Disruption of a Planetary Nitrogen Cycle as Evidence of Extraterrestrial Agriculture

Abstract

Agriculture is one of the oldest forms of technology on Earth. The cultivation of plants requires a terrestrial planet with active hydrological and carbon cycles and depends on the availability of nitrogen in soil. The technological innovation of agriculture is the active management of this nitrogen cycle by applying fertilizer to soil, at first through the production of manure excesses but later by the Haber-Bosch industrial process. The use of such fertilizers has increased the atmospheric abundance of nitrogen-containing species such as NH${}_3$ and N${}_2$O as agricultural productivity intensifies in many parts of the world. Both NH${}_3$ and N${}_2$O are effective greenhouse gases, and the combined presence of these gases in the atmosphere of a habitable planet could serve as a remotely detectable spectral signature of technology. Here we use a synthetic spectral generator to assess the detectability of NH${}_3$ and N${}_2$O that would arise from present-day and future global-scale agriculture. We show that present-day Earth abundances of NH${}_3$ and N${}_2$O would be difficult to detect but hypothetical scenarios involving a planet with 30-100 billion people could show a change in transmittance of about 50-70% compared to pre-agricultural Earth. These calculations suggest the possibility of considering the simultaneous detection of NH${}_3$ and N${}_2$O in an atmosphere that also contains H${}_2$O, O${}_2$, and CO${}_2$ as a technosignature for extraterrestrial agriculture. The technology of agriculture is one that could be sustainable across geologic timescales, so the spectral signature of such an "ExoFarm" is worth considering in the search for technosignatures.

Accepted by ApJL, 12 pages, 1 figure

References in corpus (17)

• Spectral signatures of photosynthesis I: Review of Earth organisms
• The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth
• Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life
• Pale Orange Dots: The Impact of Organic Haze on the Habitability and Detectability of Earthlike Exoplanets
• Detecting industrial pollution in the atmospheres of earth-like exoplanets
• Atmospheric characterization of terrestrial exoplanets in the mid-infrared: biosignatures, habitability & diversity
• Origins Space Telescope Mission Concept Study Report
• Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology
• The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets
• Atmospheric Beacons of Life from Exoplanets Around G and K Stars
• Concepts for future missions to search for technosignatures
• Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres
• Detectability of Chlorofluorocarbons in the Atmospheres of Habitable M-dwarf Planets
• Contact Inequality -- First Contact Will Likely Be With An Older Civilization
• Strategies and Advice for the Search for Extraterrestrial Intelligence
• Longevity is the key factor in the search for technosignatures
• Detecting Biosignatures in the Atmospheres of Gas Dwarf Planets with the James Webb Space Telescope

Cited by in corpus (8)

• Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach
• Atmospheric carbon depletion as a tracer of water oceans and biomass on temperate terrestrial exoplanets
• Fully fluorinated non-carbon compounds NF3 and SF6 as ideal technosignature gases
• Earth Detecting Earth: At what distance could Earth's constellation of technosignatures be detected with present-day technology?
• Projections of Earth's technosphere: Scenario modeling, worldbuilding, and overview of remotely detectable technosignatures
• Technosignatures longevity and Lindy's law
• Projections of Earth's Technosphere: Strategies for Observing Technosignatures on Terrestrial Exoplanets
• SETI in 2022