Degenerate Interpretations of O₃ Spectral Features in Exoplanet Atmosphere Observations Due to Stellar UV Uncertainties: A 3D Case Study with TRAPPIST-1 e

On account of their frequency and relative ease of characterization, planetary systems orbiting M dwarf stars are prime targets in the search for potentially habitable exoplanets. TRAPPIST-1 is an M8.5V star, orbited by seven terrestrial exoplanets, and each of them could be tidally locked to their host star (Gillon et al. 2017). TRAPPIST-1 e is of particular interest because with current knowledge it is more likely than the close-in planets to have retained its atmosphere due to lower predicted escape rates (Dong et al. 2018), and is probably more able to sustain surface liquid water when compared to the outer planets which receive less stellar irradiation (Gillon et al. 2017; Wolf 2017). To begin to characterize the TRAPPIST-1 exoplanets and determine the compositions of their atmospheres, at the time of writing, several observational programs with the James Webb Space Telescope (JWST) are scheduled, including the observation of four transits of TRAPPIST-1 e in 2023 (see program 1331). ⁴

In general, the detection of molecular oxygen (O₂) on an exoplanet is of profound interest because of its importance for life on Earth (Segura et al. 2003; Meadows et al. 2018). Ozone (O₃) is produced in exoplanetary atmospheres by UV radiation which is able to dissociate O₂, and it has been calculated that in some situations the detection of O₃ is easier to achieve than a detection of O₂; for example, for low O₂ concentrations like those potentially present during Earth's Proterozoic Eon, but where O₃ concentrations are still detectable (Kozakis et al. 2022). Thus, in such cases, it has been proposed that the detection of O₃ may be used as a proxy to confirm the presence of O₂ (Leger et al. 1993; Segura et al. 2003; Meadows et al. 2018; Quanz et al. 2021).

One-dimensional (1D) photochemical modeling has demonstrated that planetary atmospheric composition (including O₃ and H₂O) is influenced by the strength and shape of the incoming UV radiation from the host star (see Rugheimer et al. 2013; Grenfell et al. 2014; Meadows et al. 2018; Kozakis et al. 2022, and references therein). For example, Teal et al. (2022) used MUSCLES Treasury Survey M dwarf spectra combined with UV spectra reconstructions as stellar spectra input to Atmos (a coupled 1D photochemistry and climate model), and similarly demonstrated that UV irradiation can modulate hydrocarbon haze concentrations. Because the atmospheric composition with respect to altitude affects molecular detectability in remote sensing, the link between O₃ abundance and O₂ abundance will be difficult to ascertain because it depends on several parameters, including the catalytic cycles that remove O₃ (e.g., the HO_x , NO_x , ClO_x , and BrO_x chemical families), and atmospheric pressure. A well-characterized spectrum of the host star is required for confident modeling of planetary climate (Eager-Nash et al. 2020), atmospheric chemistry (Kozakis et al. 2022), and atmospheric escape (Dong et al. 2018). However, the host star's spectral energy distribution may not be known to high precision when analyzing and interpreting exoplanet observations.

The UV flux from TRAPPIST-1 remains uncertain because of the intrinsic faintness of the star (V = 18.798 mag; Costa et al. 2006). Peacock et al. (2019; henceforth P19) modeled the spectrum of TRAPPIST-1. To do this, they used the PHOENIX stellar atmospheric code (Hauschildt 1993; Hauschildt & Baron 2006; Baron & Hauschildt 2007), and added a treatment of the chromosphere to produce synthetic stellar spectra of cool dwarf stars, including TRAPPIST-1, whose UV light has a negligible flux contribution from the photosphere. More recently, Wilson et al. (2021b; hereafter W21) used new observations of TRAPPIST-1 to create a semiempirical spectrum for use in atmospheric modeling simulations. While neither spectrum wholly represents the true stellar irradiation environment of the TRAPPIST-1 planets, the W21 spectrum is in significantly better agreement with available observations of TRAPPIST-1.

In addition to the incoming stellar spectrum, the three-dimensional (3D) transport and chemistry of the exoplanet's atmosphere is important for understanding the distribution and abundance of chemical species. Chen et al. (2019) investigated exoplanets orbiting at the inner edge of the habitable zone using a 3D chemistry climate model (WACCM4), showing how different assumed UV spectra can influence atmospheric mixing ratios of species such as H₂O, O₃, and H. Additionally, 3D simulation studies have demonstrated the influence of UV radiation on 3D transport (Chen et al. 2019), stratospheric temperature (Godolt et al. 2015; Chen et al. 2019), and the distribution and abundance of chemical species (Chen et al. 2018, 2019, 2021; Braam et al. 2022; Ridgway et al. 2023). Tidally locked terrestrial exoplanets modeled in 3D exhibit atmospheric jets that transport heat and chemical constituents to the nightside (Showman & Polvani 2011; Eager-Nash et al. 2020; Yates et al. 2020). Proedrou & Hocke (2016), Chen et al. (2018), and Yates et al. (2020) found that O₃, which is photochemically generated on the dayside, can be transported to the nightside, where its lifetime increases due to the lack of UV irradiation and a reduction in catalytic cycle destruction. This body of previous work motivates the need to use 3D models when investigating the climate and chemistry of specific exoplanets, in particular, with respect to their molecular observability linked to the oxygenation state of the atmosphere.

