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The Exo-Life Finder (ELF) telescope: New strategies for direct detection of exoplanet biosignatures and technosignatures

S. V. Berdyugina, J. R. Kuhn, M. Langlois, G. Moretto, J. Krissansen-Totton, et al.

S. V. Berdyugina, J. R. Kuhn, M. Langlois, G. Moretto, J. Krissansen-Totton, D. Catling, J. L. Grenfell, T. Santl-Temkiv, K. Finster, J. Tarter, F. Marchis, H. Hargitai, D. Apai, "The Exo-Life Finder (ELF) telescope: New strategies for direct detection of exoplanet biosignatures and technosignatures ," Proc. SPIE 10700, Ground-based and Airborne Telescopes VII, 107004I (1 October 2018); doi: 10.1117/12.2313781 Event: SPIE Astronomical Telescopes + Instrumentation, 2018, Austin, Texas, United States Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 10/11/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Exo-Life Finder (ELF) Telescope: New Strategies for Direct Detection of Exoplanet Biosignatures and Technosignatures S. V. Berdyugina*a,b, J. R. Kuhnc,b, M. Langloisd,b, G. Morettod,b, J. Krissansen-Tottone, D. Catlinge, J. L. Grenfellf, T. Santl-Temkivg,k, K. Finsterg,k, J. Tarterh, F. Marchish,i, H. Hargitaih, D. Apaij a

Kiepenheuer Institut für Sonnenphysik, Freiburg, Germany ; bPLANETS Foundation, Kula, Maui, HI, USA; cInstitute for Astronomy, University of Hawaii, Pukalani, Maui, HI, USA; dCRAL, University of Lyon, Lyon, France; eAstrobiology Program, Department of Earth and Space Sciences, University of Washington , Seattle, WA, USA; f Department of Extrasolar Planets and Atmospheres, Institute of Planetary Research, DLR, Berlin, Germany; gStellar Astrophysics Center, Aarhus University, Aarhus, Denmark; h SETI Institute, Mountain View, CA, USA; iObservatoire de Paris, LESIA, Meudon, France; jSteward Observatory and Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA; k Department of Bioscience, Aarhus University, Aarhus, Denmark. ABSTRACT The Exo-Life Finder (ELF) will be an optical system with the resolving power of a ≥20m telescope optimized for characterizing exoplanets and detecting exolife. It will allow for direct detection of Earth-size planets in commonlyconsidered water-based habitable zones (WHZ) of nearby stars and for generic exolife studies. Here we discuss capabilities of the ELF to detect biosignatures and technosignatures in exoplanetary atmospheres and on their surfaces in the visual and near infrared. We evaluate sensitivity limits for mid- and low-resolution spectral, photometric and polarimetric measurements, analyzed using atmosphere models and light-curve inversions. In particular, we model and estimate integration times required to detect O2, O3, CO2, CH4, H2O and other biosignature gases and habitability markers. Disequilibrium biosignature pairs such as O2+CH4 or CO2+CH4–CO are also explored. Photosynthetic and nonphotosynthetic pigments are other important biosignatures that ELF will search for in atmospheres and on resolved surfaces of exoplanets, in the form of bioaerosols and colonies of organisms. Finally, possible artificial structures on exoplanet surfaces and in near-exoplanet space can be detected. Practical instrument requirements are formulated for detecting these spectral and structural biosignatures and technosignatures. It is imperative that such a study is applied first to characterize the nearest exoplanet Proxima b, then to search for exo-Earths in the Alpha Cen A and B system and other near-Sun stars, and finally to explore larger exoplanets around more distant stars. Keywords: Extremely large telescopes, exoplanets, exolife, biosignatures, technosignatures, Exo-Life Finder (ELF)

1. INTRODUCTION Finding life elsewhere in the universe is one of our greatest challenges. In the late 1950s and early 1960s, basic ideas on how to search for extraterrestrial intelligence (SETI) in the radio, optical and infrared were formulated [1,2,3]. A practical SETI was pioneered in the USA by Frank Drake [4] at the National Radio Observatory at Green Bank and paralleled by Nikolai Kardashev [5] in the USSR (CETI). Since then, SETI programs search for serendipitous or deliberately beamed alien radio communication signals [6,7,8]. More recently, radio programs have been complemented by searches for beamed optical and near-infrared signals [9,10,11,12]. The search for signatures of microbiological life in our Solar system started in the 1970s, when the Viking landers investigated Mars soil samples for the presences of organics and living microbes [13]. These and later searches have not yet provided an unambiguous detection of the presence of life elsewhere outside Earth. Since the dawning of the exoplanetary era in the mid-1990s, almost 4000 exoplanets have now been confirmed. Statistical studies based on data from the Kepler mission [14] suggest a terrestrial planetary occurrence rate of 0.77 planets per star (with uncertainty of between 0.3-1.9) for GK main sequence stars. An obvious question is whether life emerged and evolved on these planets throughout the universe. Answering this requires new Extremely Large *[email protected]

