Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology
Ravi Kopparapu, Giada Arney, Jacob Haqq-Misra, Jacob Lustig-Yaeger, Geronimo Villanueva
DDraft version February 10, 2021
Typeset using L A TEX default style in AASTeX63
Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology
Ravi Kopparapu , Giada Arney, Jacob Haqq-Misra , Jacob Lustig-Yaeger , andGeronimo Villanueva NASA Goddard Space Flight Center8800 Greenbelt RoadGreenbelt, MD 20771, USA Blue Marble Space Institute of Science,Seattle, WA, USA Johns Hopkins University Applied Physics Laboratory,Laurel, MD 20723, USA
Submitted to ApJABSTRACTNitrogen dioxide (NO ) on Earth today has biogenic and anthropogenic sources. During the COVID-19 pandemic, observations of global NO emissions have shown significant decrease in urban areas.Drawing upon this example of NO as an industrial byproduct, we use a one-dimensional photo-chemical model and synthetic spectral generator to assess the detectability of NO as an atmospherictechnosignature on exoplanets. We consider cases of an Earth-like planet around Sun-like, K-dwarfand M-dwarf stars. We find that NO concentrations increase on planets around cooler stars due toless short-wavelength photons that can photolyze NO . In cloud-free results, present Earth-level NO on an Earth-like planet around a Sun-like star at 10pc can be detected with SNR ∼ ∼ . − . µm range where NO hasa strong absorption. However, clouds and aerosols can reduce the detectability and could mimic theNO feature. Historically, global NO levels were 3x higher, indicating the capability of detecting a40-year old Earth-level civilization. Transit and direct imaging observations to detect infrared spectralsignatures of NO on habitable planets around M-dwarfs would need several 100s of hours of obser-vation time, both due to weaker NO absorption in this region, and also because of masking featuresby dominant H O and CO bands in the infrared part of the spectrum. Non-detection at these levelscould be used to place upper limits on the prevalence of NO as a technosignature. Keywords:
Exoplanet atmospheric composition, technosignatures INTRODUCTIONOver the last 25 years, more than 4000 exoplanets have been discovered from both ground and space-based surveys.We are now entering into an era of exoplanet atmospheric characterization, with the soon to be launched James WebbSpace Telescope ( JWST ), Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) space telescope,and large ground-based observatories such as the European Extremely Large Telescope (E-ELT), the Thirty MeterTelescope (TMT), and the Giant Magellan Telescope (GMT). The first detection of an exoplanet atmosphere was ona gas giant planet, HD 209458b, in 2001 (Charbonneau et al. 2002). Since then, atmospheres have been detectedon exoplanets spanning a wide range of planetary parameter space, and observers are continuing to push the limitstowards smaller worlds (Tsiaras et al. 2019; Benneke et al. 2019). The ongoing discovery of exoplanet atmospheres hasraised the prospect of eventually identifying potentially habitable planets, as well as the possibility of finding one that
Corresponding author: Ravi [email protected] https://exoplanetarchive.ipac.caltech.edu/ a r X i v : . [ a s t r o - ph . E P ] F e b Kopparapu et al. may also be inhabited. As a result, the characterization and detection of “biosignatures,”—remote observations ofatmospheric spectral features that could potentially indicate signs of life on an exoplanet—has received recent attentionas an area of priority for astrobiology (Seager et al. 2012; Kaltenegger 2017; Schwieterman et al. 2018; Meadows et al.2018; Catling et al. 2018; Walker et al. 2018; Fujii et al. 2018; O’Malley-James & Kaltenegger 2019; Lammer et al.2019; Grenfell 2017).Similar to biosignatures, “technosignatures” refer to any observational manifestations of extraterrestrial technologythat could be detected or inferred through astronomical searches. As discussed in the 2018 NASA technosignaturesworkshop report (Technosignatures Workshop Participants 2018): “Searches for technosignatures are logically continu-ous with the search for biosignatures as part of astrobiology. As with biosignatures, one must proceed by hypothesizinga class of detectable technosignatures, motivated by life on Earth, and then designing a search for that technosigna-ture considering both its detectability and its uniqueness.” Although the science of atmospheric technosignatures isless developed compared to atmospheric biosignatures, a wide class of possible technosignatures have been suggestedin the literature that include waste heat (Dyson 1960; Wright et al. 2014; Kuhn & Berdyugina 2015; Carrigan Jr.2009), artificial illumination (Schneider 2010; Loeb & Turner 2012; Kipping & Teachey 2016), artificial atmosphericconstituents (Schneider 2010; Lin et al. 2014; Stevens et al. 2016), artificial surface constituents (Lingam & Loeb 2017),stellar “pollution” (Shklovskii & Sagan 1966; Whitmire & Wright 1980; Stevens et al. 2016), non-terrestrial artifacts(Bracewell 1960; Freitas Jr & Valdes 1980; Rose & Wright 2004; Haqq-Misra & Kopparapu 2012), and megastructures(Dyson 1960; Arnold 2005; Forgan 2013; Wright et al. 2016). This breadth of topics reflects the scope of possibilitiesfor detecting plausible technosignatures, although the sophistication of technosignature science remains in its infancycompared to the rapidly evolving field of biosignatures (Wright 2019; Haqq-Misra et al. 2020).The history of life on Earth provides a starting point in the search for biosignatures on exoplanets (Krissansen-Tottonet al. 2018; Pall´e 2018), with the various stages of Earth’s evolution through the Hadean (4.6 -4 Gyr), Archean (4 - 2.5Gyr), Proterozoic (2.5 - 0.54 Gyr), and Phanerozoic (0.54 Gyr - present) eons representing