In this study we simulate the atmosphere of TRAPPIST-1 e using the WACCM6 Earth system model and the stellar spectra from P19 (Peacock 2020, model 1A, version 1) and W21 (Wilson et al. 2021a, version 7), which differ in UV flux in some wavelength bins by up to a factor of 5000. Our aim is to quantify the effects of such uncertainties in the strength of UV from the host star on the climate and composition of the atmosphere. This is the first time a 3D global climate model has been used to assess the influence of uncertain UV spectra and transport on the climate and chemistry of TRAPPIST-1 e simultaneously. Additionally, we simulate possible future observations (transmission and emission spectra) of TRAPPIST-1 e using the outputs from the WACCM6 simulations. We discuss the implications of uncertainties in the stellar UV spectrum in the interpretation of future observations of terrestrial exoplanets.

2.1. UV Spectra Input

2.2. WACCM6

TRAPPIST-1 is an ultracool M dwarf at a distance of 12.4 pc. Its stellar properties are summarized in Table 1. We assume that TRAPPIST-1 e receives 900 W m⁻² of irradiation (0.66 S_⊕, where S_⊕ is the total insolation received by the Earth). This is consistent with the value used in the TRAPPIST-1 Habitable Atmosphere Intercomparison project (THAI; Fauchez et al. 2020a; Turbet et al. 2022), although note that the latest data available in the NASA Exoplanet Archive (Akeson et al. 2013) list a value of 0.646 ± 0.025 S_⊕ (Agol et al. 2021).

Table 1. Orbital and Planetary Parameters Used in this Study, for the Exoplanet TRAPPIST-1 e and Its Star (TRAPPIST-1)

Parameter	Value
Stellar luminosity [L⊙]	0.000553
Stellar effective temperature [K]	2566
Stellar radius [R⊙]	0.119
Stellar mass [M⊙]	0.089
Stellar metallicity	0.04
Planetary orbital period [days]	6.099
Planetary radius [R⊕]	0.91
Planetary mass [M⊕]	0.772
Planetary surface gravity [m s−2]	9.14

Note. The parameters are from Delrez et al. (2018), Grimm et al. (2018), and Agol et al. (2021). The mass and radius are chosen to be consistent with the THAI project (Fauchez et al. 2020a; Sergeev et al. 2022a; Turbet et al. 2022). The ⊙ and ⊕ subscripts denote values relative to the Sun and Earth, respectively.

Download table as:  ASCIITypeset image

We use the Earth system model WACCM6 to simulate the climate of TRAPPIST-1 e with the properties indicated in Table 1, using given values from from Delrez et al. (2018) and Grimm et al. (2018). Note that the simulations were started in 2020, before Agol et al. (2021) published their work. WACCM6 is a specific configuration of the Community Earth System Model version 2 (CESM2). The model release we use in this work is CESM2.1.3. ⁵ We use a preindustrial atmosphere (approximating the atmosphere of 1850 in terms of pollutants and greenhouse gas mixing ratios) with the modern ocean and land configuration, a horizontal resolution of 1875 by 25 (latitude by longitude), and 70 vertical atmospheric levels. The ocean and atmosphere are fully interactive, meaning that they respond to physical perturbations such as temperature, or in the case of the atmosphere, chemical perturbations. Because TRAPPIST-1 e is suspected to be tidally locked (Gillon et al. 2017), we lock the substellar point by fixing the solar zenith angle in each grid cell, and we set the exoplanet's obliquity and orbital eccentricity to zero (note that the eccentricity may be nonzero, albeit <0.01; Luger et al. 2017). The substellar point is placed in the Pacific Ocean at 180° longitude and 0° latitude. The chemical mechanism, which is described in Emmons et al. (2020), has 98 chemical species and 298 chemical reactions (including photochemical reactions). Absorption by CO₂ and H₂O in the Schumann–Runge bands is included (Ji et al. 2023). The full details of the model setup, alongside simulation scripts, are available via GitHub. ⁶

We scale the P19 and W21 spectra to the irradiance received by TRAPPIST-1 e (900 W m⁻²), rebinning them to match the wavelength grid required for the WACCM6 simulations. The resulting spectra are shown in Figure 1. For the wavelength regions over which O₂ and O₃ photolyse, the integrated flux is listed in Table 2.

