Turns out Mars is not just a ball of molten metal inside. New seismic analyses from NASA’s InSight mission point to a solid inner core about 613 kilometers in radius, give or take 67.
That revision reshapes how we think about the Red Planet’s thermal history, its chemistry, and why its global magnetic field flickered out long ago.
The case hinges on signals that travel through a planet’s core and bounce off its boundaries. Researchers identified two key phases, PKKP and PKiKP, in Marsquakes recorded by InSight. PKKP samples the deep core, while PKiKP reflects off the inner core boundary. Together, they map a clear discontinuity and a speed jump of roughly 30 percent across that boundary.
In plain terms, waves run faster in the inner core. That only happens if the inner core is solid.
The inner core matters because inner core growth is a planet’s slow metronome. As metal crystallizes, it releases heat and light elements that can drive a dynamo, the planetary engine for magnetic fields. On Mars, geodesy and earlier InSight work already showed a large liquid core. This new analysis adds a solid heart within that liquid, proportionally similar to Earth’s, though scaled down. And yet Mars has no global field today. That tension is the story.
How did the team nail it with one lander? They turned an apparent limitation into leverage. Most Martian quakes sit 27 to 40 degrees from InSight, a range usually stuck in the core’s seismic shadow. But by stacking many low frequency events as a “source array,” then using polarization and slowness tricks to isolate coherent arrivals, they pulled out the faint core-bouncing and inner core reflecting phases. A Bayesian inversion of travel times did the rest, converging on an inner core radius near 613 kilometers.
There is a buried lede here. The amplitude and timing of those phases hint at a small density jump across the inner core boundary, on the order of 7 percent, which is hard to reconcile with a pure iron core. The modeling favors an oxygen rich inner core and a sulfur carbon rich outer core. And the absolute inner core wave speeds, estimated at about 7.3 to 8.3 kilometers per second, line up with that chemistry. An oxygen heavy solid, not an iron carbide heavy one, best fits the data.
That chemistry is not just a lab curiosity. It shapes the melt curve, the buoyancy of light elements, and the style of crystallization, all of which govern whether a dynamo can run. The picture that emerges is a Mars that cooled quickly early on, likely powered a magnetic field in its youth, then slid below the threshold for vigorous convection. Inner core crystallization may now be too slow, or the density contrasts too weak, to restart the engine. And in a twist, a hot outer core could even host a thin molten silicate layer at the top, which might blunt convective vigor further.
Public stakes? Plenty. A solid inner core resets constraints on Mars’s composition and heat flow, inputs that feed models of volatile loss, atmosphere stripping, and surface habitability over billions of years. It also refines the blueprint we apply to other worlds, from Mercury’s churning core to Ganymede’s hidden ocean. And, yes, there is a commercial subtext. Every time we retire uncertainty about Mars’s interior, mission designers can target instruments, or even future drilling concepts, with less guesswork. Fewer unknowns, fewer expensive surprises.
And the inner core refrain returns, because repetition suits the evidence. Inner core phases arrive earlier than expected for a purely liquid core. Inner core reflections carry opposite polarity to their companion phases, just as the models predict. Inner core size estimates remain stable across reasonable mantle profiles. But big unknowns remain, including three dimensional mantle effects and the exact partitioning of light elements at Martian pressures and temperatures. That is the kind of uncertainty that invites another mission.
Micro Explainer
How do seismologists “see” a solid inner core on Mars with one seismometer? Quakes launch compressional waves, P waves, that travel different paths. PKKP goes down, reflects within the core, and comes back up. PKiKP dives down, hits the inner core boundary, and reflects. If an inner core exists, PKiKP should pop up at a predictable time and slowness, and PKKP should arrive earlier than liquid only models allow because waves run faster in solid metal. The InSight team stacked many small quakes to boost signal, used polarization to separate vertical P motion from shear noise, then inverted the measured travel time differences relative to the first P arrival. The best fitting models require a solid inner core about 613 kilometers in radius and a roughly 30 percent jump in wave speed across the boundary, consistent with an oxygen rich solid core and a sulfur carbon rich liquid outer core.
Journal Nature (2025)
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