Evolution of Earth-like planetary atmospheres around M-dwarf stars: Assessing the atmospheres and biospheres with a coupled atmosphere biogeochemical model
S. Gebauer J. L. Grenfell R. Lehmann H. Rauer




Evolution of Earth-like  planetary atmospheres around M-dwarf stars:

Assessing the atmospheres and biospheres with a
     coupled atmosphere biogeochemical model








 S. Gebauer^1,2, J. L. Grenfell^2, R. Lehmann^3, and H. Rauer^1,2








(1) Zentrum für Astronomie und Astrophysik (ZAA), Technische Universität Berlin (TUB), Hardenbergstr. 36, 10623 Berlin, Germany



(2) Institut für Planetenforschung (PF), Abteilung Extrasolare Planeten und Atmosphären (EPA), Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherfordstr. 2, 12489 Berlin, Germany


(3) Alfred-Wegener Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Telegrafenberg A45, 14473 Potsdam, Germany











Running title: Earth-like atmosphere evolution around M-dwarf stars






Corresponding author:
 Dr. Stefanie Gebauer
 Institut für Planetenforschung (PF)
 Abteilung e Planeten und Atmosphären (EPA)

Deutsches Zentrum für Luft- und Raumfahrt (DLR)
 Rutherfordstr. 2
 12489 Berlin
 Germany
 phone: +49-(0)30-67055-454
 e-mail: stefanie.gebauerdlr.de






"Peer-review version of the manuscript. Final publication is available from Mary Ann Liebert, Inc., publishers http://doi.org/10.1089/ast.2017.1723"




Earth-like planets orbiting M-dwarf stars are prominent future targets when searching for life outside the solar system.
We apply our newly developed Coupled Atmosphere Biogeochemistry (CAB) model to investigate the coupling between the biosphere, geosphere and atmosphere in order to gain deeper insight into the atmospheric evolution of Earth-like  planets orbiting M-dwarf stars. Our main goal is to understand better atmospheric processes affecting biosignatures and climate on such worlds.
Furthermore, this is the first study to our knowledge which applies an automated chemical pathway analysis quantifying the production and destruction pathways of molecular oxygen (O_2) for an Earth-like  planet with an Archean O_2 abundance orbiting in the habitable zone of the M-dwarf star AD Leonis (AD Leo), which we take as a type-case of an active M-dwarf star.
Results suggest that the main production arises in the upper atmosphere from carbon dioxide (CO_2) photolysis followed by catalytic hydrogen oxide radical (HO_x) reactions. The strongest destruction does not take place in the troposphere, as was the case in <cit.> for an early-Earth analog planet around the Sun, but instead in the middle atmosphere where water (H_2O) photolysis is the strongest. This result was driven by the strong Lyman-α-radiation output of AD Leo, which efficiently photolyzes H_2O.
Results further suggest that early Earth-like atmospheres of planets orbiting an M-dwarf star like AD Leo are in absolute terms less destructive for atmospheric O_2 than for early-Earth analog planets around the Sun despite higher concentrations of reduced gases such as e.g. H_2, CH_4 and CO. 
Hence the net primary productivity required to produce the same amount of atmospheric O_2 at the surface is reduced. This result was due to a change in the atmospheric oxidative capacity, driven by the input stellar spectrum resulting in shifts in the intra-familiy HO_x partitioning. This implies that a possible "Great Oxidation event", analogous to that on Earth, would have occured earlier in time in analog atmospheres around M-dwarf stars, assuming the same atmospheric O_2 sources and sinks as on the Early Earth. For an Earth-like planet around an active M-dwarf, smaller amounts of net primary productivity (NPP) could therefore be required to oxygenate the atmosphere compared to Earth around the Sun.

Key words: Earth-like – Oxygen – M-dwarf stars –
   atmosphere – biogeochemistry – photochemistry – biosignatures – Earth-like planets
 






§ INTRODUCTION

The study of <cit.> (hereafter paper I) addressed the evolution of early Earth analog planets around the Sun and the implications for their atmospheres and biospheres by applying a Coupled Atmosphere Biogeochemical (CAB) model. In the present paper we investigate Earth-like  planets with differing contents of molecular oxygen (O_2) orbiting in the Habitable Zone (HZ) of the M-dwarf star AD Leonis (AD Leo), which we take as a type-case for an active M-dwarf host star. 

M-dwarf stars make up about 75% of all stars in the galaxy <cit.> and are favored observational targets. Due to their very low masses, the time which they spend on the main sequence theoretically exceeds the lifetime of the universe <cit.>. Their HZ lies close-in due to the low stellar luminosity. An increasing number of so-called super-Earths have been detected orbiting such stars (e.g. ). The newly discovered planet with 1.3 Earth masses orbiting Proxima centauri <cit.> is a promising candidate for an Earth-like  planet in the HZ of an M-dwarf star.

Due to the stellar characteristics of M-dwarf stars however, several issues arise related to the potential habitability of planets in their HZs.
These stars show a strong temporal variability and are usually very active emitting large amounts of (E)UV radiation and X-rays (see ). Coronal mass ejections are a frequent phenomenon for active M-dwarf stars which may erode the planetary atmosphere <cit.>. M-dwarf stars exhibit a dense and fast stellar wind <cit.>. Due to the very close-in HZ planets are likely to become tidally-locked within a short time after formation <cit.> leading to a possible atmospheric collapse <cit.> on the night side or evaporation on the day-side. It was shown by <cit.> that the inner HZ edge could be extended towards the star due to synchronous rotation of the planet coupled with cloud formation. However, this result was contested by a recent study of <cit.> who applied a general circulation model with self-consistent relationships between stellar metallicity, stellar effective temperature, and the planetary orbital/rotational period.  

