Figure 15. Time evolutions of the two-term linear fit offsets within the first 350 sols on Mars for (a) primary clean spots, (c) grayscale rings and (d) secondary horizontal patches. Each color corresponds to a different narrow-band filter. The dashed vertical line is the martian aphelion. (b) Offsets for the clean spots averaged over the first 100 sols of the mission (blue circles) and on the last 100 sols considered in this work (sols 250-350, red circles), represented as function of the wavelength of the corresponding narrow-band filters.
The Yellowing of the AluWhite98
One of the prominent discoveries related to the cal-targets was the “yellowing” effect of the AluWhite98 material. The evolution of this problem with time was documented since landing through the progressive visible yellowing of the white patches in the color images, and the advancing decrease of the corresponding measured short-wavelength radiance, which is reflected in the linear fits (Figure 5) and hence, the cal-target spectra (Figure 6). Therefore, the primary white patch was always excluded from the IOF calibration procedure, which relied on the other seven clean spots. Figure 16 presents a close-up on the primary white and light gray patches in four different sols. The gradual deterioration of the white color is evident as time passed, especially if compared to the light gray and to the white grayscale ring, which did not show any change. Both secondary AluWhite98 tiles were also affected. In addition, the issue was more pronounced at shorter wavelengths (L6, L5 and L4) and the effect was gradually fainter and imperceptible towards the near infrared. Curiously, the white ring, which is manufactured from the same material, was unaffected or only affected to a very limited degree.