Fully fluorinated non-carbon compounds NF3 and SF6 as ideal technosignature gases

Waste gas products from technological civilizations may accumulate in an exoplanet atmosphere to detectable levels. We propose nitrogen trifluoride (NF3) and sulfur hexafluoride (SF6) as ideal technosignature gases. Earth life avoids producing or using any N–F or S–F bond-containing molecules and makes no fully fluorinated molecules with any element. NF3 and SF6 may be universal technosignatures owing to their special industrial properties, which unlike biosignature gases, are not species-dependent. Other key relevant qualities of NF3 and SF6 are: their extremely low water solubility, unique spectral features, and long atmospheric lifetimes. NF3 has no non-human sources and was absent from Earth’s pre-industrial atmosphere. SF6 is released in only tiny amounts from fluorine-containing minerals, and is likely produced in only trivial amounts by volcanic eruptions. We propose a strategy to rule out SF6’s abiotic source by simultaneous observations of SiF4, which is released by volcanoes in an order of magnitude higher abundance than SF6. Other fully fluorinated human-made molecules are of interest, but their chemical and spectral properties are unavailable. We summarize why life on Earth—and perhaps life elsewhere—avoids using F. We caution, however, that we cannot definitively disentangle an alien biochemistry byproduct from a technosignature gas.

of Earth and is a dead remnant of a Sun-like star.This scenario would require only tens of hours of in-transit time 28,30,31 .See the SI for more details on CFCs and other suggested technosignature gases.
Here we propose nitrogen trifluoride (NF 3 ) and sulfur hexafluoride (SF 6 )-fully fluorinated non-carbon compounds-as potential technosignature gases (Fig. 1).A fully fluorinated compound is a molecule where the central atoms are only bonded to fluorine atoms.For example, in NF 3 the central nitrogen atom can bind to three other atoms.Because each of the bound atoms is fluorine, we call NF 3 fully fluorinated.NF 3 and SF 6 have been only briefly mentioned as technosignatures 34,35 , the case has not previously been developed.
We are motivated to explore non-carbon fully fluorinated compounds primarily because life on Earth never makes compounds containing the N-F and S-F bonds, nor fully fluorinated non-carbon compounds ("Results" section).S-F and N-F bonds are not even made as metabolic intermediates and are likely to be universally excluded by life, no matter its biochemistry.In fact, life on Earth very rarely uses F chemistry.Only a few species make C-F bonds at all (Petkowski et al. in prep.).While life on Earth does not make fully fluorinated carbon molecules (e.g., tetrafluoromethane, CF 4 , or CFCs that have been previously considered as potential technosignature gases 12 ), life could in principle do so without inventing completely new enzymes and other necessary biochemical repertoire (Petkowski et al. in prep.).Hence fully fluorinated non-carbon molecules may be more robust technosignature gases than (fully) fluorinated carbon molecules.
SF 6 and NF 3 on Earth are not only industrial pollutants but their relative atmospheric abundance has rapidly increased 36,37 (See Fig. 2 and SI).NF 3 has no known abiotic sources and was entirely absent from the preindustrial atmosphere 37 .Within the last half century, the NF 3 abundance in the atmosphere rose to close to 3 part-per-trillion (ppt) by volume (Fig. 2).SF 6 does not have significant abiotic sources that could mimic its rapid increase in the atmosphere.Like NF 3 , the steady and rapid relative increase of SF 6 in the atmosphere from very low background abundances of < 0.06 ppt to around 11 ppt in the last half a century combined with its relatively long atmospheric lifetime of at least a couple of hundred years 38 further supports SF 6 as a good technosignature gas candidate.However, we caution that the atmospheric chemistry of NF 3 and SF 6 has not been well studied and many potential destruction pathways for NF 3 and SF 6 may not be known (see Tables S2 and S3).

Results
No fully fluorinated molecules are made by life.Life on Earth is not known to produce any molecules with N-F or S-F bonds, and this includes fully fluorinated N and S compounds.We derived this result from our natural products database which is a curation of all known biochemicals and natural products (i.e.compounds produced by life) from an extensive literature online chemical repository search ("Methods" section and 39,40 ).Here, "natural products" means chemicals made by life.
Life does produce some compounds with N-Cl, N-Br, S-Cl, and S-Br bonds, but none are volatile.In addition, the N-Cl, N-Br, S-Cl, and S-Br compounds are typically intermediaries and not molecules that accumulate on their own.The molecules containing N-Br and N-Cl are quite reactive and therefore rare, a notable example Table 1.Summary of proposed exoplanet atmosphere technosignature gases including simulated in-transit observation times.*not explicitly stated whether this is in-transit or both in-transit and out-of-transit time.N/A means no information available.being a neurotransmitter N-bromotaurine 41 and pseudoceratonic acid 42 .The S-Cl and S-Br bond-containing molecules have been found only in proteins as intermediaries in synthesizing the N-S bonds 39 .
In comparison with the rare N-Cl, N-Br, S-Cl, and S-Br containing molecules, there are thousands of compounds containing C-Cl, C-Br and C-I bonds that are made by life (Fig. 3).Most of the life-produced halogenated compounds are Cl-containing compounds (~ 2% of all known natural products, where the current known total number of unique natural products is ~ 220,000).Br-containing compounds produced by life are a close second (~ 1.7% of all known natural products).Iodine-containing natural products are much more rare but still a significant group with approximately 200 known examples (~ 0.1% of all known natural products).The above Figure 2. Atmosphere gas abundance for some industrial pollutants including NF 3 and SF 6 .The y-axis is the fractional gas abundance in part-per-trillion (ppt) by volume and the x axis is time in years.Both NF 3 and SF 6 have a rapid increase compared to other industrial pollutants.The right panel is a zoomed in version of the left panel.Data from 36,37,95 , and notably 2013-2022 data is from the Global Monitoring Laboratory (GML) (SF 6 : https:// gml.noaa.gov/ hats/ combi ned/ SF6.html; NF 3 : https:// gml.noaa.gov/ hats/ gases/ NF3.html).

