Pre and post-landing atmospheric optical depths at the Zhurong landing site on Mars retrieved using a single-image-based approach
The atmosphere of Mars has complex photometric processes due to multiple scattering and absorption by the suspended aerosols. The atmospheric optical depth is an indicator of the aerosol concentrations and can be used to model the contribution of atmospheric scattering, thereby correcting surface spectra. The atmospheric optical depth is also important in analysing the variations of atmospheric dust and evaluating the risks facing landing and roving missions. Retrieving the atmospheric optical depth from a single image is desirable as simultaneous stereo observations of the Martian surface are not common. However, most of the existing single-image-based methods rely on shadows in the image, which can be challenging to identify when the site is smooth or when the atmosphere becomes turbid. In this paper, we present a method of retrieving the atmospheric optical depth from a single image based solely on non-shadowed surfaces. This method was validated using HiRISE images and measurements acquired by rovers on the Martian surface. The method achieved an RMSE of 2%-7% in most cases, depending on the different surface photometric models used. The results indicated that aerosol scattering parameters have less impact on the retrievals than the surface photometric properties, likely due to the fact that the data is optically thin. The optical depths at the Zhurong landing site before and after landing were estimated using the proposed method. The results show that the optical depths first decreased and then increased, with the turning point being around the landing date, indicating that the Zhurong rover landed at an appropriate time. The proposed method is of significance for the analysis of Martian atmospheric dust and surface spectra with better spatio-temporal resolutions.