Quantifying the impact of spectral coverage on the retrieval of molecular abundances from exoplanet transmission spectra

Using forward models for representative exoplanet atmospheres and a radiometric instrument model, we generated synthetic observational data to explore how well the major C- and O-bearing chemical species (CO, CO₂, CH₄, and H₂O), important for determining atmospheric opacity and radiation balance, can be constrained by transit measurements as a function of spectral wavelength coverage. This work features simulations for a notional transit spectroscopy mission and compares two cases for instrument spectral coverage (wavelength coverage from 0.5-2.5 μm and 0.5-5 μm). The simulation is conducted on a grid with a range of stellar magnitudes and incorporates a full retrieval of atmospheric model parameters. We consider a range of planets from sub-Neptunes to hot Jupiters and include both low and high mean molecular weight atmospheres. We find that including the 2.5-5 μm wavelength range provides a significant improvement in the degree of constraint on the retrieved molecular abundances: up to ~3 orders of magnitude for a low mean molecular weight atmosphere (μ = 2.3) and up to a factor of ~6 for a high mean molecular weight atmosphere (μ = 28). These decreased uncertainties imply that broad spectral coverage between the visible and the mid-infrared is an important tool for understanding the chemistry and composition of exoplanet atmospheres. This analysis suggests that the James Webb Space Telescope’s (JWST) Near-Infrared Spectrograph (NIRSpec) 0.6-5 μm prism spectroscopy mode, or similar wavelength coverage with possible future missions, will be an important resource for exoplanet atmospheric characterization.