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

TL;DR

This study synthesizes empirical transit spectra of Mars from TGO/NOMAD data, revealing seasonal variations in atmospheric features and highlighting the potential for detecting water ice signatures in exoplanet atmospheres.

Contribution

It provides the first detailed empirical transit spectra of Mars across seasons, illustrating the impact of aerosols and clouds on spectral features relevant for exoplanet characterization.

Findings

Mars atmosphere below 25 km is opaque due to dust and water ice clouds.

Seasonal variations affect CO2 and water ice spectral features.

Water ice signatures could be detectable in exoplanets with Mars-like atmospheres.

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 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.