Figure 10. Primary target imaged by the left eye of Mastcam-Z (a) on sol 23 at 100 mm focal length (seq. ZCAM03014) and (b) on sol 302 at 48 mm focal length (seq. ZCAM03014) with the presence of large sand grains indicated by arrows. (c) One of the grains in the sol 23 image is magnified for better display. Image-IDs: (a) ZL0_0023_0668983099_910IOF_N0030770ZCAM03003_1000LMA01, (b) ZL0_0302_0693748246_223IOF_N0090000ZCAM03014_048085A02.
However, the regions which experienced the greatest accumulation of dust were the magnet rings of the primary target. As anticipated in section 2, the aim of the underlying magnets was to attract magnetic dust on the external rings of the round patches. The visible condition of these rings was shown in Figure 7, where a small quantity of dust already deposited during the first 23 sols and the thickness of the dust layer was quite considerable at the end of the period under examination, in which the color of the patches was almost completely invisible under the dark red/brown coating (Figure 7d). We extracted the reflectance factor values of the magnet rings from several sols, where the ROIs of the magnet rings were selected with the template of Figure 4. Four spectra of the magnet rings are shown in Figure 11 for sols 12, 129, 265 and 346 for the eight color and grayscale materials. Generally, the dust layers decreased the reflectance of the brighter patches and raised that of the darker patches, bringing their appearance to a convergence with a reduction of the contrast and reddening. At the end of the time range under examination (Figure 11d), the magnet rings were almost coincident at shorter wavelengths, but showed a slight separation in the near infrared, suggesting that in that range the dust was less optically thick with respect to the color patches underneath.