Zoom In Zoom Out Reset image size

Table 2. For the Two Different Spectra Used in the Simulations, the Integrated flux (in Units of W m⁻²) is Given for Each Spectrum for the Total Flux and Six Different Wavelength Bands: Schumman–Runge Continuum (S-RC), Schumman–Runge Bands (S-RB), Herzberg Continuum (HC), Hartley Band (HaB), Huggins Band (HuB), and the Chappuis Band (CB)

Spectrum	Total	S-RC	S-RB	HC	HaB	HuB	CB
	10.5–99,975 nm	130–175 nm	176–192 nm	200–240 nm	200–310 nm	310–340 nm	400–650 nm
Sun	1361	0.0092	0.0373	1.2997	19.8916	23.4556	453.7313
TP-1 P19	900	0.0382	0.0239	0.0841	0.3953	0.0829	2.1355
TP-1 W21	900	0.0025	0.0001	0.0002	0.0050	0.0133	2.3045
Flux ratio $\tfrac{{\rm{P19}}}{{\rm{W21}}}$	1.00	15.54	451.81	528.85	79.59	6.21	0.93

Note. For reference, the Earth receives 1360 W m⁻² of irradiation. Photons in the S-RC, S-RB, and HC are able to photolyse O₂. Photons in the HaB, HuB, and CB, are able to photolyse O₃.

Download table as:  ASCIITypeset image

We present 10 simulations with WACCM6, five with the P19 spectrum, and five with the W21 spectrum—see Table 3 for a summary. The initial conditions for the five simulations with each spectrum consist of one with the standard initial preindustrial Earth composition (PI), then three lower O₂ simulations with a composition with 10, 100, and 1000 times less O₂ (10% of the present atmospheric level (PAL) of O₂, 1% PAL, and 0.1% PAL, respectively), and one in which the planet is not tidally locked (noTL). These simulations allow us to assess the influence of different strengths of incoming UV spectra, the influence of tidal locking, and the effects of reducing O₂ in order to quantify any degeneracies between O₂, O₃, and incident UV light. Each simulation is run for over 250 model Earth years, and the last 10 years of the simulation are used for time-averaged results (Figures 2–6 and Figures 9–11). The WACCM6 simulations were run on 120 cores at a model cost of 1332 core-hours per simulated year. A 250 yr simulation therefore costs 333,000 core-hours.

Zoom In Zoom Out Reset image size

Table 3. Ten Simulations Have Been Performed, Each with an Obliquity of 0° and a Circular Orbit, Using Two Different Spectra for TRAPPIST-1 from P19 and W21 (See Text for the UV Spectra Input Details)

Simulation	Spectrum	O2 mixing ratio [PAL]	Orbital parameters
P19 PI	P19	1	Tidally locked, 6.1 day rotational period
P19 10% PAL	P19	0.1	Tidally locked, 6.1 day rotational period
P19 1% PAL	P19	0.01	Tidally locked, 6.1 day rotational period
P19 0.1% PAL	P19	0.001	Tidally locked, 6.1 day rotational period
P19 noTL	P19	1	Not tidally locked, 1 day rotational period
W21 PI	W21	1	Tidally locked, 6.1 day rotational period
W21 10% PAL	W21	0.1	Tidally locked, 6.1 day rotational period
W21 1% PAL	W21	0.01	Tidally locked, 6.1 day rotational period
W21 0.1% PAL	W21	0.001	Tidally locked, 6.1 day rotational period
W21 noTL	W21	1	Not tidally locked, 1 day rotational period

Note. Eight simulations were set up in a tidally locked configuration with a 6.1 day rotation rate. The "PI" simulations have an initial preindustrial Earth composition and are the same apart from using the P19 and W21 spectra. The "noTL" simulations have an initial preindustrial Earth composition, are not tidally locked, and have a diurnal cycle (rotational period of 1 day). For the lower O₂ scenarios, the volume mixing ratio of atmospheric O₂ is reduced by a factor of 10, 100, and 1000 (the "10% PAL,""1% PAL," and "0.1% PAL" simulations, respectively). The PAL of oxygen is a volume mixing ratio of 0.21.

Download table as:  ASCIITypeset image

2.3. Planetary Spectrum Generator

3.1. Atmospheric Chemistry

Atmospheric O₃ is created and destroyed via the Chapman cycle (Chapman 1930), which is initiated by UV light (of wavelength λ) dissociating an O₂ molecule:

$$\begin{eqnarray}&&{{\rm{O}}}_{2}+h\nu \ (175.9\ {\rm{nm}}\lt \lambda \lt 242.4\ {\rm{nm}})\to {\rm{O}}+{\rm{O}},\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{2}+h\nu \ (\lambda \lt 175.9\ {\rm{nm}})\to {\rm{O}}{(}^{1}{\rm{D}})+{\rm{O}}{(}^{3}{\rm{P}}),\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{\rm{O}}+{{\rm{O}}}_{2}+{\rm{M}}\to {{\rm{O}}}_{3}+{\rm{M}},\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{\rm{O}}+{{\rm{O}}}_{3}\to 2{{\rm{O}}}_{2},\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{3}+h\nu \ (\lambda \geqslant 320\ {\rm{nm}})\to {{\rm{O}}}_{2}{(}^{3}{{\rm{\Sigma }}}_{g}^{-})+{\rm{O}}{(}^{3}{\rm{P}}),\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{3}+h\nu \ (\lambda \leqslant 320\ {\rm{nm}})\to {{\rm{O}}}_{2}{(}^{1}{{\rm{\Delta }}}_{g})+{\rm{O}}{(}^{1}{\rm{D}}).\end{eqnarray} \tag{ 6 }$$

