Marsquake Reveals Molten Layer Above Martian Core

NASA’s Insight lander had a major bit of luck on its 1000th Sol, or Martian day, on Sept. 18, 2021. After nearly three years of operation, an asteroid impact triggered a marsquake thousands of kilometers away, whose seismic waves bounced off the Martian core to InSight’s seismometer. For the first time, the seismic waves arrived at the right angle and with enough energy to reveal previously unrecognized layers of molten magma in the lower mantle, which surrounding Mars’s molten metal core. Two separate groups report their results in the October 26th Nature.

InSight was built to study Martian geology, and one major goal was to use seismology to compare its interior to those of other rocky planets. Its seismograph began operation in February 2019, and in 2021 Simon Stähler (ETH Zürich, Switzerland) and colleagues reported in Science that they had detected the liquid Martian core surrounded by a solid rocky mantle. They measured the core’s diameter to be 3,660 kilometers (2,274 miles).

That result left planetary scientists scratching their heads. Why was the Martian core’s diameter more than half that of the planet? The core’s large volume meant its average density had to be low, just 6 grams per cubic centimeter, compared to at least 10 g/cm³ for Earth’s molten core.

“You'd need a heck of a lot of light elements to get that,” says Amir Khan (also at ETH Zürich), who led one of the just-published Nature papers. The core would have to have formed when the solar system was young, but the magma ocean that followed would have lost most of the light elements, which are volatile. “It would be very difficult to get all those light elements into the core,” he says.

While researchers wanted more seismic data to understand the core, Mars didn’t cooperate for some time. Most marsquakes detected were too small and too close to the seismograph to yield what they needed.

Moreover, the clock was ticking for Insight: Dust had accumulated on its solar panels, and when Martian winter began in February 2021, they could only deliver 27% of their rated power. So the big, distant marsquake of September 18, 2021, and a second, closer event on Christmas Day, offered hope of solving the puzzle. And just in time — only a few months later, the loss of power prevented InSight from transmitting any more seismic data.

All told, InSight recorded 1,319 marsquakes, but only two went into the new studies. “The data from the [asteroid] impact of Sol 1000 made a huge difference,” says Henri Samuel, (Paris Cité University), who led the second Nature study.

The seismic waves from the distant impact came in “too slow to be explained by a homogeneous solid mantle,” he adds. “We learned that unlike Earth, Mars appears to have a strongly stratified mantle, with an enriched silicate layer above its core. The lower part of the layer is fully molten while the thinner upper part is partly molten.”

The discovery is key to understanding Martian geology, Samuel says. “This structure strongly influences its evolution and the interpretation of practically all available data on Mars.”

The past study in Science had not recognized the difference between the magma layer and the metallic core, so it included the lighter magma with the denser core. The new studies show the metallic core is 3,300 km in diameter, slightly smaller than previously thought. That change increases the average density to 6.6 g/cm³ . While still not nearly as dense as Earth’s iron core, the new measurement reduces amount of light elements required.

In particular, the liquid layer in the mantle would have insulated the core, preventing it from cooling. Without the internal motions associated with cooling, the core couldn’t generate a dynamo. That means external sources, such as energetic impacts, must have generated the early magnetic field recorded in the Martian crust.

The two groups started separately and used different analytic approaches to achieve the same result, showing stratification in the mantle.

“The great thing about these two papers is that we did things differently and sort of end up at the same conclusion, plus or minus [some details],” Khan says. The details are in the structure of the molten layer: Khan’s group sees only a single layer of molten magma at the bottom of the mantle, while Samuel's group sees two layers, with the thinner, top layer “mushy” and the bottom one fully molten.

Resolving the differences may be important in understanding how Mars formed as well as understanding the nature of the Martian dynamo that left magnetic fields in ancient rocks. But with InSight shut down for good, the only way to obtain more data would be to send a new seismograph to Mars, and that’s a mission not yet planned.

NASA does have a duplicate of the InSight seismograph called ForeSight, which was used in testing, but that’s now part a mission headed to the far side of the Moon in 2025. “That's going to be exciting,” says Khan, because Foresight should tell us more about the Moon's small liquid core.

Others have proposed a liquid layer at the base of the Martian mantled based on models, but they lacked hard evidence. “This shows the power of seismology,” Stähler says, “because it can sound the deep interior and give us otherwise unobtainable information.”

References:

Khan, A., Huang, D., Durán, C. et al. Evidence for a liquid silicate layer atop the Martian core. Nature 622, 718–723 (2023).

Samuel, H., Drilleau, M., Rivoldini, A. et al. Geophysical evidence for an enriched molten silicate layer above Mars’s core. Nature 622, 712–717 (2023).

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Editorial note (Nov. 30, 2023): This article contained an error in the revised estimate of the Martian core. The new studies find the Martian core to be 3,600 km in diameter. The Earth's core density was also corrected: It is more than 10 g/cm³.