Polycyclic Aromatic Hydrocarbons (PAHs) as an Extraterrestrial Atmospheric Technosignature

Dwaipayan Dubey Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, D-81679 München, Germany Exzellenzcluster ‘Origins’, Boltzmannstr. 2, D-85748 Garching, Germany Ravi Kopparapu NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA Barbara Ercolano Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, D-81679 München, Germany Exzellenzcluster ‘Origins’, Boltzmannstr. 2, D-85748 Garching, Germany Karan Molaverdikhani Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, D-81679 München, Germany Exzellenzcluster ‘Origins’, Boltzmannstr. 2, D-85748 Garching, Germany

Abstract

Polycyclic Aromatic Hydrocarbons (PAHs) 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 $\mathrm{\mu m}$ 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 $\mathrm{\pi}$ -orbital $\rightarrow$ $\mathrm{\pi^{*}}$ -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.

Exoplanet atmospheres (487); Exoplanet atmospheric composition (2021); Technosigantures (2128)

1 Introduction

The past three decades have been remarkable in exoplanetary science, led by a plethora of exoplanet discoveries and their atmospheric studies. With the current state-of-the-art space and ground-based facilities, nearly 5600 exoplanets have been detected¹¹1https://exoplanetarchive.ipac.caltech.edu/, spanning from gas giants to rocky terrestrial planets. Simultaneously, spectroscopy techniques have undergone significant refinement, emerging as a promising avenue for characterizing exoplanet atmospheres, with the key focus on understanding atmospheric pressure-temperature profiles, atmospheric compositions, and its dynamics (Crossfield, 2015). Although the initial breakthrough of atmospheric studies has happened on gas giant planets with a lot of follow-up investigations (Charbonneau et al., 2002; Benneke et al., 2019; Tsiaras et al., 2019), the characterization of the atmospheres of rocky planets is slowly coming within our reach (Greene et al., 2023; Lim et al., 2023; Lustig-Yaeger et al., 2023; Cadieux et al., 2024), offering the exciting opportunity of identifying potentially habitable worlds. Special attention is given to the search for “biosignatures”, i.e. spectral signatures of gases originating from living organisms, offering possible evidence of extraterrestrial life. The concept has earned considerable attention within the exoplanet and astrobiology²²2https://www.nationalacademies.org/our-work/astrobiology-science-strategy-for-the-search-for-life-in-the-universe communities in recent times. Significant effort is being concentrated on the identification of potential biosignatures as well as the development of strategies to detect them (Seager et al., 2012; Grenfell, 2017; Kaltenegger, 2017; Catling et al., 2018; Fujii et al., 2018; Meadows et al., 2018; Schwieterman et al., 2018; Walker et al., 2018; Lammer et al., 2019; O’Malley-James & Kaltenegger, 2019).

