Mars appears to have a solid metallic inner core surrounded by a liquid outer core, according to research based on seismic measurements from NASA's InSight lander. The Chinese-led team estimated that the solid region extends about 380 miles, or 613 kilometres, from the planet's centre.
The finding changes a previous interpretation that pointed to an entirely liquid centre. Researchers concluded that the inner core is likely made mainly of iron and nickel, the same principal materials found in Earth's core, while possibly containing a larger share of lighter elements such as oxygen. Their results were published in Nature.
Outside the solid centre, molten metal may extend from the 613-kilometre boundary to as far as 1,100 miles, or 1,800 kilometres, from Mars' centre. Lead investigator Daoyuan Sun of the University of Science and Technology of China said the inner core may have begun crystallising in the past and could still be growing. In that model, Mars initially had a fully liquid core.
The study relied mainly on 23 relatively weak marsquakes detected by InSight. Their epicentres were between 740 miles and 1,465 miles, or 1,200 to 2,360 kilometres, from the lander. InSight reached a broad plain near the Martian equator in 2018 and registered more than 1,300 quakes before operations ended in 2022.
Sun said the solid inner core accounts for about one-fifth of Mars' radius, close to the proportion occupied by Earth's inner core, although he cautioned that the similarity may be coincidental. Questions remain over whether the liquid outer region contains solid droplets or whether a partially solid, mush-like zone exists close to the inner-core boundary.
Nicholas Schmerr of the University of Maryland, who was not involved in the research, described the result positively but said the planet's interior was far from fully resolved. Because InSight is no longer operating, scientists cannot add new marsquake recordings from that instrument. Schmerr said a network of similar seismometers would be required to determine the core's exact shape and composition in greater detail.
Further modelling is also needed to understand how the inner core developed and what that evolution means for Mars' magnetic history. Mars currently has no global magnetic field. Schmerr suggested that slow crystallisation of the solid centre could be connected to that absence.
The study therefore offers evidence for an Earth-like division between solid and liquid core layers, while leaving important questions about composition, boundaries and planetary evolution open.
With only one lander supplying the relevant seismic record, the conclusion depended on extracting core-crossing information from a small group of weak events rather than a global Martian instrument network.
