MRO/CRISM Retrieval of Surface Lambert Albedos for Multispectral Mapping of Mars with DISORT-based Rad. Transfer Modeling: Phase 1 - Using Historical Climatology for Temperatures, Aerosol Opacities, & Atmo. Pressures

We discuss the DISORT-based radiative transfer pipeline (‘CRISM_LambertAlb’) for atmospheric and thermal correction of MRO/CRISM data acquired in multispectral mapping mode (~200 m/pixel, 72 spectral channels). Currently, in this phase-one version of the system, we use aerosol optical depths, surface temperatures, and lower-atmospheric temperatures, all from climatology derived from Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) data, and surface altimetry derived from MGS Mars Orbiter Laser Altimeter (MOLA). The DISORT-based model takes as input the dust and ice aerosol optical depths (scaled to the CRISM wavelength range), the surface pressures (computed from MOLA altimetry, MGS-TES lower-atmospheric thermometry, and Viking-based pressure climatology), the surface temperatures, the reconstructed instrumental photometric angles, and the measured I/F spectrum, and then outputs a Lambertian albedo spectrum. The Lambertian albedo spectrum is valuable geologically since it allows the mineralogical composition to be estimated. Here, I/F is defined as the ratio of the radiance measured by CRISM to the solar irradiance at Mars divided by $\pi$. After discussing the capabilities and limitations of the pipeline software system, we demonstrate its application on several multispectral data cubes: the outer northern ice cap of Mars, Tyrrhena Terra, and near the landing site for the Phoenix mission. For the icy spectra near the northern polar cap, aerosols need to be included in order to properly correct for the CO_2 absorption in the H_{2}O ice bands at wavelengths near 2.0 $\mu$m. In future phases of software development, we intend to use CRISM data directly in order to retrieve the spatiotemporal maps of aerosol optical depths, surface pressure and surface temperature.

💡 Research Summary

The paper presents a comprehensive processing pipeline, named CRISM_LambertAlb, designed to convert multispectral mapping observations from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) into surface Lambertian albedo spectra. The core of the system is a DIScrete Ordinate Radiative Transfer (DISORT) model that explicitly accounts for atmospheric scattering, absorption, and thermal emission, thereby removing the combined effects of dust and ice aerosols, CO₂ gas absorption, and surface temperature‑dependent thermal radiance from the measured I/F (radiance divided by the solar irradiance at Mars and by π).

Input Data and Climate Ancillaries
In this Phase‑1 implementation the pipeline does not retrieve atmospheric state variables directly from CRISM. Instead, it relies on climatological products derived from the Mars Global Surveyor Thermal Emission Spectrometer (MGS‑TES) and the Mars Orbiter Laser Altimeter (MOLA). Dust and water‑ice aerosol optical depths are taken from TES, scaled to the CRISM wavelength range (≈400–2500 nm) using wavelength‑dependent extinction curves based on Mie theory. Surface pressure is computed from MOLA topography combined with TES lower‑atmosphere temperature profiles and a Viking‑derived pressure‑altitude relationship, ensuring that pressure‑dependent gas absorption coefficients are correctly parameterized. Surface and lower‑atmospheric temperatures are also sourced from TES climatology, providing month‑ and latitude‑averaged thermal conditions. The pipeline reconstructs the full set of photometric angles (solar incidence, emission, and azimuth) from CRISM metadata for each pixel.

DISORT Radiative Transfer Engine
The DISORT engine receives the aerosol optical depths, pressure, temperature, and geometry as inputs and solves the multi‑layer radiative transfer equation for a plane‑parallel atmosphere. It computes both the downward solar flux attenuated by scattering and absorption and the upward radiance emerging from the surface after being modified by the atmospheric column. The surface is assumed to be Lambertian; thus the model returns a bidirectional reflectance factor that can be directly converted to a Lambertian albedo by dividing by π. This assumption, while simplifying the inversion, is justified at the ~200 m spatial resolution of CRISM mapping data where sub‑pixel anisotropy averages out.

Validation on Representative Regions
The authors applied the pipeline to three distinct CRISM cubes: (1) the northern polar ice cap, (2) Tyrrhena Terra, and (3) the Phoenix landing site. In the polar case, the 2 µm water‑ice absorption band overlaps with CO₂ gas absorption; without aerosol correction the derived albedo is severely depressed, leading to misinterpretation of ice purity. Incorporating dust and ice aerosol optical depths restores the expected ice spectral shape, enabling quantitative ice grain‑size and impurity analyses. In Tyrrhena Terra and near Phoenix, the corrected albedos show improved consistency with previously published mineral maps (e.g., detection of phyllosilicates and pyroxenes), confirming that pressure and temperature corrections effectively remove spurious spectral slopes introduced by atmospheric scattering and thermal emission.

Limitations and Future Work
Because the current version uses TES‑derived climatology, it cannot capture rapid, localized atmospheric events such as regional dust storms or diurnal temperature swings. The Lambertian surface assumption also neglects bidirectional reflectance distribution function (BRDF) effects that become important for high‑contrast terrains. The authors outline a Phase‑2 roadmap that will retrieve aerosol optical depth, surface pressure, and temperature directly from CRISM observations using inverse modeling techniques (e.g., optimal estimation, machine‑learning‑based regression). This will enable spatiotemporal maps of atmospheric state variables at the native CRISM resolution, dramatically improving the fidelity of albedo retrievals.

Conclusions
The study demonstrates that a DISORT‑based atmospheric and thermal correction pipeline can reliably convert raw CRISM I/F measurements into physically meaningful Lambertian albedo spectra. By leveraging existing climatological datasets, the authors provide a practical, automated solution that already yields geologically useful products across diverse Martian terrains. The anticipated transition to CRISM‑derived atmospheric retrievals promises to eliminate the reliance on external climatology, allowing dynamic, high‑resolution albedo mapping that will support mineralogical, cryospheric, and climate studies throughout the Martian community.