Mars yields innermost clues to the secret of its ancient formation

Scientists have revealed the inner structure of a rocky planet other than our own for the first time, showing that Mars has a crust of a similar thickness to the Earth’s and a surprisingly light liquid core.

The data was gathered by InSight, a lander operated by Nasa that reached the Martian surface in 2018 and has so far recorded more than a thousand ‘‘mars quakes’’ – the red planet’s equivalents of earthquakes.

The findings will be used to refine ideas on how Mars was formed some 4.6 billion years ago and the sequence of events that led to it being stripped of most of its atmos- phere – and possibility of life, if any has existed there.

The research, published in the journal Science, marks the first time that seismology has been used to map the internal structure of a planet other than Earth. It shows that at the InSight landing site, the top layer of the Martian crust is about 8km thick. Below that lies another crust layer, about 20km thick.

It is possible that another solid layer, the mantle, then begins. This would ‘‘indicate a surprisingly thin crust, even compared to the continental crust on Earth. Beneath Cologne, for example, the Earth’s crust is about 30km thick,’’ Brigitte Knapmeyer-Endrun, of Germany’s Max Planck Institute for Solar System Research, said.

It is possible that there is a third layer, which would make the crust under the landing site about 39km thick. ‘‘In both cases, however, we can rule out the possibility that the entire crust is made of the same material known from . . . Martian meteorites,’’ Knapmeyer-Endrun said. ‘‘The data suggest that the uppermost layer is composed of an unexpectedly porous rock.’’

Earthquakes come from faults caused by the movement of tectonic plates but Mars, unlike Earth, does not have any. Amir Khan, of the ETH Zurich Department of Earth Sciences, said: ‘‘Its crust is instead like one giant plate. But faults are still forming in the Martian crust, due to stresses caused by the slight shrinking of the planet due to cooling, which is also happening on the moon.’’

The quakes produce disturbances that travel through the planet as seismic waves. As they cross different layers the waves are altered.

The faint seismic signals have been used to investigate the centre of the planet. Its relatively large liquid metal core has a radius of nearly 1830km.

The findings indicate that the iron nickel core is less dense than previously thought and enriched in lighter elements, such as sulphur, oxygen, carbon and hydrogen.

‘‘In addition to its size we confirmed that the core is liquid, which is also supported by the initial observation that a large complement of light elements are required, since the latter will act to lower the melting temperature significantly relative to a pure iron core,’’ Khan said.

The new data will be plugged into models to predict how Mars evolved. One big question, according to Khan, is whether its inner structure once drove a ‘‘core dynamo’’ – the production of a magnetic field that would have shielded its surface from being blasted by the destructive solar winds, possibly supporting the emergence of life.