Meteorite impacts on Mars: seismic observations, source theory and modeling

Meteorite impacts are important actors in the evolution of the solar system and planetary surfaces. With the first seismic exploration of the Moon during the Apollo missions, it was revealed that meteorite impacts can also be a significant source of seismic signal. The interest in this type of source has been renewed since the landing of the InSight mission on Mars in November 2018, which brought a short-period and a very broadband seismometer on the surface. Eight signals of impact origin were detected by InSight seismometers during the four-year mission. As on the Moon, these signals differ from those of more classical tectonic quakes. Today, no model can fully explain the impact signal spectrum and magnitude, the P- and S- wave relative amplitudes and the seismic source mechanism. In addition to seismic waves, the fall and final impact of the meteor produces a shock wave in the Martian atmosphere. On Mars, such shock can be trapped in a low-altitude atmospheric waveguide, and can thus propagate over long distances as a guided infrasound wave. Such infrasound was recorded by InSight seismometer after coupling to the ground surface. These seismo-acoustic signals carry information about their propagation medium, the Martian atmosphere, and their coupling medium, the subsurface below InSight. Hence, they can help understand the structure of Mars. In this work, we propose to investigate both types of seismic signals. First, we model the seismic source related to the impact cratering process. We design a new analytical model of the impact seismic source using the seismic Representation Theorem and the notion of stress glut. The impact can be seen as an extended field of equivalent forces, or as a point source, combining a seismic moment tensor and a vector force. We develop a numerical method to compute the stress glut associated to a hypervelocity impact using numerical simulations based on the Finite-Discrete Element Method (FDEM). We test this numerical model thanks to a coupling method: at some distance from the crater, the signal produced by the stress glut model is compared to the FDEM signal, prolongated by coupling. Our model succeeds in representing the low-frequency amplitude of impact generated signals, but additional terms are needed to explain the signal cutoff-frequency and high-frequency energy content. We show that this model of the impact seismic source brings insight into the difference in source mechanism of vertical and oblique impacts, and reproduces key properties of Lunar and Martian recordings. In parallel, we investigate Martian impact-generated seismo-acoustic signals. Thanks to a 1D model of the propagation and coupling of guided infrasound, we show that the group velocity of these signals depends on the effective sound speed profile in the Martian atmosphere. On the other hand, the amplitude ratio of horizontal to vertical displacement is shown to depend on the shear wave velocity structure below InSight. With a Bayesian inversion and using three impact-generated seismo-acoustic signals, we infer the Martian atmospheric and subsurface structure. The obtained sound speed profiles are in agreement with simulations of the Martian Climate Database. However, the inversion of the subsurface is shown to be dependent on the choice of inversion priors. Two models are possible: one with high seismic velocities in the first 20 m below InSight, and the other with a single interface at 40 m depth. These solutions are consistent with those obtained by previous near-subsurface studies. Hence, better discrimination between models can be achieved in the future by combining multiple datasets. Zoom link: https://u-paris.zoom.us/j/88486316870?pwd=TmFSU2NGWE5BSi9vYUE4RHVZSTY2UT09