Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology
Ravi Kopparapu, Giada Arney, Jacob Haqq-Misra, Jacob Lustig-Yaeger, Geronimo Villanueva
Abstract
Abstract Nitrogen dioxide (NO 2 ) on Earth today has biogenic and anthropogenic sources. During the Covid-19 pandemic, observations of global NO 2 emissions have shown a significant decrease in urban areas. Drawing upon this example of NO 2 as an industrial byproduct, we use a one-dimensional photochemical model and synthetic spectral generator to assess the detectability of NO 2 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 NO 2 concentrations increase on planets around cooler stars because there are fewer short-wavelength photons that can photolyze NO 2 . In cloud-free results, present Earth-level NO 2 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 μ m range where NO 2 has a strong absorption. However, clouds and aerosols can reduce the detectability and could mimic the NO 2 feature. Historically, global NO 2 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 NO 2 on habitable planets around M-dwarfs would need several hundred hours of observation time, both due to weaker NO 2 absorption in this region and because of masking features by dominant H 2 O and CO 2 bands in the infrared part of the spectrum. Non-detection at these levels could be used to place upper limits on the prevalence of NO 2 as a technosignature.