For many decades, scientists assumed that Mars was geologically inactive. While its surface is covered in features that attest to its volcanic past, all evidence seemed to indicate that Mars was geologically dead. This is consistent with the fact that Mars' crust has no tectonic plates and instead consists of a single plate, making it a "Stagnant Lid" planet. However, with the arrival of NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander, indications that the planet was still "alive" began to accumulate.
Beyond "marsquakes" caused by impactors, InSight detected evidence of tectonic activity from within. In a recent study, scientists at Imperial College London and Columbia University presented seismic evidence that a marsquake originated beneath Valles Marineris. According to InSight's data, the event (S0976a) was caused by rocks slipping along fault lines deep within the crust. These findings suggest that faults formed billions of years ago, when Mars was much more active, are unstable enough to rupture under stress.
The research was led by Jung-Hun Song, an Assistant Professor at Imperial College London's Department of Earth Science and Engineering. He was joined by researchers from Columbia's Department of Earth and Environmental Sciences and the Department of Earth Science and Environmental Change at the University of Illinois Urbana-Champaign. The study describing their findings was published in Nature Communications.
One of two Mars InSight's 7-foot (2.2 meters) wide solar panels was imaged by the lander's Instrument Deployment Camera. Credits: NASA/JPL-Caltech
Like Olympus Mons and the Tharsis region, Valles Marineris is the Solar System's largest geological feature of its kind, dwarfing similar features on Earth. The vast canyon system spans 3,900 km (2,423 mi) long, up to 600 km (373 mi) wide, and more than 9 km (5.5 mi) deep. Meanwhile, the largest Canyon system on Earth, the Yarlung Tsangpo Grand Canyon, stretches roughly 505 km (313.5 mi) long, up to 113 m (371 ft) wide, and reaches a maximum depth of 6 km (3.7 mi). As Song explained:
Valles Marineris is one of the most spectacular geological structures anywhere in the Solar System, but until now we didn't know for certain that its faults are still active. Our study opens up exciting possibilities for discovering ongoing seismic activity on other planetary bodies that we might otherwise consider geologically inactive.
The most widely accepted theory is that the valley was formed by rift faults created by tensional tectonic forces that pulled the crust apart. This process is believed to be related to the intense volcanic activity that formed the Tharsis Bulge during the Noachian to Late Hesperian periods (4.1 to 3.0 billion years ago). It is also theorized that the canyon was enlarged by subsequent erosion and the collapse of the rift walls, similar to the East African Rift.
For their study, the team analyzed the S0976a seismic event that was recorded by InSight on August 25th, 2021. This was the only marsquake detected in Mars's western hemisphere, and it originated on the opposite side of the planet. Previous studies placed the marsquake near the western part of Valles Marineris. However, its cause was unclear, with possibilities ranging from meteoroid impacts to landslides and other forms of mass movement common to Mars.
Tharsis Montes, with Olympus Mons (upper left) and Valles Marineris (right). Credit: NASA/JPL-Caltech
The team built models to test these explanations, but they couldn't reproduce the observed seismic waves. The researchers also consulted satellite images of the region taken by NASA's Mars Reconnaissance Orbiter (MRO) after the event, but found no evidence of an impact crater or landslides. The best fit was a strike-slip marsquake, in which blocks of crust move horizontally past each other along a steep fault about 20 to 40 km (12 to 24 mi) beneath the valley. Said Song:
What makes the result compelling is that several completely different types of evidence point in the same direction. The seismic waves tell us that there was a sideways sliding motion on a fault well below the surface, while the geology seen from orbit shows a region crossed by long-lived fault systems.
The results also match what scientists would expect if pre-existing fault structures slipped due to long-term cooling, volcanic stresses, and local effects associated with the canyon itself. They also add to the list of regions where InSight detected evidence of active faulting on modern-day Mars. This includes the Cerberus Fossae fissures and the southern part of Elysium Planitia, which suggest that stresses in the Martian crust do not follow a single planet-wide pattern.
Lastly, the results show that four years after the mission ended because of dust buildup on its solar panels, InSight continues to provide valuable insight into the Red Planet. Said Dr. Doyeon Kim, an Assistant Professor in Earth and Planetary Science at Imperial and a senior author on the study:
S0976a has turned out to be an extraordinary event, playing an important role in our understanding of the Red Planet. Its waves traveled through the core and helped us investigate the deepest part of Mars, and now they are telling us what happened at the source itself. From a single marsquake, we can learn both about the center of the planet and about an active fault beneath one of its most remarkable landscapes.
Further Reading: Imperial College London, Nature Communications.