Why the state of Mars’ core matters

What lies at the center of Mars is more than a curiosity. The core’s size, composition, and physical state control how the Red Planet cools, how (or whether) it powers a global magnetic field, and how its surface and atmosphere have evolved over billions of years. Nailing down whether the core is solid, liquid, or layered is essential for:

• Reconstructing Mars’ early magnetic dynamo and the timing of its shutdown.
• Explaining crustal magnetization patterns preserved in ancient rocks.
• Estimating the planet’s heat budget, mantle convection, and volcanic history.
• Understanding why Mars followed a different evolutionary path than Earth and Venus.

What “solid core” means in planetary terms

In planetary interiors, “solid core” can refer to two related but distinct scenarios:

• Entirely solid metallic core: The whole metallic center has crystallized, a state favored when a planet cools substantially and the iron-rich alloy crosses its liquidus.
• Solid inner core with a liquid outer core: Like Earth today, the innermost region has crystallized while a surrounding shell remains molten; the boundary between the two grows outward over time.

The new study’s conclusion that Mars has a “solid core” most likely points to the presence of at least a solid inner core. Either case would mark a major shift from interpretations that emphasized a fully liquid core in recent years.

How scientists probe a planet’s hidden heart

No drill can reach thousands of kilometers underground. Instead, researchers combine multiple lines of evidence to infer Mars’ interior:

• Seismology: Marsquakes recorded by NASA’s InSight mission revealed how pressure (P) and shear (S) waves travel and reflect. Solid regions transmit both P and S waves; liquids largely block S waves.
• Rotational dynamics: Tiny wobbles in a planet’s spin (precession and nutation), tracked via radio signals from landers and orbiters, depend on how mass is distributed—especially the core’s size and rigidity.
• Tidal response: The way Mars flexes under the Sun’s gravity (quantified by Love numbers) constrains whether the interior deforms elastically like a solid or flows like a liquid.
• Geochemistry and density models: Meteorites of Martian origin, alongside high‑pressure lab experiments and thermodynamic calculations, restrict plausible mixtures of iron, nickel, and light elements (such as sulfur, oxygen, carbon, or hydrogen) in the core.

The new analysis reportedly marries these approaches, reinterpreting seismic paths and rotational data with updated models of temperature and composition. The result: a compelling case that at least part of Mars’ core has crystallized.

Reconciling the new result with earlier views

Prior to this work, several studies—especially early interpretations of InSight data—favored a fully liquid metallic core that was relatively large and light‑element rich. Later reanalyses introduced a key twist: a partially molten silicate layer at the base of the mantle could masquerade as a bigger, lighter core in seismic and tidal signals.

The “solid core” finding helps reconcile these threads in two ways:

• Revised layering: If a thin, molten silicate layer blankets a smaller, denser metallic region, then seismic phases once attributed to a huge liquid core may actually originate in this intervening layer, allowing the deeper metallic core to be partly or wholly solid.
• Cooling history: A solid inner core naturally emerges as Mars cools. If crystallization has already begun—or even finished—it implies the planet lost internal heat efficiently over geologic time, consistent with its modest volcanic activity in the recent past.

In short, what looked like a simple “big liquid core” picture is giving way to a more nuanced structure: mantle, possibly a thin molten layer, and a metallic core that is at least partly solid.

Implications for Mars’ magnetic past

Mars today lacks a global magnetic field, but ancient rocks preserve strong magnetization, signaling that a dynamo once operated. A solid or solidifying core reshapes that story:

• Dynamo mechanisms change over time: Early on, a hotter, liquid core could have driven convection-powered magnetism. As Mars cooled and the core began to crystallize, buoyancy from expelled light elements and latent heat might have briefly sustained magnetism before waning.
• Timing the shutdown: If the core is now solid or largely solid, the dynamo likely ceased long ago, exposing the atmosphere to the solar wind and accelerating volatile loss—factors linked to Mars’ transition from a wetter past to the arid world we see today.

Clues to composition and size

Whether fully solid or featuring a solid inner core, Mars’ center is expected to be dominated by iron with some nickel, tempered by light elements—especially sulfur, a prime candidate for lowering the core’s melting temperature. The precise recipe matters: small changes in sulfur or oxygen content can shift melting points by hundreds of degrees, toggling a core between liquid and solid at Martian pressures.

The new study’s modeling suggests a denser, more compact metallic region than some earlier estimates implied, consistent with crystallization. While exact numbers remain model-dependent, the key takeaway is qualitative: the innermost Mars is stiffer and less deformable than a fully molten core would be.

A different evolutionary path than Earth

Earth’s liquid outer core and solid inner core generate a strong, long-lived magnetic field. Mars appears to have taken a different route, cooling faster and shutting down its dynamo earlier. A solid core on Mars underscores:

• Planet size matters: Smaller planets lose heat more quickly, hastening core crystallization and dynamo cessation.
• Composition counts: Higher sulfur (or other light elements) content can delay or hasten freezing depending on pressure–temperature conditions.
• Surface consequences are profound: Without magnetic shielding, atmospheric loss and climate change accelerate, shaping habitability prospects.

What could confirm the solid core picture next?

The strongest tests will come from fresh and complementary datasets:

• More seismometers: A network (rather than a single station) would pinpoint wave speeds and core-reflected phases with far greater precision.
• Longer radio tracking: Multi-year measurements of nutation and tides refine constraints on core rigidity and size.
• Laboratory work: High-pressure, high-temperature experiments on iron–sulfur–oxygen alloys reduce uncertainties in melting relations at Martian conditions.
• Improved gravity and topography: Next-generation orbital mapping can reveal subtle signatures of deep structure when combined with thermal evolution models.

Key takeaways

• New analyses argue that Mars possesses a solid core, likely at least a solid inner core, revising earlier interpretations of a fully liquid center.
• This result integrates seismic, rotational, tidal, and geochemical constraints into a coherent interior model with layered complexity.
• A solid or solidifying core helps explain the ancient dynamo’s demise and Mars’ subsequent atmospheric and climatic evolution.
• Future missions and lab studies will be crucial to lock down the core’s exact size, composition, and the thickness of any surrounding molten layers.