Astronomy on the path of cosmic bombardment

Did changes in the orbits of large gaseous planets suddenly direct asteroids to Earth and the Moon about 3.9 billion years ago? This assumption in the context of the “Late Heavy Bombardment” theory is now contradicted by another study: through isotopic analyzes of lunar rocks, scientists have found indications that the moon’s bombardment is due to persistent impacts from asteroids still left over from the inner, rocky planetary building materials. On the other hand, researchers suspect that water-rich asteroids from the outer solar system may have collided with the Earth-Moon system earlier.

The planets of our solar system literally huddled together: the material of the loose disk of matter around the young sun coalesced to form ever-larger structures in the course of the formation of the planetary system. Then this accumulation process continued in the form of asteroid collisions that fell on the planets and their moons. Even today, as is well known, smaller and larger celestial bodies can fall to the Earth. But in the early development of the Earth-Moon system this was more than once the case. However, the terrestrial traces of this early bombardment have disappeared due to the geological activity of our planet. However, ancient structures have been preserved on the Moon, which scientists can provide evidence of the history of each of the celestial bodies.

The question is, look at the signs of influence

Investigations of lunar rocks brought to Earth by the Apollo missions led to the development of the “Late Heavy Bombardment” (LHB) theory. It relies on the dating results of some impacts of rocks, which seem to point to a special stage of cosmic bombardment: so the Moon – and thus the Earth as well – was subjected to an intense barrage of cosmic bombs around 3.9. a billion years ago. Changes in the planetary system at the time were cited as a possible explanation for this seemingly intense bombardment: instability in the orbits of gas and ice giants could have caused a sudden sharp increase in the impacts of asteroids and comets from the outer solar system. But in the meantime, there are more and more indications questioning the concept of LHB – including the interpretation of planetary change in the constellation as the cause. The results of Emily Wersham and Torsten Klein from the University of Münster now contribute to this.

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The findings are based on investigating the previously unclear origin of asteroids that once struck the moon. Researchers conducted isotope analyzes on moon rocks brought to Earth by the Apollo missions. As they explain, it contains tiny metal balls that formed during an asteroid collision 3.9 billion years ago. The focus was on isotopes of the elements ruthenium and molybdenum. By comparing it with the isotopic patterns of samples of known origin, it is possible to determine the source of the material in the solar system, the scientists explain.

Not a barrage of bombs from afar

So the rock did not have the characteristics of collisions that came from asteroids from the outer solar system: “Our investigations indicate that the bombing of the moon was done by the same bodies from which the Earth and the moon were formed.” Worsham. This finding is consistent with the interpretation that the impact of craters on the Moon can be traced back to a more or less continuous bombardment of asteroids left over from the main phase of Earth formation.

Apparently there was no sudden increase in the rate of impact due to bombardment with objects from the outer solar system. But what about the apparent accumulation