Abstract EANA2025-41

The James Webb Space Telescope (JWST) is enabling the first steps in the characterization of rocky exoplanet atmospheres. By the end of this decade, the European Extremely Large Telescope (E-ELT) and ESA’s ARIEL space mission will see their first light, further advancing our ability to explore the chemical makeup of terrestrial worlds. Interpreting these observations requires a strong understanding of the underlying photochemical networks at play in distinct planetary settings.

However, a fundamental limitation of photochemical models comes from the quality of their input data [1,2,3]. Numerous molecular species remain unconstrained in terms of their absorption cross-sections. In the absence of data, the inclusion of such molecules into photochemical models is often done by proxy [4]. Consequently, the integrity of photochemical networks relying on such assumptions is uncertain, and may compromise both the planning and the interpretation of observations performed with modern spectroscopic facilities.

The long-term solution to this problem relies on the characterization of UV-Visible molecular absorption cross-sections through high accuracy experimental or ab initio studies, which are resource-intensive. It is therefore necessary to justify the importance of characterizing opacities for specific molecules before experimental and/or theoretical groups should be expected to be willing to expend the resources required to do so. This work tests the sensitivity of photochemical models on educated assumptions for HSO and HNO absorption cross-sections – two examples of such unconstrained species – to determine whether these should be prioritized as targets for detailed characterization efforts.

HSO is a radical which takes part in photochemical networks involving sulphur-bearing compounds (e.g., SO2, H2S) which can result in the formation of optically thick hazes in N2-, CO2- and H2-dominated, anoxic atmospheres [4]. The photolysis of HSO is often included in models assuming that its cross-sections can be approximated to those of HO2 [5,6], despite not being clear the implications of such an approach.

Additionally, photochemical models have demonstrated how an O2- and CO-rich atmosphere can emerge from an initially CO2-dominated composition on rocky exoplanets orbiting M-dwarf stars [7]. Importantly, nitrogenous photochemical networks have been suggested to significantly decrease the steady-state abundances of O2 and CO from those otherwise expected to accumulate in such planetary environments [8]. The importance of understanding the photochemical processes involving nitrogen-bearing species has been further underscored by studies of early Earth analogues. In particular, HNO chemistry has been identified as a potential source of fixed nitrogen through atmospheric production of nitrate (NO3−) and nitrite (NO2−) that rain out into prebiotic oceans – a potential key process for the emergence of life [9,10]. Nonetheless, HNO’s UV-Visible absorption cross-sections remain uncharacterized and are often approximated by those of HNO2 [6].

Here we identify the most suitable proxy molecules for HSO and HNO spectra by systematically comparing it to molecules with similar spectral properties (e.g., triatomics with the same point group symmetry, molecules with the same functional group). Using the photochemical component of Atmos, a one-dimensional coupled photochemical-climate model [11], we scale the cross-sections of these proxy molecules by a broad range (from a factor of 10−3 to 103). We evaluate whether simulation results are sensitive to such variations in opacity by assessing the implications of these changes on simulated transmission spectra [12,13]. This analysis considers distinct types of planetary scenarios (e.g., varying degrees of volcanic activity) and different host star spectral types, and informs recommendations for updated cross-section prescriptions for the atmospheric species involved.

References:

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