MRO/CRISM Retrieval of Surface Lambert Albedos for Multispectral Mapping of Mars With DISORT-Based Radiative Transfer Modeling: Phase 1—Using Historical Climatology for Temperatures, Aerosol Optical Depths, and Atmospheric Pressures

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We discuss the DISORT-based radiative transfer pipeline (“CRISM_LambertAlb”) for atmospheric and thermal correction of MRO/CRISM data acquired in multispectral mapping mode (<formula formulatype="inline"><tex Notation="TeX">~</tex></formula>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 from surface altimetry derived from MGS Mars Orbiter Laser Altimeter (MOLA). The DISORT-based model takes 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 <formula formulatype="inline"><tex Notation="TeX">$I/F$</tex></formula> spectrum as inputs, and then a Lambertian albedo spectrum is computed as the output. The Lambertian albedo spectrum is valuable geologically because it allows the mineralogical composition to be estimated. Here, <formula formulatype="inline"><tex Notation="TeX">$I/F$</tex></formula> is defined as the ratio of the radiance measured by CRISM to the solar irradiance at Mars divided by <formula formulatype="inline"><tex Notation="TeX">π</tex></formula>; if there was no martian atmosphere, <formula formulatype="inline"><tex Notation="TeX">$I/F$</tex></formula> divided by the cosine of the incidence angle would be equal to the Lambert albedo for a Lambertian surface. After discussing the capabilities and limitations of the pipeline software system, we demonstrate its application on several multispectral data cubes—particularly, the outer reaches of the northern ice cap of Mars, the Tyrrhena Terra area that is northeast of the Hellas basin, and an area near the landing site for the Phoenix mission in the northern plains. For the icy spectra near the northern polar cap, aerosols need to be included in order to properly correct for the <formula formulatype="inline"><tex Notation="TeX">CO₂</tex></formula> absorption in the <formula formulatype="inline"><tex Notation="TeX">H₂O</tex> </formula> ice bands at wavelengths near 2.0 <formula formulatype="inline"><tex Notation="TeX"></tex></formula>. 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. This will allow a second level of refinement in the atmospheric and thermal correction of CRISM multispectral data. </para>