atmospheric compositionsto use as examples of spectral signatures of an inhabited planet. The use of Earth’s history as an example does notimply that these biosignatures will necessarily be the most prevalent in the galaxy, but instead this approach simplyrepresents a place to begin based on the one known example of life. By extension, the search for technosignatureslikewise can consider Earth’s evolution into the Anthropocene epoch (Crutzen 2006; Lewis & Maslin 2015; Frank et al.2017) as a template for future observing campaigns that seek to detect evidence of extraterrestrial technology. Forinstance, Lin et al. (2014) discussed the possibility of detecting tetrafluoromethane (CF ) and trichlorofluoromethane(CCl F) signatures in the atmospheres of transiting Earth-like planets around white dwarfs with JWST, which couldbe detectable if these compounds are present at 10 times the present Earth level. These chlorofluorocarbons (CFCs)are produced by industrial processes on Earth, so their detection in an exoplanet atmosphere could be strong evidencefor the presence of extraterrestrial technology. This approach does not insist that CFCs or other industrial gasesfound on Earth will necessarily be the most prevalent technosignature in the galaxy, but it represents a place to begindefining observables and plausible concepts for technosignatures based upon the one known example of technologicalcivilization.In this study, we explore the possibility of NO as an atmospheric technosignature. Some NO on Earth is producedas a byproduct of combustion, which suggests the possibility of scenarios in which larger-scale production of NO is sustained by more advanced technology on another planet. Detecting high levels of NO at levels above that ofnon-technological emissions found on Earth could be a sign that the planet may host active industrial processes. Insection 2, we describe the production reactions of NO and use a 1-dimensional photochemical model to obtain self-consistent mixing ratio profiles of nitrogen oxide compounds, on a planet orbiting a Sun-like star, a K6V spectral type(T eff = 4600K), and the two M-dwarf stars AD Leo (T eff = 3390K) and Proxima Centauri (T eff = 3000K). Usingthese photochemical results, in section 3 we calculate the observability of strongest NO features between 0 . − . µm and between 1 − µ m using a spectral generation model to produce geometric albedo and transit spectra of planets withvarious facilities like LUVOIR-15m, JWST and OST. In section 4, we discuss the implications of these observations,concluding in section 5. PRODUCTION OF NITROGEN DIOXIDE itrogen Dioxide Pollution as a Signature of Extraterrestrial Technology x = NO + NO ) are among the main pollutants in industrialized locations on the globe. Thenon-anthropogenic pathways for the production of NO x can be either due to emission from soils and wildfires, orproduced in the troposphere by lightning. The primary biogenic source of NO x is bacteria in soil through nitrification(i.e bacteria converting ammonia to nitrite and nitrate compounds), or dentrification (process of reducing nitrate andnitrite to gaseous forms of nitrogen such as N or N O). The estimated worldwide biogenic and lightning emissionsof NO x compounds are ∼ . − , (Table 1, Holmes et al. (2013)). Lightningcontributes about 5 Tg(N) yr − , which translates to 6 × NO molecules/cm /s (Harman et al. 2018).On the other hand, NO x compounds are also emitted from anthropogenic sources of combustion processes such asvehicle emissions and fossil-fueled power plants. The role of this industrial production was noted during the COVID-19pandemic, when global concentrations of NO were observed to decrease between 20 −
40% over urban areas (Bauwenset al. 2020). Indeed, these emissions dominate the production of NO x compounds in the troposphere more than thebiogenic sources with an estimated rate of 32 Tg(N) yr − (Holmes et al. 2013). NO poses harmful health effects thatcould cause impairment of lung function and respiratory problems (Faustini et al. 2014a). Typical concentrations ofNO range from 0.01 ppb (parts per billion) to ∼ x in the lower troposphere leads to a complexchemistry that results in the formation of ozone (O ), which is a harmful pollutant in the troposphere and a greenhousegas. NO x mixing ratios in excess of 10 − would cause severe damage to the O layer and could result in either a climaticwarming or cooling, depending upon the amount of NO present (Kasting & Ackerman 1985).The sinks and sources for NO in the troposphere ( ≤ photolysisis dominant in the wavelength range of 290 − photolysis produces ground state atomic oxygen, O( P), along with NO:NO + h ν ( < → O( P) + NO . (1)The O( P) then can combine with an oxygen molecule to form ozone,O( P) + O + M → O + M , (2)which gets destroyed by reoxidizing nitric oxide to nitrogen dioxide:NO + O → NO + O . (3)NO also reacts with atomic oxygen (O) and the hydroperoxy radical (HO ) to generate NO ,NO + O + M → NO + M (4)NO + HO → NO + OH . (5)However, these production mechanisms of NO are counteracted when NO reacts again with atomic oxygen to recreateNO: NO + O → NO + O , (6)The above reactions just cycle between NO and NO , so NOx is conserved. However, NO also reacts with the OHradical to form nitric acid (HNO ), which eventually is removed from the atmosphere by rainout, which is a lossprocess for NOx: NO + OH + M → HNO + M . (7)Reactions 3 and 6 form a catalytic cycle to destroy ozone with the net reaction,O + O → (8) The troposphere is the lowermost atmospheric layer from the surface up to 10-18 km, highest at the tropics and lowest near the polesduring winter. The pressure and temperature decrease with altitude, with global averages of 289 K and 1.013 millibar (mb) at the surface,and around 210K and 150 mb at a height of 15 km.
