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If you dug a hole straight down, what would you find? Earth isn’t just solid rock all the way through. There’s a thin rocky crust floating on the slowly flowing mantle, a liquid outer core and solid metal inner core. It turns out that Mars has a similar internal structure to Earth.

We know about Earth’s structure from earthquake research. By measuring a big shake from lots of different places, scientists learn about the rocks and other material between those points. So if researchers want to know what’s happening under the surface of Mars, why not put an earthquake sensor there?

That’s exactly what they did! NASA’s InSight lander has a seismometer, a machine designed to measure earthquakes. And for the past two-and-a-half years, it’s been measuring every marsquake on the red planet.

Of course, it’s not as easy to work out what’s going on when you only have one detector. The team had to use advanced techniques to peer beneath the surface. For example, they realised that there are echoes after marsquakes that are actually reflections off the core!

So what did they find? At the surface of Mars there is a rocky crust layer about 24–72 kilometres thick, which is similar to the thickness of Earth’s crust. Below that Mars has a mantle layer over a surprisingly large liquid core.

There are still plenty of mysteries hiding under the surface of Mars. Why doesn’t it have a magnetic field like Earth does? And is anything going on beneath Olympus Mons, the largest volcano in the solar system? Luckily, the InSight lander is still active, measuring every marsquake. One big shake could answer all kinds of questions!

Mars geology?

Some experts prefer not to use the word geology when talking about rocks on other planets. Geo is ancient Greek for Earth. The red planet term is Areology, as Ares is the ancient Greek name for Mars!

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