It has been discovered that fragments from massive celestial bodies that collided with Mars 4.5 billion years ago still remain in large numbers within Mars's interior. A research group from the UK, U.S., and other countries made this determination based on seismic data and other observations from the U.S. Mars subsurface exploration spacecraft "InSight." Mars lacks the plate tectonics mechanism that results in surface plates moving, and this discovery is expected to contribute to an understanding of the internal structure of similar planets.
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Unlike Earth, Mars lacks plate tectonics, so there are no earthquakes caused by strain accumulation in the crust due to plate movement. However, there are earthquakes believed to be caused by rock fracturing due to heat and pressure, as well as earthquakes caused by celestial body impacts. Waves generated by earthquakes change as they pass through various materials, and observational data from these changes serve as clues for studying planetary interiors. Mars's interior has a layered structure consisting of (from surface to center) crust, mantle, and core. The sizes and structures of these layers have been studied based on seismic data and other observations from InSight.
Annotations added to conceptual diagram provided by NASA
When the research group analyzed eight earthquakes from InSight's observational data, they found that seismic waves containing strong high-frequency energy reached deep into the underground mantle and changed distinctly. As seismic waves traveled farther through the mantle, high-frequency signals were significantly delayed.
Computer simulations also showed that these signals changed speed only when passing through very limited regions within the mantle. These regions are believed to be masses of material with different compositions from the mantle. Based on these findings, the research group concluded that when massive celestial bodies collided with Mars 4.5 billion years ago, fragments from those bodies and from Mars reached deep into the mantle and remain there today. The reason the celestial body fragments reached such depths is that the impacts melted the crust and mantle, creating vast magma oceans.
The solar system is believed to have formed 4.6 billion years ago. Gas and dust gathered to form a disk-shaped cloud, from which the sun and primitive small celestial bodies formed. Subsequently, small celestial bodies repeatedly collided and merged to evolve into planets, and rocky planets like Earth and Mars are thought to have lined up near the sun. Large and small celestial bodies appear to have frequently collided with these young planets.
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On Mars, which lacks plate tectonics, internal material circulation is much more gradual compared with Earth. Research Fellow Constantinos Charalambous at Imperial College London (UK) and member of the research group, stated: "This is the first time we've been able to observe the interior of a planet in such detail and clarity. The fact that ancient fragments still remain shows that Mars's mantle has changed slowly over billions of years. On Earth, such features have likely largely disappeared (due to crustal movements like plate tectonics)."
These massive rocks remaining in Mars's mantle provide clues for understanding Mars's interior and history. Additionally, among the planets in the solar system, plate tectonics have not been confirmed on Mercury or Venus and are absent on Mars. The current findings may also contribute to understanding the internal structures of such rocky planets.
InSight was the first spacecraft dedicated almost exclusively to investigating Mars's internal structure and was operated by the National Aeronautics and Space Administration (NASA). It was launched in May 2018 and landed in November on Elysium Planitia near Mars's equator. It was equipped with seismometers, heat flow meters, and devices to examine the interior using radio waves. The seismometer installed on Mars's surface achieved results including observing 1,319 earthquakes. Although the U.S. landers Viking 1 and 2 also carried seismometers in the 1970s, they were located on the upper part of the spacecraft, and the data was unclear. InSight offered the first clear seismic observation on a planet other than Earth. The effort to bury the heat flow meter underground failed. Operations ended in December 2022.
Provided by NASA and California Institute of Technology
The research group consists of Imperial College London, the French National Centre for Scientific Research, Johns Hopkins University, and California Institute of Technology. The results were published in the American scientific journal Science on the 28th of August, and NASA announced them on the same day.
Original article was provided by the Science Portal and has been translated by Science Japan.