SETI & Technosignatures

Polycyclic Aromatic Hydrocarbons (PAHs) As An Extraterrestrial Atmospheric Technosignature

By Keith Cowing
Status Report
astro-ph.EP
January 14, 2025
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Polycyclic Aromatic Hydrocarbons (PAHs) As An Extraterrestrial Atmospheric Technosignature
(a) Cross sections of four different PAHs in the UV region (0.2-0.4 µm). On average, Naphthalene, Anthracene, and Pyrene have stronger cross-sections among all 4 considered. (b) Collective cross sections of PAHs and other molecules on Earth (taken from Kopparapu et al. (2021)) for the wavelength coverage of a nominal 8m class Habitable Worlds Observatory (0.2-2 µm). PAHs have stronger features than other molecules in this region. The region between 0.3-0.4 µm serves as a promising window to detect PAHs due to the least overlap with other molecular cross-sections. — astro-ph.EP

Polycyclic Aromatic Hydrocarbons are prevalent in the universe and interstellar medium but are primarily attributed to anthropogenic sources on Earth, such as fossil fuel combustion and firewood burning.

Drawing upon the idea of PAHs as suitable candidates for technosignatures, we investigate the detectability of those PAHs that have available absorption cross-sections in the atmospheres of Earth-like exoplanets (orbiting G-type stars at a distance of 10 parsecs) with an 8m mirror of Habitable Worlds Observatory (HWO). Specifically, we focus on Naphthalene, Anthracene, Phenanthrene, and Pyrene. Our simulations indicate that under current Earth-like conditions, detecting PAH signatures between 0.2-0.515 μm is infeasible.

To account for the historical decline in PAH production post-industrial revolution, we explore varying PAH concentrations to assess instrumental capabilities to detect civilizations resembling modern Earth. We also evaluate telescope architectures (6m, 8m, and 10m mirror diameters) to put our results into the context of the future HWO mission. With these four molecules, PAH detection remains infeasible, even at concentrations ten times higher than current levels.

While larger mirrors provide some advantages, they fail to resolve the spectral signatures of these molecules with significant signal-to-noise ratios. The UV absorption features of PAHs, caused by π-orbital → π∗-orbital electronic transitions, serve as valuable markers due to their distinct and detectable nature, preserved by the aromatic stability of PAHs.

Additional lab measurements are necessary to gather absorption cross-section data beyond UV for more abundant PAHs. This may help further in improving the detectability of these molecules.

Dwaipayan Dubey, Ravi Kopparapu, Barbara Ercolano, Karan Molaverdikhani

Comments: 14 pages, 5 figures (Published in the Planetary Science Journal)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2501.06462 [astro-ph.EP] (or arXiv:2501.06462v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.06462
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Related DOI:
https://doi.org/10.3847/PSJ/ad98eb
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Submission history
From: Dwaipayan Dubey
[v1] Sat, 11 Jan 2025 07:26:07 UTC (1,824 KB)
https://arxiv.org/abs/2501.06462
Astrobiology,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