What if Mars’ interior was not so much a neat layered cake but a shattered dessert with shards of its former self still rooted deep within? That is the picture emerging from NASA’s InSight mission, which has uncovered seismic shadows of fossilized remnants of ancient mantle leftovers from when the planet was youthful, hot, and getting pummeled by colossal impacts.
The new research, reported in Science, based its analysis on eight well-documented marsquakes, including two that had been triggered by meteorite impacts that dug out craters roughly 150 meters across. InSight’s ultra-sensitive seismometer captured how seismic waves traveled through the Martian interior. High-frequency P-waves, which ought to have traveled smoothly through a uniform interior, arrived late and scrambled. “The seismic signals showed clear signs of interference as they traveled through Mars’ deep interior,” said Constantinos Charalambous of Imperial College London. “That’s consistent with a mantle full of structures of different compositional origins leftovers from Mars’s early days.”
Those remnants are substantial. The research indicates fragments up to 4 kilometers wide, distributed throughout the mantle like rock fossils. Their origin is 4.5 billion years ago, when Mars, in its “embryonic” state in the planet context, endured titanic collisions with objects as big as small planets. Such collisions, equivalent in magnitude to the collision thought to have formed Earth’s Moon, released enough energy to melt vast regions of crust and mantle into seas of magma. When these oceans froze and crystallized, they trapped chemically distinct shards from Mars and the impactors. “You see the same effect when a glass falls onto a tiled floor as when a meteorite collides with a planet,” said Tom Pike, also of Imperial College. “It breaks into a few big shards and a large number of smaller pieces.”
Plate tectonics would long ago have annihilated such features on Earth. The cyclical recycling of upper mantle and crust, driven by subduction and convection of the mantle, homogenizes the interior over long timescales. Mars, however, is in what planetary scientists call a stagnant lid tectonic regime with one solid, intact lithospheric shell encompassing the planet. Without the global web of plate margins that defines terrestrial tectonics, the mantle of Mars lurches agonizingly slowly, creating a time capsule of its tormented youth.
Seismic anomalies also illuminate differences in the mode of planetary rock construction. Venus, like Mars, has a stagnant lid, but its tectonomagmatic activity is more vigorous, maybe obliterating similar early architecture. Earth’s working lid plate tectonics is rare among the great silicate planets and maybe only initiated after protracted one-lid phases. Preservation of Mars’s mantle heterogeneity indicates the manner in which mode of tectonics shape a planet’s geological memory.
InSight’s results depend on the physics of seismic wave propagation. Waves decelerate, scatter, and interfere as they travel through areas of varying composition or density. The group’s global simulations told them that the observed delay could only be explained by kilometer-scale heterogeneities in the deep mantle. These are not ephemeral traits; Mars’s mantle is up to 1,550 kilometers thick and gets as hot as 1,500 degrees Celsius, but convection is too sluggish to turn it over entirely. The “fractal” size distribution from a few large blocks to many small ones is a statistical sign of catastrophic processes of fragmentation.
This discovery also assists comparative planetology. The presence of no plate tectonics on Mars suggests that its mantle cooled differently, with heat loss occurring largely through conduction along the lithosphere and local volcanism through mantle plumes. The same holds for other stagnant-lid planets, including Mercury and Venus, suggesting they as well could contain deep, unmixed remnants of ancient impacts. In the study of exoplanets, Mars is a natural laboratory analog for understanding how tectonic regime, planetary scale, and early bombardment histories all play together to produce a world’s interior structure.
“We’ve never seen the inside of a planet in such fine detail and clarity before,” Charalambous said. The clarity comes not just from InSight’s engineering the lander recorded 1,319 marsquakes before its 2022 shutdown but from the unique state of Mars itself. A smaller planet, with no active plates and a mantle frozen in slow motion, has become a vault for the Solar System’s formative chaos.