Figure 3.
Number of molecules containing C-X bonds produced by life (called "natural products"), where X is Cl, Br, I, or F. For comparison the numbers are separated into three categories.Green: all known natural products in the category (i.e.produced by life).Blue: the subset of volatile natural products, here limited to molecules with 6 non-H atoms or less.Red: the subset of fully halogenated volatile natural products, also limited to molecules with 6 non-H atoms or less.Life rarely produces fully non-F halogenated volatile natural products and does not produce any fully fluorinated products of any kind (C-F, N-F, S-F, or other).Note that one fully halogenated molecule is double counted as it contains both Cl and Br, bromotrichloromethane.Not shown is that life does not produce any molecules with N-F or S-F bonds.In contrast to the thousands of Cl-, Br-and I-containing carbon compounds made by life, the F-containing compounds are nearly excluded from life's repertoire, numbering only 34.And, only two of these 34 are volatile.Nearly all of the known biogenic F-containing natural products are fluorinated carboxylic acids (Petkowski et al.  2023

in prep.
).There are no fully fluorinated C-F compounds known to be produced by life.
Although life does not produce fully fluorinated molecules, life does actually produce at least four carbon compounds that are fully halogenated with halogens other than fluorine.This is a very small percentage of all volatile halocarbons produced by life.For some numbers, and considering molecules with 6 or fewer nonhydrogen atoms, there are 85 volatile halocarbons produced by life.There are 34 possible halomethanes, of which 14 are fully halogenated.22 halomethanes are produced by life on Earth 40 .Out of those 22, only 3 are fully halogenated: tetrachloromethane (CCl 4 ) produced by several plants and marine algae 43 , tetrabromomethane, CBr 4 , produced by from various marine algae, e.g.Asparagopsis taxiformis 44 , and bromotrichloromethane, CBrCl 3 , that contains both Cl and Br atoms, produced by marine algae 44 .For completeness, the fourth fully halogenated carbon compound produced by life is tetrachloroethene (C 2 Cl 4 ), produced by Hawaiian red seaweed Asparagopsis taxiformis 44 .
Again, the number of fully fluorinated molecules made by life is zero, no matter if they are fluorocarbons or if the F atom is attached to a non-carbon element.
We explain why life avoids F-containing compounds in Petkowski et al. 2023 (in prep.) and briefly summarize the explanation here.We have identified three challenges that F chemistry poses for life on Earth that make the use of fluorine in Earth's biochemistry a difficult prospect: 1. Relatively low bioavailability of F, which is primarily locked inside insoluble minerals, and is not available in surface water (unlike Cl, Br and I). 2. The uniquely high electronegativity of F, which means that enzymes that handle Cl, Br, peroxide and other oxidizing species cannot be repurposed to handle F. As a result of this challenge life needs to evolve a completely novel enzymatic machinery to create C-F bonds.3. The lack of reactivity of the C-F bond, which makes evolving catalysts that can handle C-F bonds a difficult task.
These substantial evolutionary barriers mean that almost all life has found ways to address its ecological requirements with chemistry other than C-F chemistry.All three factors are general properties of fluorine chemistry, are not specific to terrestrial biochemistry and therefore are likely universal.NF 3 and SF 6 have unique spectral features compared to dominant atmospheric gases.The gases NF 3 and SF 6 have unique spectral features as compared to bulk terrestrial planet atmosphere gases (Figs. 4  and 5).NF 3 and SF 6 absorption fall in the 9-12 micron spectral window where the plausible dominant super Earth or Earth-sized planet atmospheric gases CO 2 , CO, CH 4 , and the strong H 2 O vapor spectral features do not appear.Recall the gases N 2 and H 2 have no distinctive spectral features at infrared wavelengths; being homonuclear they have no net dipole moment.We note that the NF 3 and SF 6 strong absorbing power in a unique part of the spectrum compared to other major atmospheric gases is why they are potent greenhouse gases here on Earth.Trace gases of interest (such as PH 3 , NH 3 , SO 2 , H 2 S) other than the dominant terrestrial planet gases also do not have overlapping spectral features to NF 3 and SF 6 (Fig. 4).
There are gases with overlapping spectral features to NF 3 and SF 6 , and these are primarily halogenated carbon compounds (Fig. 4).The multitude of possible halogenated carbon compound gases (which could be biosignatures or technosignatures) pose a more complicated situation.Disentangling SF 6 and NF 3 from the multitude of possible chlorofluorocarbon gases and those gases from each other, depends on spectral resolution.For SF 6 there are only a few candidates with similar main spectral peaks at the same wavelengths to NF 3 and SF 6 .More work is needed to ascertain what is needed to distinguish amongst all of the halogenated gases, considering spectral features of other molecules.
Atmospheric concentrations needed for detection.On the order of 1 part-per-million (ppm) atmospheric abundance by volume of NF 3 and SF 6 produces a ~ 30 ppm signal in simulated transmission spectra for a terrestrial planet transiting an M dwarf star (Fig. 5).This is for an H 2 -dominated atmosphere.For a CO 2 -or N 2 -dominated atmosphere, the signal is much lower, owing to the much smaller scale height for high mean molecular weight gases compared to the low mean molecular weight gas H 2 (Fig. 5).This finding is consistent with other biosignature gas detection simulations, including the point that detecting a ~ 30 ppm signal will almost certainly take tens of transits or more (e.g., 2,[45][46][47][48] ), a much larger number than the typically few transits currently allocated for exoplanets with the JWST 29 .For details of simulated detectability including JWST noise floor see, for example 4,49,50 .
However, we again emphasize a major point in favor of NF 3 and SF 6 spectral distinguishability in an exoplanet atmosphere is that the 9-12 micron region has no expected major atmosphere gases with strong spectral features-although many trace gases, especially halogenated compounds, have features in this window.
The atmospheric accumulation of NF 3 and SF 6 is favorable due to their low water-solubility (for a comparison to other gases such as CO 2 and NH 3 see Fig. 6).The low water solubility means they will not dissolve in rainwater and fall to the ground or the sea.The NF 3 and SF 6 and long atmospheric lifetimes also favor their accumulation (for destruction rates see the SI).
Proper estimation of the lifetime of NF 3 and SF 6 would require experimentally measured kinetics of chemical reactions of NF 3 and SF 6 in H 2 at various temperatures.The current thermochemical literature and databases (such as NIST) have very scarce information on reactivity of those species with relevant atmospheric components, at relevant temperatures (Table S1 and S2).Measurement or detailed modeling of those values is essential for progress.
Nonetheless, we can make some estimates.Regarding the lifetime of NF 3 in an H 2 -dominated atmosphere, the rate constant [cm 3 molecule −1 s −1 ] for the reaction H + NF 3 → NF 2 + HF at 300 K has been calculated to be around 2.4 × 10 −2051 , which is four orders of magnitude lower than the rate constant of the reaction with the hydroxyl radical (OH), 4.0 × 10 −16 , (reaction OH + NF 3 → F + H 2 O + NO 2 in Table S1).This difference suggests that the reaction with OH would dominate over the reaction with an H radical as a main destruction pathway for NF 3 .As a result, the likely lifetime of NF 3 in an H 2 -dominated atmosphere would not be significantly different than it is in the oxidized, OH-rich, atmosphere of Earth.Both Earth's atmosphere and the reduced H 2 -dominated atmosphere are expected to be abundant with OH radicals due to photolysis of H 2 O.