Kopparapu et al. indicating that high stratospheric NOx can lead to ozone depletion.To study the steady-state abundances of NO x compounds in Earth-like atmospheres, we used a 1-D photochemicalmodel (described in Arney et al. 2016; Arney 2019), which is part of a coupled climate-photochemistry model called‘Atmos.’ The photochemical model is originally based on the one described in Kasting et al. (1979) and has beenupdated extensively over the years and applied to various planetary and exoplanetary conditions (e.g. Segura et al.2005; Kopparapu et al. 2012; Domagal-Goldman et al. 2014; Harman et al. 2015, 2018; Lincowski et al. 2018). Themodel version used here has been updated to correct the deficiencies identified in Ranjan et al. (2020), and the publicversion of the model is planned to be updated. This model solves a set of nonlinear, coupled ordinary differentialequations for the mixing ratios of all species at all heights using the reverse Euler method. The method is first order intime and uses second-order centered finite differences in space. The vertical grid has 200 altitude levels, ranging from0 km (lower boundary) to 100 km (upper boundary). The version used here includes updates described in Lincowskiet al. (2018) and includes 72 chemical species involved in 309 reactions to represent a modern Earth-like planet. Weconsidered a Sun-like star, a K6V stellar spectral type, and two M-stars (AD Leo and Proxima Centauri) in this study.For the Sun-like star we used the Chance & Kurucz (2010) model; for the K6V star, we used the spectrum of HD 85512from the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES)treasury survey (France et al. 2016; Loyd et al. 2016; Youngblood et al. 2016); for AD Leo and Proxima Centauri,we used stellar spectra described in Segura et al. (2005) and Meadows et al. (2018), respectively. Planets around theother stars are placed at the Earth-equivalent flux distance.For each Earth-like planet around its host star, we ran the model to steady state to obtain the mixing ratio profilesof all gaseous species, including NO . We have used a surface NO molecular flux of 8 . × molecules/cm /s asthe standard Earth-level (1x) flux in our simulations. This number comes from converting the estimated rate of 32Tg(N) yr − anthropogenic NO x compound emissions in the troposphere to the molecular flux. We also include a fixedbiogenic flux of NO as 1 . × molecules/cm /s , kept constant across all simulations. Because we do not increasethe flux of NO alongside NO , our simulations may be regarded as somewhat conservative. Other fixed boundaryconditions of N-bearing species include: a flux of 1 . × for N O, a mixing ratio of 0.78 for N , and fixed depositionvelocities of 2 . × − for HO NO and HNO .Results from our 1-D photochemical model are shown in Fig. 1, panel (a). This plot shows the NO volume mixingratio profiles of an Earth-like planet around four stellar spectral types we considered in this study: the Sun (blue), ADLeo (green), Proxima Centauri (black) and the K6V star (magenta). Two end member concentrations are shown: Thestandard Earth level flux of 8 . × molecules/cm − /s (1x, solid curves), and a flux of 172 × molecules/cm − /s(20x, dashed curves). The corresponding stellar spectra are shown in the right panel (b), highlighting the wavelengthregion of strongest NO absorption. As shown in this figure, the hotter stars provide more photons between 0.25 to0.65 µ m, which increases the rate of NO photolysis (Eq. 1, and also Table 1).However, photolysis is not the only important factor . As shown in Table 1 and Fig. 2, O also plays a major rolein determining NO concentration. Panel (a) in Fig. 2 shows the O mixing ratio profiles for various stars. For theSun (blue), the O concentration increases rapidly below ∼
20 km compared to other stars. O participates in thedominant production reaction for NO , with the help of NO as shown in Table 1. Ideally, this should increase theconcentration of NO below 20 km. However, as shown in panel (b) of Fig. 2, photolysis of NO due to photons ofwavelengths between 0 . − . µ m that penetrate into the troposphere dominate the destruction of NO , decreasingits mixing ratio (See Fig. 1, panel a). While the photolysis rates generally increase for all stars below 20 km as shownin panel b of Fig. 2, it is the rate at which O increases below this altitude that determines the slope of decrease inNO in the troposphere for planets around different stars. While the rapid increase in O is mostly negated by therapid photolysis and consequent decrease of NO below ∼ concentration increases only a little from the surface to the tropopause (black, green and magentacurves in Fig. 2). Consequently, the increasing slope of photolysis rate of NO below ∼
20 km for these other starsslightly dominates (panel b), resulting in a larger decrease in the mixing ratio of NO compared to a Sun-like star(panel a in Fig. 1).As a result, the column integrated NO abundance increases moving from hotter stars to cooler stars (Table 2). Theabsorption cross sections for NO and other key gases are shown in Fig. 3. A Sun-like star produces more photons at https://github.com/VirtualPlanetaryLaboratory/atmos × (gram/year) /(1 . × − gram × × π (6 . × m ) × × × ∼ . × molecules/cm /s itrogen Dioxide Pollution as a Signature of Extraterrestrial Technology -14 -12 -10 -8 Volume mixingratio A l t i t ud e ( k m ) SunAD LeoProx CenK6V
20x Present Earth1x Present Earth NO (a) Wavelength (micron) -4 -2 e r g s / s / c m / m i c r on Sun (5780K)K-dwarf (4600K)AD Leo (3300K)Prox Cen (3000K) -5 NO absorption (b) Figure 1. (a): Mixing ratio profiles of NO around stars of different spectral types on an Earth-like planet with 1x (solid)and 20x (dashed) present Earth NO fluxes. Below the troposphere ( ∼ concentration is higher on planets aroundcooler stars compared to the Sun because the destruction of NO is an order of magnitude more efficient around a Sun-like stardue to the availability of photons of wavelength between 0 . − . µ m that penetrate to the troposphere. See inset in panelb. (b) Spectral energy distribution of stellar spectral types used in this study, indicating wavelength region of strongest NO absorption. The inset shows the UV/Visible region where NO photolysis happens. wavelengths where NO is photolyzed (between 290 − is higher for the planetaround the Sun than for a planet around a cooler star (Table 1).The result of all the dominant production and destruction reactions discussed above is that NO and NO decreasewith altitude until ∼
