One of the key events separating modern civilization from the hunter-gatherer societies of the past is the invention of agriculture, which dates back about 10,000 years ago.
It all started with the cultivation of wild plants and the domestication of various animals – for dairy products and meat.
The main advantage of agriculture is that it can feed a much larger population than the earlier hunting and gathering. This led to the emergence of cities, the sharing of natural resources, ideas and innovations.
The development of agriculture has also had a great impact on the Earth itself. Its results are visible in the arrangement of fields and in the way light reflects off photosynthetic plants and in the chemicals they release into the atmosphere.
Now a group of astronomers and astrobiologists say this “atmospheric signature” should be clearly visible from space and that a similar signal could also be generated by aliens on another planet.
“The spectral signature of alien agriculture deserves attention when looking for technosignatures,” said Jacob Haqq-Misra of the Blue Marble Space Institute in Seattle and his colleagues.
Scientists are investigating what such a signature might look like and how easy it is to detect with the current generation of space telescopes such as James Webb.
An important element of agriculture is the application of fertilizers to increase productivity. This gives plants improved access to nitrogen, an essential element of life. Nitrogen in the form of N2 (inert gas) makes up 78 percent of the Earth’s atmosphere.
But it is difficult to convert it into a plant-friendly form, because N2 is linked by a very stable triple bond. This nitrogen is naturally broken down in a variety of ways, from lightning strikes to microorganisms in manure.
But as the population increased, especially after the Industrial Revolution, the demand for nitrogen fertilizer increased dramatically. This led to the development of artificial fertilizers and a global industry dedicated to manipulating the nitrogen cycle through the process of creating ammonia.
This allows the production of huge amounts of ammonia, some of which is released into the atmosphere, albeit for short periods of time, since ammonia usually settles to the ground after a few days. Therefore, detectable levels of ammonia in the atmosphere must be the result of significant ongoing agricultural activity.
While atmospheric ammonia is short-lived, its use as a fertilizer produces nitrous oxide (N2O), a greenhouse gas that lingers in the atmosphere for over a hundred years.
Nitrogen oxides are also formed during combustion. However, scientists are confident that other civilizations may find, like us, that combustion is not sustainable, and gradually abandon it. Thus, in the long term, the signature of nitrogen oxides is more likely to indicate agricultural activity.
Agriculture is also the main producer of atmospheric methane on Earth. So CH4 is another factor to look for, the team says.
“The signature of such an exo-farm can only occur on a planet that already supports photosynthesis, so such a planet would necessarily have spectral features due to H2O, O2 and CO2,” said Hakk-Misra and the team.
Perhaps it will be possible to detect this signature on other planets with the help of modern observatories. The researchers note that the James Webb Space Telescope, currently being commissioned, should be able to detect ammonia at levels of five parts per million in the atmosphere of a hydrogen-rich planet orbiting a nearby red dwarf. Currently, the level of ammonia on Earth is about ten parts per billion.
This makes searching for nitrogen signatures exciting for astrobiologists. It is likely that the hunt for this signal will be the next step after the discovery of photosynthetic signatures. And if we find it, then we will be sure that we are not alone in the Universe.