Earth-like planets around M-dwarf stars might exhibit possibly weak planetary magnetic fields, hence experience potentially strong fluxes of cosmic rays which could have an impact on the atmosphere <cit.> and on biosignature signals <cit.>. More recent 3D model studies of Earth-like planets orbiting M-dwarfs focus on determining habitability see e.g. <cit.> and references therein. 

M-dwarf star planets have a deficiency in Photosynthetically Active Radiation (PAR) (which lies predominantly in the wavelength range of 400 - 720 nm) because PAR emitted by such stars is weakened by e.g. photospheric absorption of TiO. The solar flux at the surface of the Earth has a maximum at 685 nm whereas for AD Leo the peak surface flux assuming Earths atmosphere lies around 1045 nm resulting in 0.125 times the Earth's 400 - 700 nm PAR band[For Earth the global, time-averaged surface incident photon flux is 2.4·10^20 photons/m^2/s.] <cit.>. That study showed that photosynthesis could also proceed at wavelengths longer than 720 nm. 



For comprehensive reviews of occurrence and habitability of planets orbiting M-dwarf stars see <cit.> and <cit.>.

In this paper we focus on Earth-like  planets orbiting in the habitable zone at 0.153 AU (Astronomical Unit) around the well-studied M-dwarf star AD Leo where the total stellar wavelength-integrated energy input equals that of 1 Solar Constant (1366 W m^-2). Fig. <ref> shows the stellar spectrum of AD Leo in comparison to the Sun illustrating that the stellar flux of AD Leo is generally higher in the IR whereas in the visible it is reduced in comparison to the Sun. Furthermore, AD Leo shows high stellar activity in the (E)UV range.











 §.§ Contribution of this Work

In this work we will investigate different evolutionary stages of hypothetical Earth-like planets orbiting AD Leo using Earth as a reference system and applying the CAB model. Ours is the first study to our knowledge which considers detailed coupled climate-photochemistry, including interactive atmospheric O_2, CO_2 and N_2 and including biogeochemical processes for Earth-like atmospheres around M-dwarf stars. 
Assuming the same biological and geological evolution as on Earth we will calculate the impact of the different stellar environment on the NPP of oxygenic photosynthesis. 

For the first time we investigate atmospheric chemical responses of O_2 in an Earth-like  planetary atmosphere for an Archean type atmosphere (with 10^-6 PAL O_2) by applying the Pathways Analysis Program (PAP) developed by <cit.>.

The paper is structured as follows:
Section <ref> and Section <ref> describe briefly the models used. Section <ref> gives an overview of the simulated Earth-like  planet scenarios around AD Leo whereas in Section <ref> the results are presented and discussed. Our main conclusions are presented in Section <ref>.



§ COUPLED ATMOSPHERE BIOGEOCHEMICAL (CAB) MODEL

In order to address the interactions between atmospheric, geological, chemical and biotic systems the Coupled Atmosphere Biogeochemical (CAB) model as discussed in paper I is applied. The CAB model is composed of two submodules: 1. atmosphere and 2. biogeochemistry (based on ). Both submodules are coupled via the stagnant boundary layer model <cit.>. 
The 1D global mean cloud-free, steady state atmospheric module based on e.g. <cit.> is composed of a climate and a photochemistry part calculating temperature, water and chemical species profiles. Unlike earlier works, the concentrations of O_2, CO_2 and N_2 are not simply held constant to fixed isoprofile values but are instead calculated interactively in the chemistry. All relevant atmospheric sources and sinks of O_2 are implemented, e.g. volcanic and metamorphic outgassing of reduced chemical species according to geological time, t_geo, and hydrogen escape at the top-of-the-atmosphere (TOA). For a more detailed description we refer to paper I.




§ PATHWAYS ANALYSIS PROGRAM

The Pathway Analysis Program (PAP) was developed by <cit.> in order to automatically identify chemical pathways in any arbitrary chemical reaction network and to quantify their efficiencies by assigning rates. PAP was applied by e.g. <cit.> and in paper I. The algorithm yields a list of all dominant pathways which produce, destroy or recycle a chemical species of interest. For a more detailed description of PAP see paper I and <cit.>.




For the analysis of large and complex reaction networks chemical pathways with a rate below a user-defined threshold rate f_min are deleted in order to avoid long computational time. In the present study, f_min=10^-12 parts per billion by volume per second (ppbv/s)) was chosen which is sufficient for finding the dominant pathways producing or consuming atmospheric O_2 in our simulated atmosphere. A PAP analysis was performed for each of the 64 vertical layers of the atmospheric column module. The resulting production and destruction rates of O_2 from each individual pathway are integrated over the atmospheric module vertical grid and are expressed as a percentage of the total column-integrated production and destruction rate from all pathways found by PAP.






§ APPLICATION TO EARLY EARTH ANALOG ATMOSPHERES AROUND AD LEO

For simulating  planets at different evolutionary stages the O_2 volume mixing ratio of the atmosphere at the surface is taken from the evolutionary path of O_2 adapted from <cit.> (shown in paper I, their Fig. 3). We do not assume any luminosity change of AD Leo because this stays relatively constant over its evolution <cit.>. 

