Scientists peering into the very heart of the planet Mars have found a deeply unexpected structure.
There, at the innermost core of the planet, InSight seismic data reveals a solid mass about 600 kilometers (373 miles) across.
This is not just in contrast to previous findings that the core is squishy all the way through – it doesn't fit with our current understanding of what the Martian core is made of.
To learn more, here's ScienceAlert's exclusive interview between our head journalist Michelle Starr and the team of researchers behind a new Nature study, led by seismologist Huixing Bi of the University of Science and Technology of China:
Was it surprising to find that the very innermost core of Mars is solid?
Indeed, having a solid inner core for Mars was something unusual. 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.
Does the result tell us anything new about the differences between the interiors of Earth and Mars?
In some ways Mars' core looks similar to Earth's, but in other ways it's quite different. Our results suggest that Mars has a solid inner core that makes up about one-fifth of the planet's radius – roughly the same proportion as Earth's inner core. But that similarity may be just coincidence.
One key difference is that Mars' core likely contains much more of the lighter elements than Earth's to satisfy our observed velocity and density jump. And the way these light elements partition between the solid and liquid parts of the Martian core is also very different from what happens inside Earth, due to the planets' distinct compositions and pressure/temperature conditions.
These might also help to explain the lack of the current global magnetic field of Mars.
Read our full story: NASA's InSight Lander Reveals a Surprise at The Very Core of Mars
Does it tell us anything about how and when Mars lost its dynamo?
Unlike Earth, Mars doesn't have a global magnetic field today. 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.
Its core may have cooled too quickly early on, and today it is cooling too slowly to drive the vigorous thermal convection needed for a magnetic field. Even if part of the inner core is crystallizing, it might be happening too slowly – or the crystallization may not produce enough density contrast, to drive convection effectively.
But to fully understand why and when Mars lost its magnetic field, scientists will need more detailed dynamic modeling and better data on the planet's core composition and mantle properties.
Can you explain how the presence of light elements relates to core crystallization?
For a core to crystallize, its temperature must fall below the solidus. The presence of light elements can strongly influence this solidus. For example, hydrogen significantly lowers the melting temperature of iron, which could prevent the formation of a solid inner core if hydrogen is the dominant light element.
In contrast, an oxygen-rich core can crystallize even at relatively high temperatures. This means that the size of the inner core provides important clues about both the core's temperature and its chemistry.
In addition, the way light elements partition between the liquid outer core and solid inner core during the crystallization would create distinct chemistry between solid and liquid core. At present, however, we still lack sufficient experimental mineral studies to fully understand these critical processes.
Was there anything unexpected about your new findings?
One of the surprises in our study was that we didn't just find a single seismic phase pointing to Mars's inner core – we found several, independent ones. For instance, 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.
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 did you do differently to discover the solid inner core?
Since Mars has only one seismic station, we treated data from 23 high signal-to-noise ratio Marsquake events as if they formed an array, which let us apply seismic array analysis techniques normally used with multiple stations 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. In doing so, we were able to detect waves that travel through the very center of Mars' core and reflection from the inner core boundary, which provide critical observations for a solid inner core.
Do you know what the inner core is likely to be made of? If so, how is this different or similar to other solid inner cores in the Solar System?
I would say the core is made of Fe mixed [with] a significant amount of light elements, such as sulfur, carbon, and oxygen. The exact amounts of light elements are still unknown. In our paper, we showed that an oxygen-enriched core, with distinct distributions of sulfur and carbon between the outer and inner cores, could provide a possible explanation.
However, because we still have limited data on how light elements partition, how the core materials behave under pressure, and how they melt under Martian inner core conditions, other possible compositions cannot be ruled out and should also be considered.
Nonetheless, the Martian core likely contains significantly more light elements than Earth's core.
Does it tell us anything new about the evolution of rocky planets?
This is an excellent question. Although we did not explore this aspect in depth in the paper, the size and properties of Mars's inner core serve as a crucial reference for understanding the planet's thermal and chemical evolution.
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.
Read our full article on the discovery here.