The latter two photolysis reactions do not contribute to O₃ destruction because the O produced rapidly recombines with O₂ to produce O₃ via reaction (3). However, the reaction between O and O₃ (reaction 4) does lead to a loss of O₃, and it can be sped up through catalytic agents, which are denoted below as X. An example of a catalytic cycle is shown here:

$$\begin{eqnarray}&&\begin{array}{l}{\rm{X}}+{{\rm{O}}}_{3}\to \mathrm{XO}+{{\rm{O}}}_{2},\\ \mathrm{XO}+{\rm{O}}\to {\rm{X}}+{{\rm{O}}}_{2},\\ {\rm{Overall:}}\ {{\rm{O}}}_{3}+{\rm{O}}\to 2{{\rm{O}}}_{2}.\end{array}\end{eqnarray} \tag{ 7 }$$

Catalytic agents may be NO, H, OH, Cl, or Br (Brasseur & Solomon 2005). These species can be produced through other photolysis reactions (e.g., H₂O photolysis producing H and OH).

Figure 2 shows the zonal mean of O production from O₂ photolysis in the P19 PI, P19 0.1% PAL, W21 PI, and W21 0.1% PAL simulations. In the two PI simulations, O₂ can be photolysed lower down in the P19 case because there is more incoming UV radiation than in the W21 case. When O₂ is reduced to 0.1% PAL, it can be seen that the peak of O₂ photolysis moves to higher pressures (downward in altitude). Where O is produced impacts the number density of O₃. Figure 3 shows the zonal mean of the O₃ number density for the same set of simulations as in Figure 2. Reducing O₂ in the P19 cases causes a drop in the O₃ number density at pressures above 1 hPa, while the opposite occurs in the W21 cases. This is because of the pressure dependency on the reaction that produces O₃ (reaction 3).

Zoom In Zoom Out Reset image size

In Figure 4 we show the global mean vertical profiles for temperature, and the mixing ratios of O, O₂, O₃, H₂O, CH₄, N₂O, and CO₂. On global average, the temperature profile shows deviations of up to 9 K in the troposphere and up to 19 K below the thermosphere between the W21 PI and P19 PI simulations. The W21 PI middle atmosphere (between the troposphere and thermosphere) is colder than the P19 PI middle atmosphere due to reduced O₃ heating because of the lower O₃ concentration. The P19 noTL simulation has a lower temperature in the troposphere by up to 23 K and in the middle atmosphere by up to 6 K, resulting in lower concentrations of H₂O compared to the P19 PI simulation. Generally, between 200 and 10 hPa, the relative H₂O number density in each simulation correlates with the relative temperature profile in each simulation (i.e., a lower temperature results in less H₂O).

Zoom In Zoom Out Reset image size

The O₃ mixing ratio profile in Figure 4 shows large deviations between the P19 PI (orange) and W21 PI (light blue) simulations, with a difference of a factor of 152 in the mixing ratio at the surface. The globally averaged O₃ number density peaks at 2.0 × 10¹⁹ molecules m⁻³ at 50 hPa in the P19 PI case and at 7.0 × 10¹⁷ molecules m⁻³ at 50 hPa in the W21 PI case. Despite the amount of O₃ present in the P19 PI simulated atmosphere, the temperature inversion in the middle atmosphere is <8 K because the UV intensity is not sufficient to provide enough O₃ UV heating to create a similar temperature inversion in Earth's stratosphere (≈60 K change between the tropopause and stratopause). The P19 noTL simulation (dark red) shows that tidal locking increases the amount of O₃ in the middle atmosphere, while reducing the O₃ concentration in the troposphere. The non–tidally locked cases are colder in the lower atmosphere, and warmer in the middle atmosphere. This reduces the rate of catalytic destruction reactions and allows O₃ formation to occur faster.

Figure 5 displays the longitudinal and latitudinal variation of the O₃ column for the simulations averaged over the last 10 years of each scenario. O₃ is inhomogeneously distributed horizontally, which highlights the importance of using 3D models. The P19 PI simulation has a global mean O₃ column of 1310 Dobson units (DU; approximately 4.4 times Earth's global mean value of ≈300 DU, where 1 DU = 2.687 ×10²⁰ molecules m⁻²), while the W21 PI simulation predicts a global mean O₃ column of 50 DU. Both of the O₃ column maxima, at 7152 DU and 117 DU for the P19 PI and W21 PI simulations, respectively, occur near the southern pole.

