One of the oldest rocks on Earth appeared on the Moon, of all places.

According to the Giant Impact Hypothesis, the Earth-Moon system was created approximately 4.5 billion years ago when an object the size of Mars collided with Earth. This impact led to the release of mbadive amounts of material that eventually came together to form the Earth and the Moon. Over time, the Moon gradually moved away from Earth and badumed its current orbit.

Since then, there have been regular exchanges between Earth and the Moon due to impacts on their surfaces. According to a recent study, an impact that took place during the Hadean Eon (approximately 4 billion years ago) may have been responsible for sending the oldest rock sample from the Earth to the Moon, where it was recovered by the Apollo 14 astronauts

The study, which appeared recently in the magazine. Letters of Earth and Planetary ScienceIt was directed by Jeremy Bellucci of the Swedish Museum of Natural History and included members of the Lunar and Planetary Institute (LPI), several universities and the Lunar Science and Exploration Center (CLSE), which is part of the Exploration Research of the Solar System of the POT. Virtual Institute

This discovery was possible thanks to a new technique developed by the study team to locate the fragments of the impactor in the lunar regolith. The development of this technique made Dr. David A. Kring, principal investigator of CLSE and scientist of the Space Research Association of Universities (USRA, for its acronym in English, USRA) in the LPI, challenge them to locate a piece of Earth on the Moon.

The resulting investigation led them to find a 2 g (0.07 oz) rock fragment composed of quartz, feldspar and zircon. Rocks of this type are commonly found on Earth, but they are very unusual on the Moon. In addition, a chemical badysis revealed that the rock crystallized in an oxidized system and at temperatures compatible with the Earth during Hadean; Instead of the Moon, it was experiencing higher temperatures at that time.

As Dr. Kring indicated in a recent LPI press release:

"It's an extraordinary find that helps paint a better image of the primitive Earth and the bombardment that modified our planet during the dawn of life."

Based on their badysis, the team concluded that the rock was formed in Hadean Eon and was launched from Earth when a large asteroid or comet hit the surface. This impact would have thrown material into space where it collided with the surface of the Moon, which was three times closer to Earth at that time. Eventually, this rocky material was mixed with the lunar regolith to form a single sample.

The moon was much closer to the Earth from what it is today when the rock fragment was produced and ejected from Earth to the Moon in a high-impact event. Credit: LPI / David A. Kring.

The team was also able to learn a lot about the history of the rock show from its badysis. On the one hand, they concluded that the rock crystallized at a depth of about 20 km (12.4 mi) below the surface of the Earth between 4.0. and 4.1 billion years ago, and then it was excavated by one or more high-impact events that sent it into the cis-lunar space.

This is consistent with the previous research carried out by the team that showed how the impacts during this period, that is, the Late Heavy Bombardment (which took place approximately 4.1 to 3.8 billion years ago) produced craters thousands of kilometers in diameter, more enough to expel material from a depth of 20 km (12.4 mi) into space.

They also determined that several other impact events affected him once he reached the lunar surface. One of them caused the sample to partially melt some 3.9 billion years ago and could have buried it below the surface. After that period, the Moon was subjected to smaller and less frequent impacts, and gave it the marked surface it has today.

The final impact event that affected this sample occurred about 26 million years ago, during the Paleogene period on Earth. This impact produced the 340 m (1082 ft) diameter Cone Crater and excavated the sample rock on the lunar surface. This crater was the landing place of the Apollo 14 mission in 1971, where mission astronauts obtained rock samples to bring back to Earth for study (which included the rock of the Earth).

Impression of the artists of the Late Heavy Bombardment period, which took place ca. 4.1 to 3.6 billion years ago. Credit: NASA

The research team recognizes that it is possible that the sample has crystallized on the Moon. However, that would require conditions that have not yet been observed in any lunar sample obtained so far. For example, the sample would have had to crystallize very deeply inside the lunar mantle. In addition, it is believed that the composition of the Moon at these depths is quite different from what has been observed in the sample rock.

As a result, the simplest explanation is that this is a terrestrial rock that ended up on the Moon, a finding that is likely to generate some controversy. This is inevitable since this is the first Hadean sample of its kind that can be found, and it is likely that the site of its discovery will be added to the disbelief factor.

However, Kring anticipates that more samples will be found, since it is likely that the Hadeas rocks have spattered the lunar surface during the Late Heavy Bombardment. Perhaps when crewed missions begin to travel to the Moon in the next decade, they will have the opportunity to find more of the oldest rock samples on Earth.

The research was possible thanks to the support provided by NASA's Virtual Solar Exploration Research Institute (SSERVI) as part of a joint venture between the LPI and NASA's Johnson Space Center.

Additional reading: LPI, Letters of Earth and Planetary Science

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