Figure 6. Calibration Target
spectra from sol 12 and 339. The circles are the observed calibrated R*
reflectances of the clean spots, while the solid lines represent the
corresponding laboratory spectra.
The Radiometric
Coefficients Files
A major portion of the cal-target sequence processing included the
generation and storage of information on the images and their ROI
selections. We refer to these as Radiometric Coefficients files, or
simply RC-files, which were created simultaneously with the linear fits
described in section 3.2. The first part of a typical RC-file included
metadata (e.g., name of the corresponding cal-target image, local
martian time of the original image). The body of the file included a
list of arrays, each of which was formed by 41 fields (one for each ROI
selection). The algorithm that selected the ROIs for each region
included the complete name of the region, its average radiance over the
pixels and uncertainty of this radiance, number of pixels selected,
illumination geometry angles and model reflectance related to the color
material and to the geometry of the region. The inverse of the slope of
the linear fit described in section 3.2 was also included in the
RC-files, along with three Boolean arrays of flags (expressed in terms
of 0 and 1) that specified which regions had been selected, which ones
were “bad” and should not be used (usually due to unwanted shadows),
and which should be used to make the linear fits for calibration. We
used a graphic interface to mark the regions appropriately. Whenever a
region had not been selected, its place in the data arrays of the
RC-files did not show any numerical value. An example of an RC-file is
given in appendix 1.
Results
All quantitative results that we present here have been obtained from
the values stored in the entire set of RC-files covering the time range
from landing to sol 350. In each color image in L0 and R0, one RC-file
was extracted individually from each of the three broad-band channels
(L0B, L0G, L0R, and R0B, R0G, R0R). Hence, the total number of RC-files
in our set was 3366. The regions of the cal-targets that were not
selected or were marked as bad were not included in the following
analysis.
Imaging of the
Calibration Targets on Mars
The considerable number of color images of the cal-targets acquired by
Mastcam-Z allowed a regular visual check of their conditions over time,
including the accumulation and displacement of airfall dust and
monitoring the color and grayscale materials.
Figure 7 is a comparison of four selected Bayer RGB images of the
cal-targets taken by the left eye of Mastcam-Z. The most evident details
that can be visually evaluated in time are the progressing change of hue
of the primary white patch (that effect is reflected in the fits shown
in section 3.2) and the accumulation of dust and sand on the surfaces.
The former is commonly referred to as “yellowing” effect that degraded
the reflectivity at shorter wavelengths of the visible layer of the
white material (AluWhite98) since landing. The term “yellowing” merely
comes from the fact that the white patch was noted to fade towards a
yellowish tone in the color images. A more detailed treatment of the
problem is reported in section 4.5.
Figure 7 also illustrates the progressive increase in the amount of
magnetic dust with time on the primary magnet rings, manifested as an
overall dark reddish color. This outcome, which was also observed on the
MER (Madsen et al. , 2009) and MSL (Bell et al. , 2017)
rovers, was expected from the cal-target design. The underlying colors,
that were already attenuated but still visible after the first sols
(Figure 7a), were not as discernible at the end of the period under
analysis in this work (Figure 7d). On other surfaces, such as the other
parts of the cal-targets or the deck surroundings, the deposition seemed
to be ruled by either original deposition of small grains during the
landing event, wind activity capable of transporting larger grains on
top of the deck, and/or airfall dust. In general, airfall dust and
larger grains were displaced frequently, allowing the cal-targets to
remain rather clean. In addition, the deposition of dust on the
secondary target was affected by its position on a vertical surface,
which would limit the fraction of dust falling on the horizontal tiles.
During the first 350 sols of the mission Perseverance experienced
several significant episodes of high wind and local dust lifting. The
strongest event occurred between sols 314 and 316 (Lemmon et al. ,
2022), with consequences visible for several sols (Figure 7d).
Appreciable layers of brownish airfall dust and sand were conspicuous on
the grayscale rings, on the deck next to the primary target (including
the vertical side of the golden base), and on the secondary horizontal
target following this event.