Regarding the lifetime of SF 6 , it is a stable and unreactive gas with a lifetime of hundreds to thousands of years in Earth's oxidizing atmosphere.SF 6 likely has a similarly long, if not longer lifetime in a H 2 -dominated exoplanet atmosphere.Experimental shock tube dissociation studies of SF 6 in the presence of H 2 gas in the temperature range of 1734-1848 K supports this conclusion.The measurements show that H 2 does not increase the dissociation of SF 6 as compared to argon gas control 52 .This result indicates that SF 6 should have at least a similar lifetime in the H 2 -dominated atmosphere as it has in Earth's atmosphere.
Abiotic sources and a false positive mitigation strategy.NF 3 has no known abiotic sources.NF 3 has no known abiotic sources.In other words, NF 3 is not known to be a product of any photochemical, volcanic, Figure 4.The molecular spectra phalanx plot compares the absorbance spectra amongst molecules.The x axis is wavenumber ranging from 4000 to 500 cm −1 (2.5-20 μm).The y axis is the order of the molecules.Color represents the intensity of the absorption peaks; yellow and green represent strong absorption, while blue and purple represent weak or no absorption.Note the absorbance is normalized to 1.The spectra phalanx provides a visualization of which molecules are clustered together in wavelength based on their spectral data.Functional groups are labeled at their clustering points.The grey dashed lines near the top of the plot mark NF 3 and SF 6 and show their relative uniqueness in wavenumber space as compared to other halogenated species.The spectra data are from 600 gas species in our All Small Molecules Database (ASM) 40 (volatile molecules with up to 6 non-H atoms) that have available spectra in NIST 64 .See "Methods" for details on construction of this figure.or other geological process.We further show formation of NF 3 is thermodynamically unfavorable for terrestrial planet conditions (Fig. 7).NF 3 is also not known to be released from any fluorine-containing minerals.The absence of false-positives for NF 3 is supported by the lack of detection of this gas in any available pre-industrial samples 37 .With no know abiotic or biotic sources, NF 3 is a, if not the, prime candidate for a technosignature gas search.www.nature.com/scientificreports/Abiotic sources of SF 6 .On Earth trace amounts of SF 6 exists in volcanic rocks in rift zones, faults, igneous intrusions, geo-thermic areas and diagenetic fluids 53 .SF 6 is predominantly present in fluorites and some granites, while, for example basalts do not contain detectable SF 6 53,54 . The exact mechanism of abiotic formation of SF 6 on Earth is unknown.It is also unclear if SF 6 is directly made by volcanoes on Earth or its release is just associated with volcanic activity.Harnisch and Eisenhauer have examined the gases from several volcanic fumaroles, e.g., Etna (Sicily), Vulcano Island (Sicily), Kuju (Japan), and Satsuma Iwojima (Japan), and find that they are not significant sources of SF 6 54 .However, the authors note that the underlying rocks of these volcanoes are not granitic and as a result might lack a source for SF 6 54 .The equilibrium pre-industrial atmospheric concentration of SF 6 on Earth is estimated to be < 0.06 ppt 53 .The dominant F-containing volcanic gas on Earth is HF with abundances reaching 0.5-15 ppb 55 .Other trace F-containing species, including NH 4 F, SiF 4 , (NH 4 )2SiF 6 , NaSiF 6 , K 2 SiF 6 , KBF 4 , and organo-fluorides, are also associated with volcanic activity or released by volcanoes, but to a much lower extent than HF [56][57][58][59] .It is therefore unlikely that SF 6 will be a significant false-positive on a water-rich terrestrial planet as Earth.
The hypothesis that volcanic SF 6 is negligible on Earth is also supported by thermodynamics of SF 6 's formation (Fig. 7).The formation of SF 6 (and NF 3 ) is highly thermodynamically unfavorable and therefore unlikely to be a source of anything but a trace product of planetary geology or volcanism.The unfavorable thermodynamic conditions of the formation of SF 6 (and NF 3 ) are in contrast to the abiotic formation of SiF 4 , another non-carbon fully fluorinated gas, that is a known volcanic product on Earth 60 (Fig. 7).
Strategies to rule out SF 6 (and NF 3 ) false positives.We first ask if SF 6 could be a significant abiotic gas on a planet with an environment different from Earth.The answer is yes, as follows.On a dry planet, or a planet that is otherwise H-depleted one would expect a different profile of F-containing volatiles erupted by volcanoes than on Earth which could lead to a potential false-positive interpretation of the source of the detected SF 6 .On a H-depleted planet, fluorine will be bound to a greater extent to elements other than H. Therefore HF, while still expected to be erupted by volcanoes, would not dominate volcanic gases.Such a scenario opens the possibility for SF 6 to be a much more abundant volcanic product on dry exoplanets than it is on wet planet Earth.
The view that SF 6 could be a volcanic product on an H-depleted world is supported by the tentative detection of 0.2 ± 0.1 ppm SF 6 in the atmosphere of Venus by Venera 14 61 .If correct, this value is five orders of magnitude larger than the amount of SF 6 detected in Earth's atmosphere.Since Venus' crust and atmosphere are significantly H-depleted (though the deeper mantle may be relatively less H-depleted 62 ), it is likely that the majority of F is erupted as other compounds than HF, e.g.SSF 2 , COF 2 , FClCO, and SOF 2 , etc. 63 .Therefore, it is not unexpected that in the H-depleted environment of Venus, with abundant sulfur, SF 6 could also be a volcanic product released in significantly higher abundance than on Earth.SF 6 could also be the result of weathering of fluorite minerals which abundance on Venus is poorly constrained.
We now turn back to non-H-depleted planets that are the focus of this paper, and propose a strategy to rule out any volcanic origin of SF 6 .We propose simultaneous observations of SiF 4 to be employed as a method to rule out volcanic sources of SF 6 (and NF 3 ).The overview reason is that SiF 4 is much more thermodynamically favored Figure 7.The free energy of formation of NF 3 , SF 6 , and SiF 4 .The y-axis is the standard free energy of formation and the x-axis is temperature (K).The unfavorable thermodynamic conditions (positive ∆G) of the formation of SF 6 and NF 3 at all relevant temperatures are in contrast to the favorable abiotic formation of SiF 4 (negative ∆G), where only SiF 4 is a known volcanic product on Earth.The formation of NF 3 and SF 6 is highly thermodynamically unfavorable and therefore unlikely to be anything but a trace product of planetary geology or volcanism.
Vol:.( 1234567890 ) such that any volcanic activity that produces SF 6 will produce significantly higher amounts of SiF 4 .In more detail, SiF 4 is a known volcanic gas on Earth.Volcanic production of SiF 4 can reach several tons per day and in some instances, such as in the Satsuma-Iwojima volcano plume, SiF 4 production can rival that of HF 60 .As with SF 6 , the formation of SiF 4 will be favored in hydrogen-depleted environments and low temperature gas sources.It is conceivable that on a planet with much more active low-temperature volcanism and lower crustal and atmospheric H abundance than Earth that a larger quantity of SiF 4 would be released by volcanoes into the atmosphere.SiF 4 , therefore, can be an indicator of geological activity on a planet with volcanic chemistry much more favorable to forming SF 6 than Earth.
In an event where SF 6 is detected but simultaneous observations of SiF 4 is not, the likelihood increases that SF 6 is biological or technological.This conclusion is supported by our calculations that suggest that there are no plausible conditions (pressure 0.1-10,000 bar, H 2 O content 0.1-95%, mantle redox state (MH vs. IW etc.)) where the amount of SF 6 (or NF 3 ) produced by volcanoes comes to within ten orders of magnitude of that of SiF 4 , effectively ruling out the possibility of volcanically-driven co-existence of SiF 4 and SF 6 (or NF 3 ) (Figure S1 and Figure S2).All of the above rationale applies to NF 3 ; unlike SF 6 there are no abiotic sources on Earth, and so volcanism is even less likely to make this gas on another planet.
There appears to be no spectral information spectral information for SiF 4 64 , getting such information is crucial for the execution of our proposed mitigation strategy.We conclude this section with a call to study the spectroscopy of the fully fluorinated non-carbon molecules.

