Agriculture is one of the oldest forms of technology in human history. Although farming today may include the use of heavy machinery and other modern tools, real innovation in farming began around 10,000 to 20,000 years ago during the Neolithic Agricultural Revolution when people established permanent settlements and learned to increase their food production by fertilizing the soil.
The earliest forms of fertilizer came from animal manure, which provided a source of nitrogen to the soil. Nitrogen is an essential nutrient for life, as nitrogen is one of the building blocks of amino acids that form our DNA. Applying manure directly to the soil was the first way people learned to increase the productivity of their fields, as the nitrogen in the manure was broken down by microorganisms to enrich the soil. The use of animal manure has led to increased animal husbandry and population growth.
Most of the nitrogen on Earth is in our air, because the molecule N2, and only certain organisms have the capacity to exploit this vast reservoir of nitrogen. Some microorganisms are able to convert nitrogen from the air into ammonia (NH3), which is a form of nitrogen that enriches the soil and can be used by plants. Some plants also have the ability to harvest nitrogen from the air – bean plants and other legumes are examples – which also enriches the soil with nitrogen that can be used by other plants. Many life forms, including humans, obtain nitrogen by eating plants or by eating other animals that eat plants.
A revolutionary discovery in agriculture took place at the beginning of the 20th century which enabled the technological production of ammonia-based fertilizers by combining nitrogen from the air with hydrogen gas. It is an industrial process that requires high temperatures as well as a metallic surface to allow the reaction, today known as the Haber-Bosch process. Fertilizer produced from the Haber-Bosch process allows farmers to spray their fields with nitrogen in the form of dissolved ammonia, which soaks into the soil so it can nourish plant roots. The use of these synthetic fertilizers multiplied the productivity of farms and allowed the population of the Earth to swell to several billion people.
The large-scale use of fertilizers also contributes to air pollution. Excess fertilizer not used by plants can return to the atmosphere as ammonia or nitrous oxide (N2O). The accumulation of these gases can be detected by satellites, especially in areas of intensive agriculture with high population density. These pollutants do not last very long in the Earth’s atmosphere before being washed away by rain or destroyed by sunlight. The fact that satellites can measure these pollutants means that they are continuously produced by human technology.
Our recent study accepted for publication in Letters from the Astrophysical Journal showed that the byproducts of agriculture could be detectable with space telescopes and suggested that the combination of ammonia and nitrous oxide would be proof of the technology if seen on another planet. We used the NASA Planetary Spectrum Generator to simulate the light that would be observed from a distant planet around another star with ammonia and nitrous oxide in the air from large-scale agriculture. We found that current levels of ammonia and nitrous oxide would be difficult to detect on another planet at interstellar distances, but higher levels of these pollutants from a population of 30 or 100 billion people would be more easy to notice.
The idea that ammonia and nitrous oxide are linked to agriculture on Earth suggests that this combination of pollutants could be remotely detectable evidence of the technology, known as technosignature. The Haber-Bosch process is a technological innovation on Earth that allowed people to harness nitrogen from our air and convert it into food, with pollution as a byproduct. The detection of these pollutants in the atmosphere of a distant planet similar to Earth would therefore be irrefutable proof of extraterrestrial technology. The Haber-Bosch process could even be carried out sustainably for millions of years or more, so alien agriculture could be a good example of a technosignature for scientists to research.
Scientists are now developing telescope designs that may be able to search for evidence of ammonia or nitrous oxide in the atmospheres of planets around other stars. It may be several decades before such telescopes are built, but studies like this help show the kind of science that could be done with future advanced telescopes.