For all runs we assume:

	
  * AD Leo input spectrum,
	
  * surface pressure p_surf=1 bar with N_2 as a fill-gas,
	
  * gravity acceleration g=981 cm s^-2,
	
  * biogenic surface fluxes of CH_4 (474 Tg/yr), CO (1796 Tg/yr), N_2O (13.5 Tg/yr), CH_3Cl (3.4 Tg/yr) (these values lead to modern Earth abundances for the given species), deposition velocities of all other chemical compounds as in paper I,
	
  * crustal mineral redox buffer is set to quartz-fayalite-magnetite (QFM) (Δ f_O_2=0),
	
  * surface vmr of CO_2 of 355 ppm.
	

Volcanic emissions of H_2, H_2S, SO_2, CO, CH_4, and CO_2 and metamorphic emissions of H_2 and CH_4 are varied according to the geological time t_geo.

 




§ RESULTS AND DISCUSSION



 §.§ Atmosphere Modeling



  §.§.§ Climate Responses

The p-T profiles for selected Earth-like  atmosphere runs around AD Leo are shown in Fig. <ref>. 
For the 1 PAL O_2 atmospheric run stratospheric temperatures are reduced
in comparison to modern Earth around the Sun (see Fig. 4 in Paper I) mainly due to the reduced UV flux longwards of about 200 nm (see Fig. <ref>) which results in less radiative heating by ozone (O_3). On reducing the atmospheric O_2 content, i.e. going backwards in time, the stratospheric temperatures shown in Fig. <ref> decrease
by about 40 K due to overall less atmospheric heating by O_3.
A weak temperature inversion develops at about 0.01 bar due to increased heating
by O_3 and less cooling by CO_2 in the upper stratosphere of Earth-like  atmosphere runs with lower O_2 contents as a result of enhanced O_3 concentrations (see Fig. <ref>) and decreasing CO_2 mixing ratios towards lower atmospheric pressures (see Fig. <ref>).

On going forwards in time, i.e. increasing O_2 surface concentrations, surface temperatures (see Fig. <ref>) increase from 296.9 K before the GOE at 10^-6 PAL O_2 to 305.1 K at 1 PAL O_2. Generally, surface temperatures are higher than for runs around the Sun firstly, due to less Rayleigh scattering and secondly, due to an enhanced greenhouse effect from molecular absorption by CH_4 and H_2O which are both enhanced in these atmospheres (see Section <ref>). Since total stellar luminosity is held constant we do not find an overall increase of surface temperature with time as observed for the early-Earth analog planets around the Sun. Furthermore, we observe a small increase in surface temperature around the Second Oxidation Event (SOE). Further analysis suggested this was related to a decrease in hydroxyl (OH) which has a complex photochemistry. This in turn led to an increase in the greenhouse gas methane (CH_4) hence surface warming and a rather strong increase in H_2O via evaporation around that time (see Section <ref>).

For the low O_2 atmosphere run around AD Leo shortwave radiation is less absorbed, whereas Rayleigh scattering is increased and greenhouse warming is reduced compared to the 1 PAL O_2 run. 

As in paper I we will now validate the net integrated TOA fluxes calculated by our long-wave radiation module RRTM <cit.> against the line-by-line (lbl) radiative transfer code SQuIRRL (Schwarzschild Quadrature InfraRed Radiation Line-by-line; developed by ). For all runs considered a maximum relative change of 1.0 % was found.






  §.§.§ Photochemical Responses



  
UV Radiation environment
Generally, for an Earth-like  planet orbiting in the HZ of the M-dwarf AD Leo the UV radiation environment is less strong than compared to Earth around the Sun. For the AD Leo scenarios at the surface, UVA radiation is virtually constant on going forwards in time. One calculates before the GOE 1.49 W m^-2 and afterwards 1.48 W m^-2. UVB radiation is enhanced at the surface before the GOE with 0.16 W m^-2 in comparison to atmospheres after the GOE with 0.02 W m^-2. For the UVC radiation[Here, UVC radiation denotes the wavelength range from 253-280 nm.] at the surface before the GOE we calculate 0.13 W m^-2 whereas afterwards it is virtually zero. 
Fig. <ref> shows the radiation shielding efficiency, R_UV, (i.e. the ratio (surface/TOA) radiation) of such atmospheres in comparison with an early-Earth analog planet around the Sun. Low R_UV values denote high shielding efficiency. UVA and UVC radiation is shielded less efficiently (higher R_UV) for the AD Leo atmosphere scenarios whereas for UVB, the shielding efficiency is higher.


In the following text we focus on the photochemical analysis of important atmospheric constituents such as proposed biosignatures (O_2, O_3, N_2O) and related compounds (CH_4, H_2O, CO_2) as in paper I. 


  
Ozone - O_3
Fig. <ref> presents selected O_3 profiles of Earth-like  atmosphere runs around the M-dwarf AD Leo.
For an Earth-like  planet around AD Leo with 1 PAL O_2 the O_3 profile is comparable to that on modern Earth. Similarly to scenarios presented in paper I, the O_3 maximum moves to higher pressures on reducing the O_2 surface mixing ratio due to the ozone-UV feedback as discussed in paper I. For atmospheres with ground level O_2 concentrations at and below 10^-3 PAL (i.e. the solid colored lines in Fig. <ref>) stratospheric O_3 starts to strongly increase towards lower pressures because of enhanced O_2 concentrations due to CO_2 photolysis (see Fig. <ref> and Section <ref> for chemical analysis). Below 2.5· 10^-5 PAL O_2 a local O_3 minimum becomes more pronounced and penetrates deeper into the atmosphere as O_2 is reduced. This is related firstly, to reduced O_2 abundances (see again Fig. <ref>) and secondly, due to enhanced HO_ concentrations (= OH + HO_2 + H, shown in Fig. <ref>) which catalyze O_3 loss in the middle atmosphere. Thereby, H is the dominant species which is increasingly produced by H_2O photolysis in the pressure regime between 0.1 and 0.01 bar via H_2O + hν → H + OH (see below). Additional photolytic destruction of CO_2 via CO_2 + hν → CO + O and subsequent reaction between OH and CO via CO + OH → H + CO_2 (not shown) represents an additional source for H.
Fig. <ref> suggests that in Earth-like atmospheres around the M-dwarf AD Leo compared to early-Earth analog planets around the Sun the O_3 column amount is generally reduced after the GOE. A small minimum arises around the SOE due to an increase in surface temperature, hence H_2O and therefore HO_. Normally, O_3 is considered to be a good proxy for O_2 (see e.g. <cit.>). Fig. <ref> however, suggests that this assumption may not always be valid since around the SOE O_3 decreases with increasing O_2 for the AD Leo scenarios. This further illustrates the sensitivity of OH photochemistry.





