Insight into the Martian underworld

What do you call an earthquake if it takes place on another planet? On the Red Planet they’re called marsquakes, and geophysicists have been using them to figure out the depth and structure of the Martian interior.

Their work has resulted in the first interior map of another planet, and it shows that Mars is markedly different to Earth, with a warm and gooey core much bigger than our own.

These insights were gleaned by the appropriately named InSight lander, a NASA mission that touched down on the planet in 2018. Unlike the rovers trundling around the surface, InSight has stayed put and set up a suite of custom-made instruments to peer into the heart of Mars. The mission’s aim is fairly simple and low-key: to understand the formation, evolution and composition of the entire planet.

Specifically, one of its jobs is to use its ultra-sensitive seismometer to measure marsquakes that rumble through the planet’s interior – so far, it has recorded 733. Three international research teams have now used the seismological data from the 35 strongest quakes to reveal the structure of Mars, publishing their results in the journal Science.

“What we’re looking for is an echo,” says Amir Khan of ETH Zurich, lead author of one study. “We’re detecting a direct sound – the quake – and then listening for an echo off a reflector deep underground.”

Khan’s study delved down to map the Martian mantle, which extends 1,560km below the surface; this mantle likely only has only one rocky layer, rather than two like the Earth.

Another team used the marsquakes to directly put numbers on the thickness of the Martian crust for the first time, finding that it’s much thinner than previously thought.

“What seismology can measure are mainly velocity contrasts – these are differences in the propagation velocity of seismic waves in different materials,” explains geophysicist Brigitte Knapmeyer-Endrun from Germany’s University of Cologne, lead author of the crust study.

“Very similar to optics, we can observe phenomena like reflection and refraction. Regarding the crust, we also benefit from the fact that crust and mantle are made of different rocks, with a strong velocity jump between them.”

So, by comparing how quickly marsquakes propagate through the interior, the structure of the crust can be determined. The team found that the crust is made up of either two or three layers; the top layer below InSight is about 8km down, with another layer around 20km below.

“It’s possible that the mantle starts under this layer, which would indicate a surprisingly thin crust, even compared to the continental crust on Earth,” says Knapmeyer-Endrun.

But there might also be a third layer below this, about 39km thick.

The final study peered right down to the core to find that it has a radius of nearly 1,830km, beginning about halfway between the surface and the centre of the planet. This is much bigger than Earth’s core. The study also confirms that the Martian core is molten, even at the centre; in contrast, Earth has a molten outer core and a solid inner core.

“This study is a once-in-a-lifetime chance,” says Simon Stähler of ETH Zurich, lead author of the third paper. “It took scientists hundreds of years to measure Earth’s core; after the Apollo missions, it took them 40 years to measure the Moon’s core. InSight took just two years to measure Mars’ core.”

Getting a clear picture of the Martian interior is key to understanding how the planet evolved, which can help illuminate why the planets in our solar system evolved to become such different worlds today. This is one of the most fascinating parts of Martian exploration, figuring out how it is – and isn’t – like Earth.

InSight is a key tool to gain this knowledge. Before the lander touched down, all of our research on Mars focused on its surface from data gathered by rovers and orbiters.

“When we first started putting together the concept of the mission more than a decade ago, the information in these papers is what we hoped to get at the end,” says InSight’s principal investigator Bruce Banerdt, of NASA’s Jet Propulsion Laboratory (JPL) in Southern California. “This represents the culmination of all the work and worry over the past decade.”

Meanwhile, InSight is still listening, and it’s waiting for an even more massive quake.

“We’d still love to see the big one,” says JPL’s Mark Panning, co-lead author of the paper on the crust. “We have to do lots of careful processing to pull the things we want from this data. Having a bigger event would make all of this easier.”

Clouds drift over the dome-covered seismometer, known as SEIS, belonging to NASA’s InSight lander, on Mars. Credit: NASA/JPL-Caltech