www.leibniz-kis.de

Ground-based and Airborne Telescopes VII, edited by Heather K. Marshall, Jason Spyromilio, Proc. of SPIE Vol. 10700, 107004I · © 2018 SPIE CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2313781 Proc. of SPIE Vol. 10700 107004I-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 10/11/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Telescopes (ELTs) that can directly image those worlds to assess their potential for habitability. We believe this requires characterizing physical and chemical properties of terrestrial-type planets, both inside and outside standard stellar habitable zones, to ultimately confirm the presence of exolife. This is difficult because of the faintness of Earth-like exoplanets in all parts of the spectrum and their small angular separation from the nearby overwhelmingly bright star. Recently a new optical system, the Exolife Finder Telescope (ELF, Fig. 1) has been proposed [15,16,17,18] for highcontrast direct imaging of Earth-size planets in liquid water habitable zones (WHZ). Being a hybrid, interferometric telescope, ELF would be a light-weight and scalable system with a partially filled aperture, potentially reaching 100’s of meters in size and with a cost per square meter that could be significantly below that of other proposed ELTs. Its design allows nulling of the stellar light background within a limited field of view down to the 10–8 contrast needed with a large effective aperture for sensitive photometry of exoplanets. In this paper, we present strategies for detection of unintentional exolife signatures using a 20m-class ELF. Such a system could be built within 5 years and provide the first complete studies of exolife on planets of 1–2 Earth masses in the solar neighborhood. WIZ Q 115 mm Off-Axis Rim 0 0.6m

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Figure 1: The ELF design principle uses an array of off-axis telescopes. Each has a 5m aperture primary segment configured from a 33m diameter parent parabolic surface. Distinct 0.6m secondary segments arranged in a circular configuration to create a Gregorian-focus. This optical system has advantages of both a large aperture telescope and an interferometer, allowing for high-contrast imaging using nulling interferometry.

The detection of life using remote sensing depends on cues that we call biosignatures (for life in general) and technosignatures (for intelligent/technological life). A biosignature is defined as any substance, group of substances, or phenomenon that provides evidence of life [19]. A technosignature can be defined as a detectible phenomenon associated with nonequilibrium abiotic energy within an exoplanetary environment. These two definitions overlap, and some life signatures can be considered as either biosignature or technosignature. Traditional technosignature searches by radio [6,7,8,20] and optical [9,10,11] SETI techniques have been ongoing for decades [21]. Exoplanet habitability and biosignature studies are a recent and active field of research [19,22,23,24,26,26,27,28]. In this paper, we consider two primary types of unintentional life signatures which may be detected by ELF: 

Atmospheric life signatures, i.e., gaseous and particle compounds of biological or technological origin in exoplanetary atmospheres, e.g., oxygen, methane, bioaerosols [25,26,29], and

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Structural life signatures, i.e., spatially resolved structures on exoplanetary surfaces or in the near-planetary space [2,30,31].

These are considered in Sections 2 and 3, respectively. Some abiotic processes can produce the same types of signatures as life’s “fingerprints”, which can lead to false-positive detections [32]. Therefore, unambiguous exolife detection probably requires multiple observations of distinct signatures. In the following, we discuss some modeling results for various signatures and evaluate sensitivity requirements for their detection. Since we have just one example of a “living” planet (Earth), the scope of life’s signatures is rather limited, even if we consider Earth’s history and use this information for the interpretation of exoplanet signals and their evolution. This may lead to false-negative observations when signatures of exolife are not recognized or overlooked. It is clear that improving our ability to recognize alien life forms and their fingerprints requires systematic study – a census of life signatures on all (terrestrial) exoplanets within a given volume, independently of whether or not they lie in the WHZ. At the same time, efforts to further develop generalized life signatures, which are chemistry-independent should be continued, e.g., chemical disequilibrium, waste heat, etc. [2,33,35].

