If the explorers from Journey to the Center of the Earth were to journey to the center of Mars instead, they definitely wouldn’t come across the subterranean oceans or live dinosaurs they encountered in the movie, but they would probably see something different from our planet’s core.

Earth has a mantle of rock that moves like a sluggish liquid. Beneath the mantle is a liquid iron outer core and solid iron inner core. Because Earth and Mars are both rocky planets, and might have even had similar surface conditions billions of years ago, does that mean we should expect the same interior on Mars? Not exactly.

When two teams of researchers used data from NASA’s InSight lander and other spacecraft to get as close to the core of Mars as they could in a lab, they found that the red planet is not much like Earth on the inside. Data from NASA’s InSight lander’s SEIS (Seismic Experiment for Interior Structure) project had previously suggested that Mars has a large core that is not very dense. But the new analysis, which included additional seismic signals, indicates that what was once thought to be the surface of the Martian core is actually a thick molten rock layer. The actual core of Mars is most likely much smaller.

Where it started…

To see why the previous InSight measurements ended up with a core estimate that was too large and not heavy enough, we have to go all the way back to the formation of Mars.

Earlier, it was thought that when Mars first formed, it was covered with an enormous magma ocean that eventually turned into a heterogeneous mantle full of silicates, iron, and radioactive elements that produced heat.

InSight’s seismic data supported this idea. The low core density that was proposed based on the lander’s observations meant that there had to be a significant amount of light elements like silicon, carbon, oxygen, and hydrogen in the core. It seemed to make sense because the Martian core was previously thought to have formed before the dispersal of all the gas that our Solar System was born in.

There is just one problem. These are all volatile elements, meaning they vaporize easily. Even some forms of silicon can evaporate when heated enough. So, much of this light material should have been lost from the magma ocean.

“There is [a] lack of knowledge as to the identity and abundance of the predominant light elements in the Martian core,” said geophysicist Amir Khan of ETH Zürich, who led one of the research teams in a study recently published in Nature.

How it’s going…

Both Khan and Henri Samuel, who led another team in a study also published in Nature, now think the mantle is homogeneous rather than heterogeneous. Its physical properties are pretty much the same throughout. By contrast, Earth’s mantle is mostly heterogeneous.

InSight had previously detected a marsquake triggered by a meteorite impact. Samuel’s team found that the seismic waves that had traveled through the planet could not be explained by a heterogeneous mantle, which would have made for a much slower wave velocity.

Both teams backed up these findings with computer simulations and models of how such waves propagate deep inside Mars. These further showed that a seismic wave velocity close to that which resulted from the quake was only possible if Mars had a small, dense core of liquid iron surrounded by a molten silicate layer—if the core was less dense, the waves would have traveled faster. Both research teams also compared the density of liquid iron to the mixture of elements that was thought to make up the surface of the core and found that liquid iron was much denser than InSight’s measurements had been suggesting.

So, what was thought to be the surface of the Martian core is actually a layer of its own that is about 1,780–1,840 km (1,106–1,143 mi) thick. The actual core is now thought to be much smaller and denser, made primarily of molten iron that might contain traces of other elements.

…Where it’s headed

This virtual dissection of the red planet could change how we approach the evolution of rocky planets—including our own. It may even tell us how Mars lost its magnetic field about 4 billion years ago. There is a possibility that the core retained too much heat to maintain a magnetic dynamo.

“Magnetic field production via a thermally driven dynamo action requires efficient convective motion in the metallic core, implying core heat loss… but [certain processes have prevented] core cooling,” Samuel and his colleagues said in their study.

Some uncertainties remain, and both Khan and Samuel agree that more investigation needs to be done in the future, but we are finally finding out what Mars is really like at its literal core.

Nature, 2023.  DOI: 10.1038/s41586-023-06601-8, 10.1038/s41586-023-06586-4.