Although some microorganisms also fix nitrogen, they do not represent significant sources of atmospheric NH3 on Earth. Likewise, the associated detection of N2O and other nitrogen-containing species would provide confidence that the production of NH3 is associated with industrial disruption of a planetary nitrogen cycle. It is worth emphasizing that NH3 or N2O alone would not necessarily be technosignatures, as either of these species could be false positives for life or could arise from nontechnological life . Rather, it is the combination of NH3 and N2O that would indicate disruption of a planetary nitrogen cycle from an ExoFarm, which may also show elevated abundances of NOx gases as well as CH4. The short lifetime of NH3 in an oxic atmosphere implies that a detectable abundance of NH3 would suggest a continuous production source. Although NH3 could be produced abiotically by combining N2 and H2, an atmosphere rich in H2 would be unstable to the O2 abundance required to sustain photosynthesis. The technosignature of an ExoFarm would therefore require the simultaneous detection of both NH3 and N2O in the atmosphere of an exoplanet along with O2, H2O, and CO2.Large-scale agriculture based on Haber–Bosch nitrogen fixation could be detectable through the infrared spectral absorption features of NH3 and N2O as well as CH4. A robust assessment of the detectability of such spectral features in an Earth-like atmosphere would ideally use a three-dimensional coupled climate–chemistry model to calculate the steady-state abundances of each of these nitrogen-containing species a function of biological and technological surface fluxes. But as an initial assessment,hydropopnic barley fodder system we consider a scaling argument to examine the spectral features that could be detectable for present-day and future Earth agriculture.

We define four scenarios for considering agriculture on an Earth-like planet, with the corresponding atmospheric abundances of nitrogen-containing species listed in Table 1. The present-day Earth scenario is based on recent measurements of NH3, N2O, and CH4 abundances . The choice of 10 ppb for NH3 is toward the higher end for Earth today and corresponds to regions of intense agricultural production. The preagricultural Earth scenario serves as a control, where the agricultural and technological contributions of NH3, N2O, and CH4 have been removed. Note that this approach assumes that eliminating the technological contributions to the atmospheric flux of these nitrogen-containing species will reduce the steady-state atmospheric abundance by a similar percentage; this approach is admittedly simplified, but the results can still be instructive for identifying the possibility of detectable spectral features. The third and fourth scenarios project possible abundances of NH3, N2O, and CH4 for futures with 30 and 100 billion people, respectively. Earth holds about 7.9 billion people today, and population projections differ on whether or not Earth’s population will stabilize in the coming century . These two population values were selected because they correspond approximately to the maximum total allowable population using all current arable land and all possible agricultural land . Most published estimates of Earth’s carrying capacity range from about 8 to 100 billion, although some estimates are less than 1 billion while others are more than 1 trillion . Theoretically, an extraterrestrial population with the energy requirements of up to 100 billion calorie consuming humans could sustain Haber–Bosch synthesis over long timescales, as long as sustainable energy sources are used . These scenarios also follow a scaling argument by assuming that the per-person contributions of these three nitrogen-containing species will remain constant as population grows. This again is a simplifying assumption that is intended as an initial approach to understanding the detectability of such scenarios.

We consider the detectability of all four of these scenarios using the Planetary Spectrum Generator . PSG is an online radiative transfer tool for calculating synthetic planetary spectra and assessing the limits of detectability for spectral features that can range from ultraviolet to radio wavelengths. The ultraviolet features of NH3, N2O, and CH4 are strongly overlapping and only show weak absorption, but mid-infrared features of all these species could be more pronounced. The mid-infrared spectral features of NH3, N2O, and CH4 calculated with PSG for preagricultural, present-day, and future Earth scenarios are plotted in Figure 1, which shows the relative intensity and transmittance spectra for observations of an Earth-like exoplanet orbiting a Sun-like star. The spectra shown in Figure 1 show the strongest absorption features due to NH3 from 10 to 12 μm, while N2O shows absorption features from 3 to 5 μm, 7 to 9 μm, and 16 to 18 μm. Absorption features due to CH4 overlap some of the N2O features from 3 to 5 μm and 7 to 9 μm. The change in peak transmittance between 10 and 12 μm for NH3 compared to the preagricultural control case is about 50% for the future Earth scenario with 100 billion people and about 25% for the scenario with 30 billion people. For N2O, the change in peak transmittance between 16 and 18 μm compared to the preagricultural control case is about 70% for 100 billion people and 50% for 30 billion people. The change in relative intensity for the 100 billion people scenario is up to about 10% compared to the preagricultural control case between 7 and 9 μm and 10 and 12 μm. Present-day Earth agriculture would exert a weakly detectable signal that might be difficult to discern from the preagricultural control case, but future scenarios with enhanced global agriculture could produce absorption features that are easier to detect. The spectral features of NH3, N2O, and CH4 could be detectable in emitted light or as transmission features for transiting planets. Specifically, the N2O line at 17.0 μm shows a strong dependency with the N2O volume mixing ratio and to a second order the NH3 line at 10.7 μm. For the future 100 billion case, both display strong enough absorption to bdetectable by the Large Interferometer for Exoplanets , Origins and Mid-InfraRed Exo-planet CLimate Explorer infrared mission concepts.