20 km, into the stratosphere. In the stratosphere, ozone can generate NO with reactionswith NO; ozone’s overlapping UV cross section with NO also provides some UV shielding. At higher altitudes,above the ozone layer, photochemical processes, including NO photolysis and reaction with OH radicals, draw downabundances. These reactions occur most markedly for the planet orbiting the Sun; NO photolysis proceeds 1-2 ordersof magnitude faster around the Sun compared to around the cooler stars. Reaction of NO with OH to form HNO occurs two orders of magnitude more efficiently around the Sun compared to the K6V star, and fully 4-6 orders ofmagnitude more efficiently around the Sun compared to the M dwarfs.It is important to note that placing constraints on a planet’s NO abundance from its spectrum would not definitivelyanswer whether the NO is biologically or abiotically produced. One would need to estimate the production ratesrequired to produce the observed NO abundance and evaluate whether abiotic sources alone can sustain the inferredproduction rate. DETECTABILITY OF NITROGEN DIOXIDE
Kopparapu et al. -9 -8 -7 -6 -5 Volume mixingratio A l t i t ud e ( k m ) SunAD LeoProx CenK6V
20x Present Earth1x Present Earth O (a) -5 -4 -3 -2 Photolysis rate (s -1 ) A l t i t ud e ( k m ) SunK6VAD LeoProx Cen (b)
Figure 2. (a): Mixing ratio profiles of O around stars of different spectral types on an Earth-like planet with 1x (solid)and 20x (dashed) present Earth fluxes of NO . Below the troposphere ( ∼ < concentration rapidly increases arounda Sun-like star (blue) compared to other star types. Because O is a dominant production mechanism for NO (See Table 1),the concentration of NO ideally should increase. (b) However, as shown in this panel, the photolysis rate of NO increasesfrom the ground to up to 10km - 20km for all star types, as photons of wavelength between 0 . − . µ m penetrate into thetroposphere. Consequently NO mixing ratio decreases between ∼ − dominates the photolysis rate (because NO photolysis is not increasing anymore), and as a result, NO mixing ratio increasesas well. The absorption cross section of NO shows a broad absorption between 0.25-0.6 µ m, which has little overlap withabsorption from other terrestrial molecular atmospheric constituents (Fig. 3, panel b). The main possible confusionwould be related to aerosols with sub-micron sizes ( ∼ . µ m), which have absorption features that could mimic theexact same shape as NO . Considering the broad nature of the NO spectral feature, a unique spectroscopic identifi-cation will be therefore ultimately challenging, and this investigation solely explores the hypothetical requirements fora possible detection for an absorption due to NO . Other absorption features are also present at ∼ . µ m, 6 . µ m and10 − µ m, but these overlap with absorption bands from H O, CO , and other species (Fig. 3, panel b). In orderto assess the detectability of NO as a technosignature, we use the mixing ratio profiles from the 1-D photochemicalmodel as input to the Planetary Spectrum Generator (PSG , Villanueva et al. (2018)) to simulate reflected light, andtransit spectra. We estimate the signal-to-noise (SNR) of detecting NO features. PSG is an online radiative transfersuite that integrates the latest radiative transfer methods and spectroscopic parameterizations, and includes a realistictreatment of multiple scattering in layer-by-layer spherical geometry. It can synthesize planetary spectra (atmospheresand surfaces) for a broad range of wavelengths for any given observatory. https://psg.gsfc.nasa.gov/index.php itrogen Dioxide Pollution as a Signature of Extraterrestrial Technology Reaction Integrated reac-tion/photolysis ratefor Sun (s − ) Integrated reac-tion/photolysis ratefor K-dwarf (s − Integrated reac-tion/photolysis ratefor 3390K star (s − ) Integrated reac-tion/photolysis ratefor 3000K star (s − )CH O + NO → CH O + NO × . × . × . × NO + O → NO + O × . × . × . × NO + O + M → NO + M 7 . × . × . × . × NO + HO → NO + OH 5 . × . × . × . × NO + NO → . × . × . × . × HO NO + M → HO + NO + M 1 . × . × . × . × NO + h ν → NO + O 2 . × . × . × . × NO + O → NO + O . × . × . × . × NO + OH + M → HNO + M 2 . × . × . × . × O + NO → NO . × . × . × . × O + NO → NO + O . × . × . × . × HO + NO + M → HO NO + M 1 . × . × . × . × NO + h ν → NO + O 2 . × . × . × . × Table 1.
Reactions that act as dominant sources and sinks for NO (first column), and column integrated reaction or photolysisrates for an Earth-like planets around Sun (second column), K-dwarf (third column), 3390K star (fourth column) and 3000Kstar (fifth column). Bold font are production mechanism for NO , and normal font are loss mechanisms. The dominant sink isNO photolysis and the dominant production mechanism is NO reaction with O (in addition to the surface flux).Species Sun K6V (4715K) AD Leo (3390K) Proxima (3000K)(molecules/cm ) (molecules/cm ) (molecules/cm ) (molecules/cm )NO . × . × . × . × O . × . × . × . × Table 2.
Column integrated number densities of NO and O (i.e, total number of molecules per unit volume of integratedalong a column of atmosphere) on an Earth-like planet with 1x NO flux around stars of different stellar spectral types. NO is more abundant on a planet around cooler stars than around a Sun-like star, despite having more O which is the dominantmolecule in producing NO , because short wavelength photons are available more around a Sun-like star than a K or M-dwarfstar. This results in higher photolysis rate (destruction) of NO around a Sun-like star (see Table 1) reducing it’s abundance . We performed simulations with PSG to generate reflected light spectra (Fig. 4) of planets around Sun-like star anda K-dwarf star. We then calculated required SNR to detect the NO feature (Fig. 5) between 0.2- 0.7 µ m. For thesesimulations, we assumed a LUVOIR-A like telescope (15 meter) observing with the ECLIPS (Extreme Coronagraphfor LIving Planetary Systems). This instrument is an internal coronagraph with the key goal of direct exoplanetobservations. It has three channels: NUV (0.2–0.525 µ m), visible (0.515–1.030 µ m) and NIR (1.0–2.0 µ m). The NUVchannel is capable of high-contrast imaging only, with an effective spectral resolution of R ∼
7. The optical channelcontains an imaging camera and integral field spectrograph (IFS) with R=140. For our spectral simulations, we useNUV (R=6) and visible (R=70) channels, as the NO cross section spans from UV into visible wavelengths (See Fig.3). Because the NO feature is quite broad in the NUV to visible region, a low resolution of R=6 and R=70 is sufficeto resolve the feature, at the same time maximizing the SNR. We calculated wavelength dependent SNR shown inFigs. 4, 5, 6, 7 and 8 as the difference between the spectra with and without the NO feature, divided by the noise Kopparapu et al.
