Photochemistry in Terrestrial Exoplanet Atmospheres II: H2S and SO2 Photochemistry in Anoxic Atmospheres
aa r X i v : . [ a s t r o - ph . E P ] F e b Photochemistry in Terrestrial Exoplanet Atmospheres II: H S andSO Photochemistry in Anoxic Atmospheres
Renyu Hu , Sara Seager , , William Bains , Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute ofTechnology, Cambridge, MA 02139 Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 Rufus Scientific, Melbourn, Royston, Herts, United Kingdom [email protected]
ABSTRACT
Sulfur gases are common components in the volcanic and biological emissionon Earth, and are expected to be important input gases for atmospheres onterrestrial exoplanets. We study the atmospheric composition and the spectra ofterrestrial exoplanets with sulfur compounds (i.e., H S and SO ) emitted fromtheir surfaces. We use a comprehensive one-dimensional photochemistry modeland radiative transfer model to investigate the sulfur chemistry in atmospheresranging from reducing to oxidizing. The most important finding is that bothH S and SO are chemically short-lived in virtually all types of atmospheres onterrestrial exoplanets, based on models of H , N , and CO atmospheres. Thisimplies that direct detection of surface sulfur emission is unlikely, as their surfaceemission rates need to be extremely high ( > -dominated atmospheres for a widerange of particle diameters (0.1 - 1 µ m), which is assumed as a free parameterin our simulations. In oxidized atmospheres composed of N and CO , opticallythick haze, composed of elemental sulfur aerosols (S ) or sulfuric acid aerosols(H SO ), will form if the surface sulfur emission is 2 orders of magnitude morethan the volcanic sulfur emission of Earth. Although direct detection of H S andSO by their spectral features is unlikely, their emission might be inferred byobserving aerosol-related features in reflected light with future generation spacetelescopes. 2 – Subject headings: radiative transfer — atmospheric effects — planetary systems— techniques: spectroscopic — astrobiology
1. Introduction
A large number of super Earths have been detected by radial velocity surveys andtransit surveys. Super Earths are exoplanets with masses no more than 10 times the massof Earth. The atmospheres of super Earths are important because characterization of superEarth atmospheres is a substantial step towards eventually characterizing truly Earth-likeexoplanets. Attempts to observe super Earth atmospheres are growing (e.g., Batahla et al.2011 for Kepler 10 b; Demory et al. 2012 and Ehrenreich et al. 2012 for 55 Cnc e), andone super Earth/mini Neptune GJ 1214b is being observed in as much detail as possible(e.g., Bean et al. 2010; Croll et al. 2011; D´esert et al. 2011; Berta et al. 2012; De Mooijet al. 2012). In particular, the transmission spectra of GJ 1214b is nearly flat from 0.6to 5 µ m, which has ruled out a planet with an H -dominated cloud-free atmosphere. Theobservational push to super Earth characterization has the potential to provide a handfulof super Earth atmospheres to study in the coming years. In the more distant future, thecommunity still holds hope that a direct-imaging space-based mission under the TerrestrialPlanet Finder concept will come to existence, with planets and their atmospheres observedin reflected light.The concentration of trace gases in super Earth atmospheres is controlled by the com-ponent gas emission from the surface and subsequent sinks in the atmosphere (chemicalreactions initiated by UV photolysis). Even with a trace amount, some gases may leavesignificant footprints in the planet’s spectra, for instance H O, CO , and O in Earth’s at-mosphere. In contrast to giant exoplanets where the atmospheric composition is mainlycontrolled by the elemental abundance, the steady-state composition of super Earth atmo-spheres is mainly controlled by photochemical processes. We have developed a comprehensivephotochemistry model that computes chemical compositions of any terrestrial exoplanet at-mospheres, ranging from reducing to oxidizing, with all key non-equilibrium processes takeninto consideration, including photolysis, chemical kinetics, vertical diffusion of molecules,atmospheric escape, dry and wet deposition, and condensation and sedimentation of con-cern condensable species (Hu et al. 2012; referred to as Paper I hereafter). The effectsof surface gas emission on different super Earth atmospheres can be investigated with thephotochemistry model.Sulfur gases emitted from the surface and their photochemical products significantlyshape the spectra of rocky bodies in the Solar System. The most striking feature of Venus’ 3 –atmosphere is a high planetary albedo due to thick H SO clouds. Photochemistry modelsof the Venusian atmosphere have been developed and the formation of H SO in the dryCO -dominated atmosphere have been simulated (e.g., Yung & DeMore 1982; Zhang etal. 2012; Krasnopolsky 2012). The latest photochemistry model of Venus’ atmosphere hasassumed a constant mixing ratio of SO (i.e., ∼
10 ppm) at the altitude of 47 km (implyingsignificant source of SO from below) and predicted the formation of H SO at the altitudesaround 66 km (Zhang et al. 2012; Krasnopolsky 2012). Io, the innermost moon of Jupiter, isbelieved to have very intensive and variable volcanic activity with SO emission, and the Io’satmosphere is dominated by photolysis of SO and subsequent formation and polymerizationof elemental sulfur (e.g., Moses et al. 2002). On early Earth, the sulfur chemistry may havebeen very different from now, featuring the formation of sulfur aerosols as well as sulfateaerosols as the atmosphere was anoxic (Kasting et al. 1989; Pavlov & Kasting 2002; Kasting& Catling 2003; Zahnle et al. 2006). The formation of insoluble sulfur aerosols is believed tobe critical for the record of mass independent fractionation that timed the rise of oxygen inEarth’s atmosphere (e.g., Farquhar et al, 2000; Pavlov & Kasting 2002; Zahnle et al. 2006).In addition, organosulfur compounds, such as dimethyl sulfide (DMS) and methanethiol(CH SH), have also been suggested to be biosignatures of the early Earth (Pilcher 2003).The greenhouse effect of SO has been suggested to have contributed to the warming of earlyMars (e.g., Halevy et al. 2007), a proposition that has been challenged by photochemistrystudies that predict sulfur and sulfate aerosol formation as a result of SO emission on earlyMars and the anti-greenhouse effect of these aerosols (Tian et al. 2010).Terrestrial exoplanets could have a wide range of sulfur gas emission. Sulfur gases arecommon volcanic gases on Earth, and in some scenarios may be more prevalent on exoplanets.Sulfur is a major building block for rocky planets and the abundance of sulfur is about onesixteenth that of carbon in the Solar System (Lodders 2003). On present-day Earth, sulfurcompounds, mainly in the form of H S and SO , are dominant volcanic gases in additionalto hydrogen, oxygen and carbon emissions. In the Earth’s magma, the dissolved sulfur has aweight percentage ranging from 10 − to 10 − (e.g., Wallace & Edmonds 2011) and degassingof sulfur compounds as the magma decompresses when rising to the surface provides a globalvolcanic sulfur flux of about 3 × S cm − s − (Seinfeld & Pandis 2006). 90% of currentEarth’s sulfur emission is in the form of SO , whereas the H S/SO ratios for individualvolcanoes vary widely between 0.01 and 1 (see Holland 2002 and references therein). Anintriguing fact is that the amounts of sulfur compounds in Earth’s atmosphere is extremelylow despite the substantial emission rates, due to very short chemical timescales. On Earth, Mixing ratio is defined as the ratio of the amount of a gas in a given volume to the total amount of allgaseous constituents in that volume. S and SO in the troposphere is only 2 days, which makes the mixing ratioof these gases in the atmosphere very small (Seinfeld & Pandis 2006). In Earth’s troposphere,the main sink of H S is the reaction with the hydroxyl radical OH (Lelieveld et al. 1997),and the main sink of SO is the removal by dry and wet deposition.Another reason that H S and SO photochemistry is interesting is that Earth-basedbiological processes involve sulfur compounds. There are multiple ways that life can produceH S, including the reduction of sulfate (e.g., Watts 2000) and the disproportionation of sulfurcompounds of intermediate oxidation states (e.g., Finster 2008). In general, 10 to 50 percentof the global H S emission on Earth is as a metabolic byproduct, whereas over 70% percentof natural SO emission is volcanic, although on modern Earth 90% of atmospheric SO isanthropogenic (Seinfeld & Pandis 2006). Understanding the atmospheric response to H Sand SO emission will allow us to examine whether or not H S is a potential biosignaturegas on a terrestrial exoplanet.Previous investigation of sulfur photochemistry in the context of super Earth character-ization has been very limited. Sulfur compounds are generally not considered in most modelsof terrestrial exoplanet atmospheres. Des Marais et al. (2002) describe the spectral featuresof certain molecules such as H O, N O, O and CH in terrestrial exoplanets. Miller-Ricci etal. (2009) present spectra of super Earths under equilibrium chemistry with photochemistryestimates and without considering sulfur compounds. Zahnle et al. (2009) investigate sulfurphotochemistry in hot Jupiters and suggest that HS and S can be generated photochem-ically from H S. Kaltenegger & Sasselov (2010) study the effect of H S and SO on theterrestrial planetary spectra and suggest that 1-10 ppm SO has potentially detectable spec-tral features that could indicate active