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Distinguishing N2-Dominated from O2-Dominated Atmospheres on Terrestrial Exoplanets with Reflected-Light Spectroscopy

  • Nick Burri
  • Mario Damiano
  • Renyu Hu
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Abstract

Reflected-light spectra of Earth-sized exoplanets promise to probe habitability and biosignatures, but interpreting atmospheric oxygen can be challenging because O2 can arise from both biological and abiotic processes. In addition, reflected-light retrievals exhibit degeneracies among atmospheric composition, clouds, surface reflectance, and gravity. We use the Bayesian retrieval framework ExoReLℜ, leveraging recent model developments that incorporate (i) O2-O2 collision-induced absorption (CIA), which produces the O4 bands, and (ii) a wavelength-dependent (step-function) surface albedo parameterization. Focusing on a modern Earth analog, we compare retrievals with and without O2-O2 CIA using spectra within 0.25-1.8 μm, and perform signal-to-noise ratio (SNR) sweeps for two observing configurations motivated by the Habitable Worlds Observatory: a resolved-visible case (RVIS=140) and a broadband-visible case (three broad bins). Omitting O2-O2 CIA allows the retrieval to fit the spectrum with an incorrect O2-dominated atmosphere. Including O2-O2 CIA leverages the O4 bands to break this degeneracy and recover the correct N2-dominated, O2-rich solution without requiring a tight mass prior. Adopting a robustness criterion of ≥95% posterior mass in the N2-dominated region, both configurations require SNR ≳20 at 0.75 μm (SNR ≈14 near the 1.27 μm O4 band for RNIR=70) to identify the correct dominant gas, with higher cloud fraction increasing this threshold. Reducing near-IR coverage to 1.4 μm prevents recovery of the correct N2-dominated solution across the explored SNR range, and instead favors a spurious O2-dominated, CO2-rich atmosphere. These results support near-IR coverage to ∼1.7 μm and SNR ≳20 at 0.75 μm for robust oxygen interpretation on Earth-like planets.

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Cite

BibTeXRIS
BibTeX
@misc{burri2026distinguishing,
  title = {{Distinguishing N$_2$-Dominated from O$_2$-Dominated Atmospheres on Terrestrial Exoplanets with Reflected-Light Spectroscopy}},
  author = {Nick Burri and Mario Damiano and Renyu Hu},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.42035},
  eprint = {2610.42035},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.42035},
}
RIS
TY  - UNPB
TI  - Distinguishing N_2-Dominated from O_2-Dominated Atmospheres on Terrestrial Exoplanets with Reflected-Light Spectroscopy
AU  - Nick Burri
AU  - Mario Damiano
AU  - Renyu Hu
PY  - 2026
DA  - 2026/10/30
PB  - arXiv
DO  - 10.48550/arxiv.2610.42035
UR  - https://arxiv.org/abs/2610.42035
AB  - Reflected-light spectra of Earth-sized exoplanets promise to probe habitability and biosignatures, but interpreting atmospheric oxygen can be challenging because O_2 can arise from both biological and abiotic processes. In addition, reflected-light retrievals exhibit degeneracies among atmospheric composition, clouds, surface reflectance, and gravity. We use the Bayesian retrieval framework ExoReL^ℜ, leveraging recent model developments that incorporate (i) O_2-O_2 collision-induced absorption (CIA), which produces the O_4 bands, and (ii) a wavelength-dependent (step-function) surface albedo parameterization. Focusing on a modern Earth analog, we compare retrievals with and without O_2-O_2 CIA using spectra within 0.25-1.8 μm, and perform signal-to-noise ratio (SNR) sweeps for two observing configurations motivated by the Habitable Worlds Observatory: a resolved-visible case (R_VIS=140) and a broadband-visible case (three broad bins). Omitting O_2-O_2 CIA allows the retrieval to fit the spectrum with an incorrect O_2-dominated atmosphere. Including O_2-O_2 CIA leverages the O_4 bands to break this degeneracy and recover the correct N_2-dominated, O_2-rich solution without requiring a tight mass prior. Adopting a robustness criterion of ≥95% posterior mass in the N_2-dominated region, both configurations require SNR ≳20 at 0.75 μm (SNR ≈14 near the 1.27 μm O_4 band for R_NIR=70) to identify the correct dominant gas, with higher cloud fraction increasing this threshold. Reducing near-IR coverage to 1.4 μm prevents recovery of the correct N_2-dominated solution across the explored SNR range, and instead favors a spurious O_2-dominated, CO_2-rich atmosphere. These results support near-IR coverage to ∼1.7 μm and SNR ≳20 at 0.75 μm for robust oxygen interpretation on Earth-like planets.
ER  -