The Goldilocks Problem For Detecting Water In Terrestrial Planets: Constraining Water Abundances In The mid-IR With LIFE

We investigate how well the Large Interferometer for Exoplanets (LIFE) mission concept can detect habitable conditions on exoplanets through the presence of atmospheric water vapor as a proxy for surface oceans.

We model the atmosphere of a pre-biotic Earth-like planet across a range of water concentrations, from water-poor to water-rich, with surface partial pressures from 10⁻⁷ to 1 bar of H₂O.

We simulate LIFE-like noise at spectral resolutions R = 50 and 100 using LIFEsim and perform Bayesian atmospheric retrievals to determine the technical requirements for LIFE to confirm habitability.

We model three vertical water distributions: a vertically constant profile, a Manabe-Wetherald based Earth-like profile, and a diffusion and photochemistry profile to test how the assumed vertical structure influences the retrieved abundances. Clouds are not modeled. We find the ability for LIFE to detect water strongly depends on the vertical profile assumed. LIFE is unable to constrain the highest water cases and provides upper limits on low water planets.

For the highest water abundances, absorption features saturate and reduce sensitivity to characterize precise H₂O levels. Water vapor is not detectable in any profile modeled for ≤10⁻⁶ bar in surface water, comparable to Mars. For an Earth-like profile, LIFE could constrain H₂O concentrations from ∼10⁻³ to 1 bar, spanning below and above present-day Earth concentrations of 10⁻² bar.

Detectable atmospheric water may imply surface oceans, as water is highly reactive and rapidly removed by surface mineral reactions. Thus, LIFE can characterize water abundances indicative of habitable surface conditions.

Sarah Rugheimer, Eleonora Alei, Björn S. Konrad, Benjamin Taysum, John Lee Grenfell, Tim Lichtenberg, Daniel Kitzmann, Floris van der Tak, Sascha P. Quanz, LIFE collaboration

Comments: 19 pages, 8 figures, in press ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2604.07461 [astro-ph.EP] (or arXiv:2604.07461v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2604.07461
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Related DOI:
https://doi.org/10.3847/1538-4357/ae5b75
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Submission history
From: Sarah Rugheimer
[v1] Wed, 8 Apr 2026 18:01:55 UTC (2,630 KB)
https://arxiv.org/abs/2604.07461

Astrobiology, Astrogeology, Astrochemistry,