Figure 8. (a) Time evolution of
the solar irradiance from landing to sol 350, in terms of slopes of the
one-term linear fits between radiance and reflectance of the
cal-targets. The colors represent the different narrow-band filters of
Mastcam-Z. The blue dashed line is the martian aphelion. (b) Solar
irradiance spectrum around the martian aphelion. The black points are
the sample spectrum of solar irradiance obtained as the average of the
slopes in each filter between sols 100 and 180 (the two black solid
lines in plot (a)). The blue solid curve is a model black body curve for
the Sun (T=5775 K) as seen from Mars.
The subsequent step of the analysis was the extraction of the spectral
irradiance of the Sun. For each narrow-band filter, we computed the
average of the slopes over the time range which showed the most flatness
of the curves. This was centered on the aphelion (sol 140) with a margin
of 40 sols in each time direction (i.e., from sol 100 to 180). The
resulting spectrum, sampled in the 11 wavelengths, is reported in the
Figure 8b together with a model black body spectrum of the Sun (T = 5775
K) as reference. Considering the atmospheric extinction, which affects
the data but is not included in the model, the data points and the model
spectrum are in good agreement at longer wavelengths but the data points
fall below at shorter wavelengths as expected due to absorption by dust
in the Martian atmosphere.
Dust Assessment and Properties
Some insights on the martian dust within the rover site can be retrieved
from Mastcam-Z images of the cal-targets. These include the deposition
of dust and other materials on the cal-target surfaces, the magnetic
properties of the dust on the magnet rings, as well as the evolving
atmospheric dust content.
Deposition of Material on the Calibration Targets
During the first 350 sols of Perseverance on Mars, we observed
variability in the dust patterns on the cal-target surfaces and on the
deck in the immediate vicinity of the primary target. Sol after sol,
layers of airfall dust and larger sand grains were transported by the
wind, deposited on those flat areas and were displaced or even cleaned
out by wind events and the motion of the rover. In order to monitor the
dust settlement and displacement more efficiently, we realized a movie
sequence of all the radiance-calibrated RGB images of the cal-targets
acquired by the left eye of Mastcam-Z from landing to sol 350, suitably
aligned by an algorithm to minimize shifts and distortions between the
frames. The movie is available in the supporting material (movie S2).
From the movie, the deposition of dust and sand was very modest up to
sol 86, when several small grains were transported on the deck. Between
sols 124 and 138 there was a more intense wind activity, which
continuously displaced larger sand grains and swept away the finer dust.
Around sols 166 and 169 fine grains deposited on the deck, the grayscale
rings and the secondary horizontal target. From sol 280 to 299 more fine
grains deposited on the grayscale rings, while the larger sand grains
were translated several times by a few millimeters. On sol 314 a major
dust event struck, bringing a large amount of dust and fine sand grains
on all the surfaces of the deck and the cal-targets, that was more
perceivable from sol 316. A considerable fraction of this dust was swept
off of all these surfaces first between sols 327 and 333, and then on
sols 349 and 350, when the deck and the grayscale rings appear quite
clean and on the secondary target the dust is concentrated along the
side between the horizontal and the vertical rows.
Figure 9 displays the distribution of a significant dust layer on sol
318, which formed during the major dust event that struck from sol 314.
This fine dust, recognizable from the darker brown tone in the color
image on the left, is more discernible in the corresponding
decorrelation stretch (DCS) in the L2, L5 and L6 filters on the right, a
technique that enhances color differences in an image (Gillespieet al. , 1986). Within the DCS image, the dust appeared as a
purple coating on the deck around the primary target, on the grayscale
rings and on the secondary horizontal tiles.