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.