Modeling of the seismic signals generated by dust devils on Mars

The solar heating of the surface of Mars during the daytime causes the formation of strong convective vortices and dust devils. However, the impact of these vortices is not limited to the atmosphere. The pressure they exert on the surface forces indeed the ground to move and in this way seismic signals are generated, as recently shown in a terrestrial field study (Lorenz et al. 2015). Hence, the very sensitive seismometer of the Seismic Experiment for Interior Structure (SEIS) of the InSight mission will likely detect dust devils passing close enough to the lander. It is therefore necessary to characterize and describe the signals generated by vortices: on the one hand to distinguish and subtract them from internally originated signals, and on the other hand to use them as a seismic source for the subsurface exploration. To model the source, we used the pressure and wind fields resulting from high-resolution Large-Eddy Simulations, which resolve the turbulence and the dynamics of the Planetary Boundary Layer of Mars. We then computed the corresponding long-period displacement fields at the surface in the quasi-static hypothesis for different models of the subsurface down to a few tens of meters depth. In the 10-100 s band, typical vortices with a pressure drop of 1-3 Pa generate seismometer accelerations in the range 5-25 nm/s2, depending on the ground model. The main effect is related to ground tilt on the horizontal components and can be detected by SEIS, with an encounter frequency of about 0.5 episode/sol. Thus, the analysis of seismic and meteorological measurements will permit to estimate the intensity and the path of the vortex, as well as the compliance of the Martian regolith. In addition, Terrestrial data show acoustic and high-frequency surface waves generated by the vortices, which may be used to determine the layering and inverse for the shear-wave profile of the subsurface at the InSight landing site.