- NASA’s InSight lander just gave scientists a peek at the make-up of Mars’s core, crust, and mantle.
- A trio of new studies reveal how the interior structure of Mars compares to that of Earth.
- Evidence also suggests Mars had a strong, protective magnetic field like Earth does, then lost it.
For the first time, we know what the interior of another planet similar to ours looks like.
By analyzing seismic data collected by NASA’s InSight lander on the red planet, the researchers estimated of the size of Mars’s core, the thickness of its crust, and the composition of its mantle (the layer in between).
“We only have this kind of information for the Earth and moon,” Brigitte Knapmeyer-Endrun, a planetary seismologist at the University of Cologne and co-author of the new research, told Insider. “But the moon we can’t compare to Earth very well because it’s so much smaller.”
Mars, however, is our tinier, terrestrial twin. The new studies suggest its core is less dense than Earth’s but scaled to size.
Beaming seismic waves through Mars’s heart
InSight, NASA’s $828 million robotic science station, landed on Mars in November 2018. Since then, the lander has used its seismometers — which detect and record earthquakes — to listen for similar quakes on Mars.
Seismic waves from 12 of these marsquakes, which travel right through the middle of the planet and bounce off the layers inside, helped Knapmeyer’s group to map the boundaries of the crust and core.
“Imagine you have a closed box and you want to find out what’s inside,” Knapmeyer said. “This method is like taking that box and putting it into an X-ray.”
Mars has a fatter core than scientists thought
InSight’s findings suggest Mars has a large liquid core that starts almost halfway to the planet’s center, nearly 1,000 miles beneath the surface.
Its 1,143-mile radius was “larger than expected,” Amir Khan, a seismologist from ETH Zurich in Switzerland and a study co-author, told Insider.
The boundary of Earth’s liquid outer core — which encircles a solid metal inner core — starts deeper than Mars’s does, at 1,800 feet.
That said, Earth is nearly twice as wide as Mars is, and its core is about twice as wide too.
Unlike Earth’s core which is dominated by nickel and iron, Mars’s less dense core contains lighter elements like hydrogen and oxygen.
The red planet appears to lack an inner core, according to Simon Stähler, another study co-author and Khan’s colleague at ETH.
“We don’t know, we’ll be looking for this,” Stähler told Insider, adding, “temperatures are likely too high for an inner core to form.”
A more engorged core also means Mars’s mantle is relatively thinner compared to Earth. It also lacks a dense layer of mineral called bridgemanite that keeps Earth’s mantle stable under high pressure, which could explain the core’s unexpected size.
“Mineralogically speaking the mantle of Mars is a ‘simpler’ version of that of Earth,” Khan said.
Mars had, and lost, its protective magnetic field
Earth’s core plays a key role in protecting the planet from dangerous solar wind and radiation. Swirling liquid iron in the outer core generates a magnetic field that stretches all the way from there to the space surrounding our planet.
That swirl happens, in part, because of a process in which hotter, lighter material from the outer core rises into the mantle above. There, it swaps places with cooler, denser mantle material, which sinks into the core below. This is known as convection.
While Mars has a liquid core, it lacks that swirling engine, known as a dynamo.
Bits of magnetized Martian crust suggest the planet did have a magnetic field once, between 4 billion and 4.5 billion years ago. The absence of the bridgemanite layer in Mars’s mantle could explain why its dynamo shut off about 300,000 years after the planet formed. Without that layer, Mars’s interior leached heat much faster, crippling the convection happening there.
Plus, “Mars is smaller and therefore cools faster than Earth,” Stähler said. “Different planet — different story.”
The red planet’s crust is as thick as Earth’s
The two planets’ crusts are similarly thick, according to Knapmeyer, though Mars’s crust has two or three layers.
The average thickness of the Martian crust is between 14 and 44 miles. Earth’s crustal thickness varies wildly — under the oceans, it can be as narrow as 3 miles — yet beneath the continents, it’s between 18 and 44 miles thick.
But Mars crust is really old and static compared to Earth’s, she said. Our planet’s shell is broken up into tectonic plates that surf on top of the mantle. Thanks to convection, these plates sometimes collide or sink under one another, meaning new bits of crust emerged all the time.
Mars lacks those plate tectonics, so there’s been no new crust generated since the planet’s crust formed some 4.5 billion years ago.
The planet was once “probably complete molten” and that hot liquid mass then differentiated into a crust, mantle, and core, Khan said.
The absence of plate tectonics is linked, in part, to sluggish convection in the mantle, according to Stähler. Lack of water plays a role too.
“Water lubricates the plate motion on Earth,” Stähler said. “On Mars, the water was probably lost very early, once there was no dynamo anymore and the atmosphere was blown away by solar wind.”
InSight’s next target? The moon
Scientists had previously used observation of Mars’s mass, diameter, and rotation from space to predict what its internal layers might look like.
Stähler was pleased InSight’s findings matched earlier predictions.
“We could confirm very rough estimates made from orbit. So now we know how much we can trust estimates of Venus’s or Mercury’s inner structure,” he said.
InSight’s mission on the red planet is expected to last until the end of 2022, but the lander’s scientific explorations won’t end next year. InSight will head to the moon as part of NASA’s Commercial Lunar Payload Services program in 2024, Stähler said.
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