Planetary Fourier spectrometer data analysis: Fast radiative transfer models

Abstract

We present fast algorithms used in IKI and IFSI for the analysis of the Martian spectra measured in the Planetary Fourier Spectrometer (PFS) experiment onboard Mars Express orbiter. The first algorithm is based on a new very general and precise approach applicable to planetary atmospheres with quite arbitrary absorption and scattering properties. The method is aimed at the simulation of high-resolution planetary spectrum in a broad spectral region with computational expenses typical to two-stream algorithms and a precision close to that of time consuming exact methods. The basic idea is to compute first a required quantity with an approximate method and then to evaluate a correction to the first approximation with just a few runs of a precise code in representative spectral channels. The correction is built as a function of the first-approximation quantity itself. The necessary input to the radiative transfer—monochromatic gaseous opacity—was evaluated by means of simple interpolation from pre-computed tables. As far as the algorithm deals with monochromatic quantities, its applications are limited to direct and even inverse problems that require the exact treatment of scattering and the absence of small-scale distortions of the spectrum. The second of the presented algorithms, based on the interpolations of the convolved transmission functions, is designed to be used in inverse problems that need a very fast evaluation of the synthetic spectrum over a broad spectral region. Performances of both methods are considered.