Zoom In Zoom Out Reset image size

Figure 6 shows the total range of O₃ columns produced across the atmospheric latitude–longitude grid against the O₂ volume mixing ratio. The O₃ column depends on the incident UV radiation, O₂ concentration, atmospheric pressure, temperature, and transport, as well as the rates of destruction. In the W21 PI case, the UV radiation is absorbed high in the atmosphere where the pressure is low, so little O₃ is produced and the O_x (O + O₃) is primarily in O. As O₂ is decreased, the O₃ column increases. In contrast, in the P19 simulations, which have larger incoming fluxes of UV, as O₂ increases above 1% PAL, the O₃ column increases, similar to the relationship on Earth. Both of these data sets are rather different when compared to previous results that have simulated the atmospheres of planets around late M dwarf stars (e.g., Rugheimer & Kaltenegger 2018; Kozakis et al. 2022; although note that the total instellation in those simulations were set to Earth's modern value). While there are likely many interacting parameters which will cause dissimilar O₃ column predictions, the very large discrepancies with 1D models could be due to 3D transport. This seems plausible because Braam et al. (2023) reported a stratospheric circulation in simulations of a terrestrial Proxima Centauri b scenario, in which the winds move O₃ from the dayside to the nightside, with similar circulation effects occurring in the WACCM6 simulations here (the large-scale dynamics of the atmosphere will be explored in future work).

Zoom In Zoom Out Reset image size

3.2. Transmission Spectra

In Figure 7 we show idealized transmission spectra between 0.1 and 11 μm generated using the WACCM6 simulations with PSG (excluding the non–tidally locked cases). The model date chosen for the transit is arbitrary. Because there are time-dependent fluctuations for several variables in the WACCM6 simulations (e.g., clouds and chemical mixing ratios), we could have investigated time variability in the transmission spectra. However, Fauchez et al. (2022) showed that such variability is within the measurement uncertainties of JWST. The spectra are binned to approximate a resolving power of R = 250 to show detail in spectral features, where R = λ/Δλ, λ is the wavelength, and Δλ is width of the wavelength bin. A 5 ppm error bar corresponding to the lowest achievable noise with JWST instruments, which may be between 5 and 20 ppm as calculated by Matsuo et al. (2019), Schlawin et al. (2020), Schlawin et al. (2021), and Rustamkulov et al. (2022), is indicated.

Zoom In Zoom Out Reset image size

The differences in the effective altitude of O₃ spectral features between the P19 PI (orange) and W21 PI (light blue) transmission spectra are −4 km, +20 km, +16 km, and +18km for the 0.25, 0.6, 4.71, and 9.6 μm O₃ features, respectively. Despite the W21 PI simulation having an O₃ column ≈ 26 times lower than the P19 PI simulation, the W21 PI UV feature (centered at 0.25 μm) due to the HaB (0.2–0.31 μm) actually has the largest effective altitude between 0.2 and 0.3 μm of all the simulations. This is because the HaB saturates quickly and the W21 PI atmosphere has more total O₃ than the P19 PI atmosphere above ≈0.5 hPa. Between 0.3 and 0.35 μm, the temperature dependence of the HaBs and HuBs reduces the effective height of the W21 PI transmission spectra due to the colder middle atmosphere in the W21 PI simulation. At 0.6 μm, a significant detection of O₃ with JWST in the W21 PI simulation scenario would be improbable given that the noise floor is comparable to the height of the feature. Therefore, assuming that the W21 spectrum is closest to the true spectrum of TRAPPIST-1, or the case that the true stellar UV emission is weaker, a null detection of the 0.6 μm O₃ feature should not rule out the presence of O₂ abundances at levels as high as the present-day Earth. For H₂O, the spectral features are stronger in the P19 PI transmission spectra compared to W21 PI by up to 8 km, which is the result of a larger number density of H₂O in the middle atmosphere. Despite the difference in temperature and O₃ number density profiles between the noTL simulation and PI simulations, the transmission spectra are remarkably similar (within a ±4 km effective altitude, not shown for clarity). The P19 0.1% PAL transmission spectrum (red) produces a quantitatively similar transmission spectrum feature at 9.6 μm to the W21 PI simulation (light blue), even though there is a 1000 times difference in the O₂ mixing ratio between the two cases. The same can be said of the following pairs at 9.6 μm: W21 1% PAL and P19 PI; W21 0.1% PAL and P19 1% PAL; and W21 10% PAL and P19 10% PAL. There is a noticeable difference between the spectra at 4.71 and 9 μm, but this would require reaching the most optimistic 5 ppm noise floor in order to demonstrate that the two O₂ scenarios are distinguishable when there exist uncertain stellar UV flux estimates. Also note that the effective height of the O₂ − X collision-induced absorption feature at 6.4 μm (see Fauchez et al. 2020b, for more details) is up to 7 km shallower when O₂ is at 0.1% PAL in the P19 and W21 scenarios, compared to the PI cases.