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2. ATMOSPHERIC BIOSIGNATURES AND TECHNOSIGNATURES Atmospheric life signatures are gaseous and particle compounds of biological or technological origin in exoplanetary atmospheres. 2.1 Biosignature gases The atmosphere of a planet can be probed by remote observations via transmission (during planetary transits), thermal emission, and scattering (reflection). The last two techniques can be employed independently of exoplanet transits or eclipses. ELF, with its high photometric sensitivity and large photon collecting aperture, will greatly expand applications of these techniques to Earth-size exoplanets. Atmospheric biosignatures, such as the presence of biogenic gases [34] and/or the presence of a so-called redox chemical disequilibrium [35,36,37,38], have been proposed as fingerprints of life. In most cases, biosignature gases characteristic to Earth at its various ages are employed. Thus, one should look into a range of gases and disequilibrium signatures in oxygen-rich and anoxic atmospheres. Using climate-photochemical models, atmospheric biosignature responses and spectral signals of terrestrial planets have been estimated assuming an Earth-like development and biomass [39,40,41,42,43,44]. In particular, the simultaneous presence of reducing gases (e.g., CH4) together with oxidizing gases (e.g., O2) could be attributed to life [35,36]. In this case, the activity of the biota that is needed to maintain the chemical disequilibrium of CH4–O2 is an important quantity to attribute to life, based on the Earth as an example [45]. It also appears that Earth’s atmosphere features strong redox disequilibrium due to the simultaneous presence of N2–O2 and liquid water [37]. Co-evolution of life and the Earth’s atmosphere may therefore help us identify when in Earth history the signal of life (including gases and surface structures) was the strongest and could be detected by ELF. Such studies may also help to minimize a risk of false-positive detections, since biogenic gases may also form due to geochemical (e.g., water-rock reactions) and photochemical processes (e.g., photolysis of water or carbon dioxide) affecting the composition of the planetary atmosphere [24,25,46]. To demonstrate the capabilities of ELF we simulated direct imaging observations of Earth-like planets to evaluate the detectability of biosignature gases for nearby targets. Observations were simulated using the coronagraph noise model described in [47] adapted for Python by J. Lustig-Yaeger (https://github.com/jlustigy/coronagraph/). The noise simulator has been modified to simulate ground-based observations and includes noise from the night sky background, thermal background, dark current, read noise, zodiacal and exozodiacal light, and speckle noise. The contrast ratio for scattered stellar light as a function of angular separation was adapted from [30] to conservatively represent the stellar nulling capabilities of ELF. Spectra were simulated with the resolution R=70, and it was assumed that telluric contribution can be subtracted using a careful calibration. Higher spectral resolution measurements using noiseless photon detectors will eventually see Doppler-shifted molecular spectral lines of the target atmosphere in between obscuring telluric lines [48]. Because the composition of the Earth’s atmosphere has evolved greatly in time and has been influenced by the biosphere for 3.5–4.0 billion years [49], we consider both modern Earth and early Earth spectral analogs in our simulations. In Fig. 2a, we show the reflectance spectrum of Proxima b assuming as a first approximation modern Earth’s atmosphere and climate [55] as observed by ELF for 100 hours. Noise is low in the NIR portion of the spectrum meaning that water vapor absorption at 0.94 µm and 1.14 µm is readily detectable, and the O2 A-band (0.76 µm) is detected with a signal to noise ratio (SNR) of ~22. Detailed simulations suggest that this is sufficient to constrain O2 abundances [50]. The O2 Aband has long been considered a possible biosignature of a photosynthetic biosphere [51]. Low stellar flux at UV–VIS wavelengths make the Rayleigh scattering slope difficult to observe in unpolarized flux, but it may still be possible to detect the O3 Chappuis band (0.45–0.85 µm). The relatively low abundances of CO2 and CH4 in modern Earth’s atmosphere coupled with their lack of strong absorption features in the visible–NIR means that they would be challenging to detect. However, CH4 abundances are likely to be much higher in oxic atmospheres of planets around Mdwarfs than around Sun-like stars for the same biogenic CH4 source flux [40], and so the O2–CH4 disequilibrium biosignature combination could be detectable for an Earth-like Proxima b (e.g. Fig. 16a in [52]). The blue and red dashed vertical lines show 3λ/D and 2λ/D inner working angle (IWA) upper wavelength constraints for observability. Even for a pessimistic IWA of 3λ/D, O2 and water vapor are still detectable. Figure 2b shows the reflectance spectrum of the modern Earth as a WHZ planet around Alpha Centauri B (a K1 V star) as observed by ELF for 100 hours. The NIR portion of the spectrum is noisier than in the Proxima b case because of the lower planet/star flux ratio. Nonetheless, O2 and H2O absorption features are still detectable with SNRs of 10 or better. The greater stellar flux at shorter wavelengths means that the Rayleigh slope and O3 Chappuis band are better

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constrained than in the Proxima b case. Additionally, a larger planet-star separation for habitable zone planets around Kdwarfs as compared to M-dwarfs means there are no IWA limitations. Instead, the spectrum is limited by the thermal background, which dominates beyond 2.5 µm. 100

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Figure 2. Simulated spectroscopic observations of Earth-like planets with ELF for 100-hour integration times assuming the telescope is observing from the Atacama Desert with the star at 30°. Purple lines show the true binned spectra with the spectral resolution R=70, whereas black dots with error bars show simulated ELF observations. Green arrows highlight atmospheric absorption features that might be detectable. The blue and red dashed vertical lines show 3λ/D and 2λ/D inner working angle (IWA) constraints, respectively. The portion of the spectra rightward of those lines would not be observable for those IWA cutoffs. (a) Proxima b as the modern Earth. (b) The modern Earth as a habitable zone planet around Alpha Centauri B. The modern Earth spectrum (at quadrature) was sourced from the validated 3D Earth model described in [55]. HzO

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Figure 3. The same as Fig. 1 but using the model of a hazy Archean Earth. The model assumes a N 2-dominated atmosphere with 5% CO2, 1.5% CH4, and a photochemically self-consistent organic haze [52,53]. (a) Proxima b at quadrature. (b) The Archean Earth as a habitable zone planet around Alpha Centauri B.

