Mars is not the pristine, stratified world that people once imagined it is a graveyard of shattered worlds, their shattered remnants encapsulated deep in its mantle for over four billion years.
It is this information that the NASA InSight mission has provided, recording 1,319 marsquakes before shutting down in 2022 with the most sensitive seismometer ever deployed on another planet. By tracing how seismic waves from these quakes slowed and scattered as they passed through the Martian interior, researchers uncovered dozens of dense structures up to 4 kilometers across, embedded at varying depths within the mantle. “What we’re seeing is a mantle studded with ancient fragments,” said Constantinos Charalambous of Imperial College London.
These are massively denser than adjacent rock and their size and distribution show evidence of violent beginnings: colossal impacts in the early years of the solar system. Mars had been attacked by asteroids and even protoplanets capable of accreting into planets themselves during that tumultuous time. The energy unleashed by such impacts would have melted vast regions of crust and mantle into seas of magma, allowing blocks of the impactors and Martian material to sink far into the planet. On Earth, the tectonically active planet, this would have been erased by mantle convection. However, Mars is a one-plate world with no subduction zones to recycle its crust, so these remnants are in place.
The blobs were singled out by InSight scientists from the high-frequency seismic signals of eight very clean marsquakes. On Earth, water and interactions with the mantle quickly damped out the type of signal these waves are, but on Mars they persist, providing scientists with an undistorted view of the deep history of the planet. As the waves traveled through certain regions, they were decelerated, as though on roads filled with rubble, indicating provinces of mixed composition and density material. Seismologists of the planet used computer modeling to match these trends and came to the conclusion that the anomalies are consistent with solidified remains of early magma oceans.
The mantle itself is around 1,550 kilometers thick and is heated to nearly 1,500 degrees Celsius. Seismic data, though, shows that it moves slowly compared to Earth’s, and this is why volcanism on the planet is weak and why delicate structures can remain intact for billions of years. Stagnant-lid regime has made the planet a time capsule containing evidence of its early evolution, according to Charalambous.
InSight’s seismometer has also gotten to know its core. Older estimates of its radius were about 1,830 kilometers, but new interpretations, some involving evidence for a molten layer of silicates over the top of the core, suggest that it might be smaller and denser and composed of iron alloyed with sulfur and oxygen. Other research has even detected seismic phases that suggest an inner solid core, a controversial finding given previous models indicating a fully molten interior. Having or not having such a core matters to Mars’ long-extinct magnetic dynamo, which in the past shielded the planet billions of years ago.
The techniques used to locate the mantle blobs utilized decades of Earth seismology but were adapted to accommodate the constraints of a single seismic station. Researchers exploited meteoroid impacts, which produce high-frequency waves with adequate penetration to probe deep into the mantle. By correlating direct and reflected phaseswaves that reflect off boundaries like the core-mantle interface they were able to chart density contrasts with unprecedented accuracy for another planet.
Beyond Mars, the discoveries illuminate rocky planet formation theories. The fossilized pieces support the theory that planets assembled on Earth through accretion by impact among planetary embryos rather than by slow agglomeration of small pieces. Unmixed residues may lie buried in other stagnant-lid planets’ mantles, such as Venus and Mercury, awaiting excavation by future missions.
As InSight data continue to be unearthed, scientists expect even more revelations on Mars’ interior structure. “We didn’t anticipate just how clearly we’d be able to see with InSight,” remarked one of the researchers, Tom Pike. For now, the dense blobs far beneath the Red Planet are a quiet reminder of when worlds were still crashing into each other, and Mars was not as peaceful, icy desert that we now know it to be.
