Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths An Integrated Biogeochemical Photochemical and Spectral Modeling Approach_2

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Evaluating the Plausible Range of N

O Biosignatures on Exo-Earths: An Integrated

Biogeochemical, Photochemical, and Spectral Modeling Approach

Edward W. Schwieterman

1,2,3,4

, Stephanie L. Olson

2,5

, Daria Pidhorodetska

1,2,3

, Christopher T. Reinhard

2,3,6

Ainsley Ganti

, Thomas J. Fauchez

3,8,9,10

, Sandra T. Bastelberger

8,10,11,12

, Jaime S. Crouse

8,10,11,13

Andy Ridgwell

1,2

, and Timothy W. Lyons

1,2,3

Department of Earth and Planetary Sciences, University of California, Riverside, CA, USA; eschwiet@ucr.edu

NASA Alternative Earths Team, Riverside, CA, USA

NASA NExSS Virtual Planetary Laboratory Team, Seattle, WA, USA

Blue Marble Space Institute of Science, Seattle, WA, USA

Department of Earth, Atmospheric, and Planetary Science, Purdue University, West Lafayette, IN, USA

School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA

The Potomac School, 1301 Potomac School Rd, McLean, VA 22101, USA

NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA

American University, Washington, DC, USA

Sellers Exoplanet Environment Collaboration (SEEC), NASA GSFC, USA

Department of Astronomy, University of Maryland, College Park, MD 20742, USA

Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD 20771, USA

Southeastern Universities Research Association (SURA), 1201 New York Avenue NW, Washington, DC 20005, USA

Received 2022 May 13; revised 2022 August 17; accepted 2022 August 24; published 2022 October 4

Abstract

Nitrous oxide (N

O)—a product of microbial nitrogen metabolism—is a compelling exoplanet biosignature gas

with distinctive spectral features in the near- and mid-infrared, and only minor abiotic sources on Earth. Previous

investigations of N

O as a biosignature have examined scenarios using Earthlike N

O mixing ratios or surface

ﬂuxes, or those inferred from Earth’s geologic record. However, biological ﬂuxes of N

O could be substantially

higher, due to a lack of metal catalysts or if the last step of the denitriﬁcation metabolism that yields N

from N

had never evolved. Here, we use a global biogeochemical model coupled with photochemical and spectral models

to systematically quantify the limits of plausible N

O abundances and spectral detectability for Earth analogs

orbiting main-sequence (FGKM)stars. We examine N

O buildup over a range of oxygen conditions (1%–100%

present atmospheric level)and N

Oﬂuxes (0.01–100 teramole per year; Tmol =10

mole)that are compatible

with Earth’s history. We ﬁnd that N

Oﬂuxes of 10 [100]Tmol yr

−1

would lead to maximum N

O abundances of

∼5[50]ppm for Earth–Sun analogs, 90 [1600]ppm for Earths around late K dwarfs, and 30 [300]ppm for an

Earthlike TRAPPIST-1e. We simulate emission and transmission spectra for intermediate and maximum N

concentrations that are relevant to current and future space-based telescopes. We calculate the detectability of N

spectral features for high-ﬂux scenarios for TRAPPIST-1e with JWST. We review potential false positives,

including chemodenitriﬁcation and abiotic production via stellar activity, and identify key spectral and contextual

discriminants to conﬁrm or refute the biogenicity of the observed N

Uniﬁed Astronomy Thesaurus concepts: Astrobiology (74);Exoplanet atmospheres (487);Exoplanets (498);

Habitable planets (695);Nitrous oxide (1114);Biosignatures (2018)

1. Introduction

To date, over 5000 exoplanetary systems have been

discovered (Christiansen 2022),

including several planets

that are rocky in composition and located within the

circumstellar habitable zone of their host star (Kane et al.

2016; Kaltenegger et al. 2019). The James Webb Space

Telescope (JWST)will allow us to probe the atmospheres of a

small number of these temperate terrestrial exoplanets, such as

the TRAPPIST-1 planets (Gillon et al. 2017; Luger et al. 2017;

Morley et al. 2017; Lincowski et al. 2018; Fauchez et al. 2019;

Lustig-Yaeger et al. 2019; Ducrot et al. 2020), while upcoming

ground-based extremely large telescopes will facilitate the

examination of nearby potentially habitable worlds, such as

Proxima Centauri b (Anglada-Escudé et al. 2016; Ribas et al.