Discussion and summary
Prospects.The JWST has been operational for science as of 2022 and is our best currently existing capability for exoplanet atmosphere observations for transiting planets via transit transmission spectroscopy.Two categories of terrestrial planet atmospheres are accessible by JWST.The first is the not yet existing terrestrial planets transiting white dwarf stars (See "Introduction" section).The second category is terrestrial planets transiting small red dwarf stars.In addition to the long-required observation times for technosignature gases for such systems ("Introduction" section), a major challenge is the red dwarf host star variability (e.g. 65and references therein).Stellar variability is a collective term for a rich set of physical phenomena caused by stellar surface inhomogeneities which come in the form of granulation and magnetic features such as spots and faculae (e.g.,

66
).The host star variability changes with time as active regions evolve and as the star rotates.Stellar spots and faculae have different temperatures from the disk-averaged photosphere, and for cooler stars, have molecular features distinct from the star itself but similar to those in a planet's atmosphere ( 65 and references therein).Stellar surface inhomogeneities can induce rogue features in stellar data which mimic signatures of exoplanet atmospheres; some studies have found that the signal from stellar inhomogeneities exceeds the signal from the planetary spectral features (e.g., 65,67,68 ).A community of over 100 experts has summarized the challenges and potential mitigation strategies of M dwarf host star variability effects on small transiting exoplanet atmospheres (NASA's Exoplanet Exploration Program Study Analysis Group 21 (SAG21) 65 ).
New large ground-based telescopes now under construction are planned to be online within the next decade: the Extremely Large Telescope (EELT, 39 m aperture diameter) 69,70 ; the Thirty Meter Telescope (TMT, 30 m aperture diameter) 71 ; and the Giant Magellan Telescope (GMT, 20 m aperture diameter) 72,73 .These large telescopes can directly image habitable-zone planets orbiting (i.e., not necessarily transiting) M dwarf stars with the right coronagraph instrumentation and extreme adaptive optics.The challenge is overcoming the high planet-star contrast at 10 7 -10 8 levels.METIS 74 on the EELT will have mid-infrared direct imaging capability via a coronagraph and extreme adaptive optics that includes low and medium resolution spectroscopy; however METIS is designed for planet discovery via imaging and its limited sensitivity means METIS is unlikely to enable reliable ~ 10 micron spectrum of a temperate, rocky world orbiting an M dwarf star ( 74 and Quanz, priv.comm.2023).Direct imaging in the thermal infrared for about five Sun-like stars may also be possible 74,75 .Near infrared imaging and spectroscopy of reflected light from rocky planets in their host star's habitable zone is anticipated to be possible for up to 100 nearby low-mass stars (primarily mid M dwarf stars) [75][76][77] ; while the near-IR is outside of the wavelength range of interest for NF 3 and SF 6 , such programs may help discover new suitable planets for follow up atmosphere observations.Other than direct imaging, the large ground-based telescopes might be capable of a combination of high-dispersion, high-spectral resolution (R ~ 100,000) spectroscopy with moderate high-contrast imaging to observe spectra of a few rocky planets orbiting Sun-like stars 78 .(Note that not all IR wavelength regions are not easily accessible from Earth's surface due to Earth's atmospheric gases).
NASA's Habitable Worlds Observatory (HWO; https:// www.great obser vator ies.org/ irouv) is a NASA Great Observatory intended to be ready for launch in the mid-2030s and will be designed to directly image exoplanets orbiting Sun-like stars for both discovery and atmospheric characterization.This telescope will have a primary mirror about 6-m in diameter and is planned to have a coronagraph to block out starlight so the planet can be directly imaged.Because HWO will operate at visible to near-IR wavelengths, HWO will not be able to access the infrared windows (> 4 microns) appropriate for NF 3 and SF 6 spectral features.
The space-based interferometer under study called the Large Interferometer for Exoplanets (LIFE; 79 ) is designed to observe at 4-18.5 microns, and this includes the infrared windows for NF 3 and SF 6 spectral features (Fig. 5).LIFE would have four elements each with aperture 2-3.5-m diameter as well as a combiner spacecraft 80 .