volcanism. Kaltenegger & Sasselov (2010), however,do not link the surface emission of sulfur gases to the mixing ratio of sulfur species via photo-chemistry involving sulfur compounds, or consider formation of aerosols. Domagal-Goldmanet al. (2011) study the chemistry of organic sulfur compounds that are strongly linked tobiology (e.g., CH SH) in anoxic N atmospheres and suggest that the most detectable featureof organic sulfur gases is their indirect photochemical product, ethane. Moreover, the fateof surface emission of sulfur compounds, e.g., H S and SO , is yet to be explored for H -dominated atmospheres. H -dominated atmospheres, like N or CO atmospheres with watervapor, could also maintain a habitable temperature at the surface through collision-inducedabsorption (Pierrehumbert & Gaidos 2011; Wordsworth 2012).In all, it is still largely unknown whether or not H S and SO spectral features can beobserved in the future in the atmosphere of an exoplanet and whether or not the emissionrate of sulfur compounds on a terrestrial exoplanet can be inferred. In this paper we investi-gate the atmospheric chemistry resulting from H S and SO surface emission in atmospheres 5 –having very different oxidation states ranging from reducing to oxidizing. Here, “emission”means the mass flux from the planetary surface to the atmosphere that may include volcanicsources and biogenic sources. We focus on terrestrial exoplanet atmospheres that includesuper Earths, since those planets hold the most interest on the path to finding and charac-terizing planets that might harbor life. In § § S and SO in both reducingand oxidizing atmospheres. In § S, SO , and photochemicalaerosols. In § S can be a biosignature gas on a planet withatmospheric conditions different from Earth’s. We present our conclusions in §
2. Model2.1. Photochemistry Model
We have developed a comprehensive photochemistry model to investigate atmospheres ofterrestrial exoplanets, and validated the model by simulating the atmospheric compositionsof current Earth and Mars (see Paper I). We now describe briefly the main features of thephotochemistry model and the specifics for this work.The purpose of the photochemistry model is to compute the steady-state chemical com-position of an exoplanetary atmosphere. The system is described by a set of time-dependentcontinuity equations, one equation for species at each altitude. Each equation describes:chemical production; chemical loss; eddy diffusion and molecular diffusion (contributingto production or loss); sedimentation (for aerosols only); emission and dry deposition atthe lower boundary; and diffusion-limited atmospheric escape for light species at the upperboundary. Starting from an arbitrary initial state, the system is numerically evolved to thesteady state in which the number densities no longer change. Because the removal timescalesof different species are very different, the implicit inverse Euler method is employed for thenumerical time stepping. The generic model computes chemical and photochemical reactionsamong 111 molecules and aerosols made of O, H, N, C, S elements, and formation of sulfur(S ) and sulfate (H SO ) aerosols. The numerical code is designed to have the capacity oftreating both reducing and oxidizing atmospheres. For the chemical and photochemical reac-tions, we use the reaction rates data from both the NIST database (http://kinetics.nist.gov)and the JPL publication (Sander et al. 2011). We have also adopted relevant reaction ratesfrom Kasting (1990), Yung & DeMore (1999), and Moses et al. (2002). Sulfur polymerizationreaction rates still lack consistent experimental measurements, and we adopt the reaction 6 –rates proposed by Kasting (1990) and those proposed by Moses et al. (2002). Both setsof sulfur polymerization reaction rates are speculative and they are widely discrepant, theeffect of which will be discussed later in section 3.1. Ultraviolet and visible radiation inthe atmosphere is computed by the δ -Eddington 2-stream method, with molecular absorp-tion, Rayleigh scattering and aerosol Mie scattering contributing to the opacity. For thestellar input spectrum we used the Air Mass Zero (AM0) reference spectrum produced bythe American Society for Testing and Materials for Sun-like stars, and used the simulatednon-active M star spectrum from Allard et al. (1997) for quiet M stars.In this paper we use the photochemistry model to study the sulfur chemistry in atmo-spheres ranging from reducing to oxidizing on terrestrial exoplanets. We use H dominatedatmospheres as the representative cases for reducing atmospheres, and we use N , and CO dominated atmospheres as the representative cases for oxidized atmospheres that could beboth reducing and oxidizing. With the photochemistry model, we simulated the chemicalcomposition of H , N , and CO -dominated atmospheres, with sulfur compounds emittedfrom the surface at various rates. Parameters of the atmospheric models are tabulated inTable 1 and details of water, carbon, and oxygen chemistry have been described in PaperI. We will focus on the sulfur chemistry and photochemistry in terrestrial exoplanet atmo-spheres in this paper.We consider in our atmospheric chemistry models the O, H, and S bearing species anda subset of C bearing species. The gaseous molecules considered in this paper are H, H , O,O( D), O , O , OH, HO , H O, H O , CO , CO, CH O, CHO, C, CH, CH , CH , CH ,CH , CH O, CH O, CHO , CH O , CH O , CH O , C , C H, C H , C H , C H , C H ,C H , C HO, C H O, C H O, C H O, C H O, S, S , S , S , SO, SO , SO , SO , SO ,H S, HS, HSO, HSO , HSO , H SO , and S , and the aerosols considered are S aerosolsand H SO aerosols. This set of species is comprised of common H, O, and C bearing speciesand photochemical products of H S and SO emission. We assume a constant H O relativehumidity at the surface of 60% to mimic the supply of water vapor from a liquid water ocean.To reduce the stiffness of the system and improve the numerical stability, “fast” species withrelatively short chemical loss timescales are computed directly from the photochemistryequilibrium. We consider in this work O( D), CH , C H, SO , and SO as fast varyingspecies. As such, the photochemistry model rigorously finds steady-state composition ofthe atmosphere starting with initial compositions without any sulfur compounds. Once themodel converged to the steady state, we checked explicitly the mass conservation of O, H,C, N, S atoms and verified the choice of fast species to have been appropriate. We have http://rredc.nrel.gov/solar/spectra/am0/ ,N , CO dominated atmospheres on an Earth-sized atmosphere in the habitable zone ofa Sun-like star. The planet has Earth-like volcanic emissions for hydrogen, oxygen, andcarbon species, and various H S and SO emissions. Note that we do not consider anybiotic contribution to the dry deposition velocities, and the volcanic carbon emission is notproportionally increased with sulfur emission. Parameters Reducing Oxidized OxidizedMain component 90%H , 10%N N , 10%N Mean molecular mass 4.6 28 42.4
Planetary parameters
Stellar type G2V G2V G2VSemi-major axis 1.6 AU 1.0 AU 1.3 AUMass M ⊕ M ⊕ M ⊕ Radis R ⊕ R ⊕ R ⊕ Temperature profile
Surface temperature 288 K 288 K 288 KSurface pressure 10 Pa 10 Pa 10 PaTropopause altitude 120 km 13.4 km 8.7 kmTemperature above tropopause 160 K 200 K 175 KMaximum altitude 440 km 86 km 51 km
Eddy diffusion coefficient
In the convective layer 6 . × cm s − . × cm s − . × cm s − Minimum 2 . × cm s − . × cm s − . × cm s − Altitude for the minimum 107 km 17.0 km 11.6 kmNear the top of atmosphere 7 . × cm s − . × cm s − . × cm s − Water and rainout
Liquid water ocean Yes Yes YesWater vapor boundary condition f (H O) = 0 . f (H O) = 0 . f (H O) = 0 . a Earth-like Earth-like Earth-like
Gas emission b CO × cm − s − × cm − s − N/AH N/A 3 × cm − s − × cm − s − CH × cm − s − × cm − × cm − SO Vary Vary VaryH S Vary Vary Vary
Dry deposition velocity c H H . × − (Assumed)CO 1 . × − cm s − (Kharecha et al. 2005)CH O 0.1 cm s − (Wagner et al. 2002)CO . × − cm s − (Archer 2010)O − (Hauglustaine et al. 1994)H O − (Hauglustaine et al. 1994)H S 0.015 cm s − (Sehmel 1980)SO − (Sehmel 1980)S (A) 0.2 cm s − (Sehmel 1980)H SO (A) 0.2 cm s − (Sehmel 1980) a Rainout rates for H , CO, CH , C H , and O are generally assumed to be zero to simulate an ocean surfacesaturated with these gases on an abiotic exoplanet. b The volcanic gas emission rates from the planetary surface are assigned for each model scenario. H Oemission is not explicitly considered because the surface has a large water reservoir, i.e., an ocean. c We here list the dry deposition velocities (with references) for emitted gases and their major photochemicalbyproducts, and dry deposition velocities that are important for the mass and redox balance of the atmo-sphere. Dry deposition velocities are assumed to be identical for the three scenarios. C H dry depositionvelocity is assumed to take into account the loss of carbon due to organic haze formation and deposition.The CO dry deposition velocity is assumed such that the steady-state mixing ratio of CO in H and N atmospheres is in the order of 100 ppm. − for convergence, and typically ourmodels balance mass flux to 10 − . We have also explicitly checked the redox (i.e., hydrogenbudget) balance (see our definition of the redox number, flux, and balance in Paper I), andrequired the models to balance redox flux to 10 − .One of the most significant controlling factors that determine the steady-state compo-sition of atmospheres on terrestrial exoplanets is the dry deposition velocities of emittedgases and their major photochemical byproducts. Of particular importance