  
Water - H_2O
H_2O profiles for the same atmospheric scenarios are presented in Fig. <ref>. In comparison to the scenarios presented in paper I around the Sun, the tropospheres of Earth-like  atmosphere scenarios around AD Leo are generally more wet due to more evaporation of H_2O via a large CH_4 greenhouse (see below). For O_2 concentrations above 0.01 PAL O_2 the H_2O mixing ratio stays approximately constant with height above the tropopause. Below 0.01 PAL O_2 H_2O mixing ratios decrease in the upper stratosphere because of strong photolytic destruction. The overall column amount of H_2O, shown in Fig. <ref>, is increased by up to an order of magnitude in comparison to the scenarios presented in paper I. After an increase around the GOE the H_2O column amounts stay relatively constant after the GOE with a second increase around the SOE.





  
Nitrous oxide - N_2O
On going forwards in time N_2O mixing ratios steadily increase as shown in Fig. <ref>. Compared to the scenarios in paper I, N_2O concentrations in atmospheres of Earth-like  atmosphere scenarios around AD Leo are enhanced which is also reflected in the total column amount shown in Fig. <ref>. This arises due to much weaker UVB emissions from AD Leo compared with the Sun (see Fig. <ref>) and weaker excited oxygen [O(^1D)] which comes from weaker O_3 photolysis (see below) since both UVB and  O(^1D) are major sinks for N_2O.





  
Methane - CH_4
Fig. <ref> shows how the CH_4 profile increases on going forwards in time. Increasing the O_2 content leads to enhanced O_3 formation, hence UV absorption and therefore to less photolytic destruction of CH_4 in the upper stratosphere. In comparison to the scenarios presented in paper I, CH_4 concentrations are increased for the AD Leo atmosphere scenarios which is directly related to less destruction by OH which is less abundant in the AD Leo atmosphere scenarios (see Fig. <ref>) because of decreased production via H_2O + O(^1D) → 2OH than for scenarios around the Sun. Although H_2O is more abundant in these atmospheres, the mixing ratio of O(^1D) is reduced in the middle and lower atmosphere because of a decreased production via O_3 + hν → O_2 + O(^1D). Also, the reduced OH concentration leads to less destruction of CO via CO + OH → CO_2 + H resulting in higher CO concentrations for Earth-like  planetary atmospheres around AD Leo compared to the Sun. The overall column amount of CH_4 is also given in Fig. <ref>. CH_4 increases (as OH decreases) on going forwards in time for the AD Leo atmosphere scenarios. The strong increase of CH_4 around the SOE is related to a rather modest decrease in OH and less photolytic destruction.





  
Carbon dioxide - CO_2
Fig. <ref> shows CO_2 profiles of Earth-like  atmosphere scenarios around the M-dwarf star AD Leo. In all atmosphere scenarios CO_2 strongly deviates from the iso-profile assumption usually applied in the literature (see e.g. ), e.g. at 1 PAL O_2 the CO_2 concentration increases from the surface to TOA by about 40%. Below pressures of 0.01 bar, CO_2 concentrations increase generally with height due to an increase in UV radiation in the wavelength range favoring OH production. This leads to stronger net production of CO_2 via the reaction CO + OH → CO_2 + H resulting in a decrease in CO towards lower pressures (not shown).
With decreasing O_2, CO_2 further increases from run to run for ground level O_2 concentrations above 2.5· 10^-3 PAL. Below this O_2 content (colored lines) the mixing ratio of CO_2 decreases towards lower pressures because of enhanced photolytic destruction.


 


  
Molecular oxygen - O_2
O_2 increasingly deviates from the usually applied iso-profile assumption
below 10^-3 PAL O_2 as seen in Fig. <ref>). The strong decrease in mixing ratio between 0.1 and 0.01 bar for the low O_2 atmospheres is correlated to enhanced H abundances which can lead to fast removal of O_2 via: H + O_2 + M → HO_2 + M in this pressure range. This also directly impacts the behavior of O_3 which is also reduced in this range. In the upper stratosphere O_2 production is initiated via the photolysis of CO_2 (see for detailed chemical analysis in Section <ref>). 

