An Impact That Rippled the Surface of Mars Has Deepened a Major Mystery

NASA’s InSight mission, which landed on Mars in 2018, has spent years gazing into the red planet’s interior by recording “marsquakes” that ripple through this alien world, providing an unprecedented view of its enigmatic subterranean layers. 

Now, just as InSight is entering its final weeks of life, it has delivered another milestone discovery: the first detection of seismic waves on the surface of another planet. In contrast to the mission’s previous recordings of so-called “body waves” that rocked Mars’ deep interior, the surface waves have exposed never-before-seen details about the upper crust of the planet, which could shed light on longstanding mysteries, according to a pair of studies published on Thursday in Science.

A team led by Liliya Posiolova, the orbital science operations lead at Malin Space Science Systems, used NASA’s Mars Reconnaissance Orbiter (MRO) to trace the origin of the surface waves to a large space rock that struck Mars on Christmas Eve of 2021, according to the first study. Another team led by Doyeon Kim, a geophysicist and senior research scientist at ETH Zurich’s Institute of Geophysics, describes how the waves “expand the current understanding of crustal structure on Mars beyond the crustal layering inferred beneath the InSight landing site,” in the second study.

“Since detecting / identifying surface waves on Mars and using surface-wave-driven information were a part of the mission goals of InSight, all of our seismologists in the team were very thrilled!” Kim said in an email to Motherboard.

“Previous to the detection of surface waves on Mars, our understanding of the Martian crust has been limited to what's underneath the InSight landing site because we were only using body waves that dive deep into the mantle,” he continued. “Because the velocity of surface waves depends on frequency, the measurement of surface wave dispersion allows us to understand how seismic velocities vary across different crustal depths averaged along its path traveling from the source to receiver. Therefore, our study provides the first glance of the crustal structure on Mars away from the lander.” 

InSight, which stands for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport, has detected more than 1,300 marsquakes during its lifespan, including a monster magnitude-5 marsquake this past May that holds the record for the biggest extraterrestrial earthquake ever. Most of these tremors were produced by shifting faults inside the planet, but a few of them were sparked by meteorites slamming into its surface.

InSight detected surface waves from one of these impacts when it occurred on December 24, 2021. Posiolova and her colleagues were then able to use MRO to pinpoint the location of the enormous 500-foot-wide crater that the space rock left in the Amazonis Planitia region of Mars, which is located more than 2,100 miles from InSight. In addition, the team was also able to link a slightly smaller crater in the Tempe Terra region, a whopping 4,600 miles from InSight, to a marsquake that the lander detected on September 18, 2021. 

“Both impacts generated craters >130 meters in diameter, making them the largest fresh craters identified since the beginning of the MRO mission 16 years ago,” Posiolova’s team noted in their study. “The seismic events have identifiable surface waves, distinguishing them from other recorded and analyzed events on Mars and indicating shallow sources. Before these events, surface waves had not been unambiguously identified on any terrestrial planet other than Earth.”

“The success in observing the formation of impact craters on Mars using instruments on several missions opens up a more detailed understanding of impact dynamics, atmospheric physics, and the exploration of planetary interiors,” the researchers added.

With this unprecedented data in hand, Kim and his colleagues were able to measure the velocity of the surface waves as they swept across thousands of miles of Martian terrain. The results show that the crustal properties of Mars are relatively similar across wide distances, even though the northern and southern hemispheres of the planet are dramatically different. 

Whereas the Martian south is mostly a highland plateau speckled with craters, the north is composed of volcanic lowlands that may have hosted a massive ocean more than three billion years ago. The mystery of how this “dichotomy” on Mars emerged has puzzled scientists for more than a century. Some scientists have suggested that the regions may have different compositions or structures, but the new study seems to cast doubt on that hypothesis, deepening the mystery behind the dichotomy. 

“As a consequence of our analysis of surface waves, we now understand that the crust of Mars north of the equatorial dichotomy (the topographic variation on Mars that divides the southern highlands and northern lowlands) has a relatively uniform crustal structure with depth, with a high shear velocity of ~3.2 kilometers/second,” Kim explained. “Moreover, the speed of which surface waves travel along the north vs. south of the dichotomy was very similar; which allowed us to think about whether the overall composition of the crust would then be perhaps fairly similar at least for those crustal depths of ~5 to 30 kilometers.”

The team also discovered that InSight’s landing site in western part of Mars’ Elysium Planitia region is unexpectedly different from almost every other area traversed by the surface waves. For some reason, the lander sits on Martian crust that is denser than other parts of the red planet, raising new questions about how this particular region evolved. 

In addition to the new studies from Science, another team published a paper in Nature Astronomy on Thursday, also based on InSight’s data, that suggests that magma may still be bubbling deep under Mars’ surface. Led by Simon Stähler, a seismologist at ETH Zurich, the study offers a tantalizing glimpse of Mars’ present-day geologic activity, which has implications for understanding planetary evolution in general.

“Across the solar system, a pattern emerges, where the present-day tectonics of the larger terrestrial planets, Mars, Venus and the Earth, is dominated by internal dynamics instead of purely passive cooling and shrinking, as is found on the smaller Moon and Mercury,” Stähler and his colleagues concluded in the study.

Given how many discoveries InSight has made in its brief time on Mars, it’s bittersweet to acknowledge that the mission will not survive to see 2023. Dust has been accumulating on InSight’s solar panels for years, starving it of power, and the lander is expected to die sometime in December. However, the mission has secured its legacy as the first seismologist on another planet, and its observations of the red planet’s interior will continue to inform our understanding of Mars, and other worlds, for years to come. 

“It is sad to face the fact that the mission will eventually come to an end at some point,” Kim said. “However, we are only starting to learn how complicated the seismic data collected from Mars are compared to similar datasets collected on Earth or the Moon that we are far more comfortable with.”

“During almost three years, the InSight team has uncovered so much about the Martian interior structure, such as the crust-mantle boundary, mantle transition zone, the iron-rich liquid outer core, etc.,” he concluded. “I am very glad that the mission lasted this far and we still have a long way to go for understanding the interior structure and dynamics of Mars which still remain largely enigmatic to date.”