in this paper isthe dry deposition velocities of H S and SO , which could vary by orders of magnitude (seeTable 1 for the fiducial values of key dry deposition velocities). The dry deposition velocitiesdepend on the properties of the lower atmosphere and the surface. For example, in a modelof the early cold Martian atmosphere, the deposition velocities are assumed to be reducedby an artificial factors of up to 1000 compared with those in warm current Earth (e.g., Tianet al. 2010), in order to account for less efficient deposition at a lower temperature. For an-other example, in modeling the early Martian atmosphere, because the putative Mars oceanis believed to be saturated with dissolved SO and other sulfur species, SO deposition isassumed to be balanced by the an equivalent return flux from the ocean (e.g. Halevy etal. 2007; Tian et al. 2010) and then V DEP of SO is assumed to be zero. In this paper,we explore the effect of varying the dry deposition velocities of H S and SO , which can bescaled down by a factor of 100 if the surface is saturated with sulfide or sulfite.Another important factor that strongly influences the atmospheric sulfur chemistry isthe formation and sedimentation of aerosols. Photochemically produced H SO and S maycondense to form aerosols if their concentrations exceed their vapor saturation concentra-tions. The parameterization of condensation and sedimentation of aerosols in our photo-chemistry model is described in detail in Paper I. Saturation vapor pressure of H SO istaken as recommended by Seinfeld & Pandis (2006) for atmospheric modeling, with a valid-ity temperature range of 150 - 360 K. S is the stable form of elemental sulfur because theS molecule has a crown-shape ring structure that puts least strain on the S-S bond amongsulfur allotropes and the crown structure allows for considerable cross-ring interaction be-tween nonbonded atoms (Meyer 1976). The saturation pressure of S is then taken as thetotal sulfur saturation pressure against liquid sulfur at T >
392 K and solid (monoclinic)sulfur at
T <
392 K tabulated by Lyons (2008). We consider the average aerosol particlediameter, a key parameter that determines the aerosols’ dynamical and optical properties, tobe a free parameter. On Earth, the ambient aerosol size distribution is dominated by severalmodes corresponding to different sources. The “condensation submode”, formed from vaporcondensation and coagulation, has an average diameter of ∼ . µ m (Seinfeld & Pandis,2006). On Venus, the Mode 1 particles with an an average diameter of ∼ . µ m dominatethe upper cloud (e.g., Carlson et al. 1993). On Titan, the photochemical aerosols in the 9 –stratosphere have mean diameters in the range of 0.1 - 1 µ m (Rages et al. 1983). We treatthe particle diameter a free parameter, and explore the effects of varying the particle diam-eter from 0.1 to 10 µ m. The dry deposition velocity of aerosols is assumed to be 0.2 cm s − ,a sensible deposition velocity of particles having diameters between 0.1 and 1 µ m (Sehmel1980; Seinfeld & Pandis 2006).Before leaving this section we provide the physical rationale of specifying temperature-pressure profiles and eddy diffusion coefficients for our atmospheric photochemistry models.We modeled the atmospheric composition from the 1-bar pressure level up to the alti-tudes of about 10 scale heights. We chose appropriate vertical resolution for each scenario sothat there are 4 layers per scale height. We adopted a temperature profile for our atmospheremodels, without considering feedback on temperature of the atmospheric composition. Thesurface temperature is assumed to be 288 K. The semi-major axis of a terrestrial exoplanetaround a Sun-like star implied by this surface temperature is 1.6 AU, 1.0 AU, and 1.3 AU, forH , N , and CO dominated atmosphere, estimated based on a similar procedure as Kastinget al. (1993). The temperature profiles are then assume to follow appropriate dry adiabaticlapse rate (i.e., the convective layer) until 160 K (H atmosphere), 200 K (N atmosphere),and 175K (CO atmosphere) and to be constant above (i.e., the radiative layer). The adoptedtemperature profiles are consistent with significant greenhouse effects in the convective layerand no additional heating above the convective layer for habitable exoplanets. We did notconsider the climate feedback of SO or sulfur aerosols in the atmosphere, which could beimportant to determine the surface temperature (e.g. Halevy et al. 2007; Tian et al. 2010).While not ideal, these temperature profiles yield the same results discussed in the below astemperature profiles varied by several tens of K. The precise temperature-pressure struc-ture of the atmosphere is less important than photochemistry for the investigation of sulfurchemistry because the most important photolysis and chemical reactions are not signifi-cantly affected by minor deviations in the temperature profile. We found that variation oftemperature profiles by a few tens of K has minor impact on the atmospheric composition.Vertical transport of gases in the atmosphere is parameterized by eddy diffusion, andthe coefficients are assumed to be those of Earth’s atmosphere scaled by the atmosphericscale height to account for H , N , and CO being the dominant species. The current Earth’seddy diffusion coefficient profile has been derived from mixing ratio profiles of several long-lived gases (Massie & Hunten 1981; also shown in Figure 1 of Paper I). We use the empiricaleddy diffusion coefficient profile for current Earth as a template, and scale the coefficientinversely with the mean molecular mass for H , N , and CO dominated atmospheres. Thejustification for such scaling is that the eddy diffusion coefficient is proportional to themixing length which is in turn a portion of the atmospheric scale height (e.g., Smith 1998). 10 –The pressure surface to pressure surface projection also ensures that the eddy diffusioncoefficient profile features a eddy diffusion minimum near the tropopause for atmosphereswith different mean molecular masses. Our approach to parameterize vertical transport isof course an approximation. An accurate representation of vertical transport would likelyinvolve circulation on the global scale (e.g., Holton 1986). We explore the effect of eddydiffusion coefficients ranging one or two orders of magnitude from the nominal value in asensitivity study (see section 4.1). We compute synthetic spectra of the modeled exoplanet’s atmospheric transmission,reflection and thermal emission with a line-by-line radiative transfer code (Seager & Sasselov,2000; Seager et al. 2000; Miller-Ricci et al. 2009; Madhusudhan & Seager 2009). Opacitiesare based on molecular absorption with cross sections computed based from the HITRAN2008 database (Rothman et al. 2009), molecular collision-induced absorption when necessary(e.g., Borysow 2002), Rayleigh scattering, and aerosol extinction are computed based on theMie theory (e.g., Van de Hulst 1981). The transmission is computed for each wavelengthby integrating the optical depth along the limb path, as outlined in Seager & Sasselov(2000). The reflected stellar light and the planetary thermal emission are computed by the δ -Eddington 2-stream method (Toon et al. 1989). We used the refractive index of S aerosolsfrom Tian et al. (2010) for the UV and visible wavelengths and from Sasson et al. (1985)for infrared (IR) wavelengths. We used the refractive index of H SO aerosols (assumed tobe the same as 75% sulfuric acid solution) from Palmer & William (1975) for UV to IRwavelengths, and Jones (1976) for far IR wavelengths.The particle size distribution of aerosols controls their optical properties. We adopt thelognormal distribution as dNdD = N t √ πD ln σ exp (cid:20) − (ln D − ln D ) σ (cid:21) , (1)where dN is the number of particles per volume in the diameter bin dD , N t is the totalnumber density of particles, D is the median diameter of the particles, and σ is the particlesize dispersion (defined as the ratio of the diameter below which 84.1% of the particles lieto the median diameter). The lognormal distribution is a reasonable assumption because itprovides a good fit to the particle size distribution measured in Earth’s atmosphere (e.g.,Seinfeld & Pandis 2006), and a sensible particle size dispersion parameter for photochemicallyproduced aerosols is in the range of 1 . ∼ . D S and the volume mean diameter D V , respectively. The mean diameters arerelated to the median diameter as D S = D exp(ln σ ) , (2) D V = D exp (cid:18)
32 ln σ (cid:19) . (3)We use the surface area mean diameter D S (referred to as “mean diameter” in the following)as the free parameter for specifying a particle size distribution, as it is relevant to theradiative properties of the particle population. The volume mean diameter D V is useful in theconversion from mass concentration of the condensed phase (computed in the photochemistrymodel) to the number of aerosol particles for radiative transfer computation. The extinctioncross sections of H SO and S molecules in aerosols for various mean particular diametersare shown in Figure 1.Elemental sulfur aerosols and sulfuric acid aerosols have different optical properties atthe visible and infrared (IR) wavelengths. In the visible, S aerosols have a larger crosssection than H SO aerosols (Figure 1). For wavelengths less than 400 nm S aerosolsare both reflective and absorptive. In the infrared, the cross section of S aerosols dropssignificantly with increasing wavelength unless the mean diameter is in the order of 10 µ m.In contrast, H SO aerosols have an enhancement of absorption at the MIR wavelengths(5-10 µ m) for all particle sizes (Figure 1).