Are SF 6 and NF 3 alien biosignatures or universal technosignature gases?
The exclusion of N-F and S-F compounds from Earth biochemistry, the low water solubility of NF 3 and SF 6 , their unique spectra, long atmospheric life time, and industrial utility on Earth all makes NF 3 and SF 6 attractive targets as technosignature gases.We discuss the reasons for and against NF 3 and SF 6 being produced by an alien biology versus by a technological society.
Life on Earth has apparently never made an N-F or an S-F bond-containing molecule, despite life independently evolving F-handling enzymes several times over the last three billion years (Petkowski et al. 2023, in  prep.).Life also rarely makes fully halogenated molecules, with only four known examples ("No fully fluorinated molecules are made by life" section).This suggests that there is at least one major evolutionary barrier to making NF 3 or SF 6 , which would require a correspondingly powerful selective benefit to overcome.SF 6 is a completely chemically inert gas, making it essentially "invisible" to biochemistry.Therefore, we have to ask why life, no matter its biochemical makeup, would invest significant metabolic machinery and energy into making a compound that it then throws away, and which, due to its chemical inertness, has a negligible effect on the organism's immediate environment or on its competitors.Biological production of SF 6 would cost an organism a large amount of energy while not giving any evolutionary advantage (i.e., does not provide any obvious fitness gain).
NF 3 is mildly reactive, and so we may speculate that NF 3 could have the same ecological role as CH 3 Br on Earth, as a mildly reactive, non-specific, diffusible toxin to intoxicate the competition in the immediate environment.Therefore, NF 3 production could increase the fitness of the NF 3 -producing organism.The toxicity of NF 3 can be to an extent similar to the toxicity of phosphine (PH 3 ) 81 .PH 3 does not react all that much with biological tissues, but it is toxic to humans, and many other O 2 -dependent organisms, because it converts hemoglobin to methemoglobin (which cannot bind oxygen), i.e.PH 3 is toxic to (large, terrestrial) oxygen-dependent organisms but not to anaerobic ones 82 .Oddly, in identifying a technosignature gas, a biological source may be a false positive.Against this, we note that there are many mildly reactive, toxic carbon-containing compounds that can be made with the enzymatic machinery that any carbon-based life is likely to possess, such as CH 3 Br, CH 3 I, cyanogen, CO, formaldehyde, nitric oxide, so NF 3 would not provide any unique advantage over these more evolutionarily accessible substances.
One could argue that the biological repertoire of gases on other planets may surprise us because we have no idea what gases non-Earth-like life might produce 40 .Production of certain biochemicals is often an evolutionary accident or depends on the planetary environmental history.An example is the gas stibine, SbH 3 , which would not be expected to be made by Earth life because Sb itself is a rare element in the Earth's crust, but nevertheless is synthesized by terrestrial anaerobic sewage sludge microflora 83 .Another example is trimethylbismuth (C 3 H 9 Bi) produced by a variety of bacteria in anaerobic conditions (reviewed in 84 ).But we favor the point that while it is possible that an alien biochemistry would find use for NF 3 and SF 6 , virtually any combination of physical and chemical properties of NF 3 or SF 6 can be duplicated with less energy and less dangerous radicals using the chemistry of other halogens and carbon.Indeed, our premise of promoting NF 3 and SF 6 as technosignatures is that F is nearly excluded by life on Earth (Petkowski et al. 2023, in prep.)) and that such an exclusion may well be universal.
We further argue that industrial use of some chemicals may be likely to be universal.SF 6 in particular has unique properties that make it useful for a technological civilization (reviewed "Results" section) and specifically its high dielectric constant and high breakdown voltage as a gas make it valuable in high voltage electrical equipment 36 .Biosignature gases are the product of evolutionary contingency and are limited by thermodynamics and the reactivity of materials to water.Industrial chemicals, however, are the result of an informed, systematic search of all possible chemicals for materials that have optimal properties for a specific application, largely regardless of the thermodynamics of their synthesis or whether their synthesis requires chemistry that is incompatible with earth surface conditions (for example, industrial production of NF 3 is done in molten ammonium fluoride 85 , an obscure material unlikely to occur on its own on any rocky planet).It is therefore plausible to suggest that an extraterrestrial civilization that wanted, for example, a gaseous product to act as a high voltage insulator and arc quencher, would choose SF 6 , no matter what the entity's own biochemistry, evolutionary history or planetary environment was.
If we in the future have a way to detect the tiny part-per-trillion amounts elsewhere that we humans have accumulated in our atmosphere, and are lucky to catch the small window where a society becomes industrial, we may observe the rapid, steady increase of the atmospheric abundance of SF 6 or NF 3 .A rapid, steady increase would favor a technological source and may be a solid discriminator between bio-and a true technosignature gas.Even the relatively rapid increase in atmospheric O 2 that led to the Great Oxygenation Event still took 1-10 million years for O 2 to accumulate in high enough concentrations to have a weathering effect on rocks (e.g., 86 ).A multi-generational observational campaign can further distinguish between SF 6 and/or NF 3 as technosignature and not biosignature gases.Observation of a planet over the time span of 2-4 generations (~ 100 years) to monitor the increase of the SF 6 and NF 3 gases could strengthen attribution to a technological source, but one would have to get lucky with timing.
We now turn to the requirement that the atmospheric abundance of NF 3 and SF 6 needed for detection with the JWST is of the order of 1 ppm, far higher than current atmospheric levels.Here on Earth, no uniquely technological gas has been produced to accumulate to such relatively high amounts in the atmosphere. 1 ppm SF 6 is 100 times the current terrestrial level, and it would take ~ 700 years of the current emission rate to build that concentration in Earth's atmosphere, taking the SF 6 atmospheric lifetime of 3200 years.The greenhouse gas effect of such concentrations on Earth would be catastrophic, which suggests that any alien technological society would curtail their emissions before they reached 1 ppm, unless their goal was substantial global greenhouse heating.Indeed SF 6 has been considered as a terraforming agent on Mars, albeit for humans at our stage of development a prohibitively expensive one 87 .
Fully fluorinated non-carbon containing gases other than NF 3 and SF 6 have little known about them (see discussion in the SI; Table S3).
Vol:.( 1234567890 ), and moreover because molecules with N-F and S-F bonds are not made by life on Earth at all.We have argued that while an alien biochemistry might find use for NF 3 and SF 6 , such use would have to provide a significant evolutionary gain that offsets e.g. the large energy expenditure for the synthesis and breakage of fluorine-containing bonds.
In contrast, industrial use may be universal due to unique physical and chemical properties of NF 3 and SF 6 gases (see SI).Therefore, NF 3 and SF 6 may be likely to be used by alien industry no matter the biochemical makeup of the alien biology or the particular environmental conditions of the alien planet.We note however that it is likely that the technological stage of the civilization, or time-span at which NF 3 and SF 6 are produced in sufficient amounts to be observed is short.NF 3 has no known sources other than industrial, while SF 6 is produced abiotically in extremely small, trace amounts.Their source can be distinguished from e.g.potential volcanic release into the atmosphere by comparison with the known volcanic gas SiF 4 .The lack of known false-positives for NF 3 and very low abundance of abiotic sources of SF 6 on Earth supports their potential as a technosignature gases on exoplanets.NF 3 and SF 6 's long atmospheric lifetimes and unique spectral features aid their potential detection.

