Forward modelling low-spectral-resolution Cassini/CIRS observations of Titan

Abstract

The Composite InfraRed Spectrometer (CIRS) instrument onboard the Cassini spacecraft performed 8.4 million spectral observations of Titan at resolutions between 0.5–15.5 cm\(^{\varvec{-1}}\). More than 3 million of these were acquired at a low spectral resolution (SR) (13.5–15.5 cm\(^{\varvec{-1}}\)), which have excellent spatial and temporal coverage in addition to the highest spatial resolution and lowest noise per spectrum of any of the CIRS observations. Despite this, the CIRS low-SR dataset is currently underused for atmospheric composition analysis, as spectral features are often blended and subtle compared to those in higher SR observations. The vast size of the dataset also poses a challenge as an efficient forward model is required to fully exploit these observations. Here, we show that the CIRS FP3/4 nadir low-SR observations of Titan can be accurately forward modelled using a computationally efficient correlated-\(\varvec{k}\) method. We quantify wavenumber-dependent forward modelling errors, with mean 0.723 nW cm\(^{\varvec{-2}}\,\)sr\(^{\varvec{-1}}\)/cm\(^{\varvec{-1}}\) (FP3: 600–890 cm\(^{\varvec{-1}}\)) and 0.248 nW cm\(^{\varvec{-2}}\,\)sr\(^{\varvec{-1}}\,\)/ cm\(^{\varvec{-1}}\) (FP4: 1240–1360 cm\(^{\varvec{-1}}\)), that can be used to improve the rigour of future retrievals. Alternatively, in cases where more accuracy is required, we show observations can be forward modelled using an optimised line-by-line method, significantly reducing computation time.