Plain Language Summary
The calibration targets of Mastcam-Z camera, mounted on the NASA’s
Perseverance rover, are used to calibrate Mastcam-Z images from units of
radiance to reflectance. Reflectance expresses how the sunlight is
reflected by geologic targets under different geometries and wavelengths
to give insights on their nature. The radiances measured within
calibration targets were combined with known reflectances to obtain
estimates of the local sunlight. These estimates were used to calibrate
Mastcam-Z images. Our analysis shows that the calibration targets
ensured a high quality of calibration over the first 350 sols (martian
days) on Mars. The sunlight derived from the measurements can be
monitored in time and shows a smooth trend with variations due to the
changing distance between Mars and the Sun, and larger disturbances due
to significant amounts of dust raised in the atmosphere by the wind.
This dust settled on the top surface of the rover, but it minimally
affected the calibration process. We also noticed that when the
atmosphere is rich in dust, less sunlight can make it to the surface,
and vice-versa. More investigation will aim at explaining small
variations in our calibration model and a weird effect affecting one of
the materials of the calibration targets.
Introduction
NASA’s Mars 2020 Perseverance Rover mission (Farley et al. , 2020)
was launched from Cape Canaveral (FL) on July 30th,
2020 and landed successfully within Jezero crater on February
18th, 2021. During its first 350 sols on Mars,
Perseverance achieved a number of significant milestones in planetary
exploration, including the completion of a campaign of scientific
investigation of the floor of Jezero crater (Mangold et al. ,
2021), the successful helicopter Ingenuity technology demonstration
(Balaram et al. , 2021), the extraction of six rock core samples,
and driving almost 4 km. Among its seven scientific instruments, the
Mastcam-Z cameras (Bell et al. , 2021) captured thousands of
images. Mastcam-Z is a multispectral, stereoscopic pair of cameras
capable of multiple focal lengths mounted on the rover’s mast. It
consists of two CCD sensors with Bayer pattern that can acquire
broad-band Red/Green/Blue color images when imaging through a broad
IR-cutoff filter. In addition, 12 narrow-band filters (six for each
sensor, or ‘eye’) span visible and near-infrared wavelengths from 440 to
1022 nm. In order to correctly calibrate Mastcam-Z images to units of
reflectance, the camera relies on a set of calibration targets (Kinchet al. , 2020), or cal-targets, fixed on the deck of the rover.
During the first 350 sols on Mars, the cal-targets were the most imaged
objects by Mastcam-Z.
In this manuscript we show the methods that were employed to extract
radiometric data from the images of the cal-targets and the main
results, including their performance based on calibration models, the
time evolution of the derived solar irradiance, a basic assessment of
the deposition of dust on the cal-target surfaces and its nature, the
observed fraction of direct sunlight versus diffuse illumination, and
its dependence on the optical depth of the martian atmosphere. We also
discuss an observed offset in the linear relation between radiance and
reflectance, and a “yellowing effect” on the white patch of the
cal-targets.
In section 2 we present the cal-targets and the basic concept of the
reflectance calibration. Section 3 is a summary of the methods used to
obtain our results, including the selection of regions of interest of
the cal-targets, the computation of the solar irradiance through linear
fits and the storage of the cal-target radiometric data. In section 4 we
report all the results concerning the calibration and performance of the
cal-targets in the first 350 sols, the dust deposition on the
cal-targets and its spectral characteristics, and the relation between
direct and diffuse solar light in time with a reference to the
atmospheric optical depth. Furthermore, we discuss the observed offset
in the linear radiance-reflectance relation. In section 5 we evaluate
the general performance of the cal-targets and the overall performance
of the current calibration procedure, based on variations in the dust
and the environment. All the figures portraying the Mastcam-Z
calibration targets are listed in a CSV file available in the supporting
material (table S1). The list includes information such as the image-ID,
sol, sequence number and the graphic values (gamma and thresholds)
employed for the display within this work.
The Mastcam-Z
Radiometric Calibration Targets
The Mastcam-Z Calibration Targets (Kinch et al. , 2020), or
cal-targets, are a pair of physical devices mounted on the Perseverance
rover deck (Figure 1) and primarily used to assist in the conversion of
Mastcam-Z images from units of radiance to reflectance (I/F). They were
designed and assembled at the Niels Bohr Institute of the University of
Copenhagen.