Figures 3a and 3b show the reflectance spectra of a hazy Archean Earth [53] as Proxima b and a WHZ Alpha Centauri B planet, respectively, as observed by ELF for 100 hours. In both cases, biogenic CH 4 and water vapor are detectable. CH4 abundances could be tightly constrained for the Proxima b case due to the high SNR in the NIR. Organic haze detection should also be possible for the Alpha Centauri B case. Detecting and constraining CO 2 from the 1.57 µm CO2 absorption band would be challenging for both the Proxima b and Alpha Centauri B cases without high spectral resolution observations to see through our atmosphere. This CO2–CH4 disequilibrium biosignature [38] might be seen on

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Proxima b [54] with other instruments, such as the James Webb Space Telescope. Other examples of potential atmospheric biosiognatures have also been modelled for Proxima b (see [52]). In these calculations, although the sky background is included as a noise source, we have assumed that telluric lines have been completely subtracted. We also assumed that observations taken across multiple nights can be co-added to produce a 100-hour integrated spectrum. Finally, we have assumed a filled (monolithic) aperture with a 20 m diameter for the purposes of these noise calculations. ELF will have a larger (unfilled) diameter with a comparable collecting area. Also, ELF is by design less sensitive to many optical wavefront phase noise sources, but this has not been modeled here. Thus our speckle noise estimates are probably conservative in these calculations. An Atacama Desert night sky background is assumed for these simulations. In summary, oxygen biosignatures on modern-Earth twins around the nearest stars should be readily detectable with ELF. Similarly, biogenic CH4 on Archean Earth-like planets would also be detectable. For both ancient and modern Earth-like exoplanets, water vapor is detectable, which would help assess overall planetary habitability and biosignature gas interpretation [46]. Detecting disequilibrium biosignature combinations will be more challenging, but the UV spectrum from M-dwarfs such as Proxima b means that CH4 could accumulate to high enough levels to enable simultaneous O2 and CH4 detection with ELF. 2.2 Biological aerosols On a planet with a significant biosphere, particles of biological origin (bioaerosols) will continuously be created injected into the atmosphere. For example, on Earth, bioaerosols that include microorganisms, plant fragments, and pollen, represent up to 25% of the total amount of aerosols in the atmosphere [56]. Organic haze in atmospheres of planets with life can also contain biological compounds. However, the density of life on anoxic planets may be significantly smaller as compared to planets that run on oxygenic photosynthesis, so detecting such life will require more sensitive methods. Here we focus on two prominent bioaerosol life signatures that might be detected in exoplanetary atmospheres: photosynthetic biopigments and homochiral molecules. Photosynthetic biopigments The origin of oxygenic, water-splitting photosynthesis on early Earth (probably from ~3.0 Ga [57]) gave access to an abundant electron source for CO2 fixation. This allowed the biosphere to achieve orders of magnitude higher O2 [xxx?] productivity [58] and support the diversity of complex life as we know it today. The evolution of oxygenic photosynthesis led to a dramatic increase of biomass, a global distribution of the biosphere, and at the same time resulted in abundant and global distribution of molecular biosignatures. Both high atmospheric oxygen concentration [34] and spectral signatures of biological pigments [59,60] arising from photosynthesis have been proposed as biosignatures on exoplanets. A polarimetric study of photosynthetic pigments in plants [61] and bacteria [62] revealed strong (up to 80%), broad-band linear polarization signatures of photosynthetic pigments. This property significantly increases the sensitivity for detection of life employing photosynthesis. Due to the accessibility and amount of energy provided by the stellar radiation, along with the development of oxidantdetoxifying enzymes that can split water molecules [63,64] 3 billion years ago, it seems natural for life to evolve the photosynthetic ability of utilizing stellar light energy also on other planets. Models of unpolarized and polarized flux spectra of Earth-like planets having different surface coverages by photosynthetic organisms, deserted land, and ocean, as well as clouds, indicate that photosynthetic life would be detectable on an unresolved exoplanetary surface if organisms cover a significant part of the planet visible surface (>50%) [61]. However, if the exoplanet surface is resolved by indirect imaging, or inversions (see Section 3), the 50% coverage is only needed for a resolution element on the planet, which can be 1-2% of the total planet surface area. In Fig. 4 unpolarized flux and polarization degree model spectra are shown for a planet with an Earth-like atmosphere and three biopigment types covering the entire visible planetary surface. This demonstrates that linearly polarized signatures of photosynthetic pigments increase the detection sensitivity by more than one order of magnitude. Also, to identify various biopigments (as well as various minerals) measurements in four broad passbands (blue, green, red, and near infrared) are sufficient. ELF can provide high SNR measurements in such broad bands with exposure times of a few hours. Homochiral molecules Some groups of organic monomers have the same molecular formula but distinct three-dimensional structures, leading to two mirror forms of a chiral molecule, i.e., enantiomers, molecules which cannot be superimposed upon each other. In organisms, one enantiomer is prioritized: on the Earth, almost only left-handed (levo, L) amino acids are incorporated

