Solar Occultation FTIR Spectrometry at Mars for Trace Gas Detection: A Sensitivity Study

A sensitivity study has been performed to estimate detection limits of various atmospheric trace gases achievable by a Mars-orbiting solar occultation Fourier transform infrared (FTIR) spectrometer. This was accomplished by first computing realistic limb transmittance spectra based on a model (T, P, VMR, and dust profiles) of the Mars atmosphere and adding appropriate noise and systematic errors based on assumed instrument design/configuration/performance. We then performed spectral fits to the resulting synthetic spectra to derive slant column abundances and their uncertainties. A profile retrieval was performed to infer limits of detection. This methodology was applied to a Mars-orbiting FTIR solar occultation spectrometer covering the 850–4,300 cm⁻¹ spectral region at 0.025-cm⁻¹ resolution. We conclude that most gases can be retrieved with a single-occultation sensitivity of 20–100 ppt. But this sensitivity varies considerably with the dust loading, especially for gases whose strongest absorption bands are toward higher wavenumbers where scattering is large. We conclude that for CH₄, the ν₄ band centered at 1,305 cm⁻¹, despite being more than 2 times weaker than the ν₃ band centered at 3,015 cm⁻¹, offers better sensitivity due to its close spectral proximity to the dust extinction minimum. We also conclude that for the purpose of CH₄ detection, a high-resolution (0.025 cm⁻¹) broadband instrument would have a substantial advantage over a medium-resolution (0.15 cm⁻¹) instrument, despite the latter having a much larger signal-to-noise ratio.