Figure
11. Reflectance spectra of the magnet rings of the primary calibration
target in four different moments of the mission: (a) sol 12, (b) 129,
(c) 265, (d) 346. Each color corresponds to a different color or
grayscale material. The scales of reflectance factor have been adapted
to the plot size for better display.
Direct and Diffuse
Sunlight on the Grayscale Rings
Suspended dust is one of the main sources of diffuse sunlight within the
thin martian atmosphere. This phenomenon affects the measured radiance
of the observed targets because the surfaces directly illuminated by the
Sun will receive the sum of direct sunlight and diffuse light, while the
shaded regions will receive only light from the diffuse component. The
diffuse light and the presence of dust in the atmosphere can be
evaluated using the direct fraction of sunlight \(F_{d}\) (Kinchet al. , 2007), defined as follows: