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.