3. Sulfur Chemistry in Reducing and Oxidizing Atmospheres
We now briefly describe the most important processes of sulfur chemistry that occurin reducing and oxidizing atmospheres on rocky exoplanets. The primary sulfur emissionfrom the planetary surface would be SO and H S; they are either deposited back to thesurface via dry or wet deposition, or converted into other forms of sulfur compounds in theatmosphere by photochemical reactions. One of the main purposes of this paper is to studythe fate of sulfur gases emitted from the surface and their possible photochemical byproductsin the atmosphere.The fate of sulfur gases emitted from the surface is mainly controlled by the redoxpower of the atmosphere - the ability to reduce or oxidize a gas in the atmosphere. Theredox power, in turn, is controlled by both the main component in the atmosphere (e.g.,H , N , and CO ) and the surface emission and deposition of trace gases (i.e., H , CH , andH S), as shown in Table 2. In the extreme cases of the atmospheric redox state, i.e., the H -dominated atmospheres and the O -rich atmospheres, the atmospheric redox power is surely 12 –reducing or oxidizing, regardless of the nature of surface emission or deposition. However,for an intermediate redox state, the atmosphere would be composed of redox-neutral specieslike N and CO , and the redox power of the atmosphere can be mainly controlled by theemission and the deposition fluxes of trace gases from the surface. The higher the emissionof reducing gases is, the more reducing the atmosphere becomes.We already know that sulfur gas emissions are effectively oxidized into sulfate, themost oxidized form of sulfur, in the oxic atmospheres such as the Earth’s (e.g., Seinfeld& Pandis 2006). In anoxic atmospheres, which include the reduced atmospheres and theoxidized atmospheres, previous studies have shown that both elemental sulfur and sulfatecould be formed (Kasting 1990; Pavlov & Kasting 2002; Zahnle et al. 2006; Hu et al. 2012).For this paper, we use H dominated atmospheres as the representative cases for reducingatmospheres, and we use N , and CO dominated atmospheres as the representative casesfor oxidized atmospheres that could be both reducing and oxidizing. We now describe thekey sulfur chemistry processes in these atmospheres as the follows.Table 2: Redox power of atmospheres on rocky exoplanets. Type Main Component Redox Power Main Reactive Species Solar-System analogs NoteReduced H , CO Reducing H NoneOxidized N , CO Weakly reducing H, OH, O None The redox power is mainly controlled by theWeakly oxidizing H, OH, O Mars, Venus surface emission of trace gases (H , CH , H S).Oxic O Highly oxidizing OH, O Earth -Dominated Atmospheres Both H S and SO emitted from the surface are efficiently converted into elementalsulfur in reducing H atmospheres. The major chemical pathways for sulfur compoundsin the H -dominated atmosphere and the results of photochemistry model simulations areshown in Figure 2. Atomic hydrogen produced from photodissociation of water vapor andH S itself is the key reactive species that converts H S and SO into elemental sulfur. Theprimary chemical loss for H S in the atmosphere is viaH S + h ν −→ HS + H , (C 1)and H S + H −→ HS + H . (C 2)The HS produced can then react with H again or with itself to produce elemental sulfur. HScan also react with S to produce S . The primary chemical loss for SO in the atmosphere is 13 –photodissociation that produces SO. SO can be either photodissociated to elemental sulfur,or be further reduced to HS via HSO by H or CHO and then converted to elemental sulfur(see Figure 2).The S and S molecules produced in the atmosphere will polymerize to form S , and S will condense to form aerosols if it is saturated in the atmosphere. Due to its ring structure,S is stable against photodissociation. S is a strong UV absorber (Kasting 1990). Therefore,S aerosols, if produced in the atmosphere, can effectively shield UV photons so that H Sand SO may accumulate beneath the aerosol layer (see the case for a sulfur emission rate300 times higher than Earth’s current volcanic sulfur emission rate shown in Figure 2).The primary source of atomic hydrogen is the photolysis of H O, which occurs abovethe altitudes of ∼ Pa pressure level. The atomic hydrogen can be then transported byeddy diffusion to the pressure level of ∼ Pa to facilitate the removal of H S and SO andthe production of elemental sulfur. Additional numerical simulations show that an increaseof the eddy diffusion coefficient by one order of magnitude can increase the yield of elementalsulfur by about 20%, because the transport of atomic hydrogen becomes more efficient. Asecondary source of atomic hydrogen is photolysis of H S (reaction C 1). This secondarysource for atomic hydrogen is particularly important when the host star is a quiet M dwarf,because a quiet M dwarf produces few photons that could dissociate water . For planetsaround quiet M dwarfs the photolysis of H S could be the main source of atomic hydrogenin their atmospheres. Additional numerical simulations show that in the habitable zone of aquiet M dwarf having an effective temperature of 3100 K, H S photolysis alone can produceenough atomic hydrogen to drive the formation of elemental sulfur in the atmosphere.We here comment on the uncertainty of photochemistry models regarding the yield ofS . In our model, we have assumed polymerization of elemental sulfur proceeds viaS + S −→ S , (C 3)S + S −→ S , (C 4)S + S −→ S , (C 5)S + S −→ S , (C 6)S + S −→ S . (C 7)We also include photodissociation for S , S , and S . However, the reaction rates of sulfurpolymerization (reactions C 3 - C 7) have not been well established by laboratory studies, Water is principally dissociated by photons in the 150 - 200 nm wavelength range, whereas H S isprincipally dissociated by photons in the 200 - 260 nm wavelength range.
