σ4Mars, a new fast radiative transfer code for the analysis of the Martian atmosphere

In the framework of the project “Earth Moon Mars” (EMM)^†, we present a novel forward model designed for the fast and accurate production of Martian radiance spectra in the longwave infrared region (100-3000 cm^-1) under nadir geometry. Building upon the flexibility and capabilities of the recently developed forward model σ-FORUM (also referred to as σ-IASI/F2N), this project seeks to extend its application, by now limited to Earth study, to the Martian atmosphere. The new model, σ4Mars, generates high-resolution spectra (with a default spectral resolution of 10^-2 cm^-1) while maintaining computational efficiency through the use of precomputed lookup tables for the computation of gas and clouds/aerosol optical depths. Multiple scattering effects are treated using scaling methods, specifically the Chou scaling approximation and the Chou adjustment (Tang correction). In addition, the code allows the computation of fast analytical derivatives of the radiance with respect to atmospheric and cloud properties, thus being suitable for the application in fast retrieval of spectrally resolved infrared observations.

We present the choices made for the parametrization of the optical depths tailored to Martian atmosphere conditions. Gas optical depths parametrizations are obtained employing the Planetary Spectrum Generator (PSG) line-by-line radiative transfer suite by NASA, using HITRAN2020 as spectroscopic database for line parameters and the Martian Climate Database version 5.3 as atmospheric database. Clouds and aerosol optical depths are parametrized as a function of the particle size distribution effective radii. The performance of the model has been evaluated using PSG as reference code by comparing gas transmittances and high-resolution radiance spectra. Preliminary tests were conducted to compare the forward model results with observed spectral radiances from the ACS TIRVIM instrument on board the ExoMars TGO, and from EMIRS on board the Emirate Mars Mission.

^† Part of the research activities described in this paper were carried out with contribution of the Next Generation EU funds within the National Recovery and Resilience Plan (PNRR), Mission 4 - Education and Research, Component 2 - From Research to Business (M4C2), Investment Line 3.1 - Strengthening and creation of Research Infrastructures, Project IR0000038 – “Earth Moon Mars (EMM)”. EMM is led by INAF in partnership with ASI and CNR.