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into proteins, and almost only right-handed (dextro, D) sugars are found in carbohydrates. This exclusive use of one enantiomer is called homo-chirality. On Earth, all metabolic pathways and information-carrying systems are homochiral, which ensures the right tertiary conformation that is crucial for molecule functionality. Homochiral molecules are optically active, imposing circular polarization on the reflected light with a very specific spectral signature. For example, in some microorganisms, homochirality in optical broad-band lab measurements revealed a weak circularly polarized signature (~0.01%) near the chlorophyll red edge [65]. In contrast, organic compounds produced by chemical processes outside living organisms are racemic, containing equal proportions of each enantiomer, and thus do not produce circular polarization. Since homochirality is a unifying property of terrestrial life, it can be used to distinguish biologically produced compounds from those that are chemically produced outside living organisms. 1.0

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Figure 4. Model flux and polarization spectra for an Earth-like planet with life samples each containing photosynthetic biopigments (chlorophyll: green line and various carotenoids: yellow and red lines). Black line in the upper panel corresponds to a planet with an oxygen-rich, Earth-like atmosphere fully covered with water clouds. Polarized spectra contain conspicuous signatures of biopigments in the visible when the planet is partially illuminated by the star [61]. They can be distinguished by a broad-band polarimetry. Molecular oxygen and water bands in the NIR are also polarized.

Bioaerosols contain homochiral molecules such as enzymes, cofactors, and biopigments. These homochiral molecules may be detected in planetary atmospheres through circular polarimetry. The homochirality signal from living microorganisms in the lab is about 0.01% [65], but it could be higher for bioaerosols containing homochiral molecules. However, the concentration of bioaerosols with homochiral molecules on exoplanets would need to be high enough to allow remote detection. In particular, their signature should be stronger than background circular polarization produced through multiple scattering by clouds and other (non-chiral) aerosols, which averages about 0.01% of total intensity of the reflected light in a homogeneous cloudy atmosphere [66]. Also, aging of bioaerosols should be taken into account, since racemization will wash out this sign of life [67,68]. 2.3 Technosignature gases The planetary atmosphere may also provide evidence for intelligent life exploiting chemistry for their technological needs. For example, chlorofluorocarbons (CFCs) resulting from anthropogenic activity were suggested as a possible technosignature. These nontoxic (but harmful to ozone), long-lived chemicals are also strong greenhouse gases, and have been proposed for warming up (terraforming) Mars, along with the super-greenhouse gas SF6 [69]. No natural processes capable of creating CFCs are known, so their detection in exoplanetary atmospheres might indicate industrial pollution or terraforming activities of an alien civilization. Other artificial super-greenhouse gases can be conceived for hypothetical terraforming of exoplanets by alien civilizations. Models were computed for CF4 and CCl3F in the mid IR for Earth-size planets transiting white dwarf stars: if the concentration of these pollutants was ~10 times the current terrestrial values, they could be detected by JWST [70]. Detection requires days of JWST observing time, but ELF could detect such signatures with high confidence within about a day of observing time.

