Mars' iron core surrounded by molten rock, seismic studies show

Two new pieces of research suggest center is smaller than previously thought

Two new studies from a NASA Mars probe suggest the red planet's core is surrounded by a layer of molten rock, and the core is smaller than previously thought.

Launched in 2016, the InSight lander's Seismic Experiment for Interior Structure (SEIS) project provided data in 2021 that suggested the presence of a large but low-density core, composed of liquid iron and lighter elements such as sulphur, carbon, oxygen and hydrogen. The results puzzled scientists because they revealed a greater number of lighter elements than theories of Mars's formation history had suggested.

The new studies published in Nature this week show that liquid iron core is actually wrapped in 150km of near-molten silicate rock and suggest a smaller core radius with a higher density than the earlier InSight study showed. The results fit more neatly with the known chemical make-up of Mars.

Research teams led by Amir Khan at Swiss university ETH Zürich and by Henri Samuel of Université Paris Cité both conclude the liquid iron–nickel core of Mars reaches no farther than halfway to the surface.

They also agree on the layer of molten rock, using three main seismological signals. Slow longitudinal seismic waves — or P waves — traverse the bottom of the mantle but do not make it into the core. At the same time, transverse seismic waves — S waves — emitting from marsquakes nearer to the lander spread through the middle and top of the mantle in a way that suggests a cool solid. Lastly, evidence of reflections of S waves from the top of the molten layer shows it is a solid.

In an accompanying article, Suzan van der Lee, professor at the Department of Earth and Planetary Sciences, Northwestern University, said: "The current results provide the most accurate and precise estimates so far of Mars' core and mantle structure. The findings show that combining seismological observations with knowledge of terrestrial-planet formation and evolution — and with data on planetary size, shape, rotation and gravitational field — offers invaluable perspectives on Mars's past, present and future dynamics."

She added: "The combination also limits the range of models that are compatible with all of the data. The inferred connections between the current interior structure and the potential evolution of Mars provide crucial context for understanding terrestrial planets, such as Earth, more generally."

However, the two results show regional snapshots of the planet, and it is possible that they might not be representative of Mars. Van der Lee said instruments at different locations would be needed to clear up the problem, although space agencies are not currently planning the missions to provide such data. ®

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