Laboratory Studies on Laser-Induced Shock Waves for LIBS Measurements on Mars

Kurzfassung

The research presented in this thesis examines the shock waves generated by laser induced breakdown spectroscopy (LIBS) measurements in the context of robotic planetary exploration. When investigating the history of planetary bodies, compositional data of geological samples is of great interest, as changing climate conditions on the surface or geological features like rivers leave traces in the composition and structure of rocks on the surface. The chemical composition of rocks and soils can be analysed with the LIBS method. This emission spectroscopy technique works by focusing a laser beam on the surface of a geological sample. A small amount of material is ablated and ionized. The plasma subsequently emits a spectrum characteristic for the material’s composition. Further information about the target properties can be inferred from a pressure wave that is generated by the expanding plasma. When probing the same spot on a geological sample multiple times, a growing crater forms from the repeated ablation process that affects both the pressure wave signal and the emitted spectrum. Acoustic recordings of the pressure wave present complementary data to the LIBS spectra, allowing for instance to deduce the material’s hardness. This data could also potentially be used for normalisation applications. Some scientific missions on the surface of Mars employ the LIBS technique already to analyse the composition of geological samples in-situ. With the successful landing of the Mars 2020 mission’s Perseverance rover, sounds such as the pressure wave emitted during the laser ablation process can be recorded for the first time with the SuperCam instrument suite. To better understand the evolution of the pressure wave and the processes involved in its generation, this work investigates the expansion of the shock wave shortly after the ignition of the plasma in a simulated Martian atmosphere experimentally, so results can be applied to in-situ data from Mars. For this, a schlieren imaging system was designed and built, allowing to directly image the density fluctuations induced by the shock wave. By fitting a Taylor-Sedov model to the temporal evolution of the shock wave position, changes in the energy of the explosion were monitored over the course of a series of shots on the same position. To the author’s knowledge, this work represents the first time that the schlieren imaging technique was used to investigate laser-induced shock waves in this low pressure regime and specifically at Martian atmospheric conditions. A data set containing acoustic recordings, spectral measurements and schlieren images was collected for several materials with different hardnesses, thermal properties and optical properties. The previously reported correlation between a decrease in acoustic energy and the material’s hardness could be confirmed for most non-metals. Comparing the evolution of the explosion energy with the development of the acoustic and spectral signals showed that target properties influence the measured parameters in different ways. The influence of the laser energy and the atmospheric pressure on the shock wave dynamics and the acoustic energy of the pressure wave were also investigated. A linear correlation between the laser’s energy and the explosion energy was found, as well as a linear correlation with the acoustic energy of the pressure wave at later stages. The explosion energy was shown to vary little with changing atmospheric pressures. Furthermore, the acoustic energy was found to grow linearly with rising ambient pressure, while the shock wave dynamics are slowed down in an environment with higher pressure. In the pressure range relevant for Martian surface operations, the data obtained in this study showed notable changes in the shock wave, suggesting that diurnal and annual variations could affect the Martian in-situ data. Moreover, changes in the laser pulse energy that are expected for data from SuperCam due to sampling targets at varying distances affect the shock wave and should be taken into account.