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3. STRUCTURAL BIOSIGNATURES AND TECHNOSIGNATURES Structural life signatures are spatially resolved structures on exoplanetary surfaces or in near-planetary space which arise due to clustering of organisms or localized activity. 3.1 Mapping surfaces of exoplanets Exolife detection prospects can be significantly improved by resolving the surfaces of exoplanets. The Earth seen as an unresolved planet shows a very marginal spectral signature of the chlorophyll red edge – a sign of the dominant photosynthetic life on present-day Earth [e.g., 71]. Modelling signals from distant Earth-like planets indicates that lifeforms should cover more than 50% of the visible planet surface to be detected spectrally from an unresolved planet [61]. In spatially resolved Earth’s albedo maps, however, one can clearly see terrestrial global life colonies, such as vegetation (on a subcontinent scale), and our civilization footprints [72] (at a higher resolution). Thus, remote sensing of surface structures on Earth can unambiguously identify their nature [73]. Obtaining resolved albedo maps of exoplanets with and without atmospheric biosignatures will be important for identifying sources of biogenic gases and studying their distribution, ecology, and evolution. A few model solutions for albedo maps and orbital parameters based on light-curves of reflected flux or polarization have been demonstrated for exoplanets [74,75,76,77,78]. Recently, a new multi-color light-curve time-series inversion algorithm ‘ExoPlanet Surface Imaging’ (EPSI) was demonstrated and described applications for planets with and without clouds, with seasonal variations, photosynthetic organism colonies and artificial structures of advanced civilizations [30]. Another detailed study by the NASA Exoplanet Exploration program’s Study Analysis Group 15 recognized and reviewed the importance of time-resolved multi-color photometry and its rich information content for characterizing the properties of rocky planets [79]. Inversions of model light-curve time-series under various assumptions indicate that useful surface feature resolution can be achieved with a broad-band flux signal-to-noise ratio of SNR ≥ 20. For example, with SNR~100 to 200, time domain samples can yield an albedo map where Earth-like (sub-)continental structures can be recognized (Fig. 5). Features of the size of Australia or the Sahara Desert on an Earth-size planet (1.5–2% of the surface area) can be well seen [30]. Phase sampling over a part of the planet orbit leads to a partially recovered map, which is still useful for investigating properties of the recovered features. In Section 4, we discuss targets which can be mapped by the 20m ELF to achieve this surface resolution. 3.2 Photosynthetic surface signatures Detecting structures associated with life in exoplanet albedo maps requires an analysis of the spectral content of the flux reflected from an exoplanet. Light-curves measured in several spectral bands (e.g., blue, green, red, infrared) with high SNR can provide colour maps of exoplanets using the EPSI technique. This is a powerful tool for detecting living organisms with specific spectral or spectropolarimetric signatures, in particular photosynthetic organisms which have dominated the Earth for billion years. Original map

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Figure 5. A test inversion [30] for an exoplanet with Earth's surface samples in “true”-colour (left) and an average spectrum (right) of a small green area at mid-latitudes in the eastern region of the planet image with the red edge chlorophyll signature near 700 nm. “Measurements” from the recovered image in four broad spectral passbands are shown with symbols, while original spectra are shown with lines.

An inversion for an exoplanet with the Earth's surface features, i.e., ice polar caps, oceans, deserts, and forests, is shown in Fig. 5 [30]. By resolving surface features, spectra of various albedo features can be extracted and explored for life signatures. A sample average spectrum (in four broad spectral passbands) of a small green area at mid-latitudes in the eastern region of the planet reproduces well the red edge chlorophyll signature near 700 nm: a deep absorption in the visible wavelengths and high albedo in the near-infrared. Since photosynthetic organisms have a variety of pigments absorbing light in broad bands, it may be possible to distinguish them using such low-resolution spectra if they cluster in relatively large areas on the planetary surface. Measuring polarized light curves helps to identify biopigments, minerals and water reservoirs [61]. In addition, polarized light can detect glint from bodies of water [80] (or other liquids, in general). Thus, spatially resolved images of exoplanets greatly increase our chances to detect exolife, because we can carry out a spectral analysis of areas with high densities of living organisms. 3.3 Artificial mega-structures EPSI will also allow the detection of artificial mega-structures (AMS) constructed by advanced civilizations either on the surface or in the near-space of an exoplanet (circumplanetary space). AMS could be of some regular shape and/or homogeneous albedo, or even in near space as "geostationary” technology (e.g., for communications or for harvesting stellar energy). Low-albedo installations similar to our photovoltaic systems can be employed on the planets’ surface and in space. High-albedo installations can redirect the incident stellar light, e.g., for heat mitigation by reflecting the light back into space. Such AMS may efficiently absorb/reflect only a particular part of the spectrum, similar to photosynthetic organisms having specific spectral edges [e.g., 81]. An example of a low-albedo installation in space (above clouds) is shown in Fig. 6 [30]. It resembles a “Dyson sphere” AMS [2], but on the planetary scale, which civilizations similar to ours could build in order to harvest stellar energy arriving to the planet (Type I civilization in Kardashev’s civilization classification [5]). As shown in Fig. 5, such AMS can be detected in reflected light using the EPSI inversion technique. 1.0 i

0.8

0 0.6 -o o.)

_o

0.4 i,

0.2

Absorbing panels_

00 0 4

0.5 0.6 0.7 Wavelength [um]

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Figure 6. A test inversion [30] for an exoplanet with artificial structures in the near-planetary space above a cloudy planet. The structures are assumed to be photovoltaic-like panels evenly distributed on an orbit around the planet, resembling a “Dyson sphere” megastructure but on a planetary scale (images on the left). The panels harvest solar energy in the visible with high absorbing efficiency (the spectrum on the right). Light curves were generated in four optical and NIR bands from the original image (at the top), and an albedo map in “true”-colour (at the bottom) was reconstructed by light-curve inversions. Technosignatures of the artificial structure are shown as regular albedo patches having identical reflection spectra. “Measurements” from the recovered image in four broad spectral passbands are shown with symbols, while the original spectrum is shown with the solid line.

