The InSight lander arrived on Mars’s surface in November 2018 with the singular purpose of taking the planet’s vital signs: its pulse, temperature, and reflexes. This largely consisted of using an advanced seismometer to measure “marsquakes,” seismic waves caused by rocks cracking under heat and pressure or meteoroid impacts. By analyzing how these waves pass through the planet, scientists were able to gain valuable insight (no pun!) into the interior structure and composition of Mars. While the InSight lander ended operations in 2022, scientists are still poring over the data it collected during its four-year primary mission.
The latest find comes from an international team of researchers who found that Mars likely experienced a series of massive impacts roughly 4.5 billion years ago. This is based on seismic evidence that revealed debris fragments beneath the Martian surface left over from these impacts. The size and energy would have been sufficient to melt much of the planet’s early crust and mantle into vast oceans of magma while also injecting material from the impactors deep in Mars’ interior. These findings imply that, like Earth, Mars’ geology was influenced by massive impacts early in its history.
The research was conducted by Research Fellow Constantinos Charalambous and his colleagues from the Department of Electrical and Electronic Engineering at Imperial College London. They were joined by researchers from the Institut de Physique du Globe de Paris, Johns Hopkins University, the University of Oxford, and NASA’s Jet Propulsion Laboratory (JPL). Their findings were presented in a study, “Seismic evidence for a highly heterogeneous martian mantle,” that appeared on August 28th in the journal Science.
A cutaway view of Mars in this artist’s concept (not to scale) reveals debris from ancient impacts scattered through the planet’s mantle. Credit: NASA/JPL-Caltech
InSight placed the first seismometer, the Seismic Experiment for Interior Structure (SEIS), on Mars in 2018. This extremely sensitive instrument recorded 1,319 marsquakes, allowing the InSight team to measure the size, depth, and composition of Mars’ crust, mantle, and core. In the early Solar System, collisions were common as protoplanets and asteroids migrated or were kicked from their orbits by encounters with larger objects. This included primordial Earth, which scientists theorize was struck by a Mars-sized object (named Theia) roughly 4.5 billion years ago.
While it is unclear how large the objects that collided with Mars were, the remains still exist as debris scattered throughout the mantle, measuring up to 4 km (2.5 mi) in diameter. These objects are basically a historical record that is not available on Earth, due to our planet actively cycling its tectonic plates through the process of convection. Said Charalambous in a NASA press release:
We’ve never seen the inside of a planet in such fine detail and clarity before. 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. [The fact that such fine structures are still visible today] tells us Mars hasn’t undergone the vigorous churning that would have smoothed out these lumps.
In their paper, the team identified eight marsquakes whose seismic waves produced strong, high-frequency signals that reached deep into the mantle, where their seismic waves were distinctly altered. They then ran planet-wide simulations, which revealed that the signals were slowed and scrambled only when they passed through small, localized regions within the mantle that had different compositions than the surrounding mantle. This led the team to conclude that the lumps were likely deposited by giant asteroids or planetoids that struck Mars during the early Solar System.
NASA’s InSight took this selfie in 2019 using a camera on its robotic arm. Credit: NASA/JPL-Caltech
This would have generated oceans of magma that scattered fragments of the impactors deep beneath the surface. Said co-author Tom Pike of Imperial College London:
We knew Mars was a time capsule bearing records of its early formation, but we didn’t anticipate just how clearly we’d be able to see with InSight. When we first saw this in our quake data, we thought the slowdowns were happening in the Martian crust. But then we noticed that the farther seismic waves travel through the mantle, the more these high-frequency signals were being delayed.
These results also suggest that similar findings could be made beneath the surface of other rocky planets like Mercury and Venus, which also lack plate tectonics, allowing them to preserve evidence of past impacts. These latest findings also show how much more there is to be learned from InSight’s data.
Further Reading: NASA