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Mars' interior: a preserved, blocky record of early impacts
An international team of planetary scientists has used seismic data to map Mars’ deep interior and found large, compositionally distinct fragments — remnants of the planet’s early crust — now embedded in the mantle. These preserved blocks, some reaching roughly 4 kilometres across, are interpreted as fossilized pieces of the planet formed and modified during an intense period of collisions in the first 100 million years of Solar System history.
Scientific background and how the discovery was made
Seismology probes the subsurface structure of planets by recording how seismic waves from quakes or impacts travel and reflect through different materials. NASA’s InSight lander, operating on Mars from 2018 to 2022, supplied a catalog of hundreds of marsquakes and impact signals that function like an acoustic CT scan. By analysing eight clear seismic events in detail, researchers led by Constantinos Charalambous (Imperial College London) reconstructed the distribution of seismic velocities in the Martian mantle and identified regions with contrasting composition and structure.
Instead of a smooth mantle, the models indicate discrete heterogeneities: large, blocky pieces of crustal material that sank or were trapped beneath the reformed surface. According to the team, these blocks likely formed when colossal impacts melted large volumes of the young planet, producing widespread magma oceans. As those melt sheets cooled and crystallized, they produced chemically distinct lithologies that were later fragmented and incorporated into the mantle during subsequent collisions and re-solidification.
"These colossal impacts unleashed enough energy to melt large parts of the young planet into vast magma oceans," Charalambous explains. "As those magma oceans cooled and crystallized, they left behind compositionally distinct chunks of material — and we believe it's these we're now detecting deep inside Mars." The preserved chunks therefore act as time capsules recording early planetary differentiation and bombardment.
Why Mars preserves these ancient structures
Mars differs significantly from Earth in its tectonic and magnetic evolution. Whereas Earth’s lithosphere is divided into moving tectonic plates that continuously recycle crustal material back into the mantle, Mars appears to have operated under a stagnant lid — a single, largely immobile outer shell. Mars also lacks a strong global magnetic field today, implying different deep-core dynamics. Without active plate tectonics to erase early heterogeneity, the Martian interior can retain relics of its formation far longer than Earth does.
The researchers write that the preserved mantle heterogeneity provides "an unprecedented window into the geological history and thermochemical evolution of a terrestrial planet under a stagnant lid," a regime thought to be common among rocky planets. Finding such preserved fragments on Mars supplies a rare data point for comparing planetary evolution across the inner Solar System, including Mercury and Venus, whose deep structures remain poorly constrained.
Implications for planetary science, habitability and future exploration
The discovery affects several areas of planetary science. First, it supports models in which early giant impacts played a major role in setting the interior structure of terrestrial worlds, a process already invoked to explain Earth’s Moon-forming collision. Second, the persistence of chemically distinct blocks in Mars’ mantle constrains heat transport, mantle convection vigor and the timescale on which planets can sustain magnetic dynamos and surface habitability conditions.
For missions and instruments, these results underscore the value of seismology for small bodies and motivate a broader, longer-lived seismic network on Mars. Denser seismic coverage would refine the size, distribution and composition of mantle blocks and test hypotheses about early magma oceans and crustal reformation.
Related technologies and next steps
Future objectives include deploying multiple seismic stations across Mars to build 3D interior maps, sample-return or in-situ geochemical analyses of ancient surface terrains to link surface rocks with deep heterogeneities, and improved models of impact-driven melting and crustal recycling.
Expert Insight
Dr. Elena Moreno, planetary geophysicist (fictional), comments: "Detecting such large, preserved fragments in Mars’ mantle is like finding pages ripped from the Solar System’s earliest history book. It tells us that Mars’ interior churned slowly after its formation, preserving evidence of processes that on Earth were erased by plate tectonics. For future missions, adding more seismic stations is the most direct way to expand this discovery into a global map of Mars’ interior evolution."
Conclusion
Seismic analysis of data from NASA’s InSight mission reveals that Mars’ mantle contains large, fossilized chunks of ancient crust produced and rearranged by intense early impacts. These fragments, some kilometres across, are preserved because Mars lacks the plate tectonic recycling that erased comparable records on Earth. The finding refines models of planetary formation, constrains thermal and magnetic histories, and makes a strong case for expanded seismic networks on Mars to further illuminate the first chapters of terrestrial-planet evolution.
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