0.7 μm range where NO2 has a strong absorption. However, clouds and aerosols can reduce the detectability and could mimic the NO2 feature. Historically, global NO2 levels were 3× higher, indicating the capability of detecting a civilization at the stage where Earth’s civilization was 40 yr ago. Transit and direct imaging observations to detect infrared spectral signatures of NO2 on habitable planets around M-dwarfs would need several hundred hours of observation time, both due to weaker NO2 absorption in this region and because of masking features by dominant H2O and CO2 bands in the infrared part of the spectrum. Non-detection at these levels could be used to place upper limits on the prevalence of NO2 as a technosignature.