Since the first mention by Tarter (2006), the concept of “technosignatures” has undergone significant development alongside the concept of “biosignatures”. Technosignatures are defined as the manifestations of extraterrestrial technology that could be detected through astronomical observations. The technosignature idea represents a logical extension of the search for biosignatures in the domain of astrobiology, leveraging Earth’s evolutionary life history and technological advancements (NASA Technosignatures Workshop Participants 2018). Similar to biosignature investigations, identifying technosignatures involves proposing detectable characteristics inspired by Earth’s perspective and designing strategies to detect them on extraterrestrial worlds. Although the classification and identification of specific technosigantures are still in the infancy stage compared to that of biosignatures (Wright et al., 2019; Haqq-Misra et al., 2020; Lingam & Loeb, 2021), a significant number of studies on possible technosigantures have paved the way to consider it as one of the important future prospects in the domain of exoplanetary science: waste heat (Dyson, 1960; Carrigan, 2009; Wright et al., 2014; Kuhn & Berdyugina, 2015), artificial illumination (Schneider et al., 2010; Loeb & Turner, 2012; Kipping & Teachey, 2016; Tabor & Loeb, 2021), artificial atmospheric constituents (Owen, 1980; Campbell, 2005; Schneider et al., 2010; Lin et al., 2014; Stevens et al., 2016; Kopparapu et al., 2021; Haqq-Misra et al., 2022), artificial surface constituents (Lingam & Loeb, 2017), non-terrestrial artifacts (Bracewell, 1960; Freitas Jr & Valdes, 1980; Rose & Wright, 2004; Haqq-Misra & Kopparapu, 2012), stellar pollution (Shklovsky & Sagan, 1966; Whitmire & Wright, 1980; Stevens et al., 2016), and megastructures (Dyson, 1960; Arnold, 2005; Forgan, 2013; Wright et al., 2015). Currently, the concept of Search for Extraterrestrial Intelligence (SETI) (Wright et al., 2018) has also been developed with a firm belief and expectation to observe Earth’s current technosigantures on planets at interstellar distances with the present technology.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in space, carrying 10–20% carbon budget of the interstellar medium (ISM) (Joblin & Tielens, 2011). Draine & Li (2007) and Tielens (2008) provide a more detailed explanation of the contributions of PAHs to the IR emission and their role in the carbon budget of the ISM. They are present in the ISM with a relative number density of 3 $\times$ $\mathrm{10^{-7}}$ respective to hydrogen nuclei (Tielens, 2008). From the astrophysical and astrobiological standpoints, PAHs are considered the most interesting complex organic structures for the following reasons: (1) they are crucial in studying the chemical and hydrodynamical evolution of protoplanetary disks and newborn planet atmospheres (Gorti et al., 2009; Ercolano et al., 2022; Dubey et al., 2023) as well as in understanding the ionization balance of gaseous atmospheres (Thi et al., 2019), and (2) they are anticipated to have significant influence on prebiotic chemistry and abiogenesis, leading an important step toward life formation (Ehrenfreund & Charnley, 2000; Ehrenfreund et al., 2006, 2007; Rapacioli et al., 2006; Wakelam & Herbst, 2008; Sandstrom et al., 2011; Kim et al., 2012; Puzzarini et al., 2017; Closs et al., 2020). A recent study by Dubey et al. (2023) shows the formation possibilities of PAHs in the thermalized atmospheres of transiting (irradiated) and directly imaged (non-irradiated) hot-Jupiters. However, their formation processes on terrestrial bodies like Earth and Saturn’s moon Titan are still under debate. The detection of PAH molecules on Titan by the space probe Cassini signified the importance of photochemistry for PAH formation (with an abundance of (2 - 3) $\times$ $\mathrm{10^{4}}$ particles per $\mathrm{cm^{3}}$ ) (Dinelli et al., 2013; López-Puertas et al., 2013). PAHs in Earth’s atmosphere, on the other hand, stem from various anthropogenic activities ranging from burning wood to incomplete combustion of fossil fuels (Ravindra et al., 2008). Burning wood could also come from forest fires, which is a natural process. The detection of PAHs could indicate either biological or technological processes, both of which would still imply the presence of life. Disparities in the development status of different countries lead to varying PAH emissions globally, with higher emissions observed in underdeveloped nations (Shen et al., 2013). Developed countries, however, have implemented measures to restrict net PAH emissions since the post-industrial revolution era, reducing the emissions by a factor of almost two.

Advancing the concept of technosignatures could unveil new avenues by revealing new classes of species further. Theoretical studies have suggested that the detection of $\mathrm{NO_{2}}$ (Kopparapu et al., 2021) and CFCs (CFC-11 and CFC-12) (Haqq-Misra et al., 2022) on Earth-like exoplanets could serve as potential technosignatures. The presence of PAHs on another planet is an addition to them. Detecting PAHs on exoplanets may provide evidence of industrial activity on their surfaces. In this work, we showcase the study on PAHs as a possible technosignature with the future Habitable Worlds Observatory (HWO) mission concept. We outline our methods in Section 2. This section explains the global variability in PAH emissions from different sources, molecular cross-sections, and simulation setups for computing the reflected spectra of a planet around a Sun-like star. Our study is confined within the habitable zone of the host star. Section 3 outlines our findings in terms of the detectability of PAH signatures from planet spectra using signal-to-noise ratio estimation. We also discuss about the suitable telescope architecture required to detect PAH with better precision. A concise summary of our result is given in Section 4.

