New seismic sleuthing suggests Mars is no tidy layer cake. Deep below its rusty plains, the planet’s mantle appears riddled with kilometer-scale fragments—likely remnants of colossal objects that slammed into the young world and left debris frozen in place.
The findings come from a fresh analysis of NASA’s InSight lander data by a team at Imperial College London, indicating that seismic waves are scattering off large, compositionally distinct clumps embedded in the Martian mantle. Some of these chunks may span up to 4 kilometers across, a scale big enough to disrupt quake waves in detectable ways.
InSight’s quakes expose a messy mantle
InSight’s ultra-sensitive seismometer recorded more than a thousand marsquakes, but a handful of exceptionally clean events proved most revealing. When researchers traced how compressional and shear waves ricocheted through the interior, they saw telltale interference and multipathing—signatures that the waves had been bouncing off dense pockets and patches rather than traversing uniform rock.
In seismology, that kind of scattering points to strong contrasts in rock properties such as composition, temperature, or porosity. On Mars, the most plausible explanation is ancient, unmixed remnants: materials with distinct mineral recipes and densities preserved since the planet’s chaotic birth. The team’s modeling shows that clusters a few kilometers wide can reproduce the observed wave behavior.
The InSight mission was managed by NASA’s Jet Propulsion Laboratory, with its French-built SEIS instrument led by CNES and the Institut de Physique du Globe de Paris, and critical contributions from ETH Zürich. That international hardware, sitting quietly on Elysium Planitia, has ended up redrawing textbooks on Martian interiors.
Remnants of colossal impacts and magma oceans
Early Mars endured repeated blows from large planetary embryos—the kind of body capable of carving basins like Hellas, Isidis, and Utopia. Each impact would have pumped enough energy into the crust and mantle to create regional or global magma oceans. As those oceans cooled, they crystallized unevenly, leaving behind chemically stratified layers and dense cumulate piles.
If pieces of those cumulates and fragments of the impactors themselves sank or were injected into the mantle, they could form the patchwork now seen in the seismic data. Imperial College London’s analysis, published in Science, argues that Mars never mixed those leftovers efficiently, preserving a heterogeneous mantle dotted with ancient material from these primordial collisions.
That picture dovetails with clues from Martian meteorites, which record striking isotopic diversity despite coming from the same planet. It also helps explain why Martian volcanism—think the Tharsis rise and Olympus Mons—appears both long-lived and geographically focused, behavior consistent with a mantle whose buoyancy and melt pathways are controlled by pockets of contrasting composition.
Why Mars kept its crumbs while Earth did not
Earth’s plate tectonics continually shuffle and recycle the crust into the mantle, stirring the planet’s interior like batter. Mars operates under “stagnant-lid” tectonics: a single, thick shell caps the planet, with no subduction to churn the mantle beneath. That lid effectively sealed in the early chaos, turning the deep interior into a time capsule of the solar system’s violent youth.
The practical upshot is that seismic anomalies on Mars can persist for billions of years. Pockets of dense, iron-rich cumulates may sit alongside lighter, more fusible patches, creating a mosaic of melt sources and heat-flow channels. Those contrasts alter seismic velocities enough to scatter quake waves, allowing scientists to map hidden relics of formation using a single seismometer station.
What the buried debris means for future missions
A chunk-filled mantle changes the stakes for sample return and future drilling. Igneous rocks hauled back by Mars Sample Return could tap melts from compositionally distinct reservoirs, revealing multiple “mantles” within one planet through isotopes of tungsten, neodymium, or oxygen. Such diversity would strengthen the case that giant impacts seeded long-lived chemical heterogeneity.
It also reshapes thermal models. Dense relics can anchor mantle flow, steer magma ascent, and influence where surface volcanism occurs. They may even help explain why Mars lost its global magnetic field comparatively early: if heat transport is patchy and inefficient, the core’s dynamo could stall sooner than simple, well-mixed models predict.
Seismology remains the key. A next-generation network of landers, or even a single station with longer operation and broader frequency coverage, could pinpoint the size, depth, and distribution of these hidden fragments. With each marsquake, scientists at JPL, Imperial College London, and partner institutions are learning to read the Red Planet’s fossilized bruises—and the story they tell is one of ancient cosmic debris, still lodged in place.