Methods
Custom natural products database.To understand the uniqueness of fully fluorinated compounds out of all chemicals made by life (called "natural products"), we use our database of natural product chemicals curated over the last decade.Our database is presented in 40 and expanded and completed as described in 39 .We created and curated our database by an extensive literature search and by searching available online natural product repositories 39 .
Our natural products database has been rigorously screened to contain only compounds that are a result of natural biochemical processes of a living organism.It also contains biological sources identified for every molecule (i.e., a list of species from which the natural product was isolated).
We emphasize how challenging it is to compile a complete list of all that is known about each natural product.First, no individual database covers more than 20% of the known natural products.Second, because most natural product databases focus on drug design, they include synthetic derivatives of natural products or drug metabolites that often mimic natural molecules while not being true natural compounds themselves.Other databases often include artificial compounds that emerge as a result of "feeding" an organism with a precursor molecule, or completely artificial compounds that have accumulated as contaminants in plants and animals.We manually excluded such compounds from our database.Third, most data sources needed extensive checking and modification due to a range of format differences and coding errors.All of the above problems motivated us to curate our own natural products database (see 39 for a full description).

Spectra visualization tool.
A key question for any atmospheric trace gas is whether or not its spectral features are distinguishable from features found in other expected atmospheric molecular gases.We introduce a new tool for analysis, detailed in 88 and summarized here.
We call our new tool a spectra phalanx plot (Fig. 4).This plot enables a visual comparison amongst the absorption peaks of each individual molecule's spectral features, with molecules that share similar spectral features grouped closer together.Each molecule occupies a horizontal line parallel to the x-axis, where the molecule's spectral features are plotted as a function of wavelength location.We use color to represent the intensity of the absorption peaks; yellow and green represent strong peaks, while blue and purple represent weak or no absorption.Note the absorbance is normalized to 1, so only the ratio between the peaks and the strongest peak is important, and comparing the absolute intensity between molecules is not possible.For this tool, we use the ~ 600 transmission/absorbance data spectra available from NIST 64 (with a small subset from HITRAN 89 ).The order of the molecules is not meaningful other than that molecules with similar wavelengths of spectral features are grouped together.
We generate the molecule ordering in the spectra phalanx plot using hierarchical clustering, a tree-based approach that builds clusters by iteratively grouping two of the closest cluster/elements into the same cluster and organized in a binary tree structure called a dendrogram 90 .We apply hierarchical clustering on molecular spectra to cluster the molecules and validate that the molecules in the same clusters share similar chemical structures using a molecular maximum common substructure search using methods described in 88 .
In more detail, comparing the molecular IR spectra of two molecules expresses how similar the two are.We use the squared Euclidean distance function to compare normalized molecular spectral signatures, each normalized to have unit peak absorbance.An agglomerative clustering process organizes the pairwise symmetric distance matrix between all pairs of molecules into a cluster hierarchy.Using the hierarchy, one can visualize and identify the molecular spectral features that contribute to the relative detectability of molecules.It is these spectral features that one would want to detect.To locate the distinguishing features, we enumerate the molecules row-by-row in the order of their cluster similarities, imaging a molecule's spectral signature.The one-dimensional image along the x-axis indicates a bright (yellow) color for high absorbance and dark (blue) for low absorbance (in contrast to a curve plot).Most interesting is the pattern that emerges when one looks at all cluster-enumerated molecular spectral signatures.This phalanx plot shown in Fig. 4 shows that normalized peaks (with some spectral width) at specific wavelengths are shared by many molecules, and these do not contribute significantly to the distinguishability and, thus, relative detectability of the molecule.But it also appears, as shown, that specific clusters share peak distributions that are relatively muted or absent in all the others.These are the distinguishing IR signature features that help detect the molecules in the group.To the degree that the number of molecules Atmospheric spectra simulator.We simulate model atmospheres to assess the approximate atmospheric abundance of gases needed for detection with simulated James Webb Space Telescope (JWST) observations.We use the computer model "Simulated Exoplanet Atmosphere Spectra" (SEAS) code from 88 .We use the molecular mixing ratio profiles and calculate the optical depth of each layer of the atmosphere 88,91 .We calculate the stellar intensity absorption along each path through the planet atmosphere by A = n i,j σ i,j l i , where A is absorption, n is number density, σ is the absorption cross-section and l is pathlength.The subscript i denotes each layer that the stellar radiation beam penetrates, and j denotes each molecule.For the height of each atmospheric layer we adopt scale height of the atmosphere.We calculate the transmittance, T, of each beam using the Beer-Lambert Law.Then, we compute the total effective height h of the atmosphere by multiplying the absorption A = 1 − T by the atmosphere's scale height.To connect to observations, we calculate the total attenuated flux as transit depth (R planet + h) 2 /R star 2 in units of ppm.We simulate transmission spectra for a hypothetical 1.5 R Earth , 5 M Earth super-Earth transiting an M dwarfstar similar to GJ 876.We choose 1.5 R Earth as it is consistent with a rocky planet 92 .We simulate three planetary atmospheres: ones dominated by H 2 , N 2 and CO 2 .We choose a relatively massive super Earth planet because a more massive planet is more likely to retain an H 2 -dominated atmosphere than a lower mass planet.We choose to simulate a rocky exoplanet with an H 2 -dominated atmosphere because such an atmosphere is favorable for detection by transmission spectroscopy than a higher mean molecular weight atmosphere such as one dominated by N 2 or CO 2 .We also simulate an N 2 and a CO 2 -dominated atmosphere.We take the temperature, pressure, and vertical gas abundance profiles from 48 , where the molecular gas abundances were computed from a photochemical equilibrium model 93,94 .We assume varying atmospheric abundances of SF 6 and NF 3 , from 1 ppb to 1 ppm.

