The old saying “every problem is an opportunity in disguise” couldn’t be more apt than here. Let’s start with the problem: to date, pollution has been one of the most obvious footprints left by humanity’s presence on Earth. Pollution is the cause of tangible consequences that afflict our planet, such as the temperature rising –a concrete indicator of climate change–, extreme cases of natural disasters, etc. For decades, now, the satellites orbiting around Earth have been sending us images showing the health condition of the layer that protects us from direct solar radiation. By the by, this is also how we discovered the existence of the ozone hole, one of the harbingers of the climate crisis that sparked the fight for the environment.

And now, let’s look at the opportunity. “On Earth, most of the nitrogen dioxide is emitted from human activity –combustion processes such as vehicle emissions and fossil-fueled power plants,” says Ravi Kopparapu, researcher at the NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Therefore, observing NO₂ [nitrogen dioxide] on a habitable planet could potentially indicate the presence of an industrialized civilization.” In a study published on the Astrophysical Journal and recently shared on Cornell University’s astrophysics website, Kopparapu and four other researchers have put forward the idea that precisely the emissions present in the lower atmosphere (10-15 km from the ground) could become a new indicator of life on other planets. The assumption is that much larger quantities of NO₂ generated by humankind –rather than naturally– become trapped at low altitudes and as such could become a reliable litmus test for activities of technologically advanced life forms. We should also consider that, to date, astronomers have identified over 4,000 planets orbiting around as many stars, which in technical jargon are called “exoplanets”. Some of these may have conditions suitable to life as we know it and, who knows –scientists wonder–, technologically evolved living beings may already be moving about on their surface.  Considering how immensely remote these celestial bodies are, we are not yet able to send spacecrafts that can directly verify what lies within their atmospheres. These are just hypotheses, at least for the moment, and this is why the term biosignatures (biological signatures) was coined –to identify all those clues that could denote the possible presence of life. One of these is the combination of gases such as oxygen and methane in the atmosphere. Likewise, the presence of a gas released as a byproduct of a widespread industrial process, such as NO₂, or other polluting molecules generated by technological activities, could be a sign of the existence of civilisations similar to ours on other planets.

This is not the first time that molecules are used as indicators: chlorofluorocarbons (CFCs), industrial products once widely used as refrigerants, had been considered in the past. Here on Earth we had to eliminate them, due to their extremely negative impact on the atmosphere, as they directly attack the ozone. Jacob Haqq-Misra, researcher at the Blue Marble Institute of Science in Seattle, Washington, contributed to a study by NASA, which stated that, “CFCs are also a powerful greenhouse gas that could be used to terraform [originally a sci-fi term to mean transforming a planet’s conditions to make it similar to Earth] a planet like Mars by providing additional warming from the atmosphere.” Indeed, despite its characteristic red colour, Mars is a planet of ice, rather than fire. “As far as we know, CFCs are not produced by biology at all, so they are a more obvious technosignature than NO₂” points out Haqq-Misra. “However, CFCs are very specific manufactured chemicals that might not be prevalent elsewhere; NO₂, by comparison, is a general byproduct of any combustion process”.

In this case, too, technological evolution has given a considerable boost to detection tools. In the NASA’s study mentioned above, the Maryland research team used mathematical models to predict whether the pollution generated by this gas could become an indicator that our current telescopes (and those under construction) would be able to detect. Basically, we start from the assumption that atmospheric nitrogen dioxide strongly absorbs certain colours (wavelengths) of visible light and can therefore be detected by observing the light reflected from an exoplanet as it orbits around its star. The theory by Kopparapu, Haqq-Misra and colleagues is that with a telescope similar to those NASA has, capable of detecting wavelengths in about 400 hours of exposure time, we should be able to discern a civilisation living on a planet similar to Earth orbiting around a star similar to the Sun up to about 30 light years away. Obviously, these are considerable research timelines, but they’re not unprecedented. One light year –the distance light travels in a year– is about 9,500 billion kilometres; by way of comparison, the stars closest to our Sun are those in the Alpha Centauri system, just over four light years away, and our galaxy stretches for about 100,000 light years across. In 2004, NASA’s well-known Hubble space telescope was kept focused on the eXtreme Deep Field, a small area close to the Big Dipper, for 552 hours, i.e. 23 days –hence, such timelines are not unfeasible.

However, NO₂ gas also occurs naturally and therefore scientists would have to carefully analyse an exoplanet to determine whether any excess could be attributed, as suggested, to a possible technologically advanced civilisation rather than to natural phenomena. According to scientific literature, researchers have found that about 76% of NO₂ emissions are due to industrial activity. In consequence, if the same gas is observed on another planet, we’ll have to develop models that can estimate its potential minimum emissions, those that may not be produced by non-industrial sources. Only once this has been corroborated, if concentration levels were to be significantly higher than suggested by the models, could any excess be attributed to industrial activity. Besides, as in any scientific analysis, there’s always the chance of false positives and the researchers themselves acknowledge that, in future, more appropriate tools will be needed to carry out this type of investigation.

Graduated in European Literature at the University La Sapienza of Rome, he is a freelance journalist and editorial translator, he has collaborated in several cultural and artistic projects as an author and writer. He currently collaborates with Media Duemila, Lonely Planet as an author and with Elliot Publishing.