Research teams have already been able to find out a great deal about the interior of Mars, but until now there has been little data on the structure of the crust. Two meteorite impacts have now changed that – one of which came as a kind of Christmas present.
On Christmas Eve last year, there was a tremendous crash on the Red Planet: a meteorite struck a 150-meter crater in the Amazon plain on Mars. The impact produced an earthquake that was registered by the seismometer of the US lander “InSight” 3532 kilometers away. The impact and another, slightly smaller one on September 18, 2021 provided valuable information about the planet’s crust, as an international team of researchers reports in the journal Science.
“It is the first time that we have been able to detect seismic surface waves on another celestial body,” says Doyeon Kim from ETH Zurich, one of the scientists involved. “Even on the moon, such evidence was not possible with the detectors of the Apollo missions.” Tectonic activities or impacts trigger two types of wave-like vibrations: sky waves that travel through the interior of a planet, and surface waves that – the The name says it all – reproduce close to the surface, i.e. in the crust.
The “InSight” probe that landed in November 2018 has already registered over 1,300 such marsquakes – but so far they have only been space waves. With this, the researchers have learned a lot about the inner structure of the red planet, and that is also the main task of the mission. The propagation of seismic waves has shown that our neighboring planet looks very similar to Earth on the inside. So far, there have only been measurements of the crust directly at the location of the “InSight” lander.
The strength of the two impacts turned out to be a stroke of luck for the researchers. From the shape of the recorded waves, the researchers immediately concluded that these were not ordinary marsquakes, but rather meteorite impacts. From the combination of space and surface waves, the “InSight” team was able to estimate fairly accurately where on Mars the impacts must have taken place. And then the search began on the high-resolution images of the “Mars Reconnaissance Orbiter” – the probe has been orbiting the red planet since 2006 – for fresh craters.
The researchers actually found what they were looking for and discovered two craters with fresh traces of ejection in the suspected regions. With the precise localization of the impacts, the team was then able to draw conclusions about the crust it was passing through from the received surface waves. And the result came as a surprise to the scientists: by and large, the crust seems to be very uniform over the entire route from the impact sites to the “InSight” lander. The researchers had actually expected significant differences between the cratered plateaus of the southern hemisphere and the lowlands of the northern hemisphere.
So far, there is no explanation as to why the northern and southern hemispheres of the red planet are so different. But the measurements of the surface waves contradict previous models, according to which the crust in the north and in the south consists of different materials. The data also shows that the crust of Mars is, on average, denser than at the InSight landing site. “This means the crust beneath the lander is likely not representative of the general structure of Martian crust,” Kim points out.
The researchers are now hoping for more impacts to learn more about the crust of our neighboring planet. However, the “InSight” mission ends this December because by then so much dust has settled on the probe’s solar cells that there is no longer enough energy to operate the measuring devices. However, the data collected will keep the scientists busy for years to come – and may provide further surprising insights.
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