Such models show that recognizing AMS in inferred images requires large areas of AMS coverage. Also, higher contrast structures with respect to the natural environment are easier to recover. In addition, an analysis of the spectral content of the reflected light is necessary. For the successful interpretation of a map with very low resolution, showing unidentified albedo features, one can learn from past mapping efforts of Solar system planetary bodies [82,83,84,85,86]. They provide numerous examples of different types of linear and patchy albedo features whose low-resolution spectra combined with low resolution shape

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and geographic location may enable their identification. These features may reveal their material and origin. In many examples, a feature of a particular geometry may be formed by a certain process, but the albedo feature may be caused by a surface veneer of a liquid or aeolian material, or a volatile-related process (fog, snow, etc.). In turn, this material may reveal a third surface process because it may have originated from elsewhere on the planet (e.g., aeolian basaltic sand deposits within a crater). Circular, regular features of any size may be caused by natural, impact processes, and linear and curved features are formed by tectonic stresses. Terrestrial anomalous geometry albedo features caused by the civilization but containing natural materials include agricultural or planted forest areas, regions where aeolian sand, snow or vegetation was removed, and artificial lakes. All such surface structures need to be studies in detail for remote sensing with ELF and EPSI. 3.4 Waste heat An Earth-like civilization, and life in general, generates heat from the energy it utilizes. The thermal radiation from this heat can be an unintentional thermodynamic marker for civilizations and any exolife, independent of its biochemistry. Dyson proposed [2] a search for the thermodynamic signature of Kardashev Type II civilizations capable of building a starenclosing biosphere (“Dyson sphere”) at about their planet’s orbit distance. Such a sphere will unavoidably (due to the First Law of thermodynamics) radiate thermal ‘waste’ energy. This excess flux may be distinguishable from that of the exoplanet and can serve as a signature of an exoplanet’s excess energy consumption. Infrared (IR) surveys have not turned up any such Type II or III candidates [87]. Some models have demonstrated that waste heat of an alien civilization utilizing only 25 times more energy than what humanity utilizes now may be detected with a 70m ELF-like telescope in a statistically interesting sample of nearby exoplanets. This assumes planetary radiation is measured simultaneously in broad passbands centered at 5 µm and 10 µm and with SNR ≥ 10 [33]. This level of energy consumption corresponds to only about 1% of the stellar energy impacting the exoplanet, which is still a Kardashev Type I civilization and only slightly more advanced than Earth’s civilization now. Assuming that an alien civilization’s energy use would be spatially clustered on the planet, perhaps similar to our urban areas, such waste heat islands could be mapped. A 20m ELF can reach this sensitivity for Earth-size exoplanets within a few ly from the Sun. With this instrumentation even potentially confounding localized abiotic sources of heat, such as volcanic activity, hot springs, etc., will be distinguishable using measurements at both 5 µm and 10 µm that allow the localized temperature of the sources to be determined. Global atmospheric warming caused by geological (e.g., Venus) or technological greenhouse effects might be distinguished by identifying the atmosphere composition (see Section 2.3). In general, we can expect unambiguous detection to depend on multiple biosignatures and technosignatures but effort needed to interpret such signals is worthwhile since the detection of an alien heat signature will have far-ranging implications. A statistically significant null detection of any Earth-like civilization will have a potentially chilling effect on our understanding of the occurrence and longevity of technological civilizations with potentially broad social implications.

4. EXOLIFE DETECTION STRATEGY 4.1 Targets The Alpha Centauri system stars A, B, and Proxima Centauri are the closest stars to the Sun (4.3 ly). A potential exoplanet discovered in this system is Proxima b [88], possibly a rocky planet in the WHZ of the M5 red dwarf. Proxima b has only been detected indirectly. The expectation value for its mass is 1.6 Earth masses and the likelihood that it is of rocky composition is 90% [89]. This exoplanet is likely the closest example of WHZ rocky planet. However, more planets are statistically expected to exist around Alpha Centauri A, B and Proxima as well as other nearby stars, such as 61 Cyg A and B (see [14]). Achieving SNR ≥ 10 in the reflected light from an exoplanet is challenging even for the nearest exoplanet. In Fig. 7 (left panel) we compute the SNR for Proxima b in the UBVRI passbands for 1h exposure time, assuming a system efficiency of 25%, illumination phase of the planet 0.5 (maximum elongation from the star), average surface albedo in all bands 0.2, and current best (VLT/SPHERE) telescope scattered light and sky background [30]. This computation demonstrates that at least a 20m ELF is needed to achieve SNR ~ 20 to 50 in the visible and near-infrared broad bands with the exposure time of a few hours. When such measurements are obtained during several orbital periods (11 days for Proxima b), this is sufficient for light-curve inversions and planetary surface albedo mapping (Section 3). Hence, the total time to acquire light-curve data useful for inversions is about 50 to 100 hours, depending on the passband. Polarization measurements with the same SNR will require a factor of two longer exposures for each Stokes parameter. This is