The altitude dependence of the net (P - L) chemical change of O_2 from chemical production and destruction is depicted in Fig. <ref>. In comparison to the atmospheric scenarios around the Sun presented in paper I, O_2 is now more strongly destroyed in the upper stratosphere (rather than the troposphere as in the case for the Sun). The net destruction peak which forms for the AD Leo atmosphere scenario with 10^-3 PAL O_2 at about atmospheric layer 50 (corresponding to 7.9 · 10^-4 bar) moves to lower altitudes on reducing the surface O_2 level concentration. This is directly correlated to a stratospheric maximum in HO_x as a sink for O_2 which also moves to lower altitudes (not shown). On reducing the surface O_2 concentration the net destruction in the troposphere becomes relatively stronger but is still overall weaker than for the atmosphere scenarios around the Sun presented in paper I. The OH radical which is crucial in the O_2 destruction pathways found by PAP for an atmosphere with a surface O_2 mixing ratio of 10^-6 PAL O_2 around the Sun presented in paper I is strongly reduced in the troposphere for the corresponding AD Leo case with 10^-6 PAL O_2 around AD Leo, although other related chemical species such as H, HO_2 and H_2O_2 (see Fig. <ref>) are enhanced. This partitioning occurs due to less photolytic destruction of reservoir species producing OH in the lower atmosphere. In addition to the stronger CO_2 photolysis in the upper stratosphere for an Earth-like  planetary atmosphere with 10^-6 PAL O_2 around AD Leo, the abundances of the HO_ radicals in the upper stratosphere are also enhanced, therefore, increasing the efficiencies of the O_2 production pathways which have been presented in paper I.
In general, the resulting atmospheric surface flux of O_2 from the column-integrated production and destruction of O_2 in the atmosphere which is needed to maintain a specific O_2 surface mixing ratio is lower by several hundreds of Tg/yr for the AD Leo atmosphere scenarios than for the Sun cases presented in paper I (the relative change between both surface fluxes varies between 6 to 39%). This suggests that Earth-like atmospheres around the M-dwarf star AD Leo could be more readily oxidized for a given photosynthetic biosphere.

A detailed chemical analysis of the production and destruction pathways of O_2 in the atmosphere of an Earth-like planet orbiting the M-dwarf star AD Leo with 10^-6 PAL O_2 now follows in Section <ref>. Here, we quantify the most dominant production and destruction pathways of O_2 by applying PAP.








  §.§.§ Pathway Analysis with respect to O_2



PAP (for details see Section <ref>) is applied to an exoplanetary atmosphere for the AD Leo scenario with a surface O_2 vmr of 10^-6 PAL O_2 (t_geo=2.688 Gyrs). Our study represents the first application of PAP in the context of the Earth-like  planet with low O_2. As in paper I, we consider only pathways with an individual contribution larger than 1.5% to the total column production (or destruction) of O_2. CO was excluded as a branching point species in order to ensure a consistent treatment of all O_2 production and destruction pathways at every atmospheric height. Hence, CO is regarded as a potential source and sink for CO_2 in every atmospheric layer.



  
Production pathways and their altitude dependence
A summary of the major column-integrated chemical O_2 production pathways P1-P7 found by PAP and their percentage contribution to the total colum-integrated O_2 production rate is given in Tab. <ref> for an Earth-like  atmosphere with 10^-6 PAL O_2 orbiting in the HZ of the M-dwarf star AD Leo in comparison to the pathways and their percentage contribution found in paper I for early Earth analog planets around the Sun.
As in paper I, these pathways can be broadly categorized into three classes, namely:

	
  * class PA: O_2 production via net reaction 2CO_2 → O_2 + 2CO (initiated by CO_2 photolysis)
		
			
  * subclass PA1: catalyzed by HO_x
				
					
  * PA1a (P1): catalyzed by O_3
					
  * PA1b (P2): without O_3
				
			
  * subclass PA2 (P3): without HO_x
		 
	
  * class PB: O_2 production via net reaction 2O → O_2
		
			
  * subclass PB1: catalyzed by HO_x
				
					
  * PB1a (P4): catalyzed by O_3 (new for AD Leo)
					
  * PB1b (P7): without O_3
				
			
  * subclass PB2 (P6): without HO_x
		
	
  * class PC (P5): O_2 production via net reaction H_2O + CO_2 → O_2 + 2H + CO (new for AD Leo)

Note that pathways of subclasses PB1a and PA1a, PB1b and PA1b and PB2 and PA2 are closely related. They differ in providing O by either in-situ photolysis of CO_2 or delivery of O by diffusion from photolyzed CO_2 in other atmospheric layers.
 
Fig. <ref> shows the relative contributions of the pathways mentioned above to the total column-integrated production rate of O_2. Pathways P1, P2, P3, P6 and P7 are also present in the paper I analysis but their relative contributions are very different. In Earth-like  atmospheres around the M-dwarf AD Leo the largest contribution originates from subclass PA1. The contribution of pathway P1 (P7 in paper I, i.e. I-P7, the same notation is used for all other pathways found in paper I) is now increased from 2.3% to 39.6% because of enhanced O_3 and HO_x in comparison to paper I (for O_3 see their Fig. 7 and for HO_x see Fig. <ref>). Pathway P2 is slightly increased whereas for P3 (I-P3 in paper I) a distinct relative decrease from 20.7% to 9.3% is observed which is related to the fact that the total O_2 production rate is 3 times higher in the case of Earth-like  planets orbiting AD Leo (compare Fig. 17 in paper I with Fig. <ref>). There are two new pathways present, namely P4 and P5. P4 contains similar reactions as in P1 but is not initiated by the photolysis of CO_2 and subsequent relaxation of O(^1D) only depending on the presence of O delivered by diffusion.
Pathway P5 contains the photolysis of CO_2 and H_2O releasing O and OH which then recombine to form O_2 and H.
The percetange contributions of pathways P6 and P7 are similar to the results of paper I.    

