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The tremors created by the impacts of two meteorites on Mars show we don’t know the Red Planet’s crust as well as we thought. Parts of it are denser than expected and this may be because of water underground.

Seismic waves from the meteorite impacts, which occurred in December 2021, were picked up by . It is the first time we have measured seismic waves moving across the surface of another planet and they give us our first glimpse of the crust of Mars beyond the immediate area of the lander’s position on the surface.

The InSight lander, which reached Mars in 2018, detects seismic waves. The speed of these waves can reveal the geological characteristics of the material they are passing through.

at ETH Zurich in Switzerland and his colleagues have found that the surface waves coming to the lander from the meteorite impact sites were moving faster than expected, at around 3.2 kilometres per second.

Until the impacts, InSight had only been able to study the seismic waves that originated inside the planet’s mantle or core and travelled up towards the lander. These deep “body waves” showed that the crust beneath the landing site was composed of three distinct layers.

However, the team’s analysis revealed that this type of structure wasn’t representative of Mars as a whole, and the crust that the surface waves was moving through was denser than what had been detected under InSight.

“It’s quite surprising because, when we look at the surface waves, the crustal structure looks really, really different away from the lander,” says Kim.

Although the exact composition of the crust couldn’t be established, Kim says a few things could have increased the wave velocities as they travelled along the surface to the probe, including the presence of a water table beneath Mars’s ancient lava flow. Having water filling gaps between the solid matter would make the whole mix denser. It is also likely that the distinct crustal layering underneath the InSight lander doesn’t exist everywhere else on Mars and might even be a unique local feature below the probe.

at the University of Bristol, UK, says the meteorites gave an unprecedented opportunity to study the shallow structure of Mars. “This paper confirms that we’re not seeing the whole of Mars by looking straight down, and this now gives us a slice of Mars instead.”

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