The Marsquake Service at ETH Zurich has been analyzing data from the NASA InSight mission’s seismometer on one of our neighboring planets.

For over three years, the only seismic waves observed on Mars were those that travelled through the planet’s depths from the each quake’s focus, or hypocenter.

But the researchers had always hoped for an event that would also cause waves to move across the planet’s surface.

On December 24, 2021, a meteorite collision on Mars produced the kind of surface waves they had been longing for, making their wait finally worthwhile.

The researchers suspected that the source of the quakes was close to the surface, so they called their colleagues who were operating a Mars-orbiting spacecraft.

In fact, photos captured by the Mars Reconnaissance Orbiter in late December 2021 revealed a massive impact crater approximately 3,500 kilometers away from InSight.

Doyeon Kim, a geophysicist and senior research scientist at ETH Zurich’s Institute of Geophysics, says, “the location was a good match with our estimates for the source of the quake.”

Kim is the main author of a study that just came out in the journal Science.

A meteorite strike just under 7,500 kilometers (approximately 5,000 miles) from InSight was also identified as the source of a second unusual tremor by the researchers.

Each earthquake’s hypocenter was at the surface, therefore in addition to producing seismic body waves resembling those of earlier marsquakes with deeper hypocenters, they also sent out waves that traveled across the surface of the planet.

“This is the first-time seismic surface waves have been observed on a planet other than Earth. Not even the Apollo missions to the Moon managed it,” Kim adds. 

Seismic surface waves are crucial to scientists because they reveal information on the structure of the Martian crust.

Although they have so far shed light on Mars’s core and mantle, seismic body waves that travel through the planet’s interior during an earthquake have so far revealed little about the crust outside of the lander.

An unexpected finding

“Until now, our knowledge of the Martian crust has been based on only a single point measurement under the InSight lander,” adds Kim. 

He was shocked by the surface wave analysis’s conclusion. The Martian crust has a generally high density and relatively consistent structure between the impact locations and InSight’s seismometer.

However, the researchers had previously found three layers of crust that suggested a lower density just beneath the lander.

The new findings are noteworthy because a planet’s crust offers crucial hints about its formation and evolution.

Since the crust itself is the consequence of subsequent magmatic processes and early dynamic processes in the mantle, it can provide information about circumstances that existed billions of years ago as well as the timing of impacts, which were extremely frequent in Mars’ early history.

“The speed at which surface waves propagate depends on their frequency, which in turn depends on their depth,” explains Kim. 

Due to the fact that each frequency is sensitive to different depths, it is possible to deduce how the velocity varies at different depths by measuring velocity changes in seismic data across multiple frequencies.

This gives us a way to figure out what the average density of the rock is, since the seismic speed also depends on how elastic the material is that the waves travel through.

With the help of this information, the researchers were able to identify the crust’s structure between 5 and 30 kilometers beneath Mars’ surface.

Explaining greater earthquake velocity

So why was the average surface wave speed now seen to be significantly greater than predicted based on the prior point measurement under the Mars InSight lander? Is the surface rock the primary cause of this, or are there other factors at work?

Volcanic rocks typically demonstrate greater seismic velocity than sedimentary rocks. Additionally, one of the greatest volcanic zones in Mars’ northern hemisphere is traversed on the pathways between the two meteorite impacts and the measuring site.

Seismic waves can travel faster due to lava flows and the closing of pore spaces as a result of the heat produced by volcanic processes.

“On the other hand, the crustal structure beneath InSight’s landing site may have been formed in a unique way, perhaps when material was ejected during a large meteoritic impact more than three billion years ago. That would mean the structure of the crust under the lander is probably not representative of the general structure of the Martian crust,” Kim adds.

Mars dichotomy solved

A long-standing enigma might also be resolved thanks to the new findings.

Since the first telescopes were directed at Mars, scientists have known that there is a significant difference between the planet’s southern and northern hemispheres.

The northern hemisphere is primarily made up of flat, volcanic plains that may have formerly been oceans, as opposed to the southern hemisphere’s prominent feature, a plateau covered in meteorite craters.

The Mars Dichotomy refers to this split between the southern highlands and the northern lowlands.

“As things stand, we don’t yet have a generally accepted explanation for the dichotomy because we’ve never been able to see the planet’s deep structure,” adds Domenico Giardini, ETH Zurich Professor of Seismology and Geodynamics. “But now we’re beginning to uncover this.”

The first results seem to disprove one of the most popular theories about the difference between the north and south of Mars. The crusts in the north and south probably aren’t made of different materials, as was often thought, and their structure may be surprisingly similar at relevant depths.

Long wave-wait

The ETH Zurich researchers anticipate more findings soon. With a magnitude of 5, InSight recorded the largest marsquake to date in May 2022.

Additionally, it captured seismic surface waves produced by this shallow incident. This was fortunate timing because the InSight mission is about to end and the lander’s solar panels are covered in dust and out of power.

A first look at the data confirms what scientists learned from the other two meteorite impacts.

“It’s crazy. We’d been waiting for so long for these waves, and now, just months after the meteorite impacts, we observed this big quake that produced extremely rich surface waves. These allow us to see even deeper into the crust, to a depth of about 90 kilometres,” adds Kim.

Source: 10.1126/science.abq7704

Image Credit: ETH Zurich, Doyeon Kim, Martin van Driel, and Christian Boehm | © IPGP – CNES – N. Starter

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