Fig. <ref> shows the altitude dependence of the O_2 production pathways for an Earth-like  planet with 10^-6 PAL O_2 around AD Leo. The total production rate of O_2 (solid red line) in the CAB model is also plotted. The sum of all pathways found by PAP amounts to 98.3% of the total O_2 production rate.
Fig. <ref> indicates that the strongest production of O_2 occurs in the upper atmosphere mainly from pathways initiated by CO_2 photolysis where UV radiation is strong. A small contribution to the total production of O_2 arises from pathway P5 which additionally photolyzes H_2O due to strong shortwave radiation penetrating deeply into the atmosphere.



  
Destruction pathways and their altitude dependence
A summary of the major column-integrated chemical O_2 destruction pathways L1-L9 found by PAP and their percentage contribution to the total column-integrated O_2 destruction rate is given in Tab. <ref> for an Earth-like  atmosphere with 10^-6 PAL O_2 orbiting around the M-dwarf star AD Leo in comparison to the pathways and their percentage contribution found in paper I for early Earth analog planets around the Sun.
As in paper I, these pathways can be categorized into 4 classes, namely:

	
  * class LA: O_2 destruction via net reaction O_2 + 2CO → 2CO_2 (CO oxidation)
		
			
  * subclass LA1 (L1): catalyzed by HO_x but without photolysis
			
  * subclass LA2: catalyzed by HO_x but with photolysis of 
			
					
  * LA2a (L2): H_2O_2
					
  * LA2b (L4, L6, L7): O_2
				
		 
	
  * class LB: O_2 destruction via CH_4 oxidation pathways
		
			
  * subclass LB1 (L3): net reaction 3O_2 + 2CH_4 → 4H_2O + 2CO
			
  * subclass LB2 (L8): net reaction O_2 + CH_4 → CH_3OOH (new for AD Leo)
		
	
  * class LC (L9): O_2 destruction via net reaction O_2 + 2H_2 → 2H_2O
	
  * class LD (L5): O_2 destruction via net reaction O_2 + H_2O + CO → H_2O_2 + CO_2


Fig. <ref> shows the relative contributions of the pathways mentioned above to the total column-integrated destruction rate of O_2. Pathways L1, L2, L3, L4, L5, L6 and L9 are also present in early-Earth analog atmosphere around the Sun presented in paper I but their relative contributions are very different.
In an Earth-like  atmosphere with 10^-6 PAL O_2 around AD Leo the largest contribution (66.3%) arises from class LA, i.e. oxidation of CO. Within this class, pathway L1 dominates the column-integrated O_2 destruction with 41.8% because of enhanced HO_x species released by strong photolysis of H_2O in this pressure range whereas this pathway, L8 in paper I, i.e. I-L8 (the same notation is used for all other pathways found in paper I), is rather minor for the scenario presented there.
The contribution of pathway L2 (I-L1) is now decreased from 28.5% to 16.7% because of less OH in the lower atmosphere compared to paper I (see Fig. <ref>). The contributions of other pathways from e.g. CH_4 and H_2O oxidation are rather small.  
There are two new pathways present, namely L7 and L8. 
L7 is additionally catalyzed by O_3 which is more abundant in this altitude range than in the early Earth analog planet with 10^-6 PAL O_2 around the Sun presented in paper I.
Pathway L8 is initialized by the photolysis of H_2O resulting in the oxidation of CH_4 by forming CH_3OOH.
 
The altitude dependance of the presented destruction pathways for an Earth-like  atmosphere with 10^-6 PAL O_2 around the M-dwarf AD Leo is given in Fig. <ref>. The total destruction rate of O_2 is also shown (solid red line). The sum of all pathways found by PAP amounts to 89.9% of the total O_2 destruction rate. The strongest destruction of O_2 occurs in middle atmosphere followed by a smaller contribution in the troposphere where destruction is composed of a large number of pathways individually contributing less than 1.5%. 
The gap between both regimes can be attributed a minimum in CO (not shown) as well as HO_2 and H_2O_2 (see Fig. <ref>) in this pressure region.

The minimum in O_2 vmr which is seen in Fig. <ref> results from low O_2 production and destruction rates (although there is a local maximum in O_2 production). From Fig. <ref> one can deduce that these rates cancel each other out in the middle atmosphere (dot-dot black line), hence we can conclude that the increase in O_2 vmr (see Fig. <ref>) with altitude is due to diffusion from above atmospheric layers where O_2 is produced abiotically from CO_2 photolysis. 

In summary, for an Earth-like  atmosphere with 10^-6 PAL O_2 around the M-dwarf AD Leo, O_2 can be produced in-situ from CO_2 photolysis followed by HO_x catalyzed recombination of O to form O_2 in the upper atmosphere. This result was already proposed by other studies such as e.g. <cit.> but has now been quantified by our work. 
The destruction of O_2 is composed of different oxidation pathways, mainly of CO as already shown in paper I. However, in the present paper the main contribution arises from the middle atmosphere where the H_2O photolysis is the strongest followed by the tropospheric pathways destroying O_2. Note too that as in paper I the biological O_2 flux originating from the surface is orders of magnitude higher than the abiotic in-situ production in the atmosphere.




 §.§ Biogeochemical Modeling

The CAB model calculates how much input from oxygenic photosynthesis, N (NPP), is needed to maintain the surface flux Φ^atm_O_2 calculated by the atmospheric chemistry module for a given surface vmr of O_2.
For M-dwarf star planets PAR is reduced resulting in less net
primary productivity by oxygenic photosynthesis (see comments above and Kiang et al. 2007).
The CAB model contains no information on the sensitivity of photosynthesis[Photosynthetic (light harvesting) pigments peak in absorbance at wavelengths where light intensity on the planetary surface is highest (Kiang et al. 2007).] to the stellar spectrum. This issue however is beyond the scope of our work.
Fig. <ref> shows the relative change in % in input from oxygenic photosynthesis, N, over geological time for constant input of Fe^2+ for anoxygenic photosynthesis, r, if one assumes Earth's biosphere evolution for an Earth-like  planet orbiting AD Leo. 

