For over a century, humanity has been on a quest to find signs of intelligent life beyond Earth. This endeavor, best illustrated by the search for extraterrestrial intelligence (SETI), gained notoriety thanks to Carl Sagan’s 1985 novel, Contact, which was later adapted into a film. Like Sagan’s protagonist, many SETI researchers utilize telescopes to capture radio signals from distant civilizations. However, radio waves are merely one of the tools scientists employ in the ongoing search for extraterrestrial life.
Astronomers look for measurable indicators of advanced technologies, known as technosignatures. In 1906, astronomer Percival Lowell mapped what he thought were numerous man-made structures, specifically Mars’ canals. Then, in 1960, physicist Freeman J. Dyson suggested that advanced civilizations might construct massive structures around stars to harvest energy, now referred to as a Dyson Sphere. Although Lowell’s canals were later attributed to natural erosion and Dyson’s idea remains a hypothesis, the quest for technosignatures persists.
Currently, astronomers analyze the chemical signatures in distant planetary atmospheres for indicators of life or advanced technologies. Researchers advocate measuring industrial gases like: CFCs or hydrofluorocarbons to help detect extraterrestrial civilizations on exoplanets. However, given their low atmospheric concentrations on Earth, detecting these gases on other worlds poses a challenge. Optimal conditions may require up to 500 hours of observation time with the James Webb Space Telescope (JWST), the largest telescope ever constructed.
The team led by Sarah Seager at MIT proposed nitrogen trifluoride (NF3) and sulfur hexafluoride (SF6) as potential technosignature gases. Both substances are industrially produced on Earth; NF3 is utilized for cleaning semiconductors and solar panels, while SF6 is used in insulating transformers and high-voltage equipment, with its atmospheric concentration increasing significantly in recent decades.
Interestingly, the research team initially ruled out biological sources for these gases, as living organisms can produce false positives for technosignatures. Their investigation into Earth’s biogenic chemical database revealed no known organisms that generate NF3 or SF6. In fact, no life forms are recognized to create molecules with nitrogen-fluorine or sulfur-fluorine bonds.
The researchers proposed that Earth’s life forms may deliberately avoid using fluorine-based molecules due to fluorine’s propensity to bind within minerals, making extraction challenging. Moreover, these molecules possess unique chemical properties that complicate their utilization by biological systems. Specifically, their strong electron affinity leads to violent reactions with other molecules, resulting in robust bonds that are hard to break. This, they argued, suggests that fluoride may be unsuitable for extraterrestrial life.
Next, they examined potential non-biological, or abiotic sources for these gases, such as tectonic and various geological processes. While NF3 has no known abiotic sources on Earth, volcanic activity does generate minute quantities of SF6. They theorized that volcanic eruptions releasing SF6 would also emit silicon tetrafluoride (SiF4), a more prevalent volcanic gas, enabling astronomers to detect both SiF4 and SF6 simultaneously, thus strengthening the case for technosignatures if SF6 is found without corresponding SiF4.
Finally, the scientists evaluated the feasibility of distinguishing these gases from other atmospheric components on exoplanets. To achieve this, astronomers monitor the exoplanet’s transit in front of its star, measuring the light’s wavelengths that pass through its atmosphere, generating patterns known as a transmission spectrum. Ideally, each peak in the spectrum corresponds to a unique atmospheric gas; however, overlapping or obscured gases can complicate detection.
Utilizing a computer model called Simulated Exoplanet Atmospheric Spectra, the research team generated a transmission spectrum for a rocky exoplanet approximately five times the mass of Earth, termed a super-Earth, orbiting a M-dwarf star. They simulated three atmospheric compositions dominated by H2, N2, and CO2. Their findings revealed that both NF3 and SF6 display spectral signatures distinct from those of the predominant atmospheric gases, and could theoretically be detected by the James Webb Space Telescope, albeit at concentrations much higher than those found in Earth’s atmosphere. Next-generation telescopes, such as the Habitable Worlds Observatory and the Large Interferometer for Exoplanets, are optimized for detecting such signatures.
While Seager and her team view NF3 and SF6 as promising technosignature gases, many uncertainties remain. Our understanding of how these gases behave in Earth’s atmosphere is limited. Additionally, the potential overlap of their transmission spectra with chlorofluorocarbon gases necessitates further studies for signal separation. Scientists also noted the unpredictability of byproducts from extraterrestrial biology. If astronomers were to observe a steady increase in technosignature gases on an exoplanet over a century, it could indicate the presence of an industrialized alien civilization. Astronomers hope to be fortunate enough to witness this evidence.
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Source: sciworthy.com
