Development and Testing of the MarSCoDe LIBS Calibration Target in China’s Tianwen-1 Mars Mission

The Mars Surface Composition Detector (MarSCoDe) is a remote sensing instrument mounted on the front deck of the Zhurong rover in China’s Tianwen-1 mission. The MarSCoDe adopts Laser-Induced Breakdown Spectroscopy (LIBS), along with Short Wave Infrared Spectroscopy (SWIR) and a telescopic micro-imager, to perform in situ detection of the chemical composition of soils, rocks, and minerals on the Martian surface. Since the MarSCoDe LIBS system works in extraterrestrial environments, it is important to equip the system with a set of onboard calibration targets, which are used for assessing the real-time performance of the instrument under various environmental conditions and conducting instrumental response calibration. Twelve dedicated LIBS reference samples were embedded as the MarSCoDe calibration target (MCCT) set, which plays a critical role in LIBS calibration before conducting LIBS analysis. This paper elaborates on the selection, development, characterization and testing of the MCCT set. The underlying scientific reasons and technical requirements that determine the selection of MCCT samples are introduced. The development procedures and mechanical performance test of both the calibration samples and the assembly holder are presented. Then, a comparison of the MCCTs and the characterization and scientific testing are described. The LIBS spectra of the MCCTs collected in three different atmospheric scenarios, namely laboratory-simulated Martian, normal terrestrial, and in situ Martian atmosphere, were investigated. The laboratory results and in situ behaviour show that the MarSCoDe instrument and the MCCT set can soundly adapt to the Martian environment with sufficient performance, as indicated by the fact that the spectral lines of the main elements in the calibration targets can be well identified and distinguished, including Ti, Si, Al, Fe, Mg, P, Ca, Na, K, O, C, H, S, etc. The MCCT samples provide a good reference for analysing Martian surface material composition and formulating the transfer relationship between the LIBS spectra measured in different atmospheric environments.