In regime I, before about t_geo=2.24 Gyrs ago (below roughly 10^-3 PAL O_2) the input from oxygenic photosynthesis, N, which is needed to maintain a specified O_2 surface mixing ratio, is less than for planets around the Sun presented in paper I. The lower the O_2 content (going further back in time) the lower the N compared to the Sun. In this regime N is sensitive to the O_2 surface flux, hence the chemical nature of the atmosphere. The reduced N implies that even for reduced NPP, the accumulation of atmospheric O_2 by photosynthesizers might still be favorable for Earth-like  planets around AD Leo because of the less destructive nature of the atmosphere for O_2 (see Fig. <ref> for net (P - L) chemical change of O_2 for Earth-like  atmospheres around star AD Leo) despite higher concentrations of reduced gases such as e.g. H_2, CH_4 and CO. This is driven by weaker tropospheric OH, hence weaker loss of O_2 via CO oxidation and stronger abiotic O_2 production due to CO_2 photolysis in the AD Leo case.

In regime II, after about t_geo=2.24 Gyrs ago (above around 10^-3 PAL O_2), N is more sensitive to the O_2 partial pressure and surface temperature rather than atmospheric chemistry, hence O_2 surface flux (as was shown in paper I). Although O_2 surface fluxes are lower than compared to the Sun, N is increased by about 7% due to higher p_ and T_ which is correlated to the greenhouse effect of gases such as H_2O and CH_4.





§ CONCLUSION

This is the first study with our newly developed CAB model <cit.> addressing Earth-like  planets with varying atmospheric O_2 content orbiting in the HZ around the M-dwarf star AD Leo.

Despite higher atmospheric CH_4, CO and H_2 concentrations than for scenarios presented in paper I for the Sun, O_2 is overall less destroyed in the AD Leo case e.g. because of smaller OH concentrations in the troposphere in comparison to the Sun scenarios. Hence this leads to a weakening in the OH catalyzed loss of O_2 via CO oxidation.

For the AD Leo case with 1 PAL O_2 the CO_2 concentrations increase towards higher altitudes by 40% implying that the usually applied iso-profile assumption (see e.g. ) is not valid in such atmospheres.

Lowering the ground level O_2 concentration led to in-situ production of O_2 from CO_2 photolysis in the upper stratosphere resulting in an increase in O_2 abundances and strong depletion of CO_2 mixing ratios towards higher altitudes in these atmospheres. A minimum in O_2 concentration becomes visible for the low O_2 atmosphere scenarios which is related to the different partitioning of HO_x species over atmospheric height. 
Generally, the atmospheres of Earth-like  planets around the M-dwarf star AD Leo with different surface O_2 concentrations are less destructive towards O_2 than for the scenarios presented in paper I for the Sun. Results therefore suggest
that the accumulation of O_2 in Earth-like  planets around AD Leo might be chemically more favorable compared to Earth-like planets orbiting around G-type stars (such as the Sun). 

The current work presents the first application of PAP quantifying the production and destruction pathways of O_2 for an Earth-like  planet with 10^-6 PAL O_2 orbiting the M-dwarf star AD Leo.
It was shown the the main production arises from CO_2 photolysis followed by catalytic HO_x reactions in the upper atmosphere whereas the strongest destruction arises not from the troposphere, as was the case in paper I, but instead in the middle atmosphere where H_2O photolysis is stronger.

If one assumes the same biosphere evolution for Earth-like  planets around the M-dwarf star AD Leo as on Earth then the net primary productivity, N, from oxygenic photosynthesis is strongly decreased for Earth-like 
planets with a surface O_2 mixing ratio below about 10^-3 PAL O_2. This implies that the productivity required for a given atmospheric O_2 abundance is less strong than in paper I. For atmospheres having ground
level O_2 concentrations above 10^-3 PAL O_2 N is slightly increased in comparison to the Sun
scenarios due to the higher surface temperatures, hence partial pressures of O_2.

In terms of next generation missions aiming at the characterization of Earth-like  planets the results of this work imply that the feasability of the presence of atmospheric O_2, hence O_3, might be enhanced because a potential "Great Oxidation Event" might have had occured earlier in geological time since the oxidative capacity driven by the stellar input spectrum is decreased. However, the distinguishabilty of biotic and abiotic O_2 and O_3 needs further investigation.




Acknowledgments:

Stefanie Gebauer acknowledges support by the DFG SPP 1833 "Building a Habitable Earth".

This research was supported by the Helmholtz Gemeinschaft
  through the research alliance "Planetary Evolution and Life".



Author Disclosure Statement:

No competing financial interests exist.

