When we picture the inside of planets like Earth and Mars, we often imagine neat layers stacked one on top of another: a solid crust, a mantle beneath it, and a dense core in the center.
It’s a tidy model that resembles a layered dessert.
But new research shows that Mars’s interior looks nothing like that. Instead, it’s more like a messy Rocky Road brownie—full of uneven chunks left over from colossal impacts that scarred the planet’s earliest history.
The discovery, published in Science, comes from data collected by NASA’s InSight lander, which listened for seismic vibrations—marsquakes—rippling through the planet.
By carefully analyzing these vibrations, researchers at Imperial College London and their collaborators found evidence that Mars’s mantle still contains massive chunks, some as large as four kilometers across, that date back more than four billion years.
These fragments are chemical “fossils” from Mars’s violent birth.
Billions of years ago, Mars formed from dust and rock orbiting the young sun. But its early growth wasn’t peaceful.
After the planet largely took shape, it endured gigantic collisions with other planetary bodies—impacts so powerful they melted much of Mars into vast oceans of magma.
As these oceans cooled and crystallized, they left behind chemically distinct chunks of material. Instead of blending smoothly, these chunks were trapped in the sluggishly moving mantle, preserving the scars of Mars’s chaotic beginning.
“Most of this chaos unfolded in Mars’s first 100 million years,” explains lead researcher Dr. Constantinos Charalambous from Imperial College London. “The fact that we can still detect traces of it today shows just how slowly Mars’s interior has been mixing ever since.”
On Earth, plate tectonics constantly recycle crust and mantle material, smoothing away ancient evidence of such violent beginnings. But Mars developed a “stagnant lid”—a thick, solid crust that sealed off the mantle.
With no tectonic activity to churn and recycle the planet’s insides, those early chaotic features remained locked in place, turning Mars into a kind of geological time capsule.
The evidence came from eight especially clear marsquakes detected by InSight, including two triggered by recent meteorite strikes that left large craters on the surface. As seismic waves passed through the mantle, researchers noticed that higher-frequency waves slowed down in ways that revealed interference patterns.
This showed the waves were traveling through a mantle full of irregular structures of different origins, rather than a smooth and uniform layer.
The fragments also follow a distinctive “fractal” pattern—similar to what happens when a glass shatters on a tiled floor.
A handful of big pieces remain, surrounded by many smaller ones. According to Professor Tom Pike, who worked with Dr. Charalambous on the study, this is exactly the pattern you would expect from the overwhelming energy of cataclysmic planetary collisions.
The findings not only reveal more about Mars but may also help scientists understand the hidden interiors of other rocky planets like Mercury and Venus, which also lack active plate tectonics. “InSight’s data continues to reshape how we think about the formation of rocky planets,” says Dr. Mark Panning of NASA’s Jet Propulsion Laboratory, which managed the InSight mission until its end in 2022.
In the end, Mars’s messy, uneven mantle isn’t just a curiosity—it’s a preserved record of the chaos that shaped our solar system billions of years ago, offering scientists a rare glimpse into planetary evolution that Earth itself has long since erased.
Source: Imperial College London.
