The current Martian mantle contains a multitude of heterogeneous fragments of kilometer-scale sizes.
These heterogeneities are believed to be the result of differentiation processes dating back more than 4 billion years, and were uncovered using data from NASA's InSight lander and its SEIS seismometer, designed in France under the scientific responsibility of the Institut de Physique du Globe de Paris (IPGP, Université Paris Cité/CNRS) with support from CNES and European partners.
Representation of Mars' evolution from a violent impact that occurred more than 4 billion years ago to the planet we know today.
@vadimsadovski / Imperial College London Ancient heterogeneous fragments distributed beneath the surface
By studying eight high-frequency Martian earthquakes, researchers have identified anomalies in the propagation of unusually slow seismic waves, revealing the presence of heterogeneities measuring 1 to 4 km (0.6 to 2.5 miles) within the mantle that "slow down" the propagation of seismic waves when they encounter these kilometer-sized fragments.
By tracing back through time, researchers concluded that these heterogeneities, whose size has gradually reduced over time, originate from very ancient processes possibly related to asteroids that collided with Mars at the dawn of the Solar System.
When impacting the planet's surface, these asteroids may have generated magma oceans whose solidification would have created compositional heterogeneities. These impacts would also have carried ancient fragments of crust and lithosphere deep into the mantle.
On Earth, plate tectonics continuously recycles oceanic crust and lithosphere, which are gradually mixed with other ancient heterogeneities by convection. On Mars, where plate tectonics is absent and mantle convection is less vigorous, this permanent recycling cannot occur, and mixing is less efficient. The fact that these fine structures have survived and are still visible today demonstrates that Mars has not experienced the same evolution as our planet.
"We had never observed the interior of a planet with such level of detail," explains Constantinos Charalambous (Imperial College). "The Martian mantle is dotted with ancient fragments, whose preservation testifies to the slow and less vigorous evolution of the red planet."
"The survival of these fragments through several billion years of convective mixing also provides valuable information about the rheology of the Martian mantle," specifies Henri Samuel, researcher at CNRS (IPGP/UPC) and co-author of the study. "The Martian mantle would be more rigid than on Earth, thus limiting the deformation and mixing of ancient heterogeneities."
"These results confirm that Mars preserves a unique geological memory, whereas Earth, through its active tectonics, has erased similar traces of its past," adds Thomas Pike (Imperial College), co-author of the study.