14 –and previous authors have adopted different rate constants for these reactions. In particular,Kasting (1990) and Pavlov & Kasting (2002) have used 3-order-of-magnitude lower rates forreactions (C 3 - C 4) and 1-order-of-magnitude lower rates for reactions (C 5 - C 7), comparedwith Moses et al. (2002). In this work, we have adopted the reaction rates of Moses et al.(2002) for elemental sulfur reactions. Our sensitivity tests show that adopting the reactionrates of Kasting (1990) would result in about 3 to 10 times less S . We have chosen ahigher sulfur polymerization rates for nominal models because: (1) the chemical pathways ofreactions (C 3 - C 7) are probably not complete and there may be other pathways to form S ;(2) S , S , and S may condense as suggested by Lyons (2008) and the polymerization maystill proceed in the condensed phase to S . Experimental studies are encouraged to settlethis important uncertainty. and CO -Dominated Atmospheres H S and SO gases emitted from the surface can be converted into both elementalsulfur (S ) and sulfuric acid (H SO ) in the oxidized (but anoxic) atmospheres such as N and CO dominated atmospheres. The major chemical pathways that lead to formation ofboth elemental sulfur and sulfuric acid, and the results of photochemistry model simulationsare shown in Figure 3. The production of elemental sulfur aerosols involves UV photons andatomic hydrogen, as does in reducing H atmospheres; whereas the production of sulfuricacid requires oxidizing species, notably OH and O . These reactive species, either reducingor oxidizing, are produced from photolysis of water and CO . In particular, the source of OH,responsible for converting SO to sulfuric acid in the atmosphere, is the photodissociation ofH O. As a result, the amount of UV photons that are capable of dissociating water controlsthe yield of H SO . For example in the habitable zone of a quiet M dwarf the yield of H SO is much reduced compared with solar-like stars by at least one order of magnitude.The photochemically produced S and H SO may condense to form aerosols in theatmosphere if saturated. As a result, aerosols in the atmospheres provide a UV shield thatenables the accumulation of H S and SO beneath the layer of aerosols. In particular foran Earth-sized planet in the habitable zone of a Sun-like star, when the surface emissionrate is more than two-orders-of-magnitude higher than the current Earth’s volcanic sulfuremission rate, photochemical aerosols in the atmosphere lead to substantial UV shielding foraccumulation of H S and SO , as shown in Figure 3. We find that only when sulfur emissionis highly elevated with respect to current Earth could H S or SO accumulate to the orderof parts per million mixing ratio in the N and CO atmospheres.The relative yield between elemental sulfur and sulfuric acid is controlled by the redox 15 –power of the atmosphere. In general, more sulfuric acid aerosols and less elemental sulfuraerosols are anticipated in a more oxidizing atmosphere. The redox power of the atmosphere,in turn, is determined by both the main constituent and the reducing gas emission. CO dominated atmospheres are mosre oxidizing than N dominated atmospheres as photodis-sociation of CO leads to atomic oxygen. Therefore the primary sulfur emission is morelikely to be converted to sulfuric acid in CO dominated atmospheres than in N dominatedatmospheres (see Figure 3). Surface emission of reducing gases, including H , CH , CO, andH S, alters the redox budget of the atmosphere and therefore increases the relative yield ofelemental sulfur versus sulfuric acid aerosols (e.g., Zahnle et al 2006). As shown in Figure3, when the sulfur emission rate increases, both N and CO atmospheres become more andmore reducing (because H S is reducing), which results in a dramatic increase of elementalsulfur production in the atmosphere. Furthermore, the H S/SO ratio in the sulfur emissionaffects its contribution to the redox power of the atmosphere and then the relative abun-dances of the two types of aerosols in the atmosphere significantly. As a result of the increasein the H S/SO emission ratio, the amount of S aerosol in the atmosphere increases, and theamount of H SO in the atmosphere decreases dramatically (see Figure 4). For an Earth-likeplanet having an N atmosphere, if the H S/SO emission ratio is less than 0.1 (as is the casefor current Earth; Holland 2002), the dominant type of aerosols in the atmosphere is sulfate;whereas elemental sulfur aerosols become the dominant type if the H S/SO emission ratiois larger than 1.
4. Results4.1. Optically Thick Aerosols from Sulfur Emission
The main finding is that on terrestrial exoplanets having atmospheres ranging fromreducing to oxidizing, the primary sulfur emission from the surface (e.g., H S and SO ) ischemically short-lived. The sulfur emission leads to photochemical formation of elementalsulfur (S ) and sulfuric acid (H SO ), which would condense to form aerosols if saturated inthe atmosphere. In reducing atmospheres (e.g., H atmospheres), S aerosols are photochem-ically formed based on H S and SO emission; and in oxidized atmospheres (e.g., N andCO atmospheres), both S and H SO aerosols may be formed (see Figure 5). In general,the higher the surface sulfur emission, the more aerosols exist in the atmosphere (see Figure5). As a result of photochemical production of elemental sulfur and sulfuric acid, terrestrialexoplanets with a habitable surface temperature (e.g., 270 ∼
320 K) and substantial sulfuremission from the surface are likely to have hazy atmospheres. In this paper, we use “hazy” 16 –to describe an atmosphere that has significant aerosol opacities at visible wavelengths (e.g.,500 nm). We find that even with an Earth-like surface sulfur emission, 1-bar H dominatedatmospheres on habitable rocky exoplanets are hazy with S aerosols (see Figure 5). We alsofind that if the sulfur emission rate is 30 ∼
300 times more than the Earth’s current volcanicsulfur emission rate, photochemical S aerosols become optically thick at visible wavelengthsin oxidized atmospheres including N and CO atmospheres (see Figure 5).The key parameters that determine the aerosol opacity in the atmosphere are the surfacesulfur emission rate, the dry deposition velocity, and the aerosol particle size. First, a highersurface sulfur emission rate leads to more sulfur and sulfate aerosols in anoxic atmospheres(e.g., Figure 5 and Figure 6). Second, larger dry deposition velocities of H S and SO cause more rapid removal of these sulfur compounds from the atmosphere, which reducesthe chance of converting them into condensable molecules (i.e., S and H SO ). Therefore,larger dry deposition velocities of H S and SO result in lower aerosol loading and aerosolopacities in the atmospheres, as shown in Figure 7. Third, we find that the particle size hasonly secondary effects on the chemical composition of the atmosphere (i.e., by increasing thepenetration of ultraviolet radiation), but has a primary effect on the aerosol optical depth.For mean particle diameter varying in the range of 0 . ∼ µ m (i.e., typical particle sizesof photochemical aerosols on Earth (e.g., Seinfeld & Pandis 2006) and Titan (e.g., Rageset al. 1983)), we do not see a notable variation in the yield of elemental sulfur, but wesee an enhancement of H SO production with large particles (Figure 6). Even with thesame aerosol abundances, however, micron-sized particles cause lower opacities at the visiblewavelengths and higher opacities in MIR compared with submicron-sized particles (Figure6). We capture the effects of the three key parameters on the aerosol opacity in anoxicatmospheres on terrestrial exoplanets by fitting the following power-law formula, i.e., τ = C (cid:18) Φ(S)10 cm − s − (cid:19) a (cid:18) V DEP V DEP (Earth) (cid:19) − b (cid:18) d P . µ m (cid:19) − c , (4)where τ is the vertical optical depth due to aerosols at 1 bar, Φ(S) is the total sulfur emissionrate, V DEP /V DEP (Earth) is the dry deposition velocities of H S and SO with respect tocurrent Earth values, d p is the mean particle diameter of aerosols, a , b , and c are positivenumbers, and C is a constant that covers other uncertainties. We have fit the empiricalrelation (4) through an extensive parameter exploration using photochemistry models (seeFigure 5 - 7 for examples) and determined the values of C , a , b and c for H dominatedreducing atmospheres and for N and CO dominated oxidized atmospheres. We summarizegraphically the parameter regime in which sulfur emission leads to a hazy atmosphere inFigure 8. Here