Mars is commonly classified as a "stagnant lid" planet because, unlike Earth, its outer shell is not divided into moving tectonic plates. On Earth, plate tectonics plays a major role in volcanism, recycling crustal material and helping build continents. Because Mars lacks this process, scientists have often assumed that its crust formed in a much simpler way.

The new study challenges that assumption. The results suggest that Mars may have developed highly evolved crust through vigorous recycling within the planet itself, without the need for Earth-style plate tectonics.

A Mysterious Boundary 24 Kilometers Down

The researchers analyzed seismic data collected by NASA's InSight mission, focusing on waves generated by meteoroid impacts and marsquakes, the Martian equivalent of earthquakes.

Scientists from Oxford's Departments of Earth Sciences and Statistics used those measurements to study an unexplained boundary roughly 24 kilometers beneath the Martian surface. Earlier research had identified the boundary, but its meaning remained unclear.

To investigate whether it represented a transition between different kinds of rock, the team compared the seismic observations with hundreds of possible rock compositions. They combined thermodynamic modeling with statistical methods to determine which materials best matched the properties detected at different depths.

The analysis showed that rocks beneath the 24 km boundary were best explained by "ultramafic" material (rich in iron and magnesium, but low in silica). Above the boundary, the seismic properties were more consistent with "mafic" rocks (containing a higher proportion of silica).

Evidence of a Vast Magma System

The researchers think the buried layer formed when molten rock accumulated deep underground and gradually separated into different materials. In this process, dense crystals would have settled near the bottom of the crust, while lighter and more chemically evolved melts moved upward.

On Earth, similar processes occur beneath volcanic arcs and are associated with the formation of continents.

Lead author Dr. Tobermory Mackay-Champion (Department of Earth Sciences, University of Oxford at the time of the study, now University of Bristol) said: "We've traditionally assumed that volcanism on Mars was relatively simple compared to that on Earth. But this discovery suggests Mars could sustain large, long-lived systems where molten rock evolved and reprocessed itself throughout the entire crust. It raises exciting possibilities for how common such systems might be on rocky planets beyond our solar system."

The buried layer may also be enormous in scale. According to the study, it could extend for hundreds or even thousands of kilometers across Mars' northern hemisphere.

That would suggest ancient Mars was not dominated only by simple, isolated volcanoes. Instead, the planet may once have contained huge, connected magmatic systems spanning large portions of its crust.

This process, known as "transcrustal magmatism," had previously been thought to be unique to Earth.

What This Could Mean for Habitable Planets

The findings could have broader implications for understanding how rocky planets become habitable.

Geological recycling can influence the development of atmospheres, oceans and environments where life might be possible. On Earth, these processes help regulate climate and support the long-term cycling of water and other volatile elements.

Because plate tectonics drives much of that recycling on Earth, scientists have often viewed it as an important requirement for creating and maintaining habitable conditions. The Mars findings suggest that complex crustal evolution and extensive geological recycling may be possible even on planets without Earth-style tectonics.

Co-author Professor Jon Wade (Department of Earth Sciences, University of Oxford) said: "One of the big questions in planetary science is whether Earth is unique. If Mars could develop this kind of complex crust without plate tectonics, then maybe the conditions needed for habitability can emerge on more planets than we realized, including those previously dismissed based on size or their apparent lack of tectonic activity."

InSight Reveals Mars' Hidden Interior

The research builds on seismic observations collected by NASA's InSight mission, which placed the first seismometer on Mars in 2018 and gave scientists an unprecedented view of the planet's internal structure.

The study was led by researchers from Oxford University's Department of Earth Sciences in collaboration with the University of Bristol and the University of Oxford's Department of Statistics.