Seismic Waves Help Map the Core of Mars for the First Time

More than a hundred years after geologists first observed how seismic waves traveled through Earth, they’ve achieved another seismic first. This time, they measured “core-transiting seismic waves” moving through Mars. The InSight lander’s seismic instrument tracked shockwaves generated by an earthquake and an impact event. Their behavior revealed for the first time that Mars very likely has a liquid core. It’s made of a single blob of molten iron alloy.

By comparison, Earth’s core is more of a combo plate. It has both a liquid outer core and a solid inner core. They contain mostly iron and nickel. The turbulent outer core is heated by radioactive decay and other processes. It also generates our planet’s magnetic field.

It turns out that Mars’s innards are a bit different from Earth’s. The Martian liquid iron core is also rich in sulfur, with smaller fractions of oxygen, carbon, and hydrogen. That mix of elements makes it much less dense than Earth’s core, and it’s more compressible.

What The Core of Mars Tells Us

According to Nicholas Schmerr of the University of Maryland and a member of a team that used InSight data to study Mars, the differences between Earth and Mars cores hint at different formation stories for each planet. “You can think of it this way; the properties of a planet’s core can serve as a summary of how the planet formed and how it evolved dynamically over time. The end result of the formation and evolution processes can be either the generation or absence of life-sustaining conditions,” he said. “The uniqueness of Earth’s core allows it to generate a magnetic field that protects us from solar winds, allowing us to keep water. Mars’ core does not generate this protective shield, and so the planet’s surface conditions are hostile to life.”

An artist’s concept of how a rocky planet forms through the accretion of smaller bodies. As it grows larger, its core gets hotter and experiences greater pressure. Layers of the planet melt. Heavier elements sink to the center (core), and the lighter ones float to the outside. Seismic waves travel differently through different layers. The InSight lander’s seismic instrument was able to “observe” those waves as they passed through the planet Mars. Courtesy NASA/JPL-Caltech.

Interestingly, despite having a liquid iron core, Mars doesn’t seem to have much of a global magnetic field. It probably did generate one in the past, however. Planetary scientists suspect that it existed because Mars’s rocks contain traces of magnetism from ancient times. That magnetic “memory” gets embedded in rock crystals as they cool in the presence of a magnetic field. That memory can last for millions or billions of years. On Earth, scientists use it to track the motions of our planet’s tectonic plates, for example. They also use it to monitor changes in Earth’s magnetic field over time—a science called “paleomagnetism.”

Ancient Rocks, Paleomagnetism, and Conditions in the Mars Core

Paleomagnetism studies of Mars rocks tell scientists about Mars’s magnetic field in the past. Although there isn’t one there now, it probably once had one similar to Earth’s. University of Maryland associate professor of geology Vedran Lekic suggests that Mars changed from a planet with a potentially habitable environment, shielded by a magnetic field, to the more “unfriendly” place it is today.

What caused it to change? Conditions in the core might have played a role, along with other factors such as violent impacts, according to Lekic. “It’s like a puzzle in some ways,” Lekic said. “For example, there are small traces of hydrogen in Mars’ core. That means that there had to be certain conditions that allowed the hydrogen to be there, and we have to understand those conditions in order to understand how Mars evolved into the planet it is today.”

No one has been able to directly image the Martian core. However, planetary scientists have made extensive models of what they think conditions are like there. The InSight seismic measurements confirm the accuracy of those models. “This was a huge effort, involving state-of-the-art seismological techniques which have been honed on Earth, in conjunction with new results from mineral physicists and the insights from team members who simulate how planetary interiors change over time,” noted Jessica Irving, a senior lecturer at Bristol University and part of the team analyzing the InSight results. “But the work paid off, and we now know much more about what’s happening inside the Martian core.”

Probing Beneath the Surface

The team used data from InSight from a marsquake that occurred on August 25, 2021, and an impact that happened on September 18, 2021. They compared the time it took waves from each event to travel through Mars to waves that stayed in the mantle. Those measurements got combined with other seismic and geophysical measurements of the Red Planet. All that data gave the team enough information to estimate the density and compressibility of the material the waves traveled through. That’s how the researchers figured out that Mars most likely has this completely liquid core.

The Mars InSight lander’s seismic detector was used to observe seismic waves from Marsquakes and impacts. Courtesy NASA

Lekic and Schmerr note that Mars gradually evolved to its current conditions, changing from a planet with a potentially habitable environment into an incredibly hostile one. Conditions in the interior play a key role in this evolution, as might violent impacts, according to the researchers. Studying the data from InSight and other missions will help them determine more about the conditions that existed in Mars’s ancient history to give it that core.

“Even though the InSight mission ended in December 2022 after four years of seismic monitoring, we’re still analyzing the data that was collected,” Lekic said. “InSight will continue to influence how we understand the formation and evolution of Mars and other planets for years to come.”

For More Information

Scientists detect seismic waves traveling through Martian core for the first time
First Observations of Core-transiting Seismic Phases on Mars
The Far Side of Mars: Two Distant Marsquakes Detected by Insight

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