we use τ and τ . µ m as the representatives for aerosol opacities at visible 17 –wavelengths and MIR wavelengths; due to the complex nature of the extinction cross sectionsof aerosol particles (Figure 1), it is not practical to fold the full wavelength dependency intothe empirical formula. Also, we find that it is always true that τ ∼ µ m and τ . µ m ∼ µ m. For H atmospheres, and mean particle diameter d P in the the range of 0 . ∼ µ m, τ = 1 ∼ (cid:18) Φ(S)10 cm − s − (cid:19) . (cid:18) V DEP V DEP (Earth) (cid:19) − . (cid:18) d P . µ m (cid:19) − . , (5)and for d P in the the range of 1 ∼ µ m, τ . µ m = 0 . ∼ (cid:18) Φ(S)10 cm − s − (cid:19) . (cid:18) V DEP V DEP (Earth) (cid:19) − . (cid:18) d P . µ m (cid:19) − . . (6)For N and CO atmospheres, and d P in the the range of 0.1 and 1 µ m, τ = 0 . ∼ (cid:18) Φ(S)10 cm − s − (cid:19) . (cid:18) V DEP V DEP (Earth) (cid:19) − . (cid:18) d P . µ m (cid:19) − . , (7)and for d P in the the range of 1 ∼ µ m, τ . µ m = 0 . ∼ . (cid:18) Φ(S)10 cm − s − (cid:19) . (cid:18) V DEP V DEP (Earth) (cid:19) − . (cid:18) d P . µ m (cid:19) − . . (8)The constant C in equations (5 - 8) spans about one order of magnitude, which coversthe variation of the following model inputs: • The H S/SO ratio in the surface sulfur emission, ranging from 0.01 to 10; • Temperature profiles deviating from the adopted temperature profile by ±
30 K thatcontrols the mixing ratio of water vapor in the atmosphere by the cold trap; • Stellar ultraviolet radiation received by the planet, ranging from the habitable zone ofsolar-like stars to the habitable zone of quiet M dwarfs with an effective temperatureof 3100 K; • Eddy diffusion coefficients ranging from 0.1 to 100 times the values of Earth’s atmo-sphere; • Sulfur polymerization reaction rates (reactions C 3 - C 7) ranging by one order of mag-nitude. 18 –To summarize, we find that the emission of H S and SO from the surface is readilyconverted into sulfur (S ) and sulfate (H SO ) in anoxic atmospheres of terrestrial exoplanets.The photochemical sulfur and sulfate would condense to form aerosols if saturated in theatmosphere, which is likely to occur on a planet in the habitable zone of either a Sun-like staror a quiet M star. The aerosol layer is optically thick at the visible and NIR wavelengthsif the surface sulfur emission is comparable to Earth’s volcanic sulfur emission in the H atmosphere, and more than 30 ∼
300 times of the Earth’s volcanic sulfur emission in otheranoxic atmospheres, depending on the dry deposition velocities of sulfur compounds andparticle size of the aerosols. , H S, and S and H SO Aerosols
The sulfur emission from surface shapes the spectra of terrestrial exoplanets at the visibleand NIR wavelengths, mostly through the photochemical formation of S and H SO aerosols.We use the model outputs from the photochemistry models to compute the transmission,reflection, and thermal emission spectra of a terrestrial exoplanet with various levels ofsulfur emission, and show examples of the computed spectra in Figure 9. We see thatsubmicron-sized S aerosols dominate the transmission and reflection spectra at wavelengthsfrom visible up to 3 µ m, if the sulfur emission is more than about two orders of magnitudehigher than Earth’s volcanic sulfur emission. In general, an atmosphere with high sulfuremission and therefore high aerosol loading generally exhibits a flat transmission spectrum(the H O features at NIR muted), and a high visible albedo (see Figure 9). Notably, S aerosols are purely reflective at 500 nm but absorptive at 300 nm. The absorption edge of S aerosols in 300 - 400 nm is evident in the reflection spectra for planets with enhanced sulfuremission (Figure 9), which is a potential diagnostic feature for S aerosols.Although opaque at visible wavelengths, the atmospheres with enhanced sulfur emissionare likely to be transparent in the MIR wavelengths ( λ > µ m). The spectral features ofaerosols depend on their particle sizes, so we now consider two possibilities: if the particlesare submicron-sized, the aerosol molecules have negligible cross sections at MIR (see Figure1); or if the particles are micron-sized, the falling velocity of aerosol particles is large enoughto rapidly remove aerosols from the atmosphere, as implied by equation (6) and equation (8)that are applicable for micron-sized particles. Therefore in both cases the aerosol opacitiesat MIR are minimal even for very high sulfur emission rates (see Figure 9 for examplesof N atmospheres, and H atmospheres are qualitatively similar). The only exception, inwhich aerosols indeed affect MIR spectra, is the case of abundant H SO aerosols. The mainspectral effect of H SO aerosols is absorption at MIR wavelengths (5 ∼ µ m; Figure 9). 19 –However, the column-average mixing ratio of H SO needs to be larger than 0.1 ppm in orderto produce significant aerosol absorption at MIR. We find with numerical exploration thatsuch a high abundance of H SO aerosols is only possible in highly oxidizing CO dominatedatmospheres without reducing gas emission (see Paper I for an example of such atmospheres).With reducing gas emission (i.e., H and CH ), it is unlikely that H SO mixing ratio exceeds0.01 ppm in anoxic atmospheres for a wide range of sulfur emission rates (see Figure 5). Insummary we expect the spectral effects of S and H SO aerosols to be minimal at MIR formost cases.We now turn to consider the direct spectral features of H S and SO . It has beenpreviously proposed that H S and SO can be detectable on terrestrial exoplanets by theirspectral features (Kaltenegger & Sasselov 2010). However, our photochemistry models showthat both H S and SO are chemically short-lived in the atmospheres, which implies thatthat substantial surface emission is required to maintain a detectable level of either H S orSO in the atmosphere. SO has diagnostic absorption features at 7.5 µ m and 20 µ m (seeFigure 9). For these features to be detectable the mixing ratio of SO needs to be largerthan 0.1 ppm, which corresponds to sulfur emission rates 1000 times more than currentEarth’s sulfur emission rates for H , N , and CO atmospheres (see Figure 5). The spectralfeature of H S is the pseudo-continuum absorption at wavelengths longer than 30 µ m, whichcoincides with the rotational bands of H O. We find that the only scenario in which H S maybe directly detected is the case with extremely high sulfur emission rates (i.e., 3000 timeshigher than the current Earth’s sulfur emission rate) on a highly desiccated planet withoutliquid water ocean so that there is no water vapor contamination. We therefore conclude thatdirect detection of H S and SO is tricky: they are chemically short-lived so that extremelylarge surface emission is required for a detectable mixing ratio in the atmosphere, and theirspectral features may be contaminated by other gases in the atmosphere.Finally, we suggest that the emission of sulfur compounds might be indirectly inferredby detecting sulfur and sulfate aerosols. Our numerical exploration reveals a monotonicrelationship between the abundance of aerosols in the atmosphere and the emission ratesof sulfur compounds (see Figure 5), and the composition of aerosols is correlated with theH S/SO ratio of the surface emission (see Figure 4). A combination of featureless lowatmospheric transmission (large planet radius viewed in transits) and high planetary albedo(large planetary flux at the visible wavelengths viewed in occultations) may establish theexistence of aerosols in the atmosphere. In particular, elemental sulfur (S ) aerosols areabsorptive at wavelengths shorter than 400 nm and therefore might be identified by theabsorption edge (see Figure 9). Sulfate aerosols (H SO ), if abundant in the atmosphere,lead to absorption features at the MIR wavelengths ( λ ∼ µ m). However, none ofthese features are uniquely diagnostic of certain types of aerosols. The identification of 20 –aerosol composition, therefore, is by no means straightforward. We learn from the SolarSystem exploration that the discriminating piece of information for aerosol identificationcomes from polarization of reflected stellar light. Historically, the bright clouds on Venuswere identified to be mainly composed of H SO droplets after the phase curve of the planetin polarized light had been observed (e.g., Young 1973; Hansen & Hovenier 1974). Wetherefore postulate that aerosol identification on terrestrial exoplanets and the inference ofsurface sulfur emission might require observation of polarized reflected light as a function ofplanetary illumination phase.
