What can seismic waves reveal about Mars’ crust?

Researchers working in the Marsquake Service at ETH Zurich have been using the NASA InSight mission’s seismometer to analyse measurements of seismic waves across Mars’ crust.

For almost three years, the only seismic waves it detected on Mars were ones that propagated from the respective quake’s focus, or hypocenter, through the depths of the planet.

Now, the research team have been able to witness an event that showed waves travelling along Mars’ surface for the first time. On 24 December 2021, a meteorite impact on Mars yielded the type of seismic waves they had been 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 Martian crust?

Atypical characteristics in the quake readings led the researchers to suspect its source was near the surface, so they contacted colleagues who were 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 kilometres from InSight.

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

The researchers were also able to pinpoint a meteorite impact at just under 7,500km (about 5,000 miles) from InSight as the source of a second atypical quake.

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

“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 stated.

What makes the 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 planet’s interior during a quake, have so far provided insights into Mars’s core and mantle, but have revealed little about the crust away from the lander itself.

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

The results of the wave analysis were surprising because the Martian crust between the impact sites and InSight’s seismometer usually has a very uniform structure and high density. However, the researchers detected three layers of crust, which implied a lower density.

Explaining the greater 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 dynamic processes in the mantle and subsequent magmatic processes, it can tell us about conditions billions of years ago and the timeline of impacts, which were particularly common in Mars’ early days.

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 changes in velocity in the seismic data across 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 of between 5-30km below the surface of Mars.

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

Generally, volcanic rocks exhibit higher seismic velocities than sedimentary rocks. Moreover, the paths between the two meteorite impacts and the measurement site pass through one of the largest volcanic regions in Mars’ northern hemisphere.

The team determined that this abnormality was due to lava flows and the closure of pore spaces from heat created by volcanic processes, as these factors can increase the velocity of seismic waves.

“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 explained.

Solving the mystery of the Mars dichotomy

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

While the dominant feature of the southern hemisphere is a plateau covered by meteorite craters, the northern hemisphere consists mostly 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 known as the Mars dichotomy.

“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. We’re only now beginning to uncover this,” said Domenico Giardini, Professor of Seismology and Geodynamics at ETH Zurich.

The initial results appear to disprove one of the widespread theories for the Mars dichotomy – the crusts in the north and in the south are probably not composed of different materials, as has often been assumed – and their structure may be surprisingly similar at relevant depths.

Could further results explain these anomalies?

The ETH Zurich researchers are expecting further results soon. In May 2022, InSight observed the largest marsquake to date, with a magnitude of five. It also recorded seismic surface waves generated by this shallow event. This happened just in time, since the InSight mission will soon be coming to an end now that the lander’s solar panels are covered in dust, and it is running out of power.

An initial analysis of the data confirms the findings that the researchers obtained 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 seismic waves. These allow us to see even deeper into the crust, to a depth of about 90 kilometres”, Kim concluded.

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