2016; Snellen et al. 2017; Meadows et al. 2018a). Ambitious

future mission concepts, such as the IR/optical/UV observa-

tory recommended by the 2020 Astronomy and Astrophysics

Decadal Survey (National Academies of Sciences, Engineer-

ing, and Medicine 2021), or ESA’s mid-IR (MIR)Large

Interferometer for Exoplanets (LIFE)concept (Defrère et al.

2018; Quanz et al. 2018,2022), would allow for the

unprecedented atmospheric characterization of a larger number

of temperate rocky planets orbiting stars in the solar

neighborhood (most of which are yet to be discovered), though

the observability of speciﬁc spectral features will be limited by

the wavelength regime and observing mode.

One of the most compelling drivers of exoplanet science is

the search for inhabited planets like Earth, which may be

identiﬁed through remote spectroscopic biosignatures (Des

Marais et al. 2002; Seager et al. 2012; Grenfell 2017;

Kaltenegger 2017; Schwieterman et al. 2018). For such

The Astrophysical Journal, 937:109 (22pp), 2022 October 1 https://doi.org/10.3847/1538-4357/ac8cfb

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distribution of this work must maintain attribution to the author(s)and the title

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https://exoplanets.nasa.gov/

inhabited worlds to be positively identiﬁed from atmospheric

spectra, they must possess global biospheres with a robust

exchange of gases between life and the atmosphere as well as

generate biosignature features that can be remotely detectable

with foreseeable technologies and are separable from false-

positive signals generated by abiotic processes (Catling et al.

2018; Meadows et al. 2018b; Schwieterman et al. 2018). Many

groups have recently undertaken studies to assess the longevity

and potential detectability of a variety of biosignature gases

with near-future observational capabilities (Reinhard et al.

2017a; Fujii et al. 2018; Kawashima & Rugheimer 2019;

Kaltenegger et al. 2020b; Pidhorodetska et al. 2020; Sousa-

Silva et al. 2020; Checlair et al. 2021; Lin et al. 2021; Phillips

et al. 2021; Wunderlich et al. 2021; Ranjan et al. 2022).

The coexistence of N

and O

in an atmosphere is one

possible biosignature. This possibility is due to both the high

chemical disequilibrium between N

, and surface liquid

water (Krissansen-Totton et al. 2016,2018)and the planetary

evolution that is required to accumulate large quantities of both

gases, including the implausibility of the abiotic generation and

atmospheric retention of these gases together given competing

atmospheric and geochemical sinks (Stüeken et al. 2016;

Lammer et al. 2019; Sproß et al. 2021). However, N

is itself

challenging to detect directly, due its status as a homonuclear

molecule with no transitional dipole moment, possessing weak

collisional-induced absorption bands at 4.3 and 2.15 μm

(Lafferty et al. 1996; Schwieterman et al. 2015).

In addition to N

gas (in the atmosphere and dissolved in the

ocean), nitrogen on the present-day Earth exists in a wide

variety of different chemical forms, ranging from reduced

to oxidized NO

−

, with many intermediate redox species

in between, including N

O. Overall, Earth’s nitrogen cycle can

be thought of as the biologically driven removal of N

from the

ocean and atmosphere, the ﬁxation of nitrogen in organic

matter, which is then followed by the recycling of nitrogen

back to the atmosphere as N

(Figure 1; Thamdrup 2012; Tian

et al. 2015). However, as a reﬂection of the diversity of

microbial metabolisms, the recycling loop contains multiple

pathways back to N

. One of these recycling routes involves

the creation N

O, which can either be further reduced

biologically to N

or escape directly across the air–sea interface

into the atmosphere.

Denitriﬁcation (the transformation of NO

−

to N

gas, with

O as an intermediate product)is a relatively ubiquitous

metabolism on Earth, and consequent N

O production can be

mediated both by bacteria as well as by some fungi (Chen et al.

2015). Additionally, the direct oxidation of ammonia by

bacteria and archaea can also produce N

O(Santoro et al.

2011; Prosser & Nicol 2012). Biological ﬂuxes of N

O into the

atmosphere are several orders of magnitude larger than abiotic

sources, such as lightning, which is estimated to produce

∼0.002% of atmospheric N

O(Schumann & Huntrieser 2007).