5. Discussion: Can H S be a Biosignature Gas?
Biosignatures are gases in an exoplanet’s atmosphere produced by life. In order toconfirm a certain gas to be a plausible biosignature, one has to verify that the gas canaccumulate in the atmosphere and that the amount of gas detected cannot be producedthrough plausible abiotic processes. It has been proposed and widely discussed that O (and its photolytic product O ), N O, CH and CH Cl are exoplanet biosignatures (e.g.,Sagan et al. 1993; Des Marais et al. 2002; Segura et al. 2005; Segura et al. 2007; Huet al. 2012). Organosulfur compounds, such as dimethyl sulfide (DMS) and methanethiol(CH SH), have also been suggested to be biosignatures of the early Earth (Pilcher 2003) andanoxic exoplanets (Domagal-Goldman et al. 2011).H S can be produced from several metabolic origins on Earth, and so is a candidatebiosignature gas. Life on Earth can produce H S through sulfate reduction (when the en-vironment is reduced) and sulfur disproportionation. Microorganisms can disproportionatesulfur compounds of intermediate oxidation states, including thiosulfate, sulfite, and elemen-tal sulfur, into H S and sulfate (Finster 2008). For example, disproportionation of sulfite inthe ocean is described by 4 SO − + H + −→ − + HS − , (C 8)in which the Gibbs free energy released is 58.9 kJ mol − sulfite. The sulfite reducers, in-cluding microorganisms in genus Desulfovibrio and
Desulfocapsa , extract energy from thedisproportionation (Kramer and Cypionka, 1989).The effect of biotic H S production is the increase of the H S/SO ratio of the surfacesulfur emission. If the H S/SO ratio in the volcanic sulfur emission is low (i.e., less than0.1), sulfur disproportionation and sulfate reduction by life may increase the H S/SO ratiosignificantly, which may lead to a change in the redox input to the atmosphere and thereforethe dominant aerosol species in the atmosphere, as suggested by Figure 4. Specifically, for a 21 –habitable terrestrial exoplanet having a weakly oxidizing N atmosphere, biotic productionof H S in excess of the geological H S emission could result in a higher amount of S aerosolsand a much lower amount of H SO aerosols in the atmosphere compared with a planetwithout life. Although it is currently not possible to distinguish different types of aerosols,H S could be a biosignature gas in the long term.The geological production of H S, and the consequent risk of a false positive mis-identification of geological H S for biological H S, will be a major obstacle of confirmingH S to be a biosignature gas. Sulfur is believed to be present in the mantle of all terrestrialplanets, and what determines the H S/SO ratio in the volcanic outgassing is the oxygenfugacity of the upper mantle, temperature of the location where magma degassing happens,water content in the conduit of magma, and gas content dissolved in the magma (e.g., Hol-land 1984; Kasting et al. 1985; Holland 2002; Burgisser & Scaillet 2007). The current Earthvolcanic emissions are oxidized, dominated by H O, CO and SO with minor contributionsof H , CO and H S. This volcanic gas composition is consistent with a magma bufferedby the quartz-fayalite-magnetite (QFM) equilibrium, i.e., a relatively oxidized upper mantle(Holland 1984). As a global average, the volcanic H S/SO emission ratio on Earth is 0.1(Holland 2002). However, if the mantle of a rocky exoplanet is much more reducing thanthat of the current Earth, significant geological source of H S can be expected (Holland 1984;Kasting 1993). As a result, it would be very hard to rule out a geological contribution tothe H S emission flux by remote sensing.In summary, although H S can be produced by energy-yielding metabolism, it is veryunlikely to be a useful biosignature gas for three reasons. Firstly, H S itself is unlikely tobe detectable directly by remote sensing because of its weak spectral features, and theircontamination by the spectral features of water. Secondly, H S has a very short atmosphericlifetime, and so unrealistic emission rates are required to build up significant levels in anyatmosphere. This second point could be overcome, in principle, by detecting S aerosolsin an anoxic atmosphere, and discriminating them from H SO aerosols. Discriminationbetween sulfur and sulfuric acid aerosols is not possible with current equipment, but may bepossible in the future through analysis of reflected light. Thirdly, however, H S suffers froma significant false positive risk, as geological sources can also produce H S, and the ratio ofH S/SO in geological emissions depends on mantle chemistry, the physical structure of theoutgassing events, and the extent of surface reprocessing of vented sulfur gases. To inferthat life was generating H S on a planet, this study shows that the observer would have todetermine the S /H SO aerosol ratio and have knowledge of the geological outgassing ratioof H S/SO and have knowledge of the surface chemistry that might modulate the primaryoutgassing rate. This seems an unreasonable requirement. 22 –
6. Conclusion
We studied the effect of H S and SO surface emission on anoxic atmospheres of ter-restrial exoplanets. With a newly established one-dimensional photochemistry model thattreats all relevant chemical reactions and photochemical processes of O, H, C, and S bearingspecies, as well as formation and sedimentation of sulfur and sulfate aerosols, we find thatH S and SO gases emitted from surface are chemically short-lived in both reducing andoxidizing atmospheres. The sulfur emission results in photochemical production of elemen-tal sulfur (S ) and sulfuric acid (H SO ), which would condense to form aerosols if theyare saturated in the atmosphere. For a planet in the habitable zone of a Sun-like star ora M star, Earth-like sulfur emission rates would result in optically thick aerosol layers inH -dominated atmospheres; and a sulfur emission rate 2-orders-of-magnitude higher thanthe Earth’s volcanic sulfur emission rate would result in optically thick aerosol layers inN and CO -dominated atmospheres. The composition of the photochemically producedaerosols mostly depends on the redox state of the atmosphere: S aerosols are formed in thereducing atmospheres (e.g., H atmospheres), and both S and H SO aerosols are formedin the oxidized atmospheres that could be both reducing and oxidizing (e.g., N and CO atmospheres). Based on extensive numerical simulations, we provide empirical formulae thatshow the dependency of the aerosol optical depth on the surface sulfur emission rates, thedry deposition velocities of sulfur compounds, and the aerosol particle sizes.We find that direct detection of H S and SO is unlikely due to the rapid photochemicalconversion from H S and SO to elemental sulfur and sulfuric acid in atmospheres having awide range of redox powers. For a terrestrial exoplanet with sulfur emitted from the surfaceat an enhanced rate, it is likely that at visible wavelengths the planet’s atmosphere appearsto be opaque due to the aerosol loading and that the planet has high visible albedo. However,for Earth-like planets with 1-bar atmospheres ranging from reducing to oxidizing, we findthe effect of photochemical sulfur and/or sulfate aerosols in the MIR wavelengths is mini-mal, because micron-sized particles that interact with MIR photons have large gravitationalsettling velocities and therefore short atmospheric lifetime. Finally, as the aerosol composi-tion is tightly related to the ratio of the H S versus SO emission, although direct detectionof H S and SO by their spectral features is unlikely, their existence might be inferred byobserving aerosol-related features in reflected light with future generation space telescopes.We thank Shuhei Ono and Jenny Suckale for helpful discussions about the sulfur-bearinggases release by the volcanism. We thank Feng Tian for providing the refractive index dataof S aerosols, and Antigona Segura for discussions regarding CO atmospheres. We thankJames Kasting for helpful suggestions about the photochemical model. We thank Linda 23 –Elkins-Tanton for helpful suggestions on the mantle degassing. We thank Vikki Meadowsfor discussions about detecting clouds on Venus. We thank the anonymous referee to theimprovement of the manuscript. RH is supported in part by the NASA Earth and SpaceScience Fellowship (NESSF/NNX11AP47H). REFERENCES
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This preprint was prepared with the AAS L A TEX macros v5.2.
27 – −1 −22 −21 −20 −19 −18 −17 −16 Wavelength [micron] σ p ’ [ c m m o l e c u l e − ] H SO Aerosols10 −22 −21 −20 −19 −18 −17 −16 σ p ’ [ c m m o l e c u l e − ] S Aerosols
