Anatomy of Empirical Transit Spectra of Mars Based on TGO/NOMAD

Abstract 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 on board ExoMars/TGO, which operates at wavelengths of 0.2–0.65 and 2–4 μ m. 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 H 2 O ice clouds, both of which substantially weaken spectral features. The spectra exhibit CO 2 absorption features at 2.7–2.8 μ m and signatures of submicron-sized mesospheric H 2 O ice clouds around 3.1 μ m, 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 CO 2 signatures and strengthen the H 2 O ice features, which serve as potential indicators of a dusty planet like Mars. If TRAPPIST-1f possessed a Mars-like atmospheric structure, both CO 2 and H 2 O ice features would be detectable at a noise level of 3 ppm, a level likely beyond current observational capabilities. Nevertheless, the 3.1 μ m feature produced by submicron-sized mesospheric H 2 O ice clouds offers a novel avenue for characterizing the atmospheres of habitable-zone exoplanets.