On Earth today, the magnitude of this biological ﬂux is ∼0.4

Tmol yr

−1

and includes contributions from both marine and

terrestrial (including agricultural and industrial)sources

(Tian 2015; Tian et al. 2020).

O is of interest here because it produces notable features

in Earth’s near-IR (NIR)and MIR spectra (Sagan et al. 1993;

Robinson & Reinhard 2018; Gordon et al. 2022). This, and its

dominant biological origin on Earth, has led previous authors to

consider N

O as a potential remote biosignature for Earthlike

planets, along with O

, and CH

(Rauer et al. 2011;

Grenfell 2017; Schwieterman et al. 2018). In general, previous

studies of N

O biosignatures have used present-day Earth’s

Oﬂux as a ﬁducial boundary condition to predict the

resulting mixing ratios on an Earthlike planet orbiting another

star or have used an inferred N

Oﬂux from Earth’s geologic

past as this surface boundary condition to predict mixing ratios

to similar ends (Segura et al. 2003,2005; Kaltenegger et al.

2007,2020a; Rugheimer et al. 2013,2015b; Grenfell et al.

2014; Tabataba-Vakili et al. 2016; Robinson & Reinhard 2018;

Rugheimer & Kaltenegger 2018; Lin et al. 2021; Alei et al.

2022). Importantly, most of these past studies have typically

predicted N

O mixing ratios on exo-Earths that are lower or

only modestly higher than modern Earth, leading to pessimistic

predictions for N

O detectability (e.g., Alei et al. 2022), though

Figure 1. A schematic nitrogen cycle as implemented in the biogeochemical model cGENIE. The faded and dashed lines signify processes that are not explicitly

included in the scheme. N

Oﬂuxes result from incomplete denitriﬁcation of ﬁxed nitrogen (NO

−

)via the nitrous oxide reductase enzyme. Denitriﬁcation is an

anaerobic process that depends on organic C ﬂuxes, which are ultimately limited by nutrient (PO

3−

)availability. A deﬁcit of enzymatic catalysts—such as copper (Cu)

—due to ocean water chemistry or initial planetary abundances would result in the partial termination of the N redox cycle and may produce large N

Oﬂuxes. In the

extreme scenario of no enzymatic catalysts, or if the nitrous oxide reductase enzyme (or an analog)had not evolved, the N

Oﬂux would equal the total denitriﬁcation

ﬂux. Photochemical reactions will eventually return N

OtoN

in the atmosphere.

The Astrophysical Journal, 937:109 (22pp), 2022 October 1 Schwieterman et al.

it has long been recognized that low stellar UV ﬂuxes promote

O accumulation (e.g., Segura et al. 2003,2005; Grenfell

et al. 2014; Rugheimer & Kaltenegger 2018)and that higher

O surface ﬂuxes would result in a more detectable

biosignature (Kaltenegger 2017; Schwieterman et al. 2018).

It has been hypothesized that biological ﬂuxes of N

O during

the Proterozoic Eon (∼2500–540 million years ago, Ma)may

have been dramatically larger than at present, due to the limited

availability of copper catalysts in euxinic (anoxic and sulfur-

rich)oceans, which would have effectively short-circuited the

last metabolic step in the denitriﬁcation cycle (Buick 2007;

Figure 1). Higher atmospheric mixing ratios of N

O could

therefore also have contributed to greenhouse warming at the

time (Roberson et al. 2011), although lower O

concentrations

would have somewhat muted the impact of higher ﬂuxes,

due to the reduced shielding of photolyzing UV radiation

(Roberson et al. 2011; Stanton et al. 2018). Under certain

conditions, N

O production on the earlier Earth could also have

been augmented by chemodenitriﬁcation—that is, the process

of N

O production via abiotic reduction of nitric oxide (NO)by

ferrous iron (Samarkin et al. 2010; Stanton et al. 2018).

Previous studies have not comprehensively examined the full

range of plausible N

Oﬂuxes over a range of pO

values and

stellar types, including the end-member scenario, in which the

nitrous oxide reductase enzyme that facilitates the last step in

the denitriﬁcation process simply does not evolve (Pauleta et al.