3.3. Emission Spectra

In Figure 8 we show the emission spectra from each simulation for the atmospheric absorption O₃ features at 4.71 and 9.6 μm at 90° orbital phase (the maximum planet–star separation as viewed in an edge-on system with a circular orbit). The 4.71 μm feature overlaps with a CO₂ feature, but O₃ is the dominant absorber at 4.71 μm. The P19 PI simulation (orange) predicts higher O₃ columns than the W21 PI case (light blue); hence, the depth of the features relative to the continuum in the W21 PI case are weaker than the P19 PI scenario by a factor of 4.2 and 4.5 at 4.71 μm and 9.6 μm, respectively. With respect to the P19 0.1% PAL emission spectrum (red), the P19 PI emission spectrum (orange) has a greater depth by a factor of 1.3 and 1.1 at 4.71 and 9.6 μm, respectively, even though the P19 PI simulation has a mean O₃ column which is 3.2 times higher than the P19 0.1% PAL simulation (1310 DU versus 405 DU). The 3D effects are important here because the largest O₃ columns are found at the poles, but PSG is integrating over the whole observable disk. At 4.71 μm, the noTL absorption features are deeper than the tidally locked PI cases by a factor of 2.0 and 1.2 in the P19 and W21 scenarios, respectively. At 9.6 μm, these values are 1.2 and 1.0. In terms of the relative depths of O₃ features shown in Figure 8, increasing depth is generally shown with increasing mean O₃ column (see Figure 5).

Zoom In Zoom Out Reset image size

The tidally locked simulations exhibit strong convection and high clouds around the substellar point, whereas the P19 noTL simulation has mainly low clouds with comparatively little high cloud coverage. Therefore, this results in the P19 noTL simulation (dark red) having the deepest O₃ emission spectral features.

It is important to note that other orbital phases may show quantitatively different results because variability in spectral features may occur due to climate variations throughout the orbit (e.g., Cooke et al. 2023). Additionally, longer-term variations may be expected due to the possible presence of a "longitudinally asymmetric stratospheric oscillation" (Cohen et al. 2022).

We have demonstrated that large differences in assumed stellar UV spectra can lead to different predictions for the strength of O₃ spectral features. These spectral features may overlap at an assumed minimum observational uncertainty of 5 ppm, despite the fact that the O₂ concentration differs by factors of up to 1000. In this section, we compare our results to previous work, consider known uncertainties, and discuss work that should be done in preparation for future exoplanet observations.

1D photochemical modeling of M dwarf terrestrial exoplanet atmospheres has demonstrated that CH₄ and N₂O could have greater abundances in the middle atmosphere compared to the modern Earth's atmosphere (e.g., Segura et al. 2005; Wunderlich et al. 2019), which we also find in our WACCM6 simulations of TRAPPIST-1 e (see Figure 4). Teal et al. (2022) showed changes of over two orders of magnitude in the middle atmosphere O₃ mixing ratios when modeling a modern Earth-like exoplanet that receives 1 S_⊕ of irradiation around GJ 176 (an M2.5V star), but with various UV irradiation scenarios. They derived transmission spectra predictions from their atmospheric simulations and found the maximum transit depth differences to be <2 ppm, which is below the noise floor (5 ppm or greater) for JWST. This is in contrast to the W21 PI and P19 PI transmission spectra results shown here, where the strengths of the estimated O₃ features here are distinct at the 5 ppm level. While different atmospheric modeling methods are used in Teal et al. (2022) compared with this work, the difference in observational significance is primarily due to the size of the stars modeled: the radius of GJ 176 is 0.45 R_⊙, and the radius of TRAPPIST-1 is 0.1192 R_⊙. Teal et al. (2022) also demonstrated that hazy Archean Earth atmospheres were more sensitive to changes in the incoming UV spectra compared to the modern Earth's atmosphere, which warrants future investigations for how uncertainties in the UV spectrum of the host star affect hazy atmospheres in 3D models.

