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.