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comparable with exposure time (up to 100 h) that is needed for detecting atmospheric biosignatures with higher (R~70) spectral resolution (Section 2), which can also be acquired during several orbits. Similar total exposure times are expected for hypothetical WHZ planets around Alpha Cen A and B, although their expected orbital periods are hundreds of days, and longer light curve measurement series would be possible. A one or two year observing program focused on Apha Cen with an ELF-like telescope would have a good success probability. Other nearby possibly rocky planets in a WHZ are Ross 128 b (11 ly) orbiting an M4 red dwarf [90] and Tau Ceti planets e and f orbiting an orange dwarf G8 V (12 ly) [91]. These and similar nearby exoplanets are excellent candidates for searching for exolife and exoplanet atmosphere characterization and surface imaging. The number of possibly detectable Earth-size planets depending on the size of the telescope is shown in Fig. 7 right panel. A 20m ELF could detect and begin to characterize more than a dozen Earth-size planets within about 20 ly distance from the Sun. When including super-Earths and Neptune-size exoplanets, this number increases up to at least 300. For comparison, a 50m ELF can characterize more than 100 Earth-size planets and at least 2000 super-Earths and Neptune-size planets. This also implies that all Jupiter-size exoplanets within about 50 ly can be characterized by the 20m ELF. 6§C°`c

1.°`0

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1000

100

100

z 10

10

1

1

10

40 20 30 50 Telescope effective diameter [m]

60

70

/ // 10

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Figure 7. Left: Proxima b reflected light-curve SNR in five photometric passbands (UBVRI) depending on the telescope aperture, assuming 0.2 geometrical planetary albedo and 1h exposure time. Right: Number of detectable WHZ Earth-size planets in the BVR bands (SNR ≥ 5) around AFGKM main sequence stars brighter than V = 13 magnitude. Here, the exposure time is 4h, the planet albedo is 0.2 in all bands, and the illumination phase is 0.5. [30].

4.2 Exolife Finding Strategy The above discussion of biosignatures and technosignatures suggests a strategy for detecting exolife on exoplanets with the ELF telescope as follows: 

acquire flux and polarization light-curve data for albedo map inversions in four broad passbands (R~5) in the visible and near infrared, at 0.4–0.5 µm, 0.5–0.6 µm, 0.6–0.7 µm and 0.8–0.9 µm,



simultaneously, acquire medium resolution spectra (R~70) in the red and infrared for atmospheric biosignatures at 0.7–0.8 µm and 1–2.5 µm, and possibly up to 5 µm,



simultaneously, acquire flux light-curve data in broad pass bands centered near 5 µm and 10 µm for chemistry, heat signatures of exolife, and exo-civilizations.

This strategy calls for several ELF datasets: broad-band optical (0.4–0.9 µm) imaging polarimetry with four channels, medium-resolution near-infrared spectroscopy (and spectropolarimetry) (0.7–2.5 µm), and mid-infrared imaging in two

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bands (5 µm and 10 µm). These are typical astronomical datasets obtainable effectively simultaneously under one major observational program – to search for primitive and technological life on nearby exoplanets. Such data are also suitable for searching for non-Earth-like life on exoplanets at various distances from the host star, i.e., not limited to WHZ distances. Signatures of exolife on such planets are yet to be understood, but by exploring a larger target sample we may encounter phenomena beyond our current framework of biosignatures and technosignatures. Such discoveries of the unknown will be a driving force for understanding life as a “natural” phenomenon.

5. CONCLUSIONS The ELF telescope will primarily search for unintentional life signatures with high sensitivity and unprecedented level of completeness within a given cosmic volume. It will address the fundamental question “Are we alone?” in a quantitative way: the occurrences of both biological and technological life will be evaluated within a certain cosmic volume in the Solar neighborhood. The biosignatures and technosignatures discussed in this paper constitute ELF’s initial observing strategy, but they will continue expanding, also beyond traditional liquid water-based life. By considering possibilities for non-Earth-like life, a larger target sample will be investigated and unknown phenomena may be encountered. However, only by measuring multiple signatures we will be able to address the probability of exolife and ultimately announce its detection. The challenging task of direct imaging of Earth-like exoplanets can be achieved from the ground with ELF using accessible technologies focused on extremely high contrast optical/infrared detection [16,17]. The ELF system will be significantly more sensitive to Earth-like planets and significantly less expensive than other broad-science ELTs which are currently planned or being built. It will be a workhorse facility for exoplanetary science and the astrobiology community [15,18] and of great interest for our public community [e.g., 92,93].

ACKNOWLEDGEMENTS S.V.B. acknowledges support from the ERC Advanced Grant HotMol (www.hotmol.eu). J.K. was supported by the Humboldt Foundation. D.C.C. acknowledges support from NASA Astrobiology Institute’s Virtual Planetary Laboratory under Cooperative Agreement Number NNA13AA93A. J.K.-T. was supported by NASA Earth and Space Sciences Fellowship NNX15AR63H. F.M.A. is supported by NASA grant NNX14AJ80G.

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