Mars is very similar to our Earth in many ways, including its internal structure - or so it was thought. But now two research teams have independently discovered an unexpected peculiarity of the Red Planet: Seismic measurement data from NASA's Mars InSight probe suggest that there is still an approximately 150 kilometer thick, liquid to semi-liquid layer of molten rock material above the liquid metal core of Mars. This finding could explain why the Martian core appeared unusually large and light in previous measurements: the newly discovered layer alters the seismic waves in a similar way to the liquid metal core itself. But the discovery also sheds new light on the thermal and magnetic evolution of the Red Planet .
Mars and Earth are unequal brothers: although they were formed from similar starting materials and in the same area of the primordial cloud, they show significant differences. The Earth has a magnetic field, plate tectonics and active volcanism, whereas Mars does not. However, whether its internal structure differs from that of Earth remained unknown for a long time. It was not until NASA's Mars InSight space probe landed on the Red Planet at the end of 2018 that the first seismic look into the interior of Mars was possible: for around four years, its seismometer recorded the propagation and speed of waves generated by earthquakes and meteorite impacts on Mars. The first analyzes of this data showed that Mars is also divided into a metal core, a rocky mantle and a crust, but there are also some differences. The Martian crust is almost twice as thick as the Earth's crust and is divided into several layers.
Mars core poses a mystery
But the findings so far about the Martian core were even more surprising. According to the InSight measurement data, this appeared to be unusually large with a radius of 1,830 kilometers. An indication of this was the reflection of shear waves at a boundary between the solid mantle rock and a hot, liquid area underneath - planetary scientists interpreted this as the boundary to the liquid metal core of the Red Planet. Measurements based on another type of seismic wave, P-waves, also suggested that the Martian core, unlike Earth's core, is completely liquid and has no solid inner part. In addition, its density would have to be significantly lower than that of the Earth's core. The planetary researchers came up with a value of a good six grams per cubic centimeter - significantly less than the nine to 13 grams per cubic centimeter of the earth's metal core. But that would mean that the iron-nickel alloy of the Martian core would have to contain many more lighter elements than the Earth's core.
“The core composition of Mars would therefore require more volatile elements such as sulfur, carbon and hydrogen than were cosmochemically available in the probable planetary building blocks of Mars,” explain Amir Khan from ETH Zurich and his colleagues. This high proportion of admixtures in the Martian core did not fit the common models of planet formation of Mars and Earth. To get to the bottom of this discrepancy, both Khan and his colleagues and a second team led by Henri Samuel from the University of Paris Cité independently analyzed the seismic data from the InSight land probe again. They benefited from the fact that shortly before the end of the mission, the probe's seismometer recorded several particularly strong earthquakes that also passed through the core region of Mars.
Liquid intermediate layer instead of core-cladding boundary
The results of both analyzes are now available – and reveal something surprising. Accordingly, the solid-liquid boundary that reflects the seismic S waves is not the core-mantle boundary of Mars, as previously thought. Instead, there appears to be another liquid layer above the liquid metal core, which consists of molten mantle rock. Using additional models, both teams determined that this liquid intermediate layer must be around 150 kilometers thick. The Martian core must therefore be correspondingly smaller - it only begins below this hot, molten rock layer. “In our scenario, this results in a core radius of 1,650 kilometers,” report Samuel and his colleagues. Khan and his team estimate the radius of the Martian core to be 1,675 kilometers. Based on the new measurement data, both teams also determined a core density that was five to eight percent higher than previously assumed. This reduces the discrepancy between the element mix in the Martian core and the composition that planetary scientists assume for the primordial cloud and planetesimals in the early solar system.
The discovery of the liquid rock layer inside Mars could also shed new light on the question of why Mars - unlike Earth - has no magnetic field. This is because the layer of molten rock on the Martian core also influences the heat flows within it and isolates the core from the rest of the planet. This also eliminates the thermal interactions with the cooler, solid jacket, which create temperature differences in the liquid metal bath of the core and trigger convection currents there. “This means that the core cannot generate the currents that drive a magnetic field - which explains why Mars no longer has an active magnetic field,” says co-author Vedran Lekic from the University of Maryland. However, on-site measurements by space probes show that there were magnetic fields on the Red Planet in its early days, at least in places. "The thermal coverage of the metal core suggests that external sources must have caused these magnetic fields in the first 500 to 800 million years of Mars' evolution," adds Samuel. “These triggers could have been high-energy impacts or nuclear movements caused by gravitational interactions with previous moons of Mars.”
The newly discovered layer in the interior of Mars not only changes the view of the core of the Red Planet, the rest of the planet and its development also have to be rethought, at least in part. However, how and why this layer of molten rock exists in Mars is still controversial: both research teams propose different scenarios. Scientists also do not yet agree on the temperature and density of the newly discovered liquid layer. It is also unclear whether this liquid layer is truly global because the seismic data so far has predominantly illuminated one side of the planet's interior. There is therefore still a need for further research here.