New clues have been added to the "origin of the moon". Where did this mysterious planet come from?

How was the Moon formed?

The most mainstream hypothesis at present is the "giant impact hypothesis".

About 4.5 billion years ago, a Mars-sized body called Theia hit the young Earth, ejecting part of the Earth (mainly the mantle) into the universe, thus creating the Moon.

Figure｜Theia hits the Earth. (Source: Wikipedia)

Previous evidence supporting the "giant impact hypothesis" came from lunar rocks brought back by Project Apollo, which contained oxygen isotope composition ratios that were almost exactly the same as those in the Earth's mantle.

Now, scientists have new clues that further support the "giant impact hypothesis" about the formation of the moon.

In a new study, geochemists, cosmochemists and geologists at ETH Zurich have discovered rare gases (also known as inert gases) from the Earth's mantle - helium and neon - in lunar meteorites.

The relevant research paper, titled "Indigenous noble gases in the Moon's interior", has been published in the authoritative scientific journal Science Advances.

(Source: Science Advances)

4 hypotheses

Regarding the origin of the moon, in addition to the "giant impact hypothesis", scientists have also successively proposed the "capture hypothesis", "homologous hypothesis" and "split hypothesis", but all of them have certain theoretical flaws.

In the "capture hypothesis", scientists believe that the moon is one of the many cosmic bodies formed in the early days of the solar system. It also floated in orbit like other celestial bodies until one day, it was attracted by the gravity of the earth and broke away from its original orbit, becoming the only natural satellite of the earth.

However, for this hypothesis to be true, the Earth would need to have a very large atmosphere to consume the energy of the Moon's passage and slow down its motion.

The "homologous origin hypothesis" tells a different story, namely that the Earth and the Moon were formed from the same primordial accretion disk.

But this hypothesis cannot explain the lack of metallic iron on the moon, nor can it explain the high angular momentum of the Earth-Moon system.

The "split hypothesis" also has certain theoretical flaws. It holds that the earth broke due to its own rapid rotation, and the broken parts became the moon.

However, to generate such a large centrifugal force, the Earth needs to rotate at super-high speed from the very beginning.

In the "giant impact hypothesis", scientists believe that huge impacts were common in the early days of the solar system. Computer simulations of giant impact models show that the binary star system produced by such an impact has sufficient angular momentum to match the orbital parameters of the current Earth-Moon system, and can also explain why the Moon has a relatively small core.

Figure｜A simple description of the "Big Impact Hypothesis". (Source: Wikipedia)

In addition, this hypothesis can also reasonably explain the difference in composition between the Earth and the Moon: most of the Moon's components come from pre-impact celestial bodies, not the original Earth.

However, this hypothesis is still not perfect. For example, studies of meteorites have shown that the oxygen and tungsten isotope compositions of other inner solar system bodies such as Mars and Vesta are different from those of the Earth, while the Earth and the Moon have very similar isotope compositions.

One plausible explanation is that the impact that led to the formation of the Earth-Moon system mixed materials that were volatilized during the formation of the Earth and the Moon, possibly causing the isotopic composition of the two bodies to become balanced, but this explanation remains controversial.

Although the "giant impact hypothesis" is not perfect, it may be the most comprehensive explanation for the formation of the moon at present, and more evidence will be needed to support it in the future.

New evidence supports the 'giant impact hypothesis'

Rare gases refer to elements belonging to Group 18 in the periodic table. They have similar properties and are colorless and odorless monatomic gases at room temperature and pressure, making it difficult for them to undergo chemical reactions. There are six naturally occurring rare gases, namely helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn).

Rare gases are mainly used in lighting equipment, welding and space exploration in industry. For example, a mixture of argon and nitrogen is the protective gas in incandescent lamps; krypton is often used in high-performance incandescent lamps with higher color temperature and efficiency because it can reduce the evaporation rate of the filament; different rare gases can be filled in discharge lamps to produce different colors of light, such as neon lamps commonly seen in neon signs.

In this work, the research team used a noble gas mass spectrometer to measure submillimeter-sized glass particles (formed when basalt cooled) in meteorite samples, ruling out the solar wind as the source of the detected gases.

Patrizia Will, the first author and corresponding author of the paper, analyzed six lunar meteorite samples collected from Antarctica. These samples were collected by NASA in the "cold desert" of Antarctica. The reasons for their formation can be speculated as follows: due to the lack of atmosphere protection, the lunar surface is constantly hit by asteroids. A high-energy impact caused rock fragments to be ejected from the middle layer of the lava flow and eventually came to the earth in the form of meteorites.

These meteorites are composed of basalt, which was formed when magma welled up from the moon's interior and cooled rapidly. Because they were covered by other basalt layers after their formation, they were protected from damage by cosmic rays, especially the solar wind.

The results showed that the glass particles retained the chemical fingerprints (isotopic signatures) of solar gases: helium and neon from the lunar interior, and the content of helium and neon detected was much higher than expected.

Figure | LAP 02436 sample slice under cross-polarized light. (Source: ETH Zurich)

The discovery strongly supports the idea that the Moon "inherits" naturally occurring noble gases from Earth. "This is an exciting result, and these basalt materials are not associated with any exposure on the lunar surface," Will said.

In this regard, Henner Busemann, one of the authors of the paper and professor at the Swiss Federal Institute of Technology, said: In future work, the research team will continue to look for other rare gases in lunar meteorites, such as xenon and krypton, as well as other volatile elements such as hydrogen or halogens.

"This discovery may help geochemical and geophysical scientists create new models and more generally show how these most volatile elements survived the formation of planets in and outside our solar system."

References:

https://www.eurekalert.org/news-releases/961341

https://en.wikipedia.org/wiki/Giant-impact_hypothesis