In terms of 3D modeling, the THAI series (Fauchez et al. 2020a; Sergeev et al. 2022a; Turbet et al. 2022) has investigated the climate of TRAPPIST-1 e using four different 3D global climate models, assuming either an N₂- or CO₂-dominated atmosphere and not including interactive chemistry, where the composition evolves depending on chemical and photochemical reactions. The surface temperatures in the WACCM6 simulations are similar although slightly lower (219–231 K global mean compared to 230–240 K in the THAI simulations. See Appendix A and Figure 9 for more details), which may be due to differences in assumptions regarding the surface (including the distribution of the continents and the fact that an interactive ocean is used here, in contrast to a slab ocean with no meridional heat transport) or the composition of the atmosphere. With a previous version of WACCM (CESM1), Chen et al. (2019) investigated a planet with a 43.87 day orbital period around a star with an effective temperature of 4000 K and an insolation of 1.9 S_⊕, as opposed to the 0.66 S_⊕ used here, and assessed the impact of uncertain host-star UV flux on the atmosphere. Chen et al. (2019) showed that two different spectra (representing a quiescent and an active M dwarf star) impacted the middle atmospheric concentrations of O₃, OH, N₂O, CH₄, and H₂O. They calculated transmission spectra for the two simulated atmospheres, finding that the only observable difference was for the O₃ feature at 9.6 μm (although the UV O₃ feature is not shown in their Figure 11). On the other hand, the transmission spectra simulations shown here in Figure 7 display noticeable spectral differences for O₃ at 0.25, 0.6, 4.7, 9.0, and 9.6 μm, as well as at 13–14 μm, and for H₂O between 5 and 6 μm. The differences in the predicted observations between our work and that of Chen et al. (2019) likely arise due to the differences in exoplanetary system setup, the different stellar spectra, and the calculated lower O₃ columns from Chen et al. (2019), compared to the simulated atmospheres here. 43.87 days is in the "slow rotator" regime (for the definition of tidally locked rotation regimes see Haqq-Misra et al. 2018), and 6.1 days for TRAPPIST-1 e can correspond to either the "Rhines rotator" or "fast rotator" regimes (Sergeev et al. 2022b). Thus, our results, alongside those from Chen et al. (2019), demonstrate that 3D modeling results are sensitive to the choice of the assumed stellar UV spectra for potentially habitable tidally locked exoplanets across early and late M dwarf stars and different rotation periods. Future work should also investigate the influence of orbital perturbations away from a synchronous 1:1 spin–orbit resonance (e.g., Chen et al. 2023) on composition.

The anisotropy of chemical molecules will affect the emission spectra and photometry of terrestrial exoplanets, by modulating when the exoplanet is brightest and dimmest at particular wavelengths (Selsis et al. 2011; Chen et al. 2018). The phase-curve photometric amplitude, and where the maximum brightness occurs during the orbit, depends on the particular rotation regime within which the exoplanet exists (Haqq-Misra et al. 2018). Thermal emission features are influenced by molecular abundance, the atmospheric temperature, and the temperature difference between the emitting and absorbing regions (e.g., for Earth, infrared emission emanates from the troposphere, while the absorbing region is the O₃ layer in the stratosphere). This temperature difference is larger for M dwarf exoplanets which exhibit a less pronounced stratospheric temperature inversion due to lower incident UV emission for the same total instellation.

Note that detecting O₃ will be difficult with JWST within the nominal 5 yr mission lifetime (although JWST is expected to continue science operations for at least 10 yr), even for a modern Earth scenario (Lin et al. 2021), and Fauchez et al. (2019) found that gases other than CO₂ may require hundreds or thousands of transits to be detectable. Simulations of high-resolution observations with the extremely large class of telescopes indicate that O₂ at 0.76 μm may be detectable in the case of TRAPPIST-1 e within ∼100 transits (Snellen et al. 2013; Rodler & López-Morales 2014; Serindag & Snellen 2019).

The derived Mega-MUSCLES spectrum of TRAPPIST-1 (W21) is constrained by more observations than the P19 spectrum, but both spectra have significant flux uncertainties. While neither spectrum used in this study is likely to represent the true stellar irradiation environment of TRAPPIST-1 e wholly, there are at least observational constraints on the "ground truth" of its parent star's spectrum. For many planetary systems, there will only be estimates from stellar models, and this will cause significant problems for predicting the photochemical environment of potentially habitable exoplanets. Furthermore, in each wavelength bin, we have assumed that the flux does not vary with time. Due to M dwarf stellar activity, such an assumption is unlikely to be accurate (Loyd et al. 2018). The O₃ abundance will be perturbed by the inclusion of incident stellar flares (Segura et al. 2010; Tilley et al. 2019; Chen et al. 2021; Ridgway et al. 2023) which we have not investigated here. Based on previous results, it seems that stellar flares will exacerbate the interpretation of observed spectra, so future work on incoming UV uncertainties could evaluate the additional impact of stellar flares. The present modeling uncertainties in the O₂–O₃ nonlinear relationship arising from differences in predictions between 1D and 3D models (Cooke et al. 2022; Kozakis et al. 2022; Yassin Jaziri et al. 2022; Ji et al. 2023) will compound this issue. In addition to previous work, our simulations, which focus on the specific target of TRAPPIST-1 e, further motivates the need for a dedicated next-generation observatory with UV capabilities to characterize exoplanet host stars.

UV flux measurements from a telescope such as the ∼6 m UV/VIS/NIR Telescope (currently referred to as the HWO) that was recommended by the Decadal Survey (National Academies of Sciences, and Engineering, & Medicine 2021) will aid the interpretation of observed exoplanet spectra and help to infer the concentration of O₂ and trace gases in the atmosphere without direct measurements (Kozakis et al. 2022). However, this telescope is not expected to be operational until the late 2030s at the earliest. Determining the EUV fluxes from a host star (which will require a dedicated observatory; Youngblood et al. 2019) will also provide important information about atmospheric escape, habitability, and help to examine the atmospheric history of the exoplanets in the system.

