Inside Mars, a ‘rocky road’ mantle reveals a violent past

Don’t let the appetizing description fool you. When planetary scientists say the interior of Mars resembles a rocky road brownie more than a piece of buttery shortbread, the tasty metaphor masks billions of years of geological violence. In a re-examination of previous observations collected by NASA’s decommissioned InSight probe, researchers have discovered that the Martian mantle is embedded with ancient fragments measuring as much as 2.5 miles wide. The data is detailed in a study published on August 28 in Nature.

The solar system’s terrestrial planets (Mercury, Venus, Earth, and Mars) first formed around 4.5 billion years ago, but the final geological results are far from uniform. On Earth, plate tectonics are continuously (if slowly) recycling between the crust and mantle. Mars is in a comparatively more stagnant situation. The Red Planet’s mantle is permanently trapped beneath an unmoving outer crust preserving the planet’s insides.

However, this peaceful present is only the result of the planet’s past cataclysmic events. Using InSight’s array of seismic detection tools, a team of planetary scientists have identified gigantic formations that break up comparatively smoother regions of mantle. These bits of planet are like the nuts and marshmallows in the rocky road brownie. Millions of years later, they serve as the leftover evidence from multiple catastrophic impacts by planet-sized objects–the same type of culprit likely responsible for smacking into Earth and generating our moon.

According to study co-author Constantinos Charalambous, these events generated enough energy to melt massive portions of the then-young planet into seas of magma.

“As those magma oceans cooled and crystallised, they left behind compositionally distinct chunks of material—and we believe it’s these we’re now detecting deep inside Mars,” he explained in a statement.

Charalambous says these collisions likely took place during the first 100 million years of Mars’ existence, and are a testament to how inert the planet is today.

“The fact that we can still detect its traces after four-and-a-half billion years shows just how sluggishly Mars’s interior has been churning ever since,” he said.

Study co-author Tom Pike called this process “fractal distribution,” which occurs when a collision “overwhelms the strength of an object.”

“You see the same effect when a glass falls onto a tiled floor as when a meteorite collides with a planet: it breaks into a few big shards and a large number of smaller pieces,” he said. “It’s remarkable that we can still detect this distribution today.”

The InSight lander registered the seismic waves of eight separate marsquakes as it traveled through the Red Planet’s mantle. Two of the events were the result of a pair of meteorite impacts that each created a 492-foot-wide crater. As these waves made their way into Mars, InSight flagged sizable interference.

“That’s consistent with a mantle full of structures of different compositional origins—leftovers from Mars’s early days,” said Charalambous.

The discoveries have ramifications beyond Earth’s nearest neighbor. While our home planet is far more geologically active, the other rocky planets may resemble Mars beneath their own crusts. Better understanding the Martian mantle may help future investigations of other worlds.

“What happened on Mars is that, after those early events, the surface solidified into a stagnant lid,” Charalambous said. “It sealed off the mantle beneath, locking in those ancient chaotic features—like a planetary time capsule.”