Constraints on Martian Core from First Observations of SKS Waves

Mars has a liquid iron alloy core at its center. Using seismic data gathered by the InSight mission, we have made the first observations of seismic waves traveling through Mars’ core. We use the travel times of core-transiting seismic waves, relative to ones which remain in the mantle, to constrain properties of the core and construct the first models of the elastic properties of the entire planet. Our results are consistent with a core rich in sulfur, with smaller fractions of oxygen, carbon and hydrogen.

This is the first-ever detection of waves that travel through the Martian core. In 2021, based on data recorded by the InSight VBB seismometer, we were able to detect waves reflecting off the core (analogous to sound echoes one can hear reflecting from a wall). These waves were produced by marsquakes that were relatively close to the InSight lander. However, during the extended mission (i.e. second martian year of the lander operating on the surface of Mars), we were able to detect seismic events much further from the lander. One of these, called S0976a, was a marsquake on the other side of Mars, located close to Valles Marineris (the largest canyon in the solar system). The other, S100a, was a large impact. Waves from these waves traveled through the martian mantle and then entered the core. By looking at the relative travel times of waves that stayed in the mantle, we were able to use these waves to determine the speed at which elastic waves travel through the martian core, which provides constraints on not just the size of the core — which we could independently estimate from the timing of the echoes I mention earlier — but also on the properties of the liquid iron alloy that makes up the core. These properties suggest that the martian core contains a large amount (about 1/5th of the core by mass) of “light elements”, including sulfur, oxygen, carbon and a dash of hydrogen. This is the first direct constraint on the properties of the materials constituting the martian core and provides important clues about how Mars formed and what the conditions in its interior are at present.

Our observations of core-transiting waves SKS indicate a core that is about 1780-1810 km in radius, which is 20-50 km smaller than the estimates based on the travel times of waves that reflect off the core (presented in Science in 2021 by Stahler et al.) and implies a denser core (average density of 6.2-6.3 g/cc) and one that has a smaller proportion of light elements (though the proportion is still about twice that in the Earth’s core).
The InSight mission has been fantastically successful in helping us decipher the structure and conditions of the planet’s interior. This is because it is the first mission to successfully deploy a seismometer on the surface, which has been able to record seismic activity on the planet — helping us understand the geologic activity on Mars at present  — and which was able to detect different kinds of seismic waves propagating through its interior. The velocity with which these waves travel tell us about the interior of the planet, and have led to multiple discoveries: the thickness of the martian crust, structural differences between the crust of the northern lowlands and southern highlands from analysis of surface waves, the detection of phase transitions of olivine about 1000 km below the surface (these pin down the pressure/temperature conditions at that depth), and of course, the first data on the exact size and properties of the core. And this was all done with a single seismometer on the surface. This success should be an impetus for future geophysical missions, including seismic networks on Mars and the Moon. 
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