The James Webb Space Telescope Near Infrared Spectrograph could potentially detect CH4 within the 0.6–5.3 μm range for transiting exoplanets . However, the detection of CH4 alone would provide no basis for distinguishing between technological, biological, or photochemical production. The detectability of these spectral features do not necessarily directly correspond to the peak transmittance, and a full accounting of the detectability of each band would need to account for the observing mode and instrument parameters. It is beyond the scope of this present paper to present detectability calculations for specific missions, as any missions capable of searching for mid-infrared technosignatures are in an early design phase, at best. One of the goals of this Letter is to highlight the importance of examining mid-infrared spectral features of exoplanets,livestock fodder system as many potential technosignatures could be most detectable at such wavelengths. Also, it demonstrates the duality of the search for bio-signatures and technosignatures. The search for passive, atmospheric technosignatures does not require the development of a dedicated instrument but can leverage the capability of instruments dedicated to the search for bio-signatures.The calculations presented in this Letter indicate the possibility of detecting a technosignature from planetary-scale agriculture from the combined the spectral features of NH3 and N2O, as well as CH4. The signature of such an ExoFarm could only occur on a planet that already supports photosynthesis, so such a planet will necessarily already show spectral features due to H2O, O2, and CO2. The search for technosignatures from extraterrestrial agriculture would therefore be a goal that supports the search for bio-signatures of Earth-like planets, as the best targets to search for signs of nitrogen cycle disruption would be planets already thought to be good candidates for photosynthetic life. A better constraint on the detectability of the spectral features of an ExoFarm would require the use of an atmospheric photochemistry model. This Letter assumed simple scaling arguments for the abundances of nitrogen containing species, but the steady-state abundance of nitrogen containing atmospheric species will depend on a complex network of chemical reactions and the photochemical impact of the host star’s UV spectrum. In such future work, the increases of NH3 and N2O, and CH4 from agriculture would be parameterized via surface fluxes instead of arbitrary fixed and vertically constant mixing ratios. A network of photochemical reactions would then determine the vertical distribution of those species in the atmosphere. A photochemical model could also capture the processes of wet and dry deposition of NH3, which is the major sink in Earth’s present atmosphere, as well as aerosol formation from NH3 and SO2/N2O that can occur in regions of high agricultural production. Past studies have predicted more favorable build-up of bio-signature gases on oxygen-rich Earth-like planets orbiting later spectral type stars due to orders of magnitude less efficient production of OH, O, and other radicals that attack trace gases like CH4 .

The photochemical lifetime of N2O and therefore its steady-state mixing ratio will be enhanced by less efficient production of O radicals that destroy it. However, because deposition is the major sink of NH3, it is not clear whether a different stellar environment would alter the atmospheric lifetime of NH3, and if so, to what extent.Examining the four scenarios in this study with such a photochemical model would require additional development work to extend the capabilities of existing models to oxygenrich atmospheres. Past photochemical modeling studies that have included NH3 considered anoxic early Earth scenarios where the focus was determining the plausible greenhouse impact of NH3 to revolve the faint young Sun paradox . More recent studies have considered NH3 bio-signatures in H2-dominated super-Earth atmospheres, which would greatly favor the spectral detectability of the gas relative to high molecular weight O2-rich atmospheres . On H2 planets with surfaces saturated with NH3, deposition is inefficient, and sufficient biological fluxes can overwhelm photochemical sinks and can allow large NH3 mixing ratios to be maintained . These “Cold Haber Worlds” are far different from the O2–N2 atmosphere we consider here, where surfaces saturated in NH3 are implausible and photochemical lifetimes are shorter. Ideally, future calculations would use a three-dimensional model with coupled climate and photochemical processes suitable for an O2–N2 atmosphere to more completely constrain the steady-state abundances, and time variation, in nitrogencontaining species for planets with intensive agriculture. Future investigation should also consider false-positive scenarios for NH3 and N2O as a technosignature. One possibility is that a species engages in global-scale agriculture using manure only; such a planet could conceivably accumulate detectable quantities of NH3 and N2O without the use of the Haber–Bosch process. The distinction between these two scenarios might be difficult to resolve, but both forms of agriculture nevertheless represent a technological innovation. Whether or not similar quantities of NH3 and N2O could accumulate on a planet by animal-like life without active management is a possible area for future work. External factors such as stellar proton events associated with flares could also produce high abundances of nitrogen-containing species in an atmosphere rich in NH3 , so additional false-positive scenarios should be considered for planets in systems with high stellar activity. This Letter is intended to present the idea that the spectral signature of extraterrestrial agriculture would be a compelling technosignature. This does not necessarily imply that extraterrestrial agriculture must exist or be commonplace, but the idea of searching for spectral features of an ExoFarm remains a plausible technosignature based on future projections of Earth today. Such a technosignature could also be long-lived, perhaps on geologic timescales, and would indicate the presence of a technological species that has managed to coexist with technology while avoiding extinction. Long-lived technosignatures are the most likely to be discovered by astronomical means, so scientists engaged in the search for technosignatures should continue to think critically about technological processes that could be managed across geologic timescales. J.H.M. gratefully acknowledges support from the NASA Exobiology program under grant 80NSSC20K0622. E.W.S. acknowledges support from the NASA Interdisciplinary Consortia for Astrobiology Research program. T.J.F and R.K.K. acknowledge support from the GSFC Sellers Exoplanet Environments Collaboration , which is supported by NASA’s Planetary Science Divisions Research Program. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of their employers or NASA.