First observations of seismic waves travelling through the Martian core

Kurzfassung

Planetary cores are key to understanding the conditions of planetary formation, composition and evolution. Recent seismic determination of the Martian core radius (e.g. Stähler et al, Science, 2021) supports the idea that Mars’ core contains a higher fraction of light elements than the Earth, and unlike Earth’s core it does not generate a global magnetic field. Directly probing its properties is therefore important for understanding differences in formation, composition and evolution between Mars and Earth. However, until now observations of Martian core-transiting seismic waves, one of the richest sources of information about the Earth’s core, have been unavailable. The InSight mission’s SEIS experiment deployed a broadband seismometer onto the surface of Mars in late 2018. In 2021 two large seismic events were detected on the antipodal hemisphere (Horleston et al, TSR, 2022). These two events were in Mars’ core shadow zone, so that waves travelling directly to the seismometer must transit the core. We present observations of such core-transiting signals, using multiple analysis techniques to ensure detection robustness, and quantify uncertainty. These seismic signals, labelled SKS waves, have travelled through the mantle as shear waves and through the core as compressional waves; they are therefore the first direct probes of the elastic properties of Mars’ core. Using the travel times of SKS waves relative to other mantle waves (PP and SS), together with other geophysical and geochemical data, we constrain models of Mars’ seismic properties, including both the seismic velocity and density of the Martian core. We use these seismic models to refine estimates of light element abundances in Mars’ core. We compare our findings to pre-detection estimates based on other geophysical observations and geochemical arguments, and discuss implications for the formation and evolution of Mars.