SI 1 A Summary of Proposed Technosignature Gases
Multiple studies have examined the detectability of CFCs in exoplanet atmospheres (e.g., 1,2 ).For instance, 2 investigated the detectability of CFC-11 (CCl3F) and CFC-12 (CCl2F2) on TRAPPIST-1e.Assuming a James Webb Space Telescope (JWST) Mid-Infrared Instrument lowresolution spectrometer (MIRI-LRS) noise floor of 10 ppm, they found that present-day Earth abundances of these two CFCs could be detected on TRAPPIST-1e, with about 100 hours of JWST observation time 2 .However, assuming a conservative JWST noise level of 50 ppm, they concluded that even CFCs five times the present-day Earth level would not be detectable by JWST, regardless of the observation time 2 .In another study, researchers investigated the detectability of CFC-11 (CCl3F) on an Earth-sized planet transiting a white dwarf star similar to Beta Persei (commonly known as Algol) 1 .They found that CFC-11 at an abundance ten times the present Earth level could be detected by JWST with about 1.2 days of observation time 1 .While no Earth-sized planets transiting bright white dwarf stars have yet been detected, they remain promising candidates for atmosphere study if they exist and can be discovered.
Other proposed technosignature gases include the simultaneous detection of NH3 and N2O in an atmosphere that also contains H2O, O2, and CO2 as a signature of extraterrestrial agriculture 3 .The gas NO2 as an atmospheric technosignature has been proposed as a sign of an industrial revolution, specifically combustion engines 4 .These technosignature gases, however, are far from unique as both the planet and life also produces them.Terrestrial agriculture is an exploitation of the biochemistry of Earth, and so its reliance of exogenous sources of NH3 (as fertilizer) and its production of N2O might be specific to terrestrial biology, and not a general sign of agriculture.NO2 from internal combustion engines assumes a very specific technological trajectory for the planet: the widespread use of a specific type of fossil-fuel-powered transport infrastructure rather than, for example, steam-or electric-powered transport.

SI 2 An Overview of NF 3 and SF 6
Both NF3 and SF6 are present in Earth's atmosphere not from intentional release but from leakage from industrial use.Here we summarize the origin and properties of NF3 and SF6.

NF 3 and SF 6 Physical and Chemical Properties
At room temperature, NF3 is a colorless and non-flammable gas.At room temperature, NF3 is only slightly soluble in water 5 and does not react with water or dilute acids [6][7][8] .NF3 is thermodynamically and chemically stable; for example it does not react with most metals below 250 °C6,9 , but it can act as a potent yet slow oxidizer 8 .
In recent years, NF3 has been widely used in the microelectronics and semiconductor industries where it is used as an etchant in producing thin-film-transistor liquid-crystal displays (TFT-LCD), semiconductors, and solar photovoltaic panels (e.g., 6,[9][10][11][12] ).Sulfur hexafluoride (SF6) is a colorless, non-toxic, and non-flammable gas 7 .At room temperature, SF6 is almost insoluble in water; the solubility of SF6 is even lower than that of helium (He), one of the least water-soluble gases 5 (Figure 6 in the main text).SF6 is completely chemically inert and non-toxic.At room temperature, it is unreactive towards most metals.It does not react with magnesium (Mg) and copper (Cu) even if they are heated to "red hot" temperatures.It does not react with phosphorus (P) or arsenic (As), nor with hydrochloric acid (HCl), sodium hydroxide (NaOH), or potassium hydroxide (KOH).SF6 does not decompose even when heated to 500 °C.In addition, SF6 does not react with liquid water or high-pressure steam.SF6 will only react with boiling sodium (Na) 7,[13][14][15][16][17][18] .SF6's only toxic effect on mammals, including humans, is when it is breathed in at greater than 80% concentration and so replaces oxygen in inspired air 19 .
The main reason behind the chemical stability and unreactivity of SF6 is a kinetic barrier to its hydrolysis 7 .SF6 has a unique molecular structure among sulfur-containing compounds.SF6 is an octahedrally-shaped molecule with six fluorine atoms symmetrically attached to the central sulfur atom (Figure 1 in the main text).As a result, the S atom is sterically shielded by the surrounding F atoms, making it inaccessible to other reacting molecules, e.g., water.
SF6 is used in a wide variety of industries, including the electrical power industry, semiconductor manufacturing, and the production of aluminum and magnesium, to name a few 7,[20][21][22] .In the electrical utility industry, SF6 is often used as an insulating gas in electrical transmission and distribution equipment, such as current/voltage transformers, circuit breakers, switchgear, and capacitors (e.g., 7,20,23 ).SF6's chemical stability, non-flammability, and non-toxicity add to its usefulness as an excellent insulating gas 7,24 .
As a result of the low reactivity of NF3 and the chemical inertness of SF6, once released to the atmosphere these gases have very long residence times.

