What can seismic waves reveal about the crust of Mars?

Researchers working at the Marsquake Facility at ETH Zurich have used the seismometer on the NASA InSight mission to analyze measurements of seismic waves throughout the Martian crust.

For nearly three years, the only seismic waves it detected on Mars were those propagating from the epicenter, or hypocenter, deep within the planet.

Now, the research team was able to see an event that showed waves traveling along the Martian surface for the first time. On December 24, 2021, a meteorite impact on Mars produced the kind of seismic waves they were hoping to witness.

The study, titled “Largest Recent Impact Craters on Mars: Orbital Imaging and Surface Seismic Co-Investigation,” was published in the journal Science.

What do seismic wave patterns reveal about the crust of Mars?

Atypical features in the earthquake measurements led the researchers to suspect that its source was near the surface, so they contacted colleagues working with a probe orbiting Mars. Images taken by the Mars Reconnaissance Orbiter in late December 2021 showed a large impact crater about 3,500 kilometers from InSight.

“The location matched well with our estimates of the source of the earthquake,” said Doyeon Kim, a geophysicist and senior researcher at the Institute of Geophysics at ETH Zurich.

The researchers were also able to identify a meteorite impact just 7,500 kilometers (about 5,000 miles) away from InSight as the source of a second atypical earthquake.

Because the epicenter of each earthquake was at the surface, they created not only seismic waves similar to previously recorded markers in which the epicenters were at greater depth, but also waves that propagated along the planet’s surface.

“This is the first time that surface seismic waves have been observed on a planet other than Earth. Even the Apollo missions to the moon didn’t make it,” Kim said.

What makes seismic surface 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 yielded information about Mars’ core and mantle, but have revealed little about the crust away from the craft itself.

“Until now, our knowledge of the Martian crust has been based on measuring only one point beneath the InSight lander,” explained Kim.

The results of the wave analysis were surprising because the Martian crust between the impact points and InSight’s seismometer is typically very uniform in structure and high in density. However, the researchers identified three crustal layers, which indicate a lower density.

Explanation of the higher seismic velocity

These 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 can tell us about conditions billions of years ago and the timing of impacts, which were particularly common in the early days of Mars.

Explaining how the new measurement was made, Kim said: “The speed at which surface waves propagate depends on their frequency, which in turn depends on their depth.”

By measuring velocity changes in seismic data at different frequencies, it is possible to infer how the velocity changes at different depths because each frequency is sensitive to different depths.

This provides the basis for estimating the average density of the rock because the seismic velocity also depends on the elastic properties of the material through which the waves travel. This data allowed the researchers to determine the structure of the crust at depths between 5-30 kilometers below the Martian surface.

The researchers aimed to answer a series of questions about the average speed of seismic waves: Why was the average speed of the recently observed surface waves significantly higher than would be expected, based on the previous point measurement under the Mars InSight lander ; Is this mainly due to the surface rock or are other mechanisms at play?

In general, volcanic rocks exhibit higher seismic velocities than sedimentary rocks. In addition, the paths between the two meteor impactors and the measurement site pass through one of the largest volcanic regions in the northern hemisphere of Mars.

The team determined that this anomaly was due to lava flows and the closing of pore spaces by heat generated by volcanic processes, as these factors can increase the speed of seismic waves.

“On the other hand, the crustal structure beneath InSight’s landing site may have formed in a unique way, perhaps when material was ejected during a large meteorite impact more than three billion years ago. This would mean that the structure of the crust beneath the craft is not representative of the general structure of the Martian crust,” explained Kim.

Solving the mystery of the dichotomy of Mars

These new discoveries may help solve a centuries-old mystery. Ever since the first telescopes were pointed at Mars, it has been known that there is a sharp contrast between the planet’s southern and northern hemispheres.

While the dominant feature of the southern hemisphere is a plateau covered in meteorite craters, the northern hemisphere consists mostly of flat, volcanic plains that may have been covered by oceans in the planet’s early history. This division into southern highlands and northern lowlands is known as the Martian bifurcation.

“As things stand, we still don’t have a generally accepted explanation for the bifurcation because we’ve never been able to see the deep structure of the planet. We are just now beginning to unravel this,” said Domenico Giardini, professor of Seismology and Geodynamics at ETH Zurich.

The initial results seem to disprove one of the widely held theories about the bifurcation of Mars – the crusts in the north and south are probably not made of different materials, as has often been assumed – and their structure can be surprisingly similar at relative depths.

Could further results explain these anomalies?

ETH Zurich researchers expect further results soon. In May 2022, InSight observed the largest earthquake to date, measuring five on the Richter scale. It also recorded seismic surface waves generated by this shallow event. That’s timely, as the InSight mission will soon come to an end now that the lander’s solar panels are covered in dust and its power is running low.

An initial analysis of the data confirms the findings the researchers got from the other two meteor impacts. “It’s crazy. We’ve waited so long for these waves and now, just months after the meteorite impact, we’ve observed this large earthquake that produced extremely rich seismic waves. These allow us to see even deeper into the crust, at a depth of about 90 kilometers,” concluded Kim.