2013). In such a scenario, we will show that N

O can

accumulate to high concentrations—even for planets orbiting

FGK stars—with implications for the detectability of this

biosignature gas with current and upcoming observatories.

Here, we conduct a systematic photochemical and spectral

investigation of N

O as an exoplanet biosignature and place

upper limits on the N

O abundances and detectability from a

productive biosphere. In Section 2, we use the Earth system

(biogeochemical)model “cGENIE”to calculate denitriﬁcation

ﬂuxes for an Earthlike marine biosphere as a function of

atmospheric oxygenation levels (pO

)and concentrations of

bioavailable phosphorous (as PO

3−

). We evaluate our results

against literature values for the Earth and generate realistic

bounds for plausible intermediate and maximum N

Oﬂuxes. In

Section 3, we calculate the photochemical stability and steady-

state mixing ratios of N

O given a large range of ﬂuxes,

inclusive of those calculated in Section 2, with a variety of

oxygenation states and for stellar hosts that span the main

sequence (F4V to M8V). In Section 4, we generate emission

and transmission spectra for a subset of the scenarios

investigated in Section 3. Finally, we calculate the number of

transits of TRAPPIST-1e that will be required to detect N

with JWST for three N

Oﬂux scenarios, and ﬁnd that detecting

O with NIRSpec is plausible for production ﬂuxes near the

biospheric maxima. We discuss the implications and potential

false positives in Section 5. We conclude in Section 6.

2. Circumscribing Plausible Global N

O Fluxes with a

Biogeochemical Model

To map out how the total oceanic denitriﬁcation (and hence

the potential maximum N

O production)rate varies as a

function of pO

and phosphate availability (PO

3−

), we use the

cGENIE Earth system model of intermediate complexity.

cGENIE consists of a 3D ocean circulation model plus a 2D

energy balance and moisture model and a 2D sea-ice model.

The 2D grid is split into 36 ×36 equal-area cells, while we

adopt 16 depth layers in the ocean, following Cao et al. (2009).

cGENIE simulates a 3D marine biosphere, including phos-

phorous and nitrogen-limited primary production, and a set of

metabolisms, including aerobic respiration, anaerobic respira-

tion, methanogenesis, and aerobic methanotrophy (Ridgwell

et al. 2007; Olson et al. 2016). A simple 2D (not vertically

resolved)gridded calculation of basic atmospheric chemical

reactions is also included (Reinhard et al. 2020). cGENIE has

been leveraged to explore the coupled evolution of Earth’s

biosphere, atmosphere, and climate system over the entire

geologic timescale, including the Archean (e.g., Olson et al.

2013), Proterozoic (e.g., Olson et al. 2016; Reinhard et al.

2016,2020), and Phanerozoic (e.g., Kirtland Turner &

Ridgwell 2016). cGENIE has most recently been used to

explore the relationship between planetary obliquity, nutrient

cycling, and the consequent potential for atmospheric oxygena-

tion on exoplanets (Barnett & Olson 2022).

The biological N cycle in cGENIE includes diazotrophy (the

biological reduction of N

to NH

, which can then be

incorporated into biomass), nitriﬁcation (the oxidation of NH

to NO

−

), and denitriﬁcation (the biological reduction of NO

−

to N

). These processes are highlighted in Figure 1.

Diazotrophy occurs only when N is scarce relative to phosphate

(PO

3−

)and N:P <16 (the “Redﬁeld Ratio”)within the photic

zone. Consequently, excess PO

3−

availability relative to N will

drive greater diazotrophy, until the global rates of N ﬁxation

balance N loss. The rates of nitriﬁcation and denitriﬁcation are

both sensitive to atmospheric pO

, which directly inﬂuences

surface and benthic oxygen concentrations, but their relation-

ships to oxygen differ dramatically. Nitriﬁcation requires O

whereas denitriﬁcation occurs in the absence of O

. Denitri-

ﬁcation additionally requires reduced organic material and is a

multistep process, with several intermediate N species between

−

and N

, such as N

O. The conﬁguration of cGENIE

employed here neglects this complexity and, by default,

assumes the complete reduction of NO

−

to N

when organic

matter and NO

−

are in sufﬁcient abundance and local dissolved

is low, following Naafs et al. (2019).

