New look at the core of Mars

Martian interior

Seismic waves traversing the core of Mars have provided new insights into its features. © NASA/JPL; Nicholas Schmerr

Seismic waves can reveal the nature of a planet's core. Measurement data from NASA's Mars InSight probe have now provided the first more detailed insights into the inner workings of Mars. Thus, the core of the red planet is most likely completely liquid and slightly smaller and denser than previously thought - it has a radius of 1780 to 1810 kilometers. In addition, the liquid iron of the Martian core contains significantly more lighter elements such as sulphur, oxygen, carbon and hydrogen than the liquid core of the Earth: they could make up 20 to 22 percent by weight, the research team reports.

Although Mars is our neighboring planet and in its early days it resembled Earth in many ways, we know very little about its inner workings. It seems clear that its composition is fundamentally similar to that of Earth and has a similar layered structure consisting of an iron-rich core, a silicate-rich mantle and a crust. For a long time, however, planetary researchers were only able to roughly estimate the thickness, composition and structure of these layers, using models and measurement data from orbital probes, among other things. This only changed with the NASA space probe Mars InSight, which landed on Mars in November 2018. Because its seismometer registered more than 1,300 marsquakes in the four years of its operation, including some tremors from meteorite impacts. The transit time and shape of such waves allow conclusions to be drawn about the nature, density and temperature of the material through which these seismic signals passed.

Two seismic events with nuclear waves

In the summer of 2021, scientists created a first model of the anatomy of Mars based on this data. At that time, however, important measurement data were missing to be able to determine the composition and size of the Martian core more precisely. This requires seismic waves from the opposite side of Mars, which have passed directly through the core on their way to the InSight seismometer. "Such farside events are difficult to detect because they lose a lot of energy along the way," explains lead author Jessica Irving from the University of Bristol. "We needed a lot of luck and experience to capture these events." Only after about three years - on mission day 976 and day 1000 - did the seismometer detect two such signals passing through the core.

"It's the first time we've captured seismic waves that have passed through the core of another planet," Irving said. The first seismic signal came from the strongest marsquake recorded during the Mars InSight mission. The second seismic surge was caused by a meteor impact on the far side of Mars. "This second signal was particularly helpful because we knew exactly where the source of the seismic waves was," explains Irving. Nevertheless, it required a lot of effort and seismological experience to extract and analyze the weak core waves from the countless background noises and complex seismograms.

More light elements in the core of Mars

The results of the seismic analyzes are now available. "Thanks to Mars InSight, we can finally understand what the center of Mars is like and why it is so similar but different from Earth," says co-author Vedran Lekic of the University of Maryland. According to the new data, the core of Mars is likely liquid and, unlike Earth, does not have a solid inner metal core. The radius of the Martian core is 1780 to 1810 kilometers - it is therefore not even half the size of the Earth's core and about 20 kilometers smaller than estimated in 2021, as the team reports. There are also new values ​​for the density of the Martian core: at 6.16 to 6.35 grams per cubic centimeter, it is slightly higher than the previously determined 6.0 grams per cubic centimeter. From this density and from the propagation times of the seismic waves, the scientists were also able to obtain new information about the chemical composition of the Martian core.

"These initial measurements of the elastic properties of the Martian core helped us determine its composition," says Irving. Until now, planetary researchers have assumed that the core of Mars consists of liquid iron mixed with small amounts of sulphur. However, the new measurement data suggest that there must also be other light elements in the Martian core - and that the total proportion of such elements is relatively high: "We conclude that the Martian core has an average total of 20.3 to 21, 4% by weight of light elements,” report Irving and her colleagues. "The liquid outer core of the Earth therefore only has about half the proportion of light elements as the core of Mars." Exactly which elements these are cannot be determined from the seismic data alone. However, based on the composition of the Martian mantle and geophysical models, the scientists assume that sulfur makes up the majority of these core iron admixtures, at around 16.5 percent by weight. There are also small amounts of oxygen, carbon and hydrogen.

"These new results are important for understanding how the formation and evolution of Mars differs from that of Earth," says Irving. Because although both planets had similar starting conditions, they show crucial differences. For example, Earth has a magnetic field and plate tectonics, while Mars does not.

Source: Jessica Irving (University of Bristol) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2217090120

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