NASA's InSight Lander Reveals a Surprise at The Very Core of Mars

Scientists probing right into the deepest depths of Mars have uncovered a structure no one was expecting.

InSight seismic data points to a solid inner core at Mars’s centre

At the planet’s very centre, InSight seismic data indicates a solid inner core roughly 600 kilometres (373 miles) wide. That finding clashes with earlier interpretations that Mars’s core was soft throughout - and it also sits awkwardly with what researchers think the Martian core is composed of.

"Having a solid inner core for Mars was something unusual," a team led by seismologist Huixing Bi of the University of Science and Technology of China told ScienceAlert.

"Early studies suggested that the Martian core contains a significant amount of light elements, which lowers the solidus temperature and makes it unlikely for the core to crystallize given its relatively high temperature."

How scientists mapped Mars’s interior with InSight

Only recently have researchers been able to outline the internal layout of the Red Planet in detail. NASA’s InSight lander carried a seismometer capable of recording vibrations from marsquakes and meteorite impacts; as those waves ricochet and refract within the planet, they behave differently depending on the density of the materials they pass through.

The effect is broadly comparable to a planet-scale “X-ray”, except it is built from sound waves rather than radiation.

Between 2018 and 2022, InSight tracked Mars’s subtle shaking for four years and logged hundreds of events. Those records produced the first high-detail picture of the planet’s interior, suggesting a broadly Earth-like arrangement: a rigid crust, a molten mantle, and a dense core at the centre.

Why Mars’s missing global magnetic field matters

Even with that overall similarity, key contrasts between Earth and Mars are tied to what is happening deep inside the planet - which is why Bi and colleagues set out to learn more about what had been described as a soft, yielding Martian core.

"Unlike Earth, Mars doesn't have a global magnetic field today," the researchers explained.

"Instead, parts of its crust are strongly magnetized, which tells us that Mars once had a magnetic field in the distant past. A planet's global magnetic field is powered by a 'dynamo' in its core, which depends on a combination of thermal and compositional convection in the liquid outer core.

"In Earth, light elements preferentially remain in the liquid during core crystallization, leading to residual buoyant liquid at the inner core boundary. This mechanism is believed to play an important role in sustaining the Earth's magnetic field today. In contrast, for Mars, things seem to work differently."

Getting around the single-station problem on Mars

On Earth, scientists usually infer internal layering using quake recordings from many seismic stations spread across the globe. Mars offered no such network: InSight listened from just one site. To make up for that limitation, the team leaned heavily on impact events, where large rocks striking the Martian surface generate waves that travel through the planet.

They selected 23 impact events with a high signal-to-noise ratio and then applied seismic array analysis methods that are typically used with multi-station datasets on Earth.

"This approach allowed us to pick out specific seismic phases based on how they arrive at the station, with their specified incident angles and arrival times," the researchers said. "In doing so, we were able to detect waves that travel through the very center of Mars's core and reflection from the inner core boundary, which provide critical observations for a solid inner core."

Why a solid core was not expected

Current thinking suggests Mars’s core differs somewhat from Earth’s in composition. Like Earth, it is thought to be dominated by iron, but with comparatively larger amounts of sulphur, oxygen, and carbon - lighter elements that, in theory, reduce the temperature at which the mixture solidifies. That threshold is set by a boundary known as the solidus.

Because the Martian core is believed to be far hotter than the solidus temperature for such a mixture, researchers had concluded that it ought to remain soft all the way in.

Reading Mars’s interior using seismic phases

Seismic waves are grouped by how they travel through a planet. P waves are the quickest and can move through the crust and mantle. K waves refer to waves that have passed through a planet’s outer core. I waves are those that have travelled through the inner core, while a lower-case i indicates a wave that has reflected off the outer boundary of the inner core.

These labels are combined to describe a wave’s full route. For instance, PKiKP waves travel through the mantle, enter the outer core, bounce off the inner core, return through the outer core, and then pass back through the mantle.

Multiple lines of evidence for a solid inner core

Within their dataset, the researchers did not see just a single indicator; instead, they identified several separate wave observations that each pointed towards a solid inner core on Mars.

"Detecting the PKiKP wave is strong evidence on its own, but we also see PKKP arriving earlier than expected, which provides further confirmation. Beyond that, our model predicts – and our data confirm – other inner-core-related phases, including PKiKP at greater distances, PKIIKP, and even a new branch of PKPPKP that travels through the inner core," they explained.

"These multiple phases are crucial because they cross-validate one another and all consistently point to the same conclusion: Mars really does have a solid inner core."

What researchers still need to explain

The mechanism that would allow a solid inner core to exist under these conditions is not yet understood. The team says further modelling will be required to test the relevant temperature, pressure, and compositional scenarios, and to examine how heavier and lighter elements separate and distribute - with the aim of reproducing the observations their analysis has revealed.

Even so, the finding opens up promising avenues. Further work could help clarify how Mars’s dynamo shut down and why the planet lost its global magnetic field. It may also shed light on the broader evolution of rocky planets - the type widely considered the most plausible hosts for life as we recognise it.

"The size and properties of Mars's inner core serve as a crucial reference for understanding the planet's thermal and chemical evolution," the researchers said.

"Gaining a clearer picture of the inner core's formation – and its implications for the history of Mars's magnetic field – will require more detailed modeling, ideally within a comparative planetology framework."

The research has been published in Nature.