Infrasound on Mars: Numerical Modelling and Application to InSight's APSS/SEIS Data

The presented work relies on ray tracing and full-wave numerical simulations to study infrasonic waves on Mars, namely at the InSight location on Elysium Planitia. Those simulations highlight key features of regional infrasound propagation (distances to lander up to ~150 km), and of conversion of pressure perturbations into Martian ground movement. Then, the previously obtained results are put in the perspective of InSight's APSS and SEIS instruments. In particular, some events of uncertain origin are studied.

The numerical tools used are SPECFEM-DG [1] for full-wave modelling, and WASP-3D [3] for ray tracing. The atmospheric models are chosen from the Mars Climate Database (MCD) [4, 5]. For the sub-surface, the pre-landing models [2] are used.

The ray tracing method is only exact in the geometric limit (f→+∞), but is computationally very light. This kind of simulation is used to comprehend the propagation patterns in several different atmospheric configurations. SPECFEM-DG is a full-wave solver: computationally heavy, but very versatile. It is based on the full Navier-Stokes equations for the atmosphere, the elastodynamics' system for the ground, and a mechanical coupling between the two. The fluid part modelling takes into account varying horizontal wind, non-linearities, viscosity, and CO2 attenuation [1]. These simulations are used on specific cases (i.e. atmospheric conditions) in order to better understand the acoustic resonances and the atmosphere-to-ground coupling.

The simulations show the existence of two modes for the conversion of pressure to ground motion. The two modes have very specific characteristics. The main conclusion is that SEIS could record ground motion caused by infrasound below the pressure sensor noise level. From this, an infrasound marker, based on APSS data (pressure, wind amplitude, azimuth) and SEIS data (ground motion/velocity), is presented. The origin of some events recorded by SEIS remain of uncertain origin. They are studied from the point of view of infrasound: the possibility of them being caused by infrasonic waves converted to ground motion is discussed.

[1] Brissaud 2017, 10.1093/gji/ggx185. [2] Delage 2017, 10.1007/s11214-017-0339-7. [3] Dessa 2005, 10.1029/2005GL022867. [4] Forget 1999, 10.1029/1999JE001025. [5] Millour 2008, 10.4271/2009-01-2395.