Evidence

Atmospheric sulfur hexafluoride (SF6) can be released into the environment via geologic means (i.e., via fracturing of silicic igneous rocks (due to faulting) and hydrothermal springs driven by volcanic activity).

SF6 is a stable gas with high greenhouse warming potential (i.e., 23,900 times that of carbon dioxide) that is mainly used to insulate high-voltage electrical devices (e.g., circuit breakers, switchgear, and transformers) and as an inert blanketing agent for molten reactive metals such as magnesium and aluminum (Maiss and Brenninkmeijer, 1998). It was first reported that fluorite and some granites contain SF6 (see Harnisch et al., 1996 and Table 1 in Harnisch and Eisenhauer, 1998) with Busenburg and Plummer (1997) finding it in natural waters (e.g., hot springs and groundwaters). In this study, the highest concentrations of SF6 were found in Illinoisan fluorite and Swedish granite, thereby supporting the results from Harnisch and Eisenhauer (1998), however other analyzed rocks of the igneous, metamorphic, hydrothermal, and sedimentary variety and their associated mineral phases contained SF6 in relatively small concentrations. Following from Harnisch and Eisenhauer (1998), the trend of increasing SF6 concentrations as bulk silica content increases stands (i.e., from mafic to felsic rock) for the samples analyzed here. Additional sources included in this paper that exceed the modern air-water equilibrium were groundwater flowing from fractured silicic igneous rocks (due to faulting) and hydrothermal springs driven by volcanic activity. SF6 can be produced two ways, naturally and via anthropogenic processes as discussed above, but in 1999 only 1.2% of the total atmospheric partial pressure was attributed to SF6. The arrived at steady state background concentration of SF6 calculated from old groundwater samples as of 2000 was 0.054 ± 0.009 pptv. This means that while SF6 may be present in an exoplanet’s atmosphere that does not unequivocally point to an advanced civilization, as there are natural sources that produce measurable pre-industrial SF6.

Authors

Eurybiades Busenberg and L. Niel Plummer

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Evidence

Atmospheric tetrafluoromethane (CF4) can be released into the environment via geologic means (i.e., via surface weathering, metamorphism or metasomatism, and continuous diffusion of gas at depth of continental crust).

Prior to Gassmann (1974) interpretation of Kranz (1966) mass spectrometry data, CF4 (a long-lived and potent greenhouse gas) was generally believed to be solely anthropogenic, produced via electrolytic primary aluminum and semiconductor production and used as a refrigerant (Freon 14). CF4 is reported to have an atmospheric lifetime of at least 50,000 years (Ravishankara et al. 1993). It was demonstrated by Harnisch et al. (1996) that around half of the current atmospheric burden accumulated slowly from lithospheric sources such as fluoride minerals (e.g., fluorite) via radiogenic production. This was corroborated by measurements of significant traces of pre-industrial CF4 in dissolved gases contained within ice core segments and encapsulated air from the 1950s and 1960. Here, the source(s) of natural CF4 are further elucidated with the addition of degassing from fluorite-bearing granites that make up the continental crust. The three primary mechanisms that should be considered for the release of CF4 are surface weathering, metamorphism/metasomatism, and continuous diffusion of gas at depth. It is calculated that 80% of crustal fluorites can be found in granites and that these fluorites have CF4 contents 50 times that of their hydrothermal counterparts. Based on the measurements and assumptions presented here, the CF4 flux is comparably small with respect to anthropogenic emissions (15,000 tons/yr versus 5,700 tons/yr). However, this paper shows that the presence of CF4 at comparable natural terrestrial levels in an exoplanet’s atmosphere would not definitely mean the surface is inhabited by a technological civilization. Additionally, dissolved CF4 concentrations have been measured elsewhere in groundwater (Deeds et al., 2008a; 2015) and the North Pacific Ocean (Deeds et al., 2008b), while firn air measurements better constrain the pre-industrial atmospheric history of CF4 (Worton et al., 2007).

Authors

Jochen Harnisch and Anton Eisenhauer

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Evidence

When functioning, the Oklo natural nuclear reactor was believed to be similar to past nuclear disasters (e.g., Three Mile Island, Chernobyl, and Fukushima Daiichi incidents) in terms of fission by-products released into the environment and resulting atmospheric anomalies; today, the areas surrounding the fossil reactors still produce fission products that influence the near atmospheric environment.

