Degenerate Interpretations Of O3 Spectral Features In Exoplanet Atmosphere Observations Due To Stellar UV Uncertainties: A 3D Case Study With TRAPPIST-1e

By Keith Cowing
Status Report
October 1, 2023
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Degenerate Interpretations Of O3 Spectral Features In Exoplanet Atmosphere Observations Due To Stellar UV Uncertainties: A 3D Case Study With TRAPPIST-1e
The transmission spectrum atmospheric effective altitude is plotted against wavelength between 0.2 µm and 11 µm for the P19 PI (orange), P19 10% PAL (brown), P19 1% PAL (yellow), and P19 0.1% PAL (red) simulations, and the W21 PI (light blue), W21 10% PAL (blue), W21 1% PAL (lilac), and W21 0.1% PAL (grey) simulations. The non-tidally locked cases are excluded for clarity, but show little differences compared to the equivalent tidally locked case. The transit depth, in terms of contrast with respect to the star, is indicated on the right vertical axis in parts per million (ppm). The spectra are binned to a spectral resolving power of R = 250. Spectral features are indicated in grey. The wavelength range of the proposed Habitable Worlds Observatory (HWO; blue shaded region), and that of the JWST NIRSpec instrument (yellow shaded region), are shown. They overlap in the green shaded region. The wavelength range of the JWST MIRI instrument is indicated in the magenta shaded region. The wavelength spacing between 0.2 – 1 µm is changed between 1 – 11 µm in order to clearly show the UV and visible regions. The black bar represents the uncertainty that would be present on a measurement that has reached the noise floor of the instrument, where the noise floor is indicated as 5 ppm. Note this error bar is an estimate of the performance of the telescope and does not indicate a measurement. — astro-ph.EP

TRAPPIST-1e is a potentially habitable terrestrial exoplanet orbiting an ultra-cool M Dwarf star and is a key target for observations with the James Webb Space Telescope (JWST). One-dimensional photochemical modelling of terrestrial planetary atmospheres has shown the importance of the incoming stellar UV flux in modulating the concentration of chemical species, such as O3 and H2O.

In addition, three-dimensional (3D) modelling has demonstrated anisotropy in chemical abundances due to transport in tidally locked exoplanet simulations. We use the Whole Atmosphere Community Climate Model Version 6 (WACCM6), a 3D Earth System Model, to investigate how uncertainties in the incident UV flux, combined with transport, affect observational predictions for TRAPPIST-1e (assuming an initial Earth-like atmospheric composition).

We use two semi-empirical stellar spectra for TRAPPIST-1 from the literature. The UV flux ratio between them can be as large as a factor of 5000 in some wavelength bins. Consequently, the photochemically-produced total O3 columns differ by a factor of 26. Spectral features of O3 in both transmission and emission spectra vary between these simulations (e.g. differences of 19 km in transmission spectra effective altitude for O3 at 0.6 μm).

This leads to potential ambiguities when interpreting observations, including overlap with scenarios that assume alternative O2 concentrations. Hence, to achieve robust interpretations of terrestrial exoplanetary spectra, characterisation of the UV spectra of their host stars is critical. In the absence of such stellar measurements, atmospheric context can still be gained from other spectral features (e.g. H2O), or by comparing direct imaging and transmission spectra in conjunction.

Gregory Cooke (1, 2), Dan Marsh (1), Catherine Walsh (1), Allison Youngblood (3), ((1) School of Physics and Astronomy, University of Leeds, UK, (2) Institute of Astronomy, University of Cambridge, UK, (3) NASA Goddard Space Flight Center, Solar System Exploration Division, USA)

Comments: 20 pages, 11 figures, accepted in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2309.15239 [astro-ph.EP] (or arXiv:2309.15239v1 [astro-ph.EP] for this version)
Submission history
From: Gregory Cooke
[v1] Tue, 26 Sep 2023 20:05:40 UTC (7,260 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