In 2018, NASA landed the first seismograph on the red planet. The InSight lander (short for Seismic Investigations, Geodesy and Heat Transport) was retired in 2022, but while it was active, measured more than 1,300 marsquakes. Scientists are still learning about Mars from the data InSight sent back.
A recent study published in the journal Science and based on InSight data suggests that Mars is lumpy inside, courtesy of a gargantuan impact 4.5 billion years ago, when the solar system was in its infancy.
The early solar system was a dangerous place
Before the sun and planets existed, there was a giant molecular cloud of diffuse gas and dust, slowly rotating in space. Then a nearby supernova went off, tossing a pebble into the cosmic pond. The death of one star can trigger the birth of others and lay the foundation for a world like ours. The supernova blast delivered heavy elements like iron and gave the molecular cloud a little push.
As supernova shockwaves rippled through the cloud, they compressed the gas and created a nursery of stars. As pockets of gas collapsed, pulling their matter together under the force of gravity, their rate of rotation sped up like an ice skater pulling their limbs in during a spin. At the same time, the proto-solar system flattened into a disk or plane, like tossed pizza dough.
Most of the matter (about 99.86%) fell into the center, becoming the Sun. What was left over became the planets, the asteroid belt, Pluto, the other dwarf planets, and everything else we find in the solar system today. But that took time.
In the beginning, everything was still whirling around, still coming together, each massive bit of rock attracting another, growing more massive, like snowballs rolling down a hill. And all the while, they were smashing into one another.
Today, dangerous impacts are blessedly rare, but that’s only because most of the planetary dust-ups already happened. As we travel further back in time, collisions were more common and some of them were staggering. Before things settled into the current configuration, there were other cosmic snowballs vying for a spot on the solar system roster. In a way, some of those protoplanets are still here, having smashed into and been absorbed by one of the 8 worlds which remain.
Finding signs of ancient impacts on Mars
One such planet-sized impactor is believed to have smashed into the early Earth about 4 billion years ago, casting off enough material in the process to create the moon.
On Earth, the fingerprints of that impact have been largely wiped away by geologic processes. Earth’s crust and mantle are alive, in motion. Tectonic shifting mixes up the interior through convection, smearing out evidence of those ancient impacts. It’s like trying to find decade-old footprints on a popular hiking trail.
On Mars, things are a lot more static. It’s a single-plate planet with incredibly slow mixing. When InSight peered into the red planet’s interior, it saw lumps up to 2.5 miles across, scattered throughout the mantle.
“We’ve never seen the inside of a planet in such fine detail and clarity before,” said the paper’s lead author, Constantinos Charalambous, in a statement. “What we’re seeing is a mantle studded with ancient fragments. Their survival to this day tells us Mars’ mantle has evolved sluggishly over billions of years. On Earth, features like these may well have been largely erased.”
Seismographs are great for detecting quakes (on Earth or Mars) but they can do a lot more. By analyzing the seismic waves as they move through a planet’s interior, scientists can map the planet’s composition. While Mars doesn’t have the sort of quakes we’re used to, because it isn’t tectonically active, InSight could detect quakes caused by rocks cracking under heat and pressure or from meteorite impacts.
Researchers looked at 8 marsquakes with strong, high-frequency seismic waves. When waves reached the mantle, they were significantly altered, suggesting they ran into material with a different composition than the surrounding rock.
“When we first saw this in our quake data, we thought the slowdowns were happening in the Martian crust,” said study co-author Tom Pike. “But then we noticed that the farther seismic waves travel through the mantle, the more these high-frequency signals were being delayed.”
Computer simulations based on the data revealed that delays happened only in small, localized areas. Researchers described it like the scattering of shattered glass, with a few large chunks and many smaller pieces. They propose the lumps were delivered by one or more massive impacts about 4.5 billion years ago. The impact or impacts carried enough energy to melt entire swaths of Mars into magma oceans the size of continents. At the same time, fragments from the impact(s) were injected into the planet’s interior, where they still remain.
The discovery not only teaches us something about the history of Mars but may also point to similar circumstances hiding beneath the surface of other tectonically static worlds. If nothing else, we know that Mars can take a punch with the best of them.
Explore more alien worlds in The Ark, streaming now on SYFY.