Wavelength (micron) -24 -22 -20 -18 -16 C r o ss sec t i on ( c m / m o l ec u l e ) -20 -18 (a) Wavelength (micron) -24 -22 -20 -18 -16 C r o ss sec t i on ( c m / m o l ec u l e ) NO CO H OO O (b) Figure 3. NO absorption cross section as a function of wavelength (panel a). The broad absorption between 0.25-0.6 µ m isthe dominant feature, and few other molecules absorb here. The inset figure focuses on the cross section in this wavelengthregion. Other features in the IR region ( ∼ . µ m, 6 . µ m and 10 − µ m) are relatively weaker and overlap with absorptionfrom other gas species, in particular H O and CO (panel b). simulated by PSG for the instrument under consideration (see section 5.3 of Villanueva et al. (2018), and also thePSG website where the noise model is discussed in detail). The “net SNR” is calculated by summing the squaresof the individual SNRs at each wavelength within a given band (either NUV or VIS), and then taking the squareroot. This methodology is largely insensitive to SNR, as long as the feature is resolved by the spectrum. See alsoAppendix A for a comparison between the PSG coronagraph noise model and a complementary noise model (Robinsonet al. 2016; Lustig-Yaeger et al. 2019), showing highly comparable results for the photon count rates and resultingspectral precision. We considered the planets around both the Sun-like and K6V stars to be located at 10 pc distance,residing in the respective habitable zones (HZs) of their host stars as calculated from Kopparapu et al. (2013, 2014),and observed at a phase angle of 45 ◦ (0 ◦ is secondary eclipse, and 180 ◦ is transit). For this feature to be detected, theplanet need not be in the HZ, as will be explained later in the discussion section (4).In both the panels of Fig. 4, the difference of the geometric albedo spectrum with and without NO are shown fordifferent levels scaled by factors of current Earth levels in a 10 hour observation with LUVOIR-15m telescope. Thecorresponding noise is shown as dashed curve. For the Sun-like star (panel a) even very high (20x) concentrations ofNO compared to the present Earth levels barely reach the 1 − σ noise level in the strongest wavelength region. Inpanel b, increasing the nominal abundance to higher concentrations on a planet around a K-dwarf produces only amarginal improvement over a Sun-like star, with the highest concentration (20x) reaching just above the noise level.This is likely because the column number density of NO on a planet around K-dwarf star seems marginally larger https://psg.gsfc.nasa.gov/helpmodel.php itrogen Dioxide Pollution as a Signature of Extraterrestrial Technology Wavelength(micron) -8 -6 -4 -2 G e o m e t r i c A l b e do ( w i t hou t N O - w i t h N O ) Noise
Sun-Earth at 10pc with LUVOIR-15m10 hour observation (a) -8 -6 -4 -2 G e o m e t r i c A l b e do ( w i t hou t N O - w i t h N O ) Noise
Kstar-Earth at 10pc with LUVOIR-15m10 hour observation (b)
Figure 4.
Geometric Albedo difference with and without NO for an Earth-like planet around a Sun-like star (panel a) andaround a K6V stellar spectral type (panel b) located at 10 pc with varying NO concentrations, assuming LUVOIR-A (15 m)observing time of 10 hours. 1 σ noise model is also shown (dashed black). The multiple factors in the legend are compared tothe concentrations of present Earth level of NO flux (8.64 × molecules/cm /s) implemented in our photochemical model ofan Earth-like planet around each stellar type. These are cloud free model results. (Table 2) due to less photolysis rate (last row, Table 1). As discussed above, the enhanced NO absorption on theK-dwarf planet compared to the planet around the Sun-like star is driven by the photochemistry.In Fig. 5, we show the calculated signal to noise ratio (SNR) values of the features shown in Fig. 4 as a function ofwavelength for 10 hour exposure times with a LUVOIR-A like telescope for wavelengths relevant to the NO feature.The “net SNR” indicated in these figures is calculated by summing up the squares of the SNR from each wavelengthband and then taking the square-root (see Eq.(6) of Lustig-Yaeger et al. (2019)). Fig. 5(a) shows that for planetsaround Sun-like stars even an increase of 10x in the NO flux is not enough to detect the feature with any meaningfulSNR within 10 hours of observation. Any lower amount of NO would need even more longer observation times.Fig. 5(b) shows SNR as a function of the same wavelength range for a planet around K-dwarf star. The combinedeffects of more NO and better planet-star contrast ratio relative to the planet orbiting the Sun (a K6V dwarf is onlyabout one tenth as luminous as a G2V dwarf) makes only a marginal difference in SNR that can be reached in thesame time as Sun-like star.While these SNR may not look promising, there is an interesting question that one can ask and explore an answer:How much LUVOIR-15 m time is needed to detect present Earth-level concentration of NO around a Sun-like starat 10 pc? Fig.6 (a) shows Geometric albedo spectrum difference with and without NO as a function of wavelengthfor 300, 600 and 1200 hours of LUVOIR-A time, respectively. Also shown in dashed lines are the corresponding noiselevels for each of these observation times. The present Earth-level NO seems to be well above the noise level after 300hours of observation time (compare the solid green curve with red-dashed line) indicating that it might be detectable.0 Kopparapu et al.
Wavelength(micron) S NR , net SNR = 0.582xNO , net SNR = 0.625xNO , net SNR = 0.7410xNO , net SNR = 0.9715xNO , net SNR = 1.92320xNO , net SNR = 1.53 Sun-Earth at 10pcwith LUVOIR-15m 10hour observation (a)
Wavelength(micron) S NR , net SNR = 0.442xNO , net SNR = 0.525xNO , net SNR = 0.7310xNO , net SNR = 1.2015xNO , net SNR = 1.9220xNO , net SNR = 2.88 Kstar-Earth at 10pc withLUVOIR-15m 10 hourobservation (b)
Figure 5.
