Figure 17. Ratio between the measured IOF and the model IOF values of (a) the primary white clean spot, (b) the light gray clean spot, (c) the white grayscale ring and (d) the secondary horizontal white patch. The colors of the points are the same for the four plots and represent the narrow-band filters. The vertical dashed line is the martian aphelion.
For the primary white patch (Figure 17a), the points were systematically just below 1 for all filters spanning the range between 677 nm and 1022 nm, while filters L4 (605 nm), L5 (528 nm) and L6 (442 nm) displayed progressively lower values. These three filters followed a similar trend: a first interval, from landing to sol 100, characterized by a linear and faster decrease, followed by a flatter decline which lasted at least until the solar conjunction of sols 217-235. After conjunction, they tended to be overall more stable (L6 seemed to increase slightly) up to the period of the major dust event of sol 315. There was not a clear relationship with the solar aphelion. For reference, Figure 17b shows unchanging ratios for the light gray primary patch. This material did not exhibit any significant deviation from the unit ratio before the dust event. Although manufactured from the same material as the other white patches, the white primary ring was not or only very slightly influenced by the yellowing effect (Figure 17c).
Discussion
In this section we discuss the performance of the Mastcam-Z radiometric calibration targets in light of the methods presented in section 3 and the results presented in section 4. Principally, we want to evaluate the performance of the cal-targets over the first 350 sols on Mars and the model used to generate the reflectance-calibrated products. In addition, we display a basic assessment of the dust on the cal-targets and the effect on the diffuse light. Eventually, we describe the tests that were aimed at understanding the visual and spectral deterioration of the AluWhite98 material.
The Cal-Target Performance and the Linear Fit Model for IOF Calibration
The performance of the Mastcam-Z calibration targets can be assessed from the results reported in sections 3.2 and 4.2, and the visual inspection of the regions involved in the IOF calibration process within the color images (section 4.1). The plots of Figure 5 show a limited dispersion of the data from the fit lines (except for the white spot), where this dispersion was quantified as a relative error on the slopes of less than 3.5% on average. This small deviation indicates that the cal-targets were successful in achieving their main goal, especially thanks to their design and the inclusion of the permanent magnets. The number of clean spots, higher than in the previous missions, considerably reduced the impact of the exclusion of one of those regions (the AluWhite98) from calibration, thanks to the other seven regions. The presence of the hollow magnets below the primary patches but not in the secondary target was a useful way to visually infer the accumulation of airfall dust on the surfaces. The magnet rings, as expected, were the first regions to be covered in dust, as shown in and Figure 11. We should also consider the action of the wind, which contributed to a frequent and efficient deposition and cleaning from dust on other parts of the cal-targets (e.g., on sols 327 and 349, as shown in the movie S2). These effects on the clean spots were critical for the radiometric calibration, because they ensured minimal dust disturbance of the clean spots’ radiances when the linear fits were made with the corresponding laboratory reflectances (Figure 5). As a consequence, the data points in those plots were well described by the one-term fit model, which was expected by the theory (equation (2 )). The ‘cleanliness’ of the clean spots due to the magnets and the wind was adequately maintained in time, as suggested by the solar irradiance time series (Figure 8a). Indeed, the irradiance \(F\) followed a quite stable and smooth trend in all filters up to the dust event of sols 314-316 that caused a significant unsettlement in images and radiance values, but that did not compromise the cal-targets or their surfaces. Further proof of the effectiveness of the clean spot-magnet ring system was the solar irradiance spectrum at the aphelion (Figure 8b), which is consistent with a solar black body model allowing for some atmospheric absorption, particularly at short wavelengths.
As mentioned in section 3.2 and fully treated in section 4.4, though the one-term linear fits characterize well the radiance-reflectance data points, we noticed a small offset in the distribution of the data points with respect to the straight line passing through the origin. The testing of a two-term model for the linear fits gave better statistical outcomes. Over the whole sets of radiance-reflectance data, the one-term model yielded a reduced \(\chi^{2}\) between 10.7 and 34.4, the two-term yielded values between 1.43 and 4.71. The exact nature of this offset is not yet known. It might be due to some computational source, such as residuals from the radiance-calibration process when the corrections (e.g., bias frames, flat fields, shutter frame subtraction) are applied to the raw images, or slight discrepancies in the reflectance model that is employed to give an estimate of the expected reflectance of each clean spot at any illumination geometry. The presence of dust on the surfaces would also tend – to first order – to result in a straight line with an offset. The slowly increasing trend of the offset in all the narrow-band filters might indicate a dust-related origin. This may imply that a small fraction of weakly or even non-magnetic airfall dust (not attracted or repelled by the magnets) gradually deposited within the clean spots and adhered electrostatically, such that it could not be swept by wind interaction. The appearance of the offset shortly after landing might suggest that dust and sand were raised by the rockets of the skycrane during landing. Deeper investigation is required to fully understand the origin and the nature of this offset.
