Mars’ Core May Be Smaller Than Thought, Wrapped in a Sea of Molten Rock

The Red Planet’s core may be much smaller and denser than researchers previously thought. New analyses of Martian seismic waves—caused by a meteorite strike—reveal its core is enveloped by a newly discovered layer of molten rock.

Previously, scientists had suggested Mars contained a large, low-density core. But the new findings paint a different picture of its interior, and researchers say that this one ties up some crucial loose ends, per two independent papers published Wednesday in the journal Nature.

The studies are “the most accurate and precise estimates so far of Mars’ core and mantle structure,” Suzan van der Lee, a space geologist at Northwestern University who was not involved with either paper, tells BBC Science Focus’ Noa Leach.

“It’s an elegant solution,” Simon Stähler, a seismologist at ETH Zurich who did not contribute to the new research, tells Nature News’ Alexandra Witze.

New @NASAInSight research published in @Nature reveals a heated "blanket" wrapped around Mars' core. @umdgeology's Vedran Lekic says this discovery leads to new clues about how the planet formed, evolved and became the barren planet it is today. https://t.co/rLt0y66b45 pic.twitter.com/oqbOD32tfg

— UMD Science (@UMDscience) October 26, 2023

In 2018, NASA’s InSight Mars lander (which retired last year) arrived on the planet and started measuring seismic activity from “Marsquakes” and meteorite impacts.

As seismic waves ripple through a planet’s interior, their behavior can reveal details about its makeup. Earlier this year, for example, scientists took advantage of an earthquake to detect the possible presence of another layer within Earth’s inner core.

Based on InSight’s detections, scientists had concluded two years ago that the Red Planet’s core had a radius of around 1,140 miles and made up about a quarter of Mars’ mass. This proposed, large core was not thought to be very dense, indicating it likely contained a lot of light elements such as sulfur, carbon, oxygen and hydrogen.

But such a large proportion of these elements didn’t seem possible—they should have been blown away by the sun before Mars formed, writes the New York Times’ Robin George Andrews.

“To get all these lighter weight elements into the core of Mars while it’s forming is just very, very difficult to do,” Amir Khan, a co-author of one of the new studies and planetary scientist at ETH Zurich, tells Space.com’s Charles Q. Choi.

Then, in 2021, InSight made a crucial observation. A meteorite struck Mars on the opposite side of the planet from the lander’s location, which allowed the robot to pick up seismic waves that had passed through the core.

“Previously, we had not observed any seismic waves that had transited the core. We had only seen reflections from the top of the core,” Khan says to Reuters’ Will Dunham.

The analyses suggested that the boundary researchers had previously identified as the top of the core was in fact a layer of molten silicate. This liquid layer stretches between 90 and 125 miles thick, per the New York Times.

Without this outer layer, Mars’ core would be significantly smaller than thought. The new research pegged the core’s volume at around 30 percent smaller than previously measured, per Space.com.

A smaller size also means the core should be denser than the previous model suggested, with fewer light elements. One of the studies found that between 9 and 15 percent of its weight would be these light elements.

“This amount of light elements is not unlike that of the Earth’s core, which is estimated to be around 10 percent,” Khan tells Reuters.

The silicate layer might be a lingering vestige of an ancient ocean of magma that once covered the Red Planet, per the research. Between Mars’ solid mantle and liquid core, it’s kept molten by heat from radioactive elements. The newly described layer would also act as an insulating blanket around the planet’s core, keeping it warm enough to prevent it from solidifying—which would help explain why Mars, unlike Earth, does not have a magnetic field, per Space.com.

Still, questions remain: One of the studies found this new rock layer would be fully molten, while the other concluded it would just be partially molten.

The discovery means “we need to go back to re-analyze and re-interpret the four-year-long seismic record and all other geophysical observables in the light of this new paradigm,” Henri Samuel, a co-author of one of the studies and a planetary scientist at Paris Cité University, tells Space.com. “This could lead to additional discoveries on the deep structure of the Martian mantle and its core.”

Will Sullivan | | READ MORE

Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.