Deep beneath the surface of Mars, scientists have detected debris left over from a massive impact event 4.5 billion years ago. These ancient impacts released immense energy, melting continent-sized regions of the early crust and mantle into vast magma oceans and simultaneously injecting fragments of the impactor and Martian debris deep into the planet's interior. Mars acts as a "time capsule," carrying a record of its early formation. This discovery provides clues to exploring Mars' internal structure and ancient history. The research was recently published in the journal Science.
On February 18, 2020, local time, the seismometer of NASA's InSight Mars probe, called the Interior Structure Seismic Experiment, was activated. (Visual China Data Map)
Earthquakes are caused by plate tectonics, but Mars lacks plate tectonics. However, two other types of earthquakes on Earth also occur on Mars: those caused by rock fracturing under high temperature and pressure, and those triggered by meteoroid impacts. NASA's InSight lander deployed the first seismometer on the Martian surface in 2018. This ultra-sensitive instrument recorded 1,319 Martian earthquakes before the InSight mission concluded in 2022.
Seismic waves generated by earthquakes change when they pass through different materials, providing scientists with a method for studying the internal structure of planets. According to data recorded by the now-retired InSight lander, eight Martian earthquakes contained strong high-frequency energy. This energy propagated deep into the mantle, causing the seismic waves to undergo significant changes.
Computer simulations revealed that the slowdown and confusion only occur when the signal passes through small, localized regions in the mantle. They also determined that these regions appear to be composed of clumps of material with a different composition than the surrounding mantle. The researchers believe these clumps are likely the remains of giant asteroids or other rocky bodies that impacted Mars in the early solar system. The enormous energy released by the impact destroyed large areas of the crust and mantle, forming a magma ocean and propelling parts of the impactor and fragments of Mars itself deep into the mantle.
The remains of the ancient impact are still scattered throughout the Martian mantle in chunks, some up to four kilometers in diameter. Researchers liken this distribution pattern to broken glass, with a few large fragments interspersed with many smaller pieces. This pattern is consistent with a process where large amounts of energy were released, dispersing the debris throughout the mantle. It is also consistent with the idea that asteroids and other objects frequently impacted the young planets in the early solar system.
Because Mars lacks plate tectonics and its interior isn't stirred and mixed by convection like Earth's, these impact remnants can be preserved on Mars. But it's currently impossible to pinpoint the exact object that struck Mars, as the early solar system was teeming with rocky objects that could have caused such an impact.
"We've never seen the interior of a planet with such detail and clarity," said lead author Constantinos Charalambous of Imperial College London. "What we see is a Martian mantle full of ancient debris. The fact that these fragments have been preserved suggests that the Martian mantle has evolved very slowly over billions of years. On Earth, these features are likely largely gone."
It is reported that this new discovery of the internal structure of Mars may reveal to humans the possible structures beneath the surface of other rocky planets without plate tectonics, such as Venus and Mercury.
