In a study published in Nature on September 3, Prof. SUN Daoyuan's group and Prof. MAO Zhu's group from the University of Science and Technology of China of the Chinese Academy of Sciences, along with international collaborators, reported the seismic evidence for the presence of a solid inner core (IC) in Mars with an estimated radius of ~600 km by analyzing seismic data acquired by NASA’s InSight mission, and revealed that the Martian IC is likely composed of a crystallized iron-nickel alloy enriched in light elements.

Mars is a terrestrial planet similar to Earth. Understanding its internal structure is of great significance for studying planetary evolution and chemical state. However, probing the deep interior of a planet is far from straightforward. For instance, Danish seismologist Inge Lehmann proposed the existence of Earth’s solid IC in 1936, but her hypothesis was not conclusively confirmed until about half a century later. Although more than a thousand marsquakes have been recorded by the InSight mission, the weak signals and high noise levels still pose challenges for investigating Martian deep interior.

In this study, through seismic array analysis, the researchers examined 23 relatively high signal-to-noise events, and identified key seismic phases that traverse the Martian core, including PKPPKP (P′P′) and PKKP, which reflect at the nearly antipodal surface and the core-mantle boundary (CMB), respectively.

Compared to predictions of travel times and slownesses derived from current Martian seismic models, the researchers found that the significantly earlier arrival of PKKP phase indicated much higher seismic velocity toward the center—a finding that cannot be accounted for by current models assuming a purely liquid core. 

In contrast, a model incorporating a solid IC provided a plausible explanation. If such an IC exists, a reflection from the inner core boundary (ICB), namely PKiKP, should also be observed. This phase was successfully identified in the vespagram, further supporting the presence of a solid IC. In addition, the polarity and amplitude ratio between PKKP and PKiKP corroborate the phase identification.

By combining constraints from multiple core phases, the researchers estimated that the Martian solid IC has a radius of ~600 km. The seismic data also revealed a sharp contrast across the ICB, with a ~30% jump in seismic velocity and ~7% density difference between the liquid outer core and the solid IC. The compositional analysis suggested that the Martian core is not composed of pure iron and nickel, but may contain 12-16% sulfur, 6.7-9.0% oxygen, and less than 3.8% carbon.

Such a light-element enriched core structure provides crucial clues for understanding the evolutionary history of the Martian magnetic filed, from its early activity to its current quiescence, and establishes a key foundation for comparing the internal evolution of Earth with other terrestrial planets.

This study identifies for the first time the existence of a solid IC in a planet other than Earth, confirming that Mars underwent a similar core-mantle differentiation process. The success of Martian seismology further demonstrates the potential of applying seismic methods to future planetary missions, such as probing the deep interior of the Moon and other planetary bodies.

"The authors have done a detailed job of using multiple working lines of evidence for their phase detection analysis. Martian seismology is notably tough, so congratulations to the authors for doing such a thorough and careful job!" commented the Nature reviewer.