Mars has a liquid metallic core with a radius of about 1,830 km.

NASA’s InSight lander gave scientists the first direct seismic measurements of the deep interior of another planet.

The basic idea is similar to earthquake seismology on Earth. A marsquake sends vibrations through the planet. Different materials bend, reflect, and slow those waves in different ways, so the arrival times recorded by the lander can reveal hidden boundaries inside Mars.

In 2021, the first major analysis placed the top of Mars’s liquid metallic core about 1,830 kilometers from the planet’s center. That implied a surprisingly large core with a relatively low density.

More marsquakes provided additional wave paths through the deep interior. In 2023, two independent studies found that the new signals were better explained by adding a layer of molten silicate rock between the solid mantle and the metallic core.

That changed the meaning of the boundary identified earlier. What had looked like the top of the metallic core could instead be the top of this molten rock layer.

With the extra layer included, the metallic core itself shrank to roughly 1,650–1,675 kilometers in radius and could be denser than first thought.

The episode did not show that InSight’s seismic method had failed. It showed how a first model of an inaccessible planetary interior can change when new wave paths reveal an extra layer.