Fig. 1.— Extinction cross sections (black lines) and scattering cross sections (orange lines)of H SO and S per molecule in the condensed phase. The dotted, solid, and dashed linesare cross sections for the mean particle diameter of 0.1, 1, 10 µ m, respectively. The sizedistribution of aerosol particles is assumed to be lognormal with a dispersion σ = 2. 28 – H Atmosphere hv, H H, HS hv, SO hv H , CH O HSS −10 −8 −6 −4 −2 −1 Mixing Ratio [ppm] P r e ss u r e [ P a ] SO H SH SO (A)S (A) Fig. 2.— Formation of elemental sulfur aerosols in reducing H -dominated atmosphereson an Earth-sized rocky planet in the habitable zone of a Sun-like star. The upper panelschematically illustrates the chemical pathways from the primary sulfur emission (i.e., H Sand SO ) to elemental sulfur, in which sulfur compounds are located according to theiroxidation states labeled on the top of the figure. The thin arrows show the major chemi-cal pathways in the atmosphere, and the thick arrows show the major surface-atmosphereinteractions. The lower panel shows the results of photochemistry simulations, with totalsurface sulfur emission of 10 (solid lines) and 10 (dashed lines) cm − s − , i.e, 3 and 300times higher than the Earth’s volcanic sulfur emission rate. The H S/SO ratio in the sulfuremission is 0.5 and the particle mean diameter is 0.1 µ m. Other model parameters are tab-ulated in Table 1. UV photons and atomic hydrogen effectively convert H S and SO intoelemental sulfur, and elemental sulfur aerosols shield UV photons so that H S and SO mayaccumulate below the aerosol layer if the sulfur emission is more than 300 times higher thanthe Earth’s volcanic emission rate. 29 – −10 −8 −6 −4 −2 −1 Mixing Ratio [ppm] P r e ss u r e [ P a ] CO −1 P r e ss u r e [ P a ] N SO H SH SO (A)S (A) O H2OO O H O, OHhv H , S , CH OO , O H O O O , O H S , H H O , O hv, H, OH h v , S O S S h v , H , S CH O Fig. 3.— Formation of sulfuric acid aerosols in oxidized N and CO dominated atmosphereson an Earth-sized rocky planet in the habitable zone of a Sun-like star. Similar to Figure2, the upper panel schematically illustrates the chemical pathways from the primary sulfuremissions (i.e., H S and SO ) to elemental sulfur and sulfuric acid. For double arrows thelabel above the arrow indicates the oxidizing agents, and the label below the arrow indicatesthe reducing agents. The lower two panel shows the results of photochemistry simulations forN and CO dominated atmospheres, respectively. The total surface sulfur emission is 10 (solid lines) and 10 (dashed lines) cm − s − , i.e, 3 and 300 times higher than the Earth’svolcanic sulfur emission rate. The H S/SO ratio in the sulfur emission is 0.5 and the particlemean diameter is 0.1 µ m. Other model parameters are tabulated in Table 1. Both elementalsulfur aerosols and sulfuric acid aerosols are formed in the oxidized and anoxic atmospheres.The origins of the principle reducing agents (H and CHO) and the principle oxidizing agents(OH, O and O ) is photodissociation of H O and CO . The apparent depletion of SO at thepressure level of 10 - 100 Pa in the N atmosphere (the blue solid line in the middle panel)is due to the production of atomic hydrogen by methane photodissociation at this pressurelevel. 30 – −2 −1 −13 −12 −11 −10 −9 −8 −7 −6 H S/SO RATIO M i x i ng R a t i o H SSO H SO (A)S (A) N Atmosphered P = 0.1 µm Φ (S) = 1.0X10 cm −2 s −1 Fig. 4.— Correlation between the aerosol composition and the composition of sulfur emis-sions in the weakly oxidizing N atmosphere on an Earth-sized rocky planet in the habitablezone of a Sun-like star. The total surface emission rate is 10 cm − s − , or 30 times theEarth’s volcanic sulfur emission rate, and the particle mean diameter is 0.1 µ m. Other modelparameters are tabulated in Table 1. As a larger fraction of surface sulfur emission is in theform of H S, the amount of S aerosols in the atmosphere increases, and the amount ofH SO aerosols in the atmosphere decreases dramatically. 31 – N Atmosphere CO AtmosphereH Atmosphere10 −13 −12 −11 −10 −9 −8 −7 −6 −5 M i x i ng R a t i o H SSO H SO (A)S (A) H SSO H SO (A)S (A) H SSO H SO (A)S (A) −4 −3 −2 −1 Total Sulfur Emission Flux [S cm −2 s −1 ] τ ae r o s o l
500 nm7.5 µm Total Sulfur Emission Flux [S cm −2 s −1 ]
500 nm7.5 µm Total Sulfur Emission Flux [S cm −2 s −1 ]
500 nm7.5 µm
Fig. 5.— The relationship between the aerosol mixing ratios and aerosol opacities andthe total sulfur emission rate. Column-integrated aerosol opacities at the 1-bar pressurelevel including both elemental sulfur aerosols and sulfuric acid aerosols at 500 nm (solidlines) and 7.5 µ m (dashed lines) are shown in the lower panel. The planet is an Earth-sized rocky planet orbiting a Sun-like star, with reducing (H -dominated), weakly oxidizing(N -dominated), or highly oxidizing (CO -dominated) atmospheres. The aerosol particlemean diameter is assumed to be 0.1 µ m, and the H S/SO ratio of the surface emissionis 0.5. Other model parameters are tabulated in Table 1. Sulfur emission 2-orders-of-magnitude larger than current Earth’s volcanic sulfur emission ( ∼ × S cm − s − ) leads tosubstantial aerosol opacities in the visible wavelengths in N and CO atmospheres, and sulfuremission comparable with current Earth’s volcanic sulfur emission leads to substantial aerosolopacities in the visible wavelengths in H atmospheres. The wiggle in the concentration ofsulfuric acid aerosols reflects the competition between two effects: more sulfur is available tobe converted into sulfuric acid as the sulfur emission increases, but the atmosphere becomesmore reducing and less oxidizing as the sulfur emission increases. 32 – Solar-type star, d = 0.1 µmQuiet M star, d = 0.1 µmSolar-type star, d = 1 µm pp p −4 −3 −2 −1 Total Sulfur Emission Flux [S cm −2 s −1 ] τ . µ m −4 −3 −2 −1 τ m −12 −11 −10 −9 −8 −7 −6 H S O A e r o s o l M i x i ng R a t i o −12 −11 −10 −9 −8 −7 −6 S A e r o s o l M i x i ng R a t i o Total Sulfur Emission Flux [S cm −2 s −1 ] N Atmosphere H Atmosphere
Fig. 6.— Aerosol mixing ratios and optical depths at the surface (1-bar pressure level) at500 nm and 7.5 µ m as a function of total sulfur emission rates, for an Earth-sized rockyplanet orbiting a Sun-like star at 1 AU (black lines), a habitable planet around quiet Mdwarf having effective temperature of 3100 K (red lines), and an Earth-sized rocky planetorbiting a Sun-like star at 1 AU with particle mean diameter of 1 µ m (blue lines). The leftcolumn shows the case of weakly oxidizing N atmospheres, and the right column shows thecase of reducing H atmospheres. The H S/SO ratio in the sulfur emission is 0.5 and othermodel parameters are tabulated in Table 1. We see that decreasing UV photon flux has littleeffect on the S formation, but results in a decrease of the amount of sulfuric acid aerosols,and therefore a decrease of MIR optical depth. We also see that particle diameter variationin 0 . ∼ µ m has little effect on the chemical composition, but for similar mass abundancea larger particle size results in a smaller optical depth in the visible wavelengths and a largeroptical depth in the MIR wavelengths. 33 – −4 −3 −2 −1 τ ae r o s o l
500 nm7.5 µm H Atmosphere Φ (S) = 3.3X10 cm −2 s −1 H S /SO = 0.1d P = 0.1 µm −2 −1 −4 −3 −2 −1 Deposition Velocity of H S and SO / Current Earth Values τ ae r o s o l
500 nm7.5 µm N Atmosphere Φ (S) = 3.3X10 cm −2 s −1 H S /SO = 0.1d P = 0.1 µm Fig. 7.— The relationship between the aerosol opacities (both S and H SO aerosols) at500 nm and 7.5 µ m in the H and N atmospheres and the H S and SO dry depositionvelocities. Model parameters are shown in the figure and tabulated in Table 1. 34 – −2 −1 Total Sulfur Emission Rate [cm −2 s −1 ] V D EP o f H S and S O [ E a r t h V a l ue s ] SO FeatureOpticallythickaerosolsClearAtmosphere
Earth Venus H N , CO VenusVenusEarthEarth H VenusVenusVenus , CO Fig. 8.— Formation of optically thick aerosols in atmospheres on rocky exoplanets inthe habitable zone of their host star as a result of surface sulfur emission and deposition.The shaded areas are the parameter regime boundaries between a clear atmosphere and anoptically thick atmosphere (defined as aerosol optical depth at 500-nm wavelength τ > ) and oxidized (N and CO ) atmospheres. The upper-left corner ofthe parameter regime (small sulfur emission rates, large deposition velocities) leads to clearatmospheres; whereas the lower-right corner of parameter regime (large sulfur emission rates,small deposition velocities) leads to optically thick aerosols in the atmosphere composed ofsulfur (S ) and sulfate (H SO ). The widths of the shaded boundary regime between clearatmospheres and optically thick atmospheres contain the uncertainties of: (1) the meanaerosol particle size ranging from 0.1 to 1 µ m, (2) the H S/SO ratio of the sulfur emissionranging from 0.01 to 10 (i.e., more H S leads to thicker haze), (3) the spectral type of the hoststar ranging from G2V to M5, (4) the strength of vertical mixing in the atmosphere by eddydiffusion ranging from 0.1 to 10 times Earth’s value, and (5) the surface temperature rangingfrom 270 to 320 K. Earth and Venus are shown for a reference in the Solar System: Earth’svolcanic sulfur emission and H S deposition velocity are plotted; and Venus’ equivalentupward SO flux and SO deposition velocity at the altitude of 58 km are plotted (adaptedfrom Krasnopolsky 2012). We note that the equivalent SO flux is a transfer rate across the58-km altitude, and does not imply a surface emission rate. The SO features at 7.5 µ mand 20 µ m requires a mixing ratio on the order of ppm to be spectrally significant, whichcorresponds to a sulfur emission flux of more than 10 cm − s − due to rapid photochemicalremoval of SO in the atmosphere. 35 – R e f l e c t i v i t y N Atmosphered P = 0.1 µmS Aerosol ReflectionS Absorption E a r t h X E a r t h X E a r t h X E a r t h X E a r t h X T r an s m i ss i on N Atmosphered P = 0.1 µmS Aerosols E a r t h X E a r t h X E a r t h X E a r t h X E a r t h X H SO AerosolsCO CO SO H SH OSO B r i gh t ne ss T e m pe r a t u r e [ K ] H SH OH O CO SO SO SO CH N Atmosphered P = 0.1 µm S Aerosols
Fig. 9.— Transmission, reflection and thermal emission spectra of a terrestrial exoplanetwith N -dominated atmosphere with various surface sulfur emission up to 3000 times Earth’scurrent volcanic emission (labeled in colors). The planet is an Earth-sized planet at the 1-AUorbit a Sun-like star, having surface temperature of 288 K. The H S/SO emission ratio is0.5, the aerosol particle diameter is assumed to be 0.1 µ m, and other model parameters aretabulated in Table 1. The cross sections of S and H SO4