Two large meteorite impacts shed light on the crust of Mars

Following two large meteorite impacts on Mars, researchers have observed, for the first time, seismic waves that spread across the surface from a planet other than Earth. The earthquake data was recorded by NASA’s InSight lander and analyzed at ETH Zurich, Switzerland in collaboration with the InSight science team and provides new knowledge on the structure of the Martian crust, as published in the journal Science.

Researchers working at ETH Zurich’s Mars Earthquake Service have been analyzing measurements made by NASA’s InSight mission seismometer on one of our neighboring planets. For nearly three years, the only seismic waves it detected on Mars were those thate propagated from the focus or hypocenter of the respective earthquakethrough the depths of the planet.

However, researchers were hoping for an event that would also generate waves traveling across the planet’s surface, and their wait was finally rewarded on December 24, 2021, when a meteorite impact on Mars produced the kind of surface waves that have been waiting for a long time.

The atypical characteristics of the earthquake readings led the researchers to suspect that su origin was near the surface, so they got in touch with colleagues who were working with a probe in orbit around Mars. And indeed, the images taken by the Mars Reconnaissance Orbiter they showed a large impact crater about 3,500 kilometers from InSight.

“The location coincided with our estimates of the origin of the earthquake,” he says. Doyeon Kim, geophysicist and senior research scientist at the ETH Zurich Institute for Geophysics and lead author of the study. The researchers were also able to point to a meteorite impact just under 7,500 kilometers from InSight as the source of a second atypical earthquake.

Since the hypocenter of each earthquake was at the surface, they not only generated seismic body waves similar to those of previously recorded earthquakes in which the hypocenters were deeper, but also waves that propagated along the surface of the planet. “This is the first time surface seismic waves have been observed on a planet other than Earth. Even the Apollo missions to the Moon didn’t do it,” says Kim.

What makes surface seismic waves so important to researchers is that they provide information about the structure of the Martian crust. Seismic body waves, which travel through the interior of the planet during an earthquake, have so far provided information about the core and mantle of Marsbut have revealed little about the crust far from the lander itself.

“Until now, our knowledge of the Martian crust was based on a single point measurement under the InSight lander,” explains Kim. The result of the surface wave analysis surprised him. On average, the Martian crust between the impact sites and InSight’s seismometer It has a very uniform structure and a high density. However, just below the lander, the researchers had previously detected three layers of crust implying a lower density.

The new findings are remarkable because a planet’s crust provides important clues about how that planet formed and evolved. Since the crust itself is the result of early dynamical processes in the mantle and subsequent magmatic processes, it may inform us about the conditions billions of years ago and about the timing of impacts, which were especially frequent on early Mars.

“The speed at which surface waves propagate depends on their frequency, which in turn depends on their depth,” explains Kim, and by measuring the speed changes in seismic data across different frequencies, it is possible to infer how velocity changes at different depthsbecause each frequency is sensitive to different depths.

This provides the basis for estimating the average rock density, since the seismic velocity also depends on the elastic properties of the material through which the waves travel. These data allowed the researchers to determine the structure of the crust at depths of approximately five to 30 kilometers below the surface of Mars.

In general, volcanic rocks tend to show higher seismic velocities than sedimentary rocks. In addition, the trajectories between the two meteorite impacts and the measurement site pass through one of the largest volcanic regions in the northern hemisphere of Mars.

Lava flows and the closing of pore spaces by the heat created by volcanic processes can increase the speed of seismic waves. “On the other hand, the crustal structure under the InSight landing site may have formed in a unique way, perhaps when the material was ejected during a large impact meteorite more than three billion years ago. That would mean that the structure of the crust under the lander is probably not representative of the general structure of the Martian crust,” explains Kim.

The new research could also help solve a centuries-old mystery: Ever since the first telescopes were pointed at Mars, it’s been known that there is a stark contrast between the planet’s northern and southern hemispheres. While the southern hemisphere is characterized by being a plateau covered with meteor craters, the Northern Hemisphere is made up primarily of flat, volcanic lowlands that may have been covered by oceans in the planet’s early history. This division into southern highlands and northern lowlands is called the Mars dichotomy.

“As things stand, we still don’t have a generally accepted explanation for the dichotomy because we’ve never been able to see the deep structure of the planet,” he acknowledges. Domenico Giardiniprofessor of seismology and geodynamics at ETH Zurich, but now we are starting to find out.”

The first results seem to refute one of the most widespread theories about the dichotomy of Mars: the northern and southern crusts are probably not composed of different materials, as has often been assumed, and their structure can be surprisingly similar at relevant depths.

Researchers at ETH Zurich hope to get more results soon. In May 2022, InSight observed the largest Martian earthquake to date, with a magnitude of five. It also recorded the surface seismic waves generated by this shallow event. This happened just in time, as the InSight mission will soon come to an end now that the lander’s solar panels are covered in dust and it’s running out of power.

A first analysis of the data confirms the findings that the researchers obtained from the other two meteor impacts. “It’s crazy. We had been waiting for these waves for a long time, and now, just a few months after the meteorite impacts, we see this great earthquake that produced extremely rich surface waves. These allow us to see even deeper into the crust, down to a depth of about 90 kilometres,” Kim concludes.