The first paper to discuss remote sensing of the upper atmosphere (i.e., the ionosphere) to ascertain near-surface radioactive pollution was Boyarchuk et al. (1997), where it is suggested that variations in the electric field near the Earth’s surface result in observable perturbations in the ionosphere. This basic concept evolved into the lithosphere-atmosphere-ionosphere-magnetosphere model coupling (LAIMC) (Pulinets and Ouzounov, 2011; see Figure 1 in Pulinets et al., 2015 for schematic). Using the 2011 Fukushima Daiichi Nuclear Power Plant (FDNPP) accident as a recent exemplar case study, Ouzounov et al. (2011) demonstrated the atmospheric response to this incident via satellite and ground-based observations. The National Oceanic and Atmospheric Administration’s Advanced Very High Resolution Radiometer (AVHRR), which measures Earth’s emitted thermal radiation in the spectral range of 10.3 to 12.5 microns, recorded an anomalous increase in the outgoing longwave radiation at the top of the atmosphere over Japan (14W/m2 at maximum) temporally coinciding with the leakage of radioactive gas (e.g., cesium-137 and iodine-131) into the atmosphere. These events were accompanied by elevated in situ radioactivity and atmospheric temperature measurements near the FDNPP. Similar treatments have been applied to historical AVHRR data sets for the Three Mile Island and Chernobyl incidents with OLR anomalies of ~3 and ~7 W/m2 (Laverov et al., 2011). While OLR anomalies are only one of the effects caused by the influx of radiogenic material into the near surface environment, there is also the resulting change in boundary layer conductivity mentioned above. This change in conductivity is conferred to the upper atmosphere where it influences the ionospheric potential and can then be observed (Markson, 2007; Pulinets and Davidenko, 2014). These observable, quantifiable effects are not solely related to anthropogenic activities. It has been shown by Pulinets and Ouzounov (2018) that this model also fits for extinct natural nuclear reactors (Gauthier-Lafaye et al., 1989; 1996) that are still degassing; however, it can be imagined that when fission was actively occurring the by-products would have been the same (i.e., cesium and iodine). Observed ionospheric anomalies originating from dust storms, volcanic eruptions, tropical cyclone formation, and cloud formation can also be understood by applying the LAIMC model. Thermal and electrical disturbances initiated near the surface of a terrestrial body (citing Earth as an example) due to the presence of radiogenic material, both natural and artificial, are transferred to the ionosphere where they can be observed remotely. Nuclear weapons testing higher in the atmosphere induces a greater ionospheric response.

Authors

K. A. Boyarchuk, A. M. Lomonosov, S. A. Pulinets, and V. V. Hegai

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Evidence

The photometric properties of astrophysical (or interstellar) dust, (e.g., brightness and color temperature) can mimic those of high-luminosity waste heat.

Astrophysical (or interstellar) dust and extrasolar civilizations could interact similarly with incident light from a nearby star (i.e., both could absorb the incoming light and reradiate primarily it in the mid-infrared, so that it can then be detected). However, the emission spectra associated with the dust is believed to be consistent with a three component system at a minimum, typically exhibiting non-thermal spectral features corresponding to polycyclic aromatic hydrocarbons in the near-infrared and to big grains dominated by amorphous silicates around 10 µm. Very small grains (spectrally consistent with carbonaceous material) are the constituent responsible for the continuum emission in the mid-infrared (C. Meny et al., 2007). With this in mind, a broadband search would not be able to spectrally distinguish dust from other mid-infrared sources and additional information (i.e., targeted spectral analysis or location within star forming region) would be required to differentiate a civilization with a low-level waste heat output from natural astrophysical sources. Human cities and/or natural sources with the highest infrared flux ratio (similar to the “advancement metric” from Kuhn and Berdyugina, 2015) are suggested as a baseline for the emissions from extraterrestrial civilizations we should be sensitive to.

Authors

J. T. Wright, R. L. Griffith S. Sigurdsson, M. S. Povich, and B. Mullan

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Evidence

It is hypothesized that most civilizations manage their growth as to follow the same trajectory of humans (i.e., slowdown population growth and energy consumption) and eventually would be in equilibrium with the exoplanet they inhabit (i.e., indistinguishable from an uninhabited world) or go extinct.

In terms of detecting communications from advanced civilizations, here it is hypothesized that most civilizations manage their growth as to follow the same trajectory of humans (i.e., slowdown population growth and energy consumption), slow their growth exponentially, or self-destruct. In any one of these cases contact would not come until thousands of years in the future due to our current technological abilities and/or the die off of a quicker-growing, unchecked civilization. Some sustainable civilizations may not even experience associated artificial warming of their environment but instead interact with incident radiation as their planet would; intercepting it, using it, and reradiating it at natural wavelengths. As an example, Wright et al. (2014) makes the point that if the sustainability hypothesis is valid, then expected extraterrestrial intelligence resource consumption would be low. The assumption is then made that long-lived civilizations will be more efficient at harnessing energy from their environment (e.g., geothermal or solar) thereby decreasing their waste heat production and voiding the prospect of detectable “heat islands” (Kuhn and Berdyugina, 2015). Extrasolar sustainable civilizations may advance at a slower rate than humans and be in greater equilibrium with their world (i.e., using energy more efficiently and producing less environmental contaminants) and as a result be less likely to produce strong technosignatures that would be indicative of an “advanced” civilization.

Authors

B. Mullan and J. Haqq-Misra

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