Seismic Experimentation to Advance Understanding of Martian Regolith-Ice

Prior workers have hypothesized the various states and stability regimes of regolith-bound H₂O-ice on Mars based on the diffusion of atmospheric water vapor into the porous regolith of Mars. Such models rely upon the orbital dynamics of Mars, especially obliquity variations, to determine periods of thermophysical stability of ice within the lower latitudes. As the obliquity of Mars changes, the stability regimes for ice change as well, which results in latitudinal transfer of ice through vapor diffusion. However, ice may remain metastable at shallow depths within lower latitudes (~55°) even during low obliquity, as verified by satellite observations. In addition to poorly constrained metastability, the quantity of ice found at lower latitudes exceeds what could have accumulated solely through available atmospheric vapor through the previous ~1.4-2.5 Ma obliquity cycle.

In order to understand seasonal controls and identify key regions of regolith ice growth, future in situ missions must have established methodology to investigate the martian subsurface. A novel and effective, yet non-invasive, method uses high frequency seismic waves, because they reveal subsurface physical properties. While many physical parameters of the martian regolith have been constrained, no research to date uses these seismic data to define the shallowest velocity structure of the martian regolith with the addition of H₂O in any state. Further, the burgeoning nature of martian near-surface seismology still has many unknowns regarding the physical and chemical parameters that could potentially affect seismic velocity profiles. Before any in-situ seismic surveys are made, we must first understand how known physical properties will affect seismic wave propagation on Mars. We are examining the applicability of terrestrial-based seismic models, such as modified contact theory, to explain the observed velocities. We find that a velocity increase with depth, as predicted through contact theory, is a valid model for the martian regolith, through comparisons with recent InSight findings. Further, our preliminary results show that surface waves dominate in our experiments, which are useful for deriving accurate velocity/depth models. Future work is planned to examine the validity of contact theory with the addition of H₂O in pore space.