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 c c c c c

Summary of dominant chemical production (P) pathways of O_2 found by PAP and their percentage contribution to the total column-integrated O_2 production rate for an Earth-like  atmosphere with a surface O_2 vmr of 10^-6 PAL O_2. Only pathways that contribute >1.5% of the total column-integrated production rate are shown. For an explanation of classes and subclasses see text. Values in parentheses indicate the pathway number, i.e. I-Px, and percentage contribution from paper I.



class     subclass     pathway number     contribution [%]     pathway


class     subclass     pathway number     contribution [%]     pathway













 PA      PA1a     P1 (I-P7)     39.6 (2.3)     2·(CO_2 + hν → CO + O(^1D))
			
                    2·(O(^1D) + N_2 → O + N_2)

         	        O + O_2 + M → O_3 + M
			
         	        OH + O → H + O_2
			
         	        H + O_3 → OH + O_2
			
         	        net: 2CO_2 → O_2 + 2CO
    PA1b     P2 (I-P1)     30.8 (28.4)     2·(CO_2 + hν → CO + O(^1D))
			
         	        2·(O(^1D) + N_2 → O + N_2)
			
         	        HO_2 + O → OH + O_2
			
         	        OH + O → H + O_2
			
         	        H + O_2 + M → HO_2 + M
			
         	        net: 2CO_2 → O_2 + 2CO






  	
     PA2     P3 (I-P3)     9.3 (20.7)     2·(CO_2 + hν → CO + O(^1D))
		
         	        2·(O(^1D) + N_2 → O + N_2)
			
         	        O + O + M → O_2 + M
			
         	        net: 2CO_2 → O_2 + 2CO




  	 	  
 PB     PB1a     P4 (-)     6.4 (-)     O + O_2 + M → O_3 + M
				
         	        H + O_3 → OH + O_2
					
         	        OH + O → H + O_2
		
         	        net: 2O → O_2
    PB1b     P7 (I-P6)     1.7 (3.9)     OH + O → H + O_2
					
         	        H + O_2 + M → HO_2 + M
					
         	        HO_2 + O → OH + O_2
		
         	        net: 2O → O_2
    PB2     P6 (I-P5)     2.8 (5.3)     O + O + M → O_2 + M
			
         	        net: 2O → O_2


 PC          P5 (-)     2.9 (-)     CO_2 + hν → CO + O
				
         	        H_2O + hν → H + OH
					
         	        OH + O → H + O_2
		
         	        net: H_2O + CO_2 → O_2 + 2H + CO








 c c c c c

As for Tab. <ref> but for O_2 destruction (L) pathways.



class     subclass     pathway number     contribution [%]     pathway


class     subclass     pathway number     contribution [%]     pathway













 LA     LA1     L1 (I-L8)     41.8 (3.4)     H + O_2 + M → HO_2 + M

                   H + HO_2 → OH + OH
			
                   2·(CO + OH → CO_2 + H)
					
                   net: O_2 + 2CO → 2CO_2
    LA2a     L2 (I-L1)     16.7 (28.5)     2·(H + O_2 + M → HO_2 + M)
		
                   HO_2 + HO_2 → H_2O_2 + O_2
			
                   H_2O_2 + hν → OH + OH
			
                   2·(CO + OH → CO_2 + H)
			
                   net: O_2 + 2CO → 2CO_2


       LA2b     L4 (I-L5)     3.9 (5.0)     O_2 + hν → O + O(^1D)

                   O(^1D) + N_2 → O + N_2
			
                   2·(HO_2 + O → OH + O_2)
			
                   2·(CO + OH → CO_2 + H)
			
                   2·(H + O_2 + M → HO_2 + M)
			
                   net: O_2 + 2CO → 2CO_2


       LA2b     L6 (I-L3)     2.0 (7.4)     O_2 + hν → O + O
			
                   2·(HO_2 + O → OH + O_2)
			
                   2·(CO + OH → CO_2 + H)
			
                   2·(H + O_2 + M → HO_2 + M)
			
                   net: O_2 + 2CO → 2CO_2


       LA2b     L7 (-)     1.9 (-)     O_2 + hν → O + O(^1D)
			
                   O(^1D) + N_2 → O + N_2

                   2·(O + O_2 + M → O_3 + M)
			
                   2·(H + O_3 → OH + O_2)
			
                   2·(CO + OH → CO_2 + H)
			
                   net: O_2 + 2CO → 2CO_2








  







    	
LB     LB1     L3 (I-L4)     4.6 (5.4)     H_2O_2 + hν → OH + OH
		
                  2·(CH_4 + OH → CH_3 + H_2O)
	
                  2·(CH_3 + O_2 + M → CH_3O_2 + M)
	
                  2·(CH_3O_2 + HO_2 → CH_3OOH + O_2)
	
                  2·(CH_3OOH + hν → H_3CO + OH)
	
                  2·(H_2CO + OH → H_2O + HCO)
	
                  2·(HCO + O_2 → HO_2 + CO)
	
                  HO_2 + HO_2 → H_2O_2 + O_2
	
                  2·(H_3CO + O_2 → H_2CO + HO_2)
	
                  net: 3O_2 + 2CH_4 → 4H_2O + 2CO
    LB2     L8 (-)     1.8 (-)     H_2O + hν → H + OH
		
                  H + O_2 + M → HO_2 + M
	
                  CH_4 + OH → CH_3 + H_2O
	
                  CH_3 + O_2 + M → CH_3O_2 + M
	
                  CH_3O_2 + HO_2 → CH_3OOH + O_2
	
                  net: O_2 + CH_4 → CH_3OOH





















   
LC         L9 (I-L9)     1.6 (3.0)     2·(H + O_2 + M → HO_2 + M)

                   HO_2 + HO_2 → H_2O_2 + O_2

                   H_2O_2 + hν →	OH + OH
		
                   2·(H_2 + OH → H_2O + H)
					
                   net: O_2 + 2H_2 → 2H_2O
LD         L5 (I-L11)     2.2 (1.7)     CO + OH → CO_2 + H

                   H_2O + hν → H + OH

                   2·(H + O_2 + M → HO_2 + M)
		
                   HO_2 + HO_2 → H_2O_2 + O_2
					
                   net: O_2 + H_2O + CO → H_2O_2 + CO_2




    
    
    







  
  

  





  
 






 





























	