2 Methods

2.1 Global PAH emission and its variability

PAHs are very crucial in the context of Earth due to their significant contribution to global pollution and human health hazards (Laflamme & Hites, 1978; Perera, 1997). Based on the PKU-FUEL-2007 database (Wang et al., 2013), Shen et al. (2013) calculated the historical and future trends in country-specific PAH emissions from 1960 to 2030. We obtained their data on global PAH emissions, including specific data from four major contributing countries: India, China, Russia, and the US (see Figure 1). While India and China represent the developing countries, Russia and the US provide an understanding of the developed regions. Figure 1 illustrates the relative contributions of different anthropogenic sources to the net PAH production for them. The variability in PAH source profiles across different countries is particularly attributed to their different developmental position, energy infrastructure, and vegetation coverage. Globally, including India, China, and the US, indoor firewood burning is the major contributor to the net PAH emission. While it is one of the major energy sources in rural areas of India (90%) and China (70%) (Balakrishnan et al., 2011; Zhou et al., 2017), the situation in the US differs significantly, with the prevalence of certified wood stoves in rural households (U.S. Environmental Protection Agency 1987). Russia, on the contrary, is mostly dominated by the emissions from motor vehicles. The coke production sector (extraction of carbon-rich fuel from coal which is further used in the steel-making industry and other industries) has historically held significance in Russia and China, yet it is presently declining due to the discontinuation of the use of beehive coke ovens. Deforestation is also quite significant for Russia and the US.

Refer to caption — Figure 1: Contributions of different anthropogenic sources to the net global PAH emission including four major PAH producing countries in the year 2007 (Shen et al., 2013). The pie charts illustrate the total emissions for each country as a whole. These countries exemplify the conditions found in both developed and developing regions globally. Firewood and crop residue burn, deforestation/wildfire, and motor vehicles are the dominating source profiles across the globe.

For this study, we have selected four PAHs: Naphthalene ( $\mathrm{C_{10}H_{8}}$ ), Anthracene ( $\mathrm{C_{14}H_{10}}$ ), Phenanthrene ( $\mathrm{C_{14}H_{10}}$ ), and Pyrene ( $\mathrm{C_{16}H_{10}}$ ). Our selection criteria are twofold: firstly, these compounds represent the simplest PAHs ranging from those composed of 2 benzene rings to 4 benzene rings (easily formed and more abundant); secondly, their UV cross sections are publicly available in the community. Measured global emissions (in Mg year^-1 unit) of these PAHs in 2007 are shown in Table LABEL:tab:density. For further calculations, we utilize the following mathematical prescription to convert the PAH emissions to volume mixing ratio unit (parts per billion (ppb)). We have made two key assumptions relevant to this analysis: (1) Emissions are uniformly distributed within a defined volume of air in the atmosphere. Since emissions occur close to the ground, we assume that the PAH mass is concentrated within the first 5 to 10 km of altitude, primarily within the troposphere, where these heavier molecules are likely more prevalent. This region encompasses most of the atmospheric mass. (2) According to Shen et al. (2013), the global trend in PAH emissions has remained relatively consistent over the past decades since 1960. However, the atmospheric lifetime of Naphthalene, Anthracene, Phenanthrene, and Pyrene on Earth is on the order of hours to a few days (Naphthalene: El-Masri (2005); Anthracene: Shahsavar et al. (2023); Phenanthrene: Zhao et al. (2016), and Pyrene: NRC (1983) (National Research Council)). A significant portion of these molecules is expected to be degraded through reactions with reactive species such as OH radicals, ozone, and other oxidizing agents. Recombination processes are relatively slower than their destruction mechanisms, leading to a net decrease in atmospheric concentrations over time. To account for this loss mechanism, we have incorporated a decay term into the PAH concentration calculations (see Equation 2). Given the absence of a detailed PAH kinetic network tailored for an Earth-like atmosphere, this approach represents the most realistic estimation of present-day PAH concentrations.

