Using Mastcam Multispectral Data from Curiosity to Correct Orbital Observations of Gale Crater, Mars

Atmospheric corrections of spectral and image data are important as they are required to remove the effects of the atmosphere to isolate surface composition. Several studies have developed methods to correct for both CO₂ and mitigate atmospheric scattering effects arising from time-variable aerosols (e.g. McGuire et al., 2009; Wolff et al., 2009; Doute et al., 2024; Tornabene et al., 2024). While the volcano-scan method is used to atmospherically correct CO₂ for Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) observations, it does not address the additive and multiplicative effects from scattering components in the VNIR. CRISM has an excellent spectral resolution, and its data has been ground-truthed by missions like the Curiosity and Perseverance rovers (e.g. Johnson et al., 2017; Fraeman et al., 2020; Horgan et al., 2023). However, its spatial resolution ranges from 18 to 180 m/pixel depending on its observation mode. In our previous studies, we have attempted to ground truth higher spatial resolution color data (25 cm/pixel, 3 color bands) from the High-Resolution Imaging Science Experiment (HiRISE) and 4-band spectra from the Colour and Stereo Surface Imaging System (CaSSIS; 4 m/pixel, 4 bands) with Curiosity data. We found that the relative relationships between HiRISE band ratios of different geologic members were similar to that of Curiosity’s Mastcam (445 nm-1012 nm). However, these findings were not consistent across multiple observations which have different viewing geometries and vary w.r.t. aerosol optical depths, prompting the need for a more robust method that allows for co-analysis of rover- and orbital- based data.

Here we present a new bootstrap atmospheric correction method that relates the reflectance received by rover multispectral cameras (Mastcam and Mastcam-Z) to that of orbital imaging systems. By implementing Mastcam multispectral observations taken at optimal geometries and optical depths (see Lemmon et al. 2024), we can effectively correct CaSSIS observations over Curiosity’s traverse for photometric and atmospheric effects. This method yields CaSSIS spectral data comparable to that of the rover cameras. As expected, higher optical depths increase the contribution from scattered radiance. Moreover, the scattered radiance shows the least significant effects in the BLU filter of CaSSIS, and more in the RED and NIR filters which is in line with findings from Landis & Hyatt (2006). Spectra from uncorrected CaSSIS cubes show similar ferric dust-like shapes regardless of the feature they are extracted from. After correction, there is more spectral variability originating from the surface and a clearer distinction between ferric and ferrous materials. The Bagnold Dunes show olivine-bearing spectra, and there is evidence for hematite-bearing signatures at locations where Curiosity has identified it. The lower Stimson formation is also more distinct. In future work, acquiring more orbital observations of Curiosity’s traverse with different optical depths and geometries could allow for the application of this method beyond rover localities. Preliminary results using our atmospheric model allows for consistent comparison between CaSSIS and rover spectral data. In turn, this could allow for quantitative extension of geologic members defined by the rovers, their enhanced context, and thus better constraints on their paleoenvironments.