Calculated SNR values to detect various levels of NO concentrations as a function of wavelength, around a Sun-likestar (panel a) and for a K6V spectral type star (panel b) located at 10 pc. The calculation assumed a LUVOIR-A (15m) typetelescope with 10 hour observation time. While these plots show that at any given wavelength, NO of any concentration isdetected comparatively at a higher SNR around a K6V star than a Sun-like star, it will still be challenging to detect the featurewithin 10 hours. The NO concentrations are generally higher around the K-dwarf star compared to an Earth-like planet arounda Sun-like star, giving rise to this marginal increment in SNR around a K-dwarf star. These are cloud free model results. To find out with what SNR it would be detectable, Fig. 6 (b) shows the “net SNR” to detect present Earth-level NO as a function of observation time. To achieve a net SNR of 5 (dashed red line), it would take LUVOIR-15 m about400 hours. For comparison, to obtain the Hubble Ultra Deep Field image, ∼
400 hours of actual observation time( ∼ ∼
900 hours of observation time assuming ∼ ∼ ∼ feature on a Earth-Sun system at 10 pc. An even more interesting aspect is that, we can place upperlimits on the amount of NO available on that planet as we spend more observation time on a prime HZ candidate.This could potentially indicate the presence or absence or the level of technological civilization on that planet. DISCUSSION itrogen Dioxide Pollution as a Signature of Extraterrestrial Technology -8 -6 -4 -2 G e o m e t r i c A l b e do ( w i t hou t N O - w i t h N O ) NO signal10hour noise300hour noise1200hour noise Sun-Earth at 10pc with LUVOIR-15m (a)
10 50 100 300 600
Observation time (hours) n e t S NR Current Earth-level NO LUVOIR-15m, Sun-Earth @ 10pc (b)
Figure 6. (a) Geometric albedo difference with and without NO as a function of wavelength for different observation timeswith LUVOIR-15m telescope to detect a present Earth-level NO amount on a Sun-Earth system at 10pc. (b) IntegratedSNR (over the wavelengths) versus the amount of observation time needed for the same system configuration. For example, todetect an Earth level NO with SNR ∼ ∼
400 hours of observation time. Forcomparison, Hubble’s large programs such as the Ultra Deep Field (UDF) and CANDLES surveys used between ∼ − While the results from the previous section provide a preliminary study of NO as a potential technosignature,some caveats need to be mentioned. First, we have performed 1-D photochemical model calculations using a modernEarth template generated from a 1-D radiative-convective, cloud-free climate model from Kopparapu et al. (2013).Clouds can significantly effect the observed spectrum and potentially alter the calculated SNR. To test this, we haveprescribed water-ice clouds (particle size 25 µ m) between 0.001 - 0.01bar, and liquid water clouds (particle size 14 µ m)between 0.01bar - 0.1bar in PSG. Fig. 7 shows SNR as a function of wavelength for an Earth-like planet around aSun-like star at 10 pc distance observed with the LUVOIR-15m telescope for 10 hours with (blue solid) and without(red solid) clouds. The absorption cross-sections of water and ice clouds are in the same wavelength region as the peakNO absorption which further masks the NO feature in this band. We should caution that this is all based on ad-hocprescription of clouds at a certain height, and a more rigorous analysis using 3-D climate models which can simulateself-consistent and time-varying cloud cover need to be performed. We leave that for future study.Secondly, we have used the 15 m architecture of LUVOIR-A, and the SNR values we report are a best case scenarioowing to its large mirror size. Other telescope architectures such as LUVOIR-B (8 m) or HabEX (a 4 m mirroraccompanied by a coronagraph and a starshade) may need more observation time than shown in Fig. 5 and Fig. 7 todetect NO features. Kopparapu et al. S NR With clouds, net SNR = 0.55without clouds, net SNR = 0.58Earth-Sun at 10pc withLUVOIR-15m 10 hourobservation, 1xNO (a)
10 50 100 300 600 1200
Observation time (hours) n e t S NR Without clouds 1xNO With clouds 1xNO Current Earth-level NO LUVOIR-15m, Sun-Earth@ 10pc (b)
Figure 7.
Effect on the SNR of a geometric albedo spectrum with (blue solid) and without (red solid) clouds on an Earth-likeplanet (1xNO ) around Sun-like star. Water clouds absorb in the same wavelength region as NO absorption bands, thusreducing the SNR and potentially causing confusion source. (b) Similar to Fig.6b, integrated SNR (over the wavelengths) versusthe amount of observation time with (green dashed) and without (blue solid) clouds. The time to reach a SNR = 5 is slightlylonger with clouds. As shown in Fig. 3, NO also absorbs in the infrared (IR) part of the spectrum, particularly between 3.2-3.7 µ m,5.2-8.9 µ m and 9.7-18 µ m. However, the absorption in these regions is either weak across the band compared tothe 0.25-0.65 µ m visible band, or limited to a very narrow region of the spectrum. Consequently, detecting NO intransit spectroscopy with either JWST or the flagship mission concept study Origins Space Telescope (OST) wouldbe challenging. Nevertheless, we tested this with PSG, and the results are shown in Fig. 8. We placed a planet likeProxima Cen b around it’s host star at 10 pc assuming that it transits, with 20x NO abundance to maximize thesignal. We used JWST NIRSpec and OST MISC-Transit instrument for the ∼ µ m and ∼ µ m wavelength regions forthe detection of NO . While OST has greater performance than JWST, the observations here are limited by maskingfeatures from H O and CO in the near-IR. Even at 20x NO from our photochemical model H O features completelydominate the ∼ µ m region of NO absorption (Fig.8(a)). The SNR for a 10 hour and 500 hour observation times forboth telescopes is shown in Fig.8(b). Even with large observation times, it would be very challenging to detect theNO feature with any meaningful SNR.A space-based nulling interferometer such as ESA’s LIFE (Large Interferometer for Exoplanets) mission concept(Defr`ere et al. 2018; Quanz et al. 2018) could potentially detect mid-IR (5 − µ m) features in direct imaging spectra.While we are unable to assess quantitative limits on SNR at this time for this mission, we speculate that phasedependent thermal emission spectroscopy (Wolf et al. 2019; Suissa et al. 2020) may be another way to detect NO feature. itrogen Dioxide Pollution as a Signature of Extraterrestrial Technology P l a n e t- S t a r C on t r as t ( pp m ) JWST 500hrOST 500hrJWST no NO OST no NO (a) Wavelength(micron) -4 -3 -2 -1 S NR Transiting Earth around 3000K star @10pc
JWST 10hrOST 10hrJWST 500hrOST 500hr (b)
