The Mastcam-Z Radiometric Calibration Targets on NASA’s Perseverance
Rover: Derived Irradiance Time-Series, Dust Deposition, and Performance
over the First 350 Sols on Mars.
Abstract
The Mastcam-Z radiometric calibration targets mounted on the NASA’s
Perseverance rover proved to be effective in the calibration of
Mastcam-Z images to reflectance (I/F) over the first 350 sols on Mars.
Mastcam-Z imaged the calibration targets regularly to perform
reflectance calibration on multispectral image sets of targets on the
Martian surface. For each calibration target image, mean radiance values
were extracted for 41 distinct regions of the targets, including patches
of color and grayscale materials. Eight strong permanent magnets, placed
under the primary target, attracted magnetic dust and repelled it from
central surfaces, allowing the extraction of radiance values from eight
regions relatively clean from dust. These radiances were combined with
reflectances obtained from laboratory measurements, a one-term linear
fit model was applied, and the slopes of the fits were retrieved as
estimates of the solar irradiance and used to convert Mastcam-Z images
from radiance to reflectance. Derived irradiance time series are
smoothly varying in line with expectations based on the changing
Mars-Sun distance, being only perturbed by a few significant dust
events. The deposition of dust on the calibration targets was largely
concentrated on the magnets, ensuring a minimal influence of dust on the
calibration process. The fraction of sunlight directly hitting the
calibration targets was negatively correlated with the atmospheric
optical depth, as expected. Further investigation will aim at explaining
the origin of a small offset observed in the fit model employed for
calibration, and the causes of a yellowing effect affecting one of the
calibration targets materials.