It started with a quiver deep down in the Martian crust weak, barely detectable, yet bearing 4.5-billion-year-old echoes. Those seismic waves, recorded by NASA’s InSight lander from 2018 to 2022, have revealed a remarkable discovery: giant preserved fragments of Mars’ primordial crust, trapped in the planet’s mantle since the formation of the Solar System.
The discovery emerged from the painstaking analysis of eight exceptionally clear marsquake events by a team led by Constantinos Charalambous of Imperial College London. Using the Seismic Experiment for Interior Structure (SEIS), InSight recorded how primary (P) and secondary (S) waves traveled through the planet, reflecting and refracting off internal boundaries. Variations in wave speed and attenuation allowed researchers to map the mantle’s composition, revealing rocky blocks up to 4 kilometers across, relics of the planet’s earliest crust.
Their existence indicates a cataclysmic origin. “These colossal impacts unleashed enough energy to melt large parts of the young planet into vast magma oceans,” Charalambous said. As these cooled, chemically different crustal rafts solidified and subsequently sank into the mantle, where Mars’ stagnant-lid tectonics one immobile crustal shell trapped them. Plate tectonics on Earth would have already recycled such material into the mantle by now, wiping out the record.
The situation is remarkably similar to the giant impact hypothesis for Moon formation, where a body of Mars size, Theia, impacted the proto-Earth. Numerical simulations of such impacts, typically employing smoothed particle hydrodynamics, reveal how energetic impacts can produce global magma oceans and redistribute crustal material. In Mars’ case, the impactor may have been large enough to excavate and melt vast swaths of crust but insufficient to drive long-term tectonic recycling, locking the evidence in place.
InSight’s seismic data also refined the planet’s layered structure. The crust beneath the lander is 15–25 miles thick, with three distinct sublayers. Below is a mantle of about 969 miles thick, covering a molten core of approximately 1,120 miles radius more and lighter than predicted, suggesting admixtures of lighter elements with iron. The lithosphere, Mars’ solid outer layer, is a whopping 310 miles thick, indicative of the geologic lassitude of the planet relative to Earth.
Conservation of these chunks of embedded crust within the mantle provides a unique glimpse at the process of early planetary differentiation. The Earth’s record of the initial 100 million years is virtually erased, but Mars’ tectonically inert crust is a time capsule. This difference is essential to comparative planetology: stagnant-lid planets such as Mars, Mercury, and Venus are proposed to potentially harbor deep geochemical records unavailable on tectonically dynamic planets.
The intense, violent bombardment that imbedded these fragments took place in the Solar System’s turbulent infancy, when giant planet and leftover planetesimal gravitational sorting caused severe collisional events. Asteroid-belt meteorite studies suggest that such bombardments, perhaps tied to early giant planet migration, might peak impact rates during the first 100 million years, creating the type of mantle heterogeneity that is now being observed on Mars.
InSight’s purpose also caught related phenomena: marsquakes concentrated in Cerberus Fossae, an active volcanic area; seismic signatures of meteoroid impacts that unearthed subsurface ice; and magnetic “ghosts” rocks in the crust that carry remanent magnetization from a fossilized global field. These discoveries, along with the new data on the mantle, weave together a history of a planet that formed hot, was subjected to titanic impacts, cooled in a stiff shell, and has since developed with geological restraint.
“Much of this turmoil must have occurred in Mars’s initial 100 million years,” Charalambous said. That its record still can be found after 4.5 billion years is a testament to how glacially the Martian interior has churned. To planetary scientists, those preserved pieces are more than fossilized rock they are lasting witnesses to the shaping violence that sculpted not only Mars, but the geometry of the inner Solar System as a whole.