Figure 8. (a) Transit spectrum of NO in the near-IR and mid-IR region on a HZ Earth-like planet around Proxima Cen-likestar ( T eff = 3000K) located at 10pc with 20x present Earth-level NO fluxes using JWST NIRSpec (blue) and OST MISC(red) observations. No clouds were included. The vertical solid black line indicates the error bar, and dashed lines indicateNO absorption bands in the IR (b) Both 10 hour (dashed) and 500 hour (solid) observations indicate it is very challenging toobserve even 20x NO abundance in transit observations in the IR because of several other overlapping gases in this region thathave a stronger absorption than NO . See Fig. 3(b). Historically, the United States NO concentrations have varied (gone down) by a factor of 3 over a period of 40years, from 1980-2019. Therefore, we can expand the possibilities of detecting a technological civilization at thestage where Earth civilization was 40 years ago. It is possible to imagine a more highly industrialized society thatcould possibly operate in the regime of 5 × Earth NO level making it possible to detect it with LUVOIR-15m witheven less observation time than for present-Earth conditions. We should stress here that when we mean a technologicalcivilization, it does not necessarily mean a much more advanced society than current Earth level. Just like the searchfor biosignatures encompass ‘Earth-through time’ with different stages of Earth’s biosphere evolution, we could do asimilar search for a ‘technosphere’ at different stages of a technological civilization.It is possible that atmospheric technosignatures, in particular industrial pollutants like NO , are short-lived. How-ever, this is comparable to searches for radio technosignatures where the transient nature of the radio communicativecivilizations may also be short-lived. Furthermore, it may be that an industrialized society that is prone to emit NO as a byproduct of their combustion technology may also have radio communication capabilities, just like us. In thisrespect, a search for radio technosignatures can be performed if NO is detected on a potential habitable planet.If we are looking for NO as a technosignature, and not as a biosignature, then it may appear that one need notlimit the search to known planets in the HZ. A technological civilization can possibly inhabit even an adjacent barren Kopparapu et al. planet (like Mars in our Solar System), and use the atmosphere as a waste dump of NO emissions. Or they mayprefer to live sub-surface on a HZ planet and release “waste” NO into the atmosphere. Speculations are endless.However, industrial NO on Earth is produced by essentially burning biomass (coal, petroleum products) that havebeen excavated to fuel the civilization (We note that NO can also be produced by nuclear detonations.) The vastmajority of burnable organic matter is directly or indirectly derived from oxygenic photosynthesis, meaning an abioticor anoxic world would not have abundant preserved organic matter. To burn this biomass, one needs an atmospherewith oxygen. The observation of high abundances of NO on an exoplanet atmosphere would indicate a sustained sourceof industrial production, likely requiring an oxic atmosphere and indicating a significant source of biomass to sustainlong-term industrial activity. While NO can exist in abundant quantities on planets around K-dwarf stars, it may notnecessarily be a desirable thing for the inhabitants if they have biology similar to humans, because exposure to NO could cause impairment of lung function and/or recurrent respiratory problems (Faustini et al. 2014b). Conversely, ifextraterrestrial biology is sufficiently different from Earth life, then it could be impervious to NO toxicity. In thisrespect, NO on K-dwarfs is similar to the likely accumulation of abiotic and biologically produced CO on Earth-likeplanets orbiting mid-to-late M-dwarfs, in addition to the accumulation of biosignature gases (Schwieterman et al.2019).Missions like LUVOIR, HabEX, and OST may have biosignature targets as a priority, so it may be untenable to seekdedicated observing time for exclusive technosignature detection. However, in the search for exo-Earth candidates, wewill undoubtedly detect other planets within the stellar system (Stark et al. 2014; Kopparapu et al. 2018). LUVOIRand HabEX will be able to simultaneously obtain the spectra of the other bright planets in the system, while performingtheir observations on a prime HZ target. Consequently, there may not be a need to schedule separate observationtime for technosignature detection, as such efforts could “piggy back” on a routine survey to observe both HZ andnon-HZ planets (Lingam & Loeb 2019). However, this assumes that the NO detection will likely occur within thetotal integrated observational time spent on the prime HZ candidate for the biosignature detection, whereas Fig. 5indicates lower NO abundances may require longer search times. CONCLUSIONThe presence of NO on Earth today results in part from sustained industrial processes in urban areas. This papersuggests that the detection of NO in an exoplanet atmosphere could serve as a technosignature, as Earth-level biogenicsources would be unable to generate detectable atmospheric abundances of NO . Using a 1-D photochemical modelthat uses present Earth atmospheric temperature profile, we find that it would be challenging to detect Earth-levelNO around G and K-dwarf stars through direct imaging with only 10 hours of observation time. To detect presentEarth-level NO concentration with a SNR ∼
5, it would take ∼
400 hours of LUVOIR-15m telescope. Such largeprograms may be possible considering several hundred hours of observing time spent on Hubble UDF and CANDLESsurveys. Historically, the United States NO emission varied (gone down) by roughly a factor of ∼ abundance. The advantage of searching K-dwarf planets has alreadybeen noted in the search for biosignatures (Cuntz & Guinan 2016; Arney 2019), and our results indicate that K-dwarfplanets could similarly be advantageous when searching for technosignatures like NO .However, when we prescribe water-ice and liquid water clouds, there is a moderate decrease in the SNR of thegeometric albedo spectrum from LUVOIR-15 m, with present Earth-level NO concentration on an Earth-like planetaround a Sun-like star at 10 pc. Clouds and aerosols can reduce the detectability and could mimic the NO fea-ture, posing a challenge to the unique identification of this signature. This highlights the need for performing thesecalculations with a 3-D climate model which can simulate variability of the cloud cover and atmospheric dynamicsself-consistently.While NO absorbs even in the near-IR and mid-IR, we find that transit observations in this region with JWSTand OST may prove challenging to detect NO because of the weaker absorption and also due to overlapping gasabsorption of potent greenhouse gases such as H O, CO and CH .Further work is needed to explore the detectability of NO on Earth-like planets around M-dwarfs in direct imagingobservations in the near-IR with ground-based 30 m class telescopes. NO concentrations increase on planets aroundcooler stars due to reduced availability of short-wavelength photons that can photolyze NO . Non-detectability at itrogen Dioxide Pollution as a Signature of Extraterrestrial Technology present on M-dwarf HZ planets like Prox Cenb.The serendipitous detection of NO , or any other potential artificial atmospheric spectral signature (CFCs, forexample) may become a watershed event in the search for life (biological or technological). Is it likely that biosignaturesare more prevalent than technosignatures? We will not know for certain until we search. Our aim in this study is topoint out that both biosignatures and technosignatures are two sides of the same coin, and the search for both canco-exist together with upcoming observatories. It is worth pointing out the obvious in this concluding statement: thequestion “Are we alone?”