Scientists peering into the very heart of hearts 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.
"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."
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It's only in the last few years that scientists have been able to map the red planet's interior structure. That's because NASA's Insight lander features a seismometer that can record waves generated by quakes and meteorite strikes as they bounce around inside the planet, reacting differently to different matter densities.
The result is somewhat like a planet-sized 'X-ray', only made of acoustic waves.
InSight spent four years, from 2018 to 2022, monitoring the trembles in the belly of Mars, collecting data on hundreds of events. This data provided the first detailed internal map of Mars, revealing a structure similar to that of Earth: a hard crust, a molten mantle, and a dense core at the center.
But there are some crucial differences between Earth and Mars that have to do with the planetary interior, and that's why Bi and colleagues wanted to obtain more information about Mars's putatively soft and squishy 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."
Investigations of Earth's layers rely on quake data from multiple seismic stations. On Mars, InSight spent its time in just a single location. To compensate for this, the researchers relied on impact events, in which large rocks smacking into the Martian surface send acoustic waves rippling through the planet.
They identified 23 high signal-to-noise ratio impact events and used seismic array analysis techniques usually applied to data from multiple stations here 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."
The composition of the Martian core seems to be a little different from that of Earth's. Mars's core is also mostly made of iron, but with higher proportions of sulfur, oxygen, and carbon mixed in – lighter elements that should theoretically lower the temperature at which the mixture solidifies, defined by a limit referred to as the solidus.
Since the core of Mars is significantly hotter than this temperature, scientists thought the core should be soft all the way through.
Seismic waves are categorized based on how they move through a planetary interior. P waves are the fastest, traveling through the crust and mantle. K waves are waves that have traveled through a planetary outer core. I waves are those that have traveled through the inner core, while a lower-case i represents a wave that has bounced off the outer boundary of the inner core.
These letters can be put together to describe a wave's path; for example, PKiKP waves travel through the mantle, enter the outer core, bounce off the inner core, come back out through the outer core, and then the mantle.
In their data, the researchers found not just one but multiple waves that separately indicated the presence of a solid inner core of 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."
Exactly how this can happen is currently unclear. Modeling will need to be done to explore the temperature, pressure, and compositional conditions involved, as well as the way the heavy and light elements are partitioned, to try to replicate what the team's results have revealed.
The results, nevertheless, are exciting. This further exploration may lead to deeper insights into how Mars lost its dynamo and its global magnetic field. It may also reveal something about the way rocky planets – those scientists believe most likely to host life as we know it – evolve.
"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.