Marsquakes have so far provided NASA's InSight probe with information about the underworld of our neighboring planet. However, the tremors caused by two strong meteorite impacts have now provided very special data, researchers report. Because in contrast to previous earthquakes from the depths, they generated shock waves near the surface. Previously unknown features of the Martian crust are now reflected in their signature. This information could shed further light on the planet's formation history, the scientists say.
It listens to Mars like a stethoscope: NASA's Mars InSight space probe, which landed on the surface of Mars in November 2018, uses a seismometer to record underground vibrations. In the meantime, she has detected hundreds of marsquakes, which in turn contain further information: The course of the waves enabled planetary researchers to draw conclusions about the structures in the depths of the planet. These so-called sky waves, which propagate through deep Mars from the focus of the earthquake, made it possible to gain insights into the Martian core and mantle. Unfortunately, they could provide little information about the crust. Seismic waves that propagate laterally in the rock near the surface would be favorable for this. However, the previous marsquakes were too weak to cause them.
Impacts cause surface waves
But on December 24, 2021, the scientists from the Marsquake Service at the Swiss Federal Institute of Technology Zurich (ETH) were delighted and surprised at the data transmitted by InSight: the signature of surface waves became apparent. They then contacted colleagues for more information. They then provided images from the Mars Reconnaissance Orbiter showing a fresh large impact crater around 3500 kilometers from InSight on Christmas Day 2021. "The location matched well with our estimates for the source of the quake," says lead author Doyeon Kim. A second stroke of luck then followed: In the case of another, atypical earthquake, the researchers were also able to identify a meteorite impact as the source, which had occurred around 7500 kilometers away from InSight.
Because the origins of the two tremors were on the surface, not only sky waves were generated, but also waves that propagated along the planet's surface, the scientists explain. "It's the first time anyone has observed surface seismic waves on any planet other than Earth," Kim said. As he explains, there is a variety of information in the waves. "The speed at which the surface waves propagate depends on their frequency, which in turn depends on the depth." The average density of the rock can also be estimated because the seismic velocity depends on the elastic properties of the material through which the waves travel, the researchers said.
Using the new data, the team has now obtained information about the structure of the crust at a depth of around 5 to 30 kilometers below the surface of Mars. "Until now, our knowledge of the crust was based only on a point measurement under the InSight lander," explains Kim. On the other hand, there was no more extensive information. The researchers were now able to compare the previous data with the new data. Surprisingly, it turned out that the Martian crust between the impact sites and the InSight seismometer has a very uniform structure and high density on average, which is reflected in comparatively high seismic velocities. Directly below the probe, on the other hand, the researchers had previously found rather inhomogeneous crustal features and a lower density.
Notes on the crustal structures
As the team explains, it's not entirely clear what the differences are due to. One explanation would be the effect of volcanic rock. Because in such material there are usually comparatively high seismic velocities. The paths between the two meteorite impacts and the measurement site also pass through one of the largest volcanic regions in the northern hemisphere of Mars. "On the other hand, the crustal structure beneath InSight's landing pad may have formed in a unique way, such as when material was ejected during a large asteroid impact over three billion years ago. Then the crustal structure under the probe is probably not representative of the general crustal structure on Mars,” explains Kim.
The new data could also shed light on a mystery called the Mars dichotomy, the researchers say: Since the first telescopes were pointed at Mars, the southern hemisphere has been known to be dominated by a high plateau covered with meteorite craters. In contrast, the northern part consists mostly of flat, volcanic lowlands. "So far there is no accepted explanation for this division because we have never been able to see its deep structure," says co-author Domenico Giardini from ETH. "Now we begin to uncover this structure." The first results refute a previous explanation for the Mars dichotomy: the crusts in the north and south are probably not made of different materials as previously assumed, and their deep structure could be surprisingly similar.
Finally, the team reports on a promising new development: In May 2022, InSight recorded the largest marsquake detected to date. It reached a magnitude of 5 and this could apparently also lead to surface waves, which are evident in the data. The researchers are currently working on the analysis, but the preliminary results appear to confirm what they learned from the two meteorite impacts.
Source: ETH Zurich, specialist article: Science, doi: 10.1126/science.abq7157