Recording laser-induced sparks on Mars with the SuperCam microphone

The SuperCam instrument suite onboard the Mars 2020 Perseverance rover includes a microphone used to complement Laser-Induced Breakdown Spectroscopy investigations of the surface of Mars. The potential of the SuperCam microphone has already been demonstrated for laser ablation under Earth atmosphere in our preliminary study with a small set of samples and fixed experimental conditions. This new experimental study, conducted under Mars atmosphere, explores all the main environmental, instrumental and target dependent parameters that likely govern the laser-induced acoustic signal that will be generated on Mars. As SuperCam will observe targets at various distances from the rover, under an atmospheric pressure that follows diurnal and seasonal cycles, this study proposes a sequence of corrections to apply to Mars data in order to compare acoustic signal from targets sampled under different configurations. In addition, 17 samples, including pure metals but also rocks and minerals relevant to Mars' surface were tested to study the influence of target properties and laser-matter interactions on the acoustic signal and the ablated volume. A specific behavior is reported for metals and graphite, which rapidly disperse the incoming laser energy through heat diffusion. However, for other minerals and rocks, the growth of the crater is seen to be responsible for the shot-to-shot decrease in acoustic energy. As a consequence, it is confirmed that monitoring the acoustic energy during a burst of laser shots could be used to estimate the laser-induced cavity volume. Moreover, the amount of matter removed by the laser is all the more important when the target is soft. Hence, the decreasing rate of the acoustic energy is correlated with the target hardness. These complementary information will help to better document SuperCam targets.