Figure 13. Plots of measured
radiance versus model reflectance factor for 3 sols (51, 210 and 302)
and 3 filters (L6, R1 and R5), showing the related one-term fit model
(solid line) and the two-term fit model (dashed line). The values of the
slopes and offset are reported for both models in the lower right corner
of each box. The white clean spot was not used for the computation of
the fits.
The slightly greater flatness of the two-term fits that came out of the
slope computation also resulted from the relative difference between the
slopes in the two models. We quantify this relative difference\([(\text{slope}_{\text{two}\_\text{term}}-\text{slope}_{\text{one}\_\text{term}})/\text{slope}_{\text{one}\_\text{term}}]\)in Figure 14a for four filters (L6, L3, R2, R6). The data points at very
low values on sol 9 and 23 were due to a low resolution (26 mm and 34
mm, respectively). If we exclude these two observations and those after
sol 314, that were strongly perturbed by the dust event, the differences
were negative and slightly increasing in absolute value with time. The
difference in filter L6 (442 nm, purple circles) had a net decrease from
-0.09 to -0.12 (-35%), while R6 (1022 nm, black circles) went from
-0.17 to -0.20 (-13%). All the other filters followed a similar trend,
with rates between -15% and -28%. In addition, the relative
differences in slopes did not follow any linear dependency on
wavelength. The plot of Figure 14b shows the averages of the relative
differences for each filter over the sol range 100-180 (across the
martian aphelion, highlighted by the two vertical solid lines in Figure
14a). All the points are included between -0.2 and -0.07, with L6 being
the highest and R6 the lowest. The spectrum of the relative difference
is characterized by a decrease from -0.09 in L6 to -0.15 in L4 (442 nm
to 605 nm), followed by a rough slight increase up to -0.14 in R5 (978
nm) and a drop to -0.19 in R6.