Before next-generation telescopes are online, there are other clues available to characterize oxygenated terrestrial atmospheres if the interpretation of the spectral features (e.g., O₃) leaves degeneracies in the parameter space between O₂ concentration, O₃ concentration, UV irradiation, and O₃ depleting catalytic cycles. For example, the major differences between the P19 PI and the P19 0.1% PAL transmission spectra are between the H₂O, O₂, and O₃ features. Moreover, the estimated intersimulation trends with wavelength in transmission spectra are not mirrored in emission spectra predictions. Namely, the depth relative to the continuum in emission spectra for O₃ at 4.71 and 9.6 μm contrasts with the relative strength of associated transmission spectrum features between the simulations. This means that if both transmission spectra and emission spectra are acquired with adequate precision, multiwavelength observations combined with atmospheric retrieval methods (Quanz et al. 2021) will be useful when delineating between possible atmospheric composition scenarios. Nevertheless, Batalha et al. (2018) showed that confident estimates on atmospheric composition from emission spectra observed with JWST MIRI LRS will prove difficult to achieve for temperate exoplanets, using TRAPPIST-1 f as an example.

Finally, future modeling work should explore the impact of chemical boundary conditions on simulated transmission and emission spectra. For example, the choice of boundary conditions, such as the upward flux and abundances of species in the HO_x , NO_x , ClO_x , and BrO_x families, will not only affect the O₃ distribution but also the atmospheric temperature and incoming radiation. Provided the atmospheric profile of O₃ derived from atmospheric retrievals is well constrained, by assuming minimal and maximal O₃ catalytic cycle destruction, limits could be placed on O₂.

For the first time using a 3D chemistry climate model (WACCM6) to simulate TRAPPIST-1 e (assuming an initial Earth-like composition) and including two different incoming UV spectra, we demonstrated that using a single observed O₃ feature outside of the UV range to extrapolate to undetected molecules, such as O₂, will lead to degeneracies over multiple orders of magnitude in the parameter space for atmospheric composition. The UV spectrum in both of the incoming stellar spectra varies by up to a factor of ≈500 for important photolysis bands, and up to ≈5000 for individual wavelength bins. While the atmospheric columns of many species (including O₂ and CO₂) are virtually unaffected by the difference between the two spectra, for an O₂ mixing ratio of 0.21, the O₃ columns differ by a factor of 26 due to different O₃ production rates that are sensitive to the incoming spectrum. Consequently, the associated O₃ transmission spectral features differ in effective altitude by up to 20 km, while the O₃ features in emission spectra differ by a factor of up to 4.5 in relative depth. One implication is that a nondetection of O₃ at visible wavelengths may not indicate the absence of an oxygenated atmosphere. Furthermore, tidal locking of the model results in substantially different emission spectral features which are shallower relative to the emission continuum.

Without the direct detection of O₂, additional context for determining the oxygenation state of the atmosphere can be gained from either (1) future missions that are able to characterize the UV spectra of faint stars better, or (2) sensitive direct imaging observations combined with transmission spectra observations targeting individual features.

G.J.C. acknowledges the studentship funded by the Science and Technology Facilities Council of the United Kingdom (STFC, grant No. ST/T506230/1). C.W. acknowledges financial support from the University of Leeds and from the Science and Technology Facilities Council (grant No. ST/X001016/1 and MR/T040726/1). This work was undertaken on ARC4, part of the High Performance Computing facilities at the University of Leeds, UK.

We would like to acknowledge high-performance computing support from Cheyenne (doi:10.5065/D6RX99HX) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the National Science Foundation. The CESM project is supported primarily by the National Science Foundation (NSF). This material is based upon work supported by the National Center for Atmospheric Research (NCAR), which is a major facility sponsored by the NSF under Cooperative Agreement 1852977.

This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.

The time-averaged surface temperatures for all the simulations (see Table 3 for simulation details) are shown in Figure 9. On time average, the global mean surface temperatures are between 219 and 231 K, with the minima ranging between 171 and 180 K, and the maxima ranging between 252 and 277 K. The P19 and W21 noTL simulations are everywhere colder than 273 K, while all other simulations do have some areas of the surface which are warmer than 273 K (the freezing point of water under 1 bar of atmospheric pressure).

Zoom In Zoom Out Reset image size

Figure 10 shows the water vapor (H₂O) column for the PI and 0.1% PAL simulations for both the P19 and W21 scenarios. The H₂O column is predominantly situated around and above the substellar point. This is because the incoming solar radiation heats the surface, and water vapor evaporates. The ascending air parcels carry the water vapor upwards, where the parcels cool and form clouds.

Zoom In Zoom Out Reset image size

Figure 11 shows the high cloud fraction for the PI and 0.1% PAL simulations for both the P19 and W21 scenarios. High clouds are defined as clouds at pressures less than 400 hPa. Clouds, through shortwave (incoming solar radiation) and long-wave (outgoing terrestrial radiation) forcing, provide a net warming of 8–12 W m⁻² in the tidally locked scenarios, and 5–7 W m⁻² in the non–tidally locked scenarios.

Zoom In Zoom Out Reset image size