With the coming telescopes we can look more for biosignatures for all exoplanets found. Much of the biosignature conversation focuses on biogenic chemistry, such as atmospheric gases produced by simple unicellular creatures. But what if we want to look for technological civilizations that might be out there? Could we find them by looking for their air pollution?
When a distant civilization gave our planet a glimpse at their own overview of alien worlds and technosignatures, they couldn’t help but notice our air pollution.
Could we turn the tables on them?
Air pollution as a techno signature is not a brand new idea. A new paper deals with a specific chemical pollutant that is both biogenic and anthropogenic on earth: NO2 or nitrogen dioxide. The authors say nitrogen dioxide could be detected as a technosignature on an Earth-like planet orbiting a sun-like star using a 15-meter telescope, similar to NASA’s proposed Large UV / Optical / IR Surveyor (LUVOIR). However, it would take several hundred hours of observation.
The paper in which these results are presented is entitled “Nitrogen Dioxide Pollution as the Signature of Alien Technology”. It is available on the prepress website arxiv.org and has not yet been reviewed by experts. The main author is Dr. Ravi Kopparapu, a scientist at NASA’s Goddard Space Flight Center.
“Is it likely that biosignatures are more common than technosignatures? We won’t know for sure until we search. “
From “Nitrogen Dioxide Pollution as Signature of Alien Technology”
Thanks mainly to the Kepler mission and the TESS mission, we are living in an age of discovery of exoplanets. We now know several thousand exoplanets and the number continues to grow. Astronomers have characterized the mass, density, potential habitability, and other properties of many of them.
The next step is to study the atmosphere of some of the thousands of confirmed exoplanets. Exoplanet scientists eagerly await the upcoming James Webb Space Telescope (JWST) launch. The JWST can examine exoplanet atmospheres in detail.
Other upcoming facilities such as the Atmospheric Remote Sensing Infrared Exoplanet Space Telescope (ARIEL) and large ground-based observatories such as the European Extreme Large Telescope (E-ELT), the Thirty Meter Telescope (TMT) and the Giant Magellan Telescope (GMT) can also examine exoplanets more closely.
Researchers have prepared in advance for all of this observation to understand what to look for and what to see when studying atmospheres. This new study focuses on NO2 and how it can be detected in these atmospheres. They focused on NO2 because it is man-made through combustion and it is one of the most important technological pollutants. Not everything is created by combustion, but part of it is.
“Some NO2 on earth is created as a by-product of combustion, which suggests the possibility of scenarios where NO2 is produced on a larger scale
is backed by more advanced technology on another planet, ”they write. “The evidence of high levels of NO2 via non-technological emissions on earth could be a sign that active industrial processes may be taking place on the planet.”
To see how it could be detected, the researchers used a simple photochemical model and generated synthetic atmospheric spectra. The atmospheric spectra mimicked what astronomers might see on a distant exoplanet as the planet passed through its sun.
This figure from the study shows the NO2 absorption cross-section as a function of wavelength. The broad absorption is between 0.25 and 0.6 µm
the dominant feature, and few other molecules absorb here. Photo credit: Kopparapu et al., 2021.
One of the concepts in the study is the geometric albedo. It is the ratio of the brightness of a celestial body at a phase angle of zero to an idealized, perfectly reflective disk. With a phase angle of zero, an observer would look directly at the light source. But on an exoplanet with an atmosphere, the light is scattered, and we would never see it at a phase angle of zero. The geometric albedo helps astronomers circumvent this problem.
This figure from the field shows the difference in the geometric albedo with and without NO2 for an Earth-like planet around a sun-like star (field a) and around a K6V star spectral type (field b) at 10 pc with different NO2 concentrations, provided LUVOIR-A (15 m)
Observation time of 10 hours. The dashed line represents the noise, so the highest NO2 concentrations hardly exceed the noise. Photo credit: Kopparapu et al., 2021.
Although NO2 is detectable, it will take a long time to be detected. The 15-meter-long LUVOIR telescope would take about 400 hours to capture NO2 on an Earth-like planet around a Sun-like star 10 parsecs away. While that’s a lot of observation time to devote to a goal, it’s not entirely unprecedented. For comparison, the Hubble Ultra Deep Field lasted about 11.5 days or 276 hours of observation time.
With a 10-second Sun-Earth system, observation with the LUVOIR telescope would take about 400 hours to detect NO2 levels at earth level above the signal-to-noise ratio (SNR). Photo credit: Kopparapu et al., 2021.
The study has a special feature. The team works with atmospheric NO2 levels from around 40 years ago, when the concentration in the earth’s atmosphere was higher. So if such results were ever actually found, and if ever confirmed that they came from a technological source, it would be a civilization that was on Earth 40 years ago.
“Historically, NO2 concentrations in the US changed (decreased) by a factor of 3 over a period of 40 years from 1980 to 2019,” the authors write in the publication. “Therefore, we can expand the possibilities of discovering a technological civilization at the stage that Earth’s civilization was 40 years ago. One can imagine a more highly industrialized society that
could potentially operate in the range of 5 × Earth NO2 levels, allowing LUVOIR-15m to capture it with even less observation time than under current Earth conditions. “
But that’s a step ahead of things.
Simply finding NO2 in an exoplanet’s atmosphere doesn’t tell astronomers how it was made. “It is important to note that restricting a planet’s NO2 abundance from its spectrum would not definitively answer whether the NO2 is produced biologically or abiotic. One would have to estimate the production rates required to produce the observed NO2 abundance and assess whether abiotic sources alone can sustain the inferred production rate. “
It will take a lot more work to both observe and model to determine if an NO2 signal has a technological source. However, there is no doubt that a clear detection of technological NO2 would be a big event.
“The accidental detection of NO2 or some other potential man-made atmospheric spectral signature (e.g. CFC) can mark a turning point in the search for life (biological or technological),” the researchers conclude. “Is it likely that biosignatures are more common than technosignatures? We won’t know for sure until we search. “