We estimate an upper bound on the possible N

Oﬂux arising

from incomplete denitriﬁcation for a given atmospheric pO

and ocean nutrient inventory by assuming that the entire

denitriﬁcation ﬂux results in the evolution of N

O(rather than

going directly to N

). This could occur, for instance, if the

nitrous oxide reductase, the enzyme that facilitates the last step

of the denitriﬁcation process (Pauleta et al. 2013), has not

evolved or if dissolved copper, which is key to the functioning

of this catalyst, was severely rate-limiting in abundance

(Buick 2007). Nitrous oxide reductase can also be substantially

inhibited in community settings by other biological products,

including C

, CO, NO, N

−

, and CN

−

(Kristjansson &

Hollocher 1980; Koutný & Kučera 1999). Our goal here is not

to be overly prescriptive of the speciﬁc scenarios in which

denitriﬁcation is incomplete, but instead to examine plausible

maxima in N

O production by Earthlike biospheres.

Figure 2shows the total denitriﬁcation ﬂux from Earth’s

marine biosphere as a function of pO

(relative to the present

atmospheric level, or PAL)and phosphate availability (PO

3−

relative to the present ocean level, or POL). We simulate pO

levels of 0%–100% and phosphate availability between one

and 2 times POL. The upper range of phosphate availability

represents a planet with higher nutrient availability from

enhanced continental weathering or a larger crustal abundance

The Astrophysical Journal, 937:109 (22pp), 2022 October 1 Schwieterman et al.

of P compared to Earth. Continental weathering could be

enhanced by a more robust hydrological cycle, greater

topographic relief, or a larger extent of coastal depositional

settings. Indeed, enhanced weathering in the wake of snowball

deglaciation may have resulted in PO

3−

concentrations

transiently exceeding 2 times POL in the late Neoproterozoic,

roughly coincident with the evidence for increases in oxygen

levels (Planavsky et al. 2010). Steady-state Plevels are

unlikely to have dramatically exceeded ∼2 times POL at any

point in Earth’s history (Reinhard et al. 2017b; Lenton et al.

2018; Reinhard & Planavsky 2022).

The denitriﬁcation rates increase with P availability, which

controls the organic C ﬂuxes. The relationship between

denitriﬁcation and atmospheric oxygen is more complex. At

low levels of oxygen, nitriﬁcation, which requires oxygen, is

limited. The denitriﬁcation rates are thus limited as well. At

very high levels of oxygen, nitriﬁcation occurs readily, but

denitriﬁcation, which requires low levels of oxygen, is

suppressed. Oxygen is heterogeneously distributed in the

ocean, such that both metabolisms may occur simultaneously,

despite being spatially separated. Nitriﬁcation is favorable in

well-oxygenated surface waters where oxygenic photosynthesis

occurs, whereas denitriﬁcation is most favorable in oxygen

minimum zones underlying productive regions of the surface

ocean, where high organic C ﬂuxes deplete O

via aerobic

respiration. This possibility may be particularly true for

atmospheric pO

that is lower than the present-day abundance.

Our results show a maximum denitriﬁcation ﬂux of

∼40 Tmol yr

−1

(1×POL P)to 100 +Tmol yr

−1

(2×POL P)

around an atmospheric pO

of ∼50% PAL O

. At this

intermediate oxygen level, the surface ocean is in equilibrium

with the atmosphere and is well oxygenated, but the deep ocean

remains poorly ventilated—optimizing the rates of both

nitriﬁcation and denitriﬁcation. However, the sensitivity to

is asymmetric around this level. At the lowest O

levels

(<20% PAL), denitriﬁcation is strongly attenuated (due to

limited nitriﬁcation; see, e.g., Anbar & Knoll 2002; Fennel

et al. 2005). Toward higher O

levels, denitriﬁcation falls off,

with a shallower linear decrease, as deep ocean oxygenation

increases. However, even under very high oxygen levels, such

as those of modern Earth, oxygen minimum zones persist and

allow denitriﬁcation in an otherwise well-oxygenated ocean.