—which has been the driving force behind the search for extraterrestrial biosignatures—is aquestion posed by a technological civilization.ACKNOWLEDGMENTSThe authors would like to thank an anonymous reviewer whose comments greatly improved the manuscript. Wewould also like to thank Sandra Bastelberger, Chester “Sonny” Harman, Thomas Fauchez, James Kasting and AviMandell for discussions that helped in this work. R. K. would like to acknowledge Vivaswan Kopparapu, his 11 yearold son, who helped R. K. to realize that increasing the LUVOIR-15 m observation time by a factor of 4 doubles theNO SNR for an Earth-Sun system at 10 pc. Goddard affiliates acknowledge support from the GSFC Sellers ExoplanetEnvironments Collaboration (SEEC), which is supported by NASA’s Planetary Science Division’s Research Program.J.H.M. gratefully acknowledges support from the NASA Exobiology program under grant 80NSSC20K0622. Thiswork was performed as part of NASA’s Virtual Planetary Laboratory, supported by the National Aeronautics andSpace Administration through the NASA Astrobiology Institute under solicitation NNH12ZDA002C and CooperativeAgreement Number NNA13AA93A, and by the NASA Astrobiology Program under grant 80NSSC18K0829 as part ofthe Nexus for Exoplanet System Science (NExSS) research coordination network.APPENDIX A. COMPARISON OF LUVOIR NOISE MODELSWe conducted a comparison between the LUVOIR coronagraph noise model included in PSG and the Pythonimplementation of the Robinson et al. (2016) coronagraph noise model from Lustig-Yaeger et al. (2019) (henceforthCG). We found the two models to agree very well (Figure 10), with both models implementing very similar formalismsfor computing sensitivities.We define the end-to-end throughput for the planetary fluxes as: T total = T T ele × T cor × T opt × T read × T QE , where T T ele accounts for light lost due to contamination and inefficiencies in the main collecting area, T cor is the coronagraphicthroughput at this planet-star separation, T opt is the optical throughput (the transmissivity of all optics), T QE is theraw quantum efficiency (QE) of the detector, and T read is the read-out efficiencies. The left panel in Figure 9 shows theoptical throughput ( T opt ) from Stark et al. (2019) and the right panel shows the coronagraph throughput as a functionof planet-star separation ( T cor ). Although the design of the LUVOIR-A coronagraph has multiple different maskswith slightly different IWAs, both coronagraph models use a combined mask (shown in Figure 9) to approximate theoptimal use of the coronagraph for any simulated target. Importantly, the coronagraph throughput already accountsfor the fraction of the exoplanetary light that falls within the photometric aperture, denoted f pa in Robinson et al.(2016), so we manually set f pa = 1 in the CG model to properly account for this factor. The number of stellarphotons is defined by the contrast at the core throughput, and thus the number of stellar photons is calculated as C · max( T cor ) ≈ − · .
27. For T T ele , we adopt 0.95 for all wavelengths, on par with the particulate coverage fractionfor JWST’s mirrors. EMCCD detectors are expected to have T read near 0.75 (Stark et al. 2019), while for NIR andother detectors, read-out inefficiencies and bad-pixels may account to a similar value, and we adopt T read =0.75 acrossall detectors as a conservative estimate. The reported quantum efficiency of the different detectors ranges from 0.6 to0.9, yet technological improvements in several of these detectors could be expected in the near future, and we adopta general T QE =0.9 for all detectors.The Signal-to-Noise ratio (SNR) is effectively defined by the different sources of noise, quantified as count rates.We not only compared resulting SNRs between the two models, but also the simulated count rates for the different6 Kopparapu et al.
Figure 9.
Coronagraph throughputs used for LUVOIR-A noise modeling. The left panel shows the wavelength dependentoptical throughput. The right panel shows the coronagraph throughput as a function of planet-star separation.
Table 3.
LUVOIR-A coronagraph model input parametersParameter Description Value D Mirror Diameter 15 m C Contrast 10 − T opt Optical Throughput Figure 9 (left) T cor Coronagraph Throughput Figure 9 (right) R e − Read Noise (UVIS/NIR) 0 / 2.5 D e − Dark Current (UVIS/NIR) 3E-5 / 2E-3 s − X Circular Photometric Aperture Radius 0 . λ/DN ez Number of Exozodis 4.5 components, and found very good agreement (Figure 10). For these simulations, we assumed a circular aperturedefined by diffraction (1.22 λ/D ), an exo-zodiacal level of 4.5 times the one of our solar system (22 mag/arcsec ), anda local zodi level of 22.5 mag/arcsec . The noise term was computed as C noise = (cid:112) C p + C s + 2 C b (A1)where C p is the total number of planet photons, C s is the stellar photon noise (e.g. “leakage” through the coronagraph),and C b is the total background, which includes all other noise sources such as zodi, exozodi, dark current, thermal, andread noise. Observations are normally performed as on-off, meaning one with the planet, and one without. As such,the background sources of noise need to be counted twice (equation A1). Depending on the observational procedure,the stellar photons can be assumed to be present in the “off” position or not. Robinson et al. (2016) assumes by defaultthat the star is also in the “off” position, and therefore doubles C s , while the default in PSG is the “off” positionwithout star leakage (so only counted once, equation A1). We explored the impact on the SNR of this assumption inthe observational procedure, and only observe small ( < Arney, G., Domagal-Goldman, S. D., Meadows, V. S., et al.2016, Astrobiology, 16, 873, doi: 10.1089/ast.2015.1422 Arney, G. N. 2019, ApJL, 873, L7,doi: 10.3847/2041-8213/ab0651 itrogen Dioxide Pollution as a Signature of Extraterrestrial Technology Figure 10.
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