“Uncovering the Secrets of Rocky Planets: InSight’s Discoveries of the Core of Mars and Implications for Earth”

2023-05-05 02:00:20

Planets like the Earth are called “rocky planets”, and their surfaces are composed mainly of silicate rocks. (core)” is believed to exist.

It is believed that the Earth’s core has a two-layer structure and is divided into a liquid “outer core” on the outside and a solid “inner core” on the center side. Of course, we can’t directly see what’s going on in the Earth’s core, which is too deep, but we can infer such structures by analyzing seismic waves that pass through the Earth’s interior. This is because seismic waves have the property of changing depending on the physical properties of the materials they pass through, such as density and the difference between solids and liquids. This is just like looking inside the womb of a pregnant woman with an ultrasound.

Now, do rocky planets other than Earth have the same structure as Earth? Theoretically, rocky planets of some size are presumed to have a metallic core at their core. However, the theory is just a theory, so it was unknown what kind of structure the inside of the actual celestial body had.

Differences in the internal structure of rocky planets are very interesting data, as they may reflect differences in the way planets were formed and their environments. The Moon is the only celestial body other than the Earth whose internal structure has been inferred from seismic waves so far. However, the Moon is thought to have undergone a special formation process called a giant impact, so a simple comparison with the Earth is not possible.

It is the exploration of “Mars” that can improve such a situation. NASA (National Aeronautics and Space Administration) operated the Mars lander “Insight” from 2018 to 2022. Its goal is to capture Martian earthquakes with high sensitivity and acquire data for estimating the internal structure1. was one.InSight is the first Mars rover to accurately measure Martian earthquakes (※) 。

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*As a past example, NASA’s “Viking 1” and “Viking 2” (1976-1980) were also equipped with seismometers. was not possible. Although No. 2 was able to measure vibrations thought to be caused by an earthquake, the possibility of wind-induced vibrations could not be ruled out because the main body was not sufficiently fixed and comparisons with No. 1 could not be made.

[▲ Figure: Both of the two seismic waves analyzed this time pass through the center of Mars. This reveals that the core of Mars is entirely liquid (Credit: NASA/JPL & Nicholas Schmerr.)]

A research team such as Jessica CE Irving of the University of Bristol focused on and analyzed two seismic waves named ‘S0976a’ and ‘S1000’ detected by Insight. Both of these seismic waves are believed to have occurred on the opposite side of the InSight landing site. Since the seismic waves may have traveled from the epicenter through the center of Mars to reach InSight, it is thought to be suitable for investigating the state of the center of Mars.

As a result of the analysis, the properties of the Martian core have been clarified. The core has an estimated radius of 1780 to 1810 km, which is about half the size of Mars. It also turned out that the core of Mars is almost entirely liquid, and it is unlikely that there is a solid core in the center like Earth. Considering that Mars, which is a smaller celestial body than Earth, could cool and harden inside more quickly than Earth, it is a surprising discovery that the whole is still melting.

Furthermore, it was found that the core of Mars is rich in elements that are lighter than iron and nickel, likely reaching 20% ​​to 22% by weight. This is a big difference compared to the estimated less than 10% in the Earth’s core. About three-quarters of the light elements are estimated to be sulfur, with the remainder made up of small amounts of oxygen, carbon, and hydrogen. Considering that light elements tend to float on the surface of celestial bodies in the same way that oil floats on water, but do not easily sink in the core, it is interesting that there are many light elements in the core of Mars.

The richness of light elements shown here may reflect differences in planet formation processes at the time of the birth of the solar system. In addition, there is a possibility that we can get an answer that can explain why both plate tectonics and a strong magnetic field are maintained on the earth, even after 4.6 billion years since its birth, on Mars.

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A comparison of the internal structures of Earth and Mars provides clues to the similarities and differences in the formation processes of rocky planets, and is also important data for estimating the internal structures of other rocky planets such as Venus. Although InSight is no longer operational, there is a large amount of unanalyzed data that further study may reveal more about the internal structure of Mars and other planets.

Source

• Jessica C. E. Irving, et.al. “First observations of core-transiting seismic phases on Mars”. (Proceedings of the National Academy of Sciences)
• Georgia Jiang. “Scientists Detect Seismic Waves Traveling Through Martian Core for the First Time”. (The University of Maryland)

Text: Riri Ayae

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