A Steady and Rapid Increase of Atmospheric Concentration of SF 6 and NF 3 in an Industrialized World
The widespread use of NF3 and SF6 due to their unique advantages and pivotal roles across multiple industries has nonetheless raised concerns about their environmental impact.Both NF3 and SF6 are very potent greenhouse gases (e.g., [25][26][27] ).NF3 has a 100-year Global Warming Potential (GWP) of about 16100, while SF6 has a GWP of about 23500 28 .NF3 and SF6 are more than five orders of magnitude more efficient greenhouse gases than CO2.Since NF3 and SF6 are very stable, they are difficult to break down and remove from the atmosphere, and hence NF3 and SF6 have very long atmospheric lifetimes.The atmospheric lifetime of NF3 is about 500 years, and that of SF6 is 850 -3200 years 28,29 .
Since the use of NF3 in specialized industry began in the 1970s, and was later used in more widespread applications in the electronic industry in the late 1990's, the amount of atmospheric NF3 has been rapidly increasing and has doubled every five years since the late 20th century (https://gml.noaa.gov/hats/gases/NF3.html).In contrast to SF6, NF3 does not have any known non-human-made source and measurements of air entrapped in ancient ice Antarctica as well as measurements of NF3 in archived air tanks filled before 1975, i.e., before the introduction of NF3 to human industry, have resulted in undetectable levels of NF3.The undetectable levels of NF3 in pre-industrial samples suggest that background NF3 levels were essentially zero or no greater than the limit of detection of 0.008 ppt 30 .The rapid rise in the global atmospheric abundance and projected future demands of human industry place NF3 as the fastest growing contributor to radiative forcing of all the synthetic greenhouse gases by the mid-XXI century 31 .Lack of detectable pre-industrial NF3 supports its potential as a technosignature gas and strongly suggests that NF3 does not have any significant abiotic source.
The increase in atmospheric SF6 follows the same trend as NF3.Over the last 70 years, since its first application in industry in 1953, SF6's concentration in the troposphere increased dramatically from approx.0.05 ppt to > 4 ppt (Figure 2 in the main text) 32 .
There is limited atmospheric chemistry reactivity data for NF3 and SF6 with potential destruction pathways possibly unknown.The known rates show very low reaction rates with the dominant atmospheric radical OH, and the only significant rates are for gases of low atmosphere abundance (Tables S1 and S2).

SI 3 Other Fully Fluorinated Non-Carbon Molecules?
Given how compelling NF3 and SF6 are as technosignature gases, one should ask if other volatile fully fluorinated molecules are equally promising as technosignature gases.
Other fully fluorinated human-made gases do exist and are worth further exploration as technosignature gases because they do not exist in nature.However, there are a limited number of possibilities, and the ones that have been synthesized have limited data (e.g., reactivity, water solubility, or gas phase spectral feature information).The list includes PF3, P2F4, SeF6, or S2F10 (Table 1) 33 .Out of the list of possibilities (Table S3), SiF4 is made by Earth's volcanoes and therefore not a suitable technosignature gas (see 5.3) and PF3 slowly hydrolyzes in water.Despite its reactivity to water, PF3 is worth further exploration, but so far, perhaps because it is not useful for industry, there is almost no spectral or any other relevant information on its potential as a technosignature gas.The other gas candidates listed in Table S3 are all highly reactive in water and not worth further comment, except to note that for some gases life does produce the hydrogenated versions, such as NH3, H2S, PH3, GeH4, but never partially fluorinated versions.

Reactants Products
Rate Law [cm

Figure 5 .
Figure 5. Simulated spectra of an exoplanet with transiting an M5V star with an atmosphere abundance of 1 part-per-million of NF 3 and SF 6 .Top panel: the y-axis shows transit depth (ppm), and the x-axis shows wavelength (μm).The spectra are simulated from 1 to 23 μm, covering the wavelength span of most of JWST's observation modes.The yellow, green, and blue regions show the spectral coverage of NIRSpec, and the red region shows coverage of MIRI LRS.Bottom panel: absorption cross sections in cm 2 as a function of wavelength for key molecules of interest.For context, the contemporary Earth abundance levels are 3 ppt 37 and 11 ppt 38 for NF 3 and SF 6 respectively.

Figure 6 .
Figure 6.Solubility of various atmospheric gases in water.The x-axis shows the chemical species' names, and the y-axis shows the Henryʼs law constant on a log scale.Henry's law is defined as H CP (X) = C(X)/p, where H CP (X) is Henry's law constant for a species X in mol Pa −1 m −3 .P is the partial pressure of that species in Pascal, and C(x) is the dissolved concentration (in mol m −3 ) under the equilibrium condition.The larger H CP , the more soluble the species is.NF 3 and SF 6 have very low water solubility. https://doi.org/10.1038/s41598-023-39972-zwww.nature.com/scientificreports/ In summary, NF 3 and SF 6 are appealing technosignature gases primarily because use of F is nearly excluded by the chemistry of life on Earth for fundamental reasons(Petkowski et al. 2023, in prep. ) Scientific Reports | (2023) 13:13576 | https://doi.org/10.1038/s41598-023-39972-zwww.nature.com/scientificreports/Summary.
88 a group is small and the spectral features shared by them are exclusive relative to the others in our All Small Molecule (ASM) database40, they are highly detectable.See88for more details.