To contextualize our calculations, the total denitriﬁcation

ﬂux on modern Earth is about ∼20 Tmol N

−1

, with

substantial uncertainties (Canﬁeld et al. 2010). As shown in

Figure 2, a planet with twice the nutrient P availability could

maintain a denitriﬁcation ﬂux of ∼100 Tmol yr

−1

even at near-

modern levels of O

. This value is similar to a study of

potential late Cretaceous (93 Ma)marine nitrogen cycling, for

which Naafs et al. (2019)calculated a denitriﬁcation ﬂux of

∼100 Tmol yr

−1

, given 2 times POL, modern oxygen, and 4

times modern CO

. We therefore adopt three ﬁducial N

ﬂuxes in our subsequent photochemical calculations of N

abundances: 1, 10, and 100 Tmol yr

−1

.Aﬂux of 1 Tmol yr

−1

represents 5%–10% of the global denitriﬁcation ﬂux of modern

Earth having evolved as N

O rather than N

, which is

approximately a factor of 2 higher than Earth’s estimated

global N

Oﬂux (Tian et al. 2020).Aﬂux of 10 Tmol yr

−1

represents 50%–100% of the global denitriﬁcation ﬂux of

modern Earth (i.e., all NO

−

that is consumed in denitriﬁcation

becomes N

O), while a ﬂux of 100 Tmol yr

−1

represents a

global biosphere with lower oxygen and higher P than modern

Earth, but consistent with periods of Earth’s history. We regard

the latter case as a reasonable upper bound for a weakly or fully

oxygenated Earthlike world.

Figure 2. Results from GENIE showing (a)denitriﬁcation rates, (b)surface oxygen concentrations, and (c)benthic oxygen concentrations as a function of atmospheric

oxygen (in terms of % of PAL; x-axis)and phosphate availability (in terms of POL; y-axis). Denitriﬁcation is optimized at intermediate atmospheric oxygen levels,

where there is sufﬁcient O

in the surface waters to stimulate surface nitrate production via nitriﬁcation, but insufﬁcient O

to oxygenate the deep ocean, which would

suppress denitriﬁcation.

The Astrophysical Journal, 937:109 (22pp), 2022 October 1 Schwieterman et al.

3. Calculating N

O Abundances for FGKM Stars with a

Photochemical Model

Here we test the N

Oﬂux–abundance photochemical

relationships for a comprehensive range of N

Oﬂuxes, which

include the bounds described above (1, 10, and 100 Tmol yr

−1

but also extend to much lower ﬂuxes. Note that we do not

explicitly distinguish ﬂuxes from the ocean and ﬂuxes from a

terrestrial biosphere in our atmospheric calculations. For

comparison, the total primary production of the land-based

denitriﬁcation is about one half that of the ocean (with

substantial uncertainty; see, for example, Falkowski et al. 2000;

Gruber & Galloway 2008). This difference is relatively small in

comparison to the increase in denitriﬁcation when increasing

the oceanic P availability from 1 to 2×POL, and we thus

consider plausible terrestrial ﬂuxes to be broadly included

within these original bounds.

3.1. Photochemical Model and Inputs

To calculate ﬂux–abundance relationships for Earthlike

planets as a function of N

Oﬂux and pO

, we use the

photochemical model component of the Atmos code

(Arney

et al. 2016). The code was originally developed by Kasting

et al. (1979)and has been improved by successive authors

(Pavlov et al. 2001; Zahnle et al. 2006; Arney et al. 2016;

Lincowski et al. 2018). The photochemical code uses the

reverse Euler method to solve the ﬂux and continuity equations

at each vertical layer, providing stable solutions at steady state.

The model uses a δtwo-stream method to calculate the

radiative transfer (Toon et al. 1989)and includes vertical

transport via molecular and eddy diffusion. The atmosphere is

divided into 200 layers of 0.5 km in altitude. The model

contains NO production by lightning (Harman et al. 2018)and

the H

O cross-sectional and sulfur gas reaction rate updates

recommended by Ranjan et al. (2020).

To enhance reproducibility, we use the publicly available

Atmos “ModernEarthSimple”template. This template includes

50 species and 238 photochemical reactions and is appropriate

for modeling major trace species (O

,CH

, CO, and N

O)on

high-oxygen Earthlike planets (e.g., Meadows et al. 2018a).

Table 1contains our assumed surface boundary conditions,

including the deposition rates and volcanic ﬂuxes. These

boundary conditions are consistent overall with those of the

modern Earth and those used in previous studies of O

-rich

planets (e.g., Segura et al. 2005; Schwieterman et al.

