Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology

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 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 due to less 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 10pc can be detected with SNR ~5 within ~400 hours with a 15 meter LUVOIR-like telescope when observed in the 0.2 - 0.7micron 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 3x higher, indicating the capability of detecting a 40-year old Earth-level civilization. Transit and direct imaging observations to detect infrared spectral signatures of NO${}_2$ on habitable planets around M-dwarfs would need several 100s of hours of observation time, both due to weaker NO${}_2$ absorption in this region, and also 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.

Accepted to Astrophysical Journal (in press). 1-D photochemical code from 'Atmos' is available at: https://github.com/VirtualPlanetaryLaboratory/atmos

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