The Earth’s radius is approximately 6371 km. By considering an atmospheric shell extending 10 km above the Earth’s surface, where emissions are assumed to be present, the volume of this atmospheric shell can be calculated as follows:

\mathrm{V_{air}}=\mathrm{\frac{4}{3}\pi(R^{3}_{outer}-R^{3}_{inner})}\approx 5.11\times\mathrm{10^{18}}\text{ $\mathrm{m^{3}}$}

(1)

where $\mathrm{R_{inner}}$ = Earth radius $\approx$ 6371 km and $\mathrm{R_{outer}}$ = Earth radius with the atmospheric shell $\approx$ 6381 km.

If the emission rate of an individual PAH is measured to be F Mg each year (specific values provided in Table LABEL:tab:density), then, based on assumption 2 (i.e., the global emission rate has remained consistent since 1960), the remaining atmospheric density of PAHs ( $\mathrm{C_{g/m^{3}}}$ [now]) at the current times can be calculated using Equation 2:

\mathrm{C_{g/m^{3}}}[\text{now}]=\sum_{i=1960}^{2024}\Big{[}\mathrm{\frac{F_{i}}{5.11\times 10^{18}}\times 10^{6}\Big{]}e^{-(2024-i)/\tau}}\approx\mathrm{\frac{F_{2024}}{5.11\times 10^{18}}\times 10^{6}}\approx\mathrm{\frac{F_{2007}}{5.11\times 10^{18}}\times 10^{6}}

(2)

In Equation 2, the concept of atmospheric lifetime is applied, analogous to an exponential decay process. Here, $\tau$ represents the lifetime of the specific molecules, typically ranging from hours to a few days, during which they are degraded through chemical reactions and other loss mechanisms. Suppose the atmospheric lifetime of a molecule is x days (= $\mathrm{\frac{x}{365}}$ years). In that case, the exponential term in the decay process will lead to the near-complete depletion of PAH concentrations from previous years, as the exponent becomes largely negative over time. Therefore, focusing on the PAH concentration for the current year provides the most realistic estimate for assessing the detectability.

At standard temperature and pressure (STP: 0^∘C and 1 atm), 1 mole of any ideal gas occupies 22.4 liters of volume = 0.0224 $\mathrm{m^{3}}$ . Therefore, 1 $\mathrm{m^{3}}$ of air contains $\mathrm{\frac{1}{0.0224}}$ $\approx$ 44.64 moles of air. Now, let’s say the molecular mass of the PAH is M g/mol. We can convert the density from $\mathrm{C_{g/m^{3}}}$ [now] to $\mathrm{C_{moles/m^{3}}}$ using Equation 3:

\mathrm{C_{moles/m^{3}}}=\mathrm{\frac{C_{g/m^{3}}[\text{now}]}{M}}

(3)

Finally, the volume mixing ratios in ppb are obtained by:

\text{volume mixing ratio (ppb)}=\mathrm{\frac{C_{moles/m^{3}}}{44.64}}\times 10^{9}

(4)

These steps led to a global average volume mixing ratio of 0.0078 ppb for Naphthalene, 0.00024 ppb for Anthracene, 0.0014 ppb for Phenanthrene, and 0.00041 ppb for Pyrene.

PAHs	Global Net Emission	PAHs	Global Net Emission
	(Mg year^-1)		(Mg year^-1)
\hlineB3.5 Naphthalene (Nap)	2.3 $\times$ $\mathrm{10^{5}}$	Phenanthrene (Phe)	5.3 $\times$ $\mathrm{10^{4}}$
Anthracene (Ant)	1.0 $\times$ $\mathrm{10^{4}}$	Pyrene (Pyr)	1.9 $\times$ $\mathrm{10^{4}}$
References	Shen et al. (2013)

Polycyclic Aromatic Hydrocarbons (PAHs) as an Extraterrestrial Atmospheric Technosignature

Abstract

1 Introduction

2 Methods

2.1 Global PAH emission and its variability

2.2 Molecular cross-sections

2.3 Radiative transfer calculation and simulation setup

3 Detectability of PAHs

3.1 Different level of atmospheric PAH concentration

3.2 Telescope architecture

4 Discussion and Conclusion

References

Appendix A Cross-sectional requirement for PAHs