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 have 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 ${\mu}$m and 0.5-5 ${\mu}$m. 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 ${\mu}$m 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 (${\mu}$=2.3) and up to a factor of ~6 for a high mean molecular weight atmosphere (${\mu}$=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 JWST/NIRSpec 0.6-5 ${\mu}$m prism spectroscopy mode, or similar wavelength coverage with possible future missions, will be an important resource for exoplanet atmospheric characterization.