2019a,2019b; Wunderlich et al. 2020). We assume a variety

of O

abundances, ranging from 0.01–1.0 PAL, which is

equivalent to 0.002 to 0.21 bar. Our N

O surface ﬂuxes range

from 0.01 to 100 Tmol yr

−1

(3.7 ×10

to 3.7 ×10

molecules

−2

−1

). To further enhance reproducibility and isolate the

sensitivity to varied molecular ﬂuxes and stellar spectra, we

assume a surface pressure of P

=1 bar and a surface

temperature of 288 K for all cases with Earth’s modern

temperature–pressure proﬁle. N

is used as a ﬁller gas. We

compared our results to those obtained with the “Moder-

nEarthComplex”template (based on Lincowski et al. 2018),

which includes 71 additional reactions (309 total reactions)and

23 additional (73 total)chemical species, and found the

predicted N

O mixing ratios to be consistent between

templates. We also examined the sensitivity to stratospheric

temperature proﬁles and found minimal differences that are

small compared to our range of considered pO

levels, N

ﬂuxes, and stellar spectra. Tropospheric temperature proﬁles

should be relatively unaffected, assuming the same surface

temperature of 288 K. Substantially different surface tempera-

tures would impact H

O abundances (depending on the relative

humidity), which can have downstream impacts on CH

and

other trace gases, but will have smaller inﬂuences on N

given its major photochemical sinks (see below).

We sourced stellar spectra directly from the existing Atmos

library, including the additions from Arney (2019). Figure 3

shows the stellar spectra used in our simulations, with the

bottom panel zooming in on the UV component of each

spectrum and the molecular cross sections for N

O, O

, and

. Our solar spectrum was sourced from Thuillier et al.

(2004). The original source of the spectrum for the star HD

85512 (K6V)is the Measurements of the Ultraviolet Spectral

Characteristics of Low-mass Exoplanetary Systems treasury

survey (Youngblood et al. 2016; France et al. 2016; Parke

Loyd et al. 2018), and the original source for the Proxima

Centauri (M5V)spectrum is the Habitable Zones and M dwarf

Activity across Time program (Shkolnik & Barman 2014;

Parke Loyd et al. 2018; Peacock et al. 2020). The TRAPPIST-1

spectrum was the median average from the three-activity

models simulated by Peacock et al. (2019a,2019b). Note that

for TRAPPIST-1 and Proxima Centauri speciﬁcally, we

adopted ﬂux scaling consistent with TRAPPIST-1e and

Table 1

Photochemical Boundary Conditions

Chemical

Species

Deposition Velo-

city (cm s

−1

)

Flux (Molecules

−2

−1

)

Mixing

Ratio

O1LL

LLVariable

5×10

−5

6.9 ×10

OLLFixed

H1LL

OH 1 LL

1LL

0.2 LL

2.4 ×10

−4

CO 1.2 ×10

−4

3.0 ×10

HCO 1 LL

CO 0.2 LL

01×10

1LL

NO 1.6 ×10

-2

1×10

3×10

−3

HNO 1 LL

S 0.2 2 ×10

19×10

17×10

HSO 1 LL

0.07 LL

HNO

0.2 LL

OLVariable L

0.2 LL

OCS 0.01 1.57 ×10

Notes.

The tropospheric H

O proﬁle is ﬁxed to an Earth average (Manabe &

Wetherald 1967).

The species included in the photochemical scheme with a deposition velocity

and ﬂux of 0 include C

, HS, S, SO, S

,SO

,N,NO

, and N

https://github.com/VirtualPlanetaryLaboratory/atmos

The Astrophysical Journal, 937:109 (22pp), 2022 October 1 Schwieterman et al.

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EvaluatingthePlausibleRangeofN2OBiosignaturesonExo-Earths:AnIntegratedBiogeochemical,Photochemical,andSpectralModelingApproachEdwardW.Schwieterman1,2,3,4,StephanieL.Olson2,5,DariaPidhorodetska1,2,3,ChristopherT.Reinhard2,3,6,AinsleyGanti7,ThomasJ.Fauchez3,8,9,10,SandraT.Bastelberger8,10,11,12,JaimeS...

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Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths An Integrated Biogeochemical Photochemical and Spectral Modeling Approach_2

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