Preliminary dust assessment
We observed the martian dust both directly (by deposition on the surfaces from which we measured the radiance and their surroundings) and indirectly (by analyzing its effect on the light that illuminated the cal-targets). As shown in the irradiance time series in Figure 8a, which only refer to the clean spots (expected to be the cleanest regions of the cal-targets), in absence of larger events dust never had a significant impact on the cal-targets or their ability to correctly perform the IOF calibration. On the other hand, as mentioned above, the frames of the movie S2 express a frequent change in the material that accumulated on the deck of the rover. We could recognize patterns of fine dust layers, which were more evident on the deck and on the bright materials due to the higher light/dark-toned contrast, and single larger grains of sand or regolith raised from the surface, which in some cases moved a few mm or even cm across two consecutive sols. Whereas the peripheral regions of the primary target were likely ruled by the magnetic field of the permanent magnets (in particular the round patches and the outermost grayscale rings), which produced an optically thick buildup of dark reddish dust on the magnet rings, the variations in all the other regions were probably controlled by wind (e.g., several dust devils were observed by Mastcam-Z in the rover site; Newman et al. , 2022) and possibly aided by vibrations caused by the motion of the rover, which traveled almost 4 km in the first 350 sols. This fine dust not only deposited on horizontal surfaces, but also adhered to vertical sides, such as the lower part of the golden base of the primary target where the magnetic field from the permanent magnets is prominent and on the cylindrical structure of the gnomon (the reddish dust coating is just perceivable on its boundaries in Figure 7).
The model of the direct fraction of sunlight on the illuminated and shaded grayscale rings was a powerful method to follow the presence of dust suspended within the martian atmosphere. The two time series shown in Figure 12 display a roughly inverse correlation in the sense that\(F_{d}\) rises when \(\tau_{I}\) falls and vice-versa. This can be interpreted in a way that a growth in the density of dust in the atmosphere leads to an increase of the optical depth measured from Mastcam-Z images and increased diffuse scattering of sunlight, leading to a decrease in \(F_{d}\). In addition, \(F_{d}\) and \(\tau_{I}\) show a shallow local maximum and minimum, respectively, around the martian aphelion. The correlation can be recognized after the solar conjunction, when stronger perturbations caused a higher variability in \(F_{d}\) and\(\tau_{I}\), and upon the major dust event from sol 314.
The spectra of dust accumulated on the magnet rings (Figure 11d) are consistent with previous observations of martian dust attracted to permanent magnets (e.g., Madsen et al. , 2009). The dust is brownish-red with very low reflectance factor at ultraviolet and blue wavelength, a characteristic rise from green to red consistent with the presence of ferric iron and higher reflectance factors in the red and infrared. Overall, though, the spectrum is darker than a typical spectrum of martian bright dust, again consistent with expectations for magnetically attracted material that can be expected to be richer in magnetite and with a larger average grain size (Kinch et al. , 2006).
The AluWhite98
We have not yet been able to identify a root cause of the observed yellowing of the white patches of the cal-targets. The white material is different (AluWhite98 manufactured by Avian Technologies) from the seven other materials (glazed ceramic material manufactured by Lucideon) (Kinch et al ., 2020). This explains why this effect was only observed in the white patches but it remains enigmatic why the white ring was not or almost not affected. One hypothesis suggested that a possible cause was the different type of epoxy adhesive employed to fix on one hand the primary and secondary patches and tiles (Henkel/Loctite EA-9309NA) and on the other hand the white ring (3M-2216B/A Gray) to their supports.
Several tests of martian environment simulation were carried out on four spare AluWhite98 samples. Three of these samples were fixed onto an Aluminum support using the two types of epoxies mentioned above, whereas the fourth was put in place without any adhesive. The samples underwent the same baking process of preparation as those currently within the cal-targets on Mars, and were treated with UV irradiation at the University of Winnipeg. However, the tests did not reproduce the visible, radiometric, and spectral outcomes of the in-flight materials. This issue therefore remains unsolved.
Conclusions and future work
In this work we assessed the performance of the Mastcam-Z radiometric Calibration Targets (or cal-targets; Kinch et al. , 2020), regularly employed to convert Mastcam-Z images (Bell et al. , 2021) from units of radiance to reflectance, over the first 350 sols on Mars.
The cal-targets proved to be efficient not only for calibration, but also to retrieve information on the environment and the dust dynamics within Jezero crater. The design of the regions of interest for calibration, surrounded by strongly magnetized hollow cylindrical magnets, allowed accurate measurements of the local radiances (Hayeset al. , 2021) involving low disturbance due to dust, which was mostly attracted or repelled by the magnets, or deposited and cleaned off by the wind. Linear fits between model reflectances and observed radiances are of good quality with only limited dispersion of data points around the fit line. A continuous monitoring of the linear fits, as well as their slopes (equal to the instantaneous local solar irradiance), will ensure a correct application of the reflectance calibration procedure in the future, as significant natural events (such as e.g., the major dust event of sols 314-316) can directly affect the atmospheric optical thickness and the cal-target materials, hence perturbing the stability of the calibration process. This includes the implementation of a dust model, which was already performed in the MER mission (Kinch et al. , 2007). Due to the satisfying results from Mastcam-Z cal-targets within the first 350 sols, we do not contemplate the urgency of a dust model.
We did observe that the linear fits could consistently be improved by the inclusion of a slight offset term. The origin of this offset is not yet understood but plausible hypotheses include residuals of the radiance calibration, imperfections of the reflectance model, or dust, or some combination of the three.
Finally, the yellowing effect of the AluWhite98 patches of the primary and secondary cal-targets could not yet be reproduced by experiments and therefore for now it remains an unexplained phenomenon. Determining the physical trigger of this effect could help establish the starting point and provide a useful reference in the design of the new calibration targets for the cameras of coming planetary missions.