Recently, a research team led by Professors Sun Daoyuan and Mao Zhu from the University of Science and Technology of China (USTC), in collaboration with international scholars, has made significant advancements in the field of planetary science. The team has confirmed for the first time the existence of a solid inner core on Mars, with a radius of approximately 600 kilometers, through an in-depth analysis of Marsquake data recorded by NASA's InSight lander. The findings suggest that the main composition of this inner core may be an oxygen-rich crystalline iron-nickel alloy. This achievement was published in Nature on September 3, Beijing time, and was selected for a press release by the journal.

Mars, as the terrestrial planet most similar to Earth in the solar system, has long been a key subject in the study of planetary internal structures and evolution, as well as one of the core objectives of deep space exploration. Investigating the deep internal structure of planets has always been a challenge. For instance, scientists did not confirm the existence of Earth's inner core until 1936, based on seismic waves, and it took nearly half a century to fully validate its solid state, which was completed in the 1980s. In comparison, exploring Mars's internal structure is even more difficult; direct observational data was only obtained in 2018. To date, although thousands of Marsquake events have been recorded, issues such as weak signals and noise interference continue to severely limit research on Mars's deep structure.

To overcome this challenge, the research team innovatively introduced the Marsquake array analysis method. By analyzing data from 23 relatively high signal-to-noise ratio Marsquake events, they successfully extracted key seismic phases that traverse Mars's core, such as the PKPPKP (P'P') reflected at the surface and the PKKP reflected at the core-mantle boundary. Notably, the observed arrival time of PKKP was found to be 50 to 200 seconds earlier than predicted by current models that only consider a liquid core. This difference indicates that seismic waves travel faster in solids than in liquids, suggesting a layered structure for Mars's core: an outer liquid core and a deeper solid inner core with a higher wave speed.

In further analysis, the research team identified the PKiKP seismic phase signal, regarded as a "solid inner core signature," in the Marsquake data for the first time. This finding provides evidence for the existence of a solid inner core on Mars. By combining different seismic phases, the team measured the radius of Mars's solid inner core to be approximately 600 kilometers, accounting for one-fifth of Mars's total radius. If Mars were scaled to the size of Earth, its inner and outer core structure would be highly similar to that of Earth.

Additionally, the Marsquake data indicate a wave speed jump of about 30% and a density difference of approximately 7% between the outer core and inner core. On this basis, the research team further analyzed the mineral composition of the inner core. The results reflect that Mars's core is not composed solely of iron and nickel; it may also contain 12-16% sulfur, 6.7-9.0% oxygen, and no more than 3.8% carbon. This core structure containing lighter elements provides important clues for understanding the evolution of Mars's magnetic field from a previously active state to its current quiet phase, and establishes a critical foundation for comparing the internal evolutionary differences between Earth and other terrestrial planets.

This research marks the first confirmation of a solid inner core outside of Earth, validating the similar core-mantle differentiation structures of Mars and Earth. The innovative Marsquake Seismology Methods developed by the research team offer important references for future missions, such as lunar exploration, aimed at probing the deep structures of celestial bodies using seismology. Furthermore, this achievement signifies a key step for Chinese research teams in the field of planetary internal structure exploration, demonstrating China's innovative capabilities and international influence in the intersection of planetary science and geophysics.

Reviewers of Nature highly praised this work, noting: "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!" They also commented, "To me, this is a really interesting paper and I think that it will generate discussion for years to come!"

Source: anhuinews.com