Anatomy Of Empirical Transit Spectra Of Mars Based On TGO/NOMAD

Transit spectroscopy is a powerful tool for probing atmospheric structures of exoplanets. Accurately accounting for the effects of aerosols is key to reconstructing atmospheric properties from transit spectra, yet this remains a significant challenge. To advance this effort, it is invaluable to examine the spectral features of well-characterized planetary atmospheres.

Here, we synthesize empirical transit spectra of Mars across different seasons based on data from the NOMAD’s Solar Occultation channel onboard ExoMars/TGO, which operates at wavelengths of 0.2-0.65 and 2-4 micron. In the generated empirical transit spectra, the atmosphere below 25 km is found to be largely opaque due to the presence of micron-sized dust and water ice clouds, both of which substantially weaken spectral features.

The spectra exhibit CO₂ absorption features at 2.7-2.8 micron and signatures of sub-micron-sized mesospheric water ice clouds around 3.1 micron, accompanied by a continuum slope. The amplitudes of these spectral features are found to vary with the Martian seasons, where the dust storms weaken the CO₂ signatures and strengthen the water ice features, which serve as potential indicators of a dusty planet like Mars.

If TRAPPIST-1f possessed a Mars-like atmospheric structure, both CO₂ and water ice features would be detectable at a noise level of 3 ppm, a level likely beyond current observational capabilities.

Nevertheless, the 3.1 micron feature produced by sub-micron-sized mesospheric water ice clouds offers a novel avenue for characterizing the atmospheres of habitable-zone exoplanets.

Shohei Aoki, Yuka Fujii, Hideo Sagawa, Geronimo L. Villanueva, Ian Thomas, Bojan Ristic, Frank Daerden, Miguel Angel López-Valverde, Manish R. Patel, Jonathon Mason, Yannick Willame, Giancarlo Bellucci, Ann Carine Vandaele

Comments: 24 pages, 10 figures, Submitted to AAS Journals
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2601.08397 [astro-ph.EP] (or arXiv:2601.08397v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2601.08397
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Submission history
From: Shohei Aoki
[v1] Tue, 13 Jan 2026 10:05:41 UTC (4,619 KB)
https://arxiv.org/abs/2601.08397

Astrobiology,