Scientists confirm that Mars' core is more similar to Earth's than previously thought

Mars keeps its secrets well, but it just gave up a big one. A new study reports the first seismic evidence that the planet has a solid inner core, a part of Mars many scientists long suspected was missing.

The team found signals in marsquake data that only make sense if a solid center exists. The inner core appears to span about 18 percent of the planet’s radius, which puts Mars in more familiar company with Earth.

The research was led by Huixing Bi of the University of Science and Technology of China (USTC). The work builds on four years of seismic listening by NASA’s InSight lander.

How the signal was found

NASA’s InSight carried an ultra sensitive seismometer called SEIS that listened for marsquakes from late 2018 until dust ended the mission in 2022.

Those recordings, gathered by the InSight mission, became a gold mine for tracking waves that crossed the core.

Waves move through a planet in specific ways that depend on what is inside. The team identified patterns that point to a solid center and a surrounding liquid outer core.

What PKiKP and PKKP mean

Seismologists sort waves into named types called seismic phases, each with a known path and speed. Two of those phases, PKiKP and PKKP, were key here.

PKiKP is a compressional wave that reflects off the inner core boundary, a sharp interface between the liquid outer core and the solid inner core.

PKKP passes through the outer core and, in this case, shows travel time shifts that only fit if a faster, solid center is present.

What the paper actually says

“Here we present an analysis of seismic data acquired by the InSight mission, demonstrating that Mars has a solid inner core,” wrote Bi. The authors were precise about their evidence. 

The wave patterns also reveal important chemical details. They suggest that the inner core contains a distinct mix of lighter elements that separated from the outer core during crystallization.

This separation provides scientists with a reference point for modeling the thermal and chemical state of Mars, offering a clearer picture of how the planet has evolved internally over time.

Why Mars’ core matters

A solid inner core forms when part of a planet’s core begins to crystallize out of a liquid metal mix. That process releases heat and separates light elements, which changes the way the whole core moves and cools.

Those changes shape the past and future of the planet’s magnetic activity. They also set limits on how fast the interior loses heat, which ties into volcanism and long term climate history.

Mars once had a global magnetic field, but it faded long ago. Analyses of crustal magnetism indicate an early active dynamo that later shut down, leaving only pockets of remanent magnetization in old rocks.

A solid inner core does not guarantee a living dynamo today. The core may be cooling too slowly to drive vigorous motion, or the crystallization may not create enough density contrast to stir the liquid layer.

Before this work, researchers had already confirmed that Mars has a liquid outer core. Earlier work used InSight data to detect reflections from the core mantle boundary, which required a liquid layer.

That picture lacked one crucial piece, the solid center. The new detections fill that gap by catching core reflected and core transiting phases that require a solid interior.

Mars’ core and PKiKP

In seismology, waves travel faster through solids than through liquids, and some wave types cannot pass through liquids at all.

Careful timing and polarization analysis separate faint core related arrivals from louder crust and mantle signals.

Array style methods boost weak features by stacking many events. The results in this case isolate the subtle PKiKP reflection and a PKKP arrival that arrives earlier than a liquid only core would allow.

The wave speeds across the inner core boundary jump by roughly one third, which hints at composition. That jump suggests a solid center that is not pure iron nickel, but likely enriched in light elements such as oxygen.

Partitioning of these light elements between the liquid and solid phases can match the observed velocity jump. That points to ongoing crystallization that has been active for a long time.

Earth, Mars, and planet cores

PKiKP is a workhorse in deep Earth studies, and it has settled debates on other worlds.

Similar phases helped confirm the Moon’s solid inner core and fluid outer core over a decade ago, a result first shown using Apollo era data from the Moon.

Seeing the same phase family on Mars is a strong sign that planetary interiors can be read with a single station if the data quality is high enough. It raises the odds that future small networks could map other bodies in detail.

Confirming a solid inner core with one station is a feat, but a network would sharpen the picture.

Multiple sites would let scientists track changes in wave speed with direction, a property called anisotropy that often betrays how crystals align as they grow.

Future landers could also co-locate seismometers with magnetometers to watch how the core and any residual field interact in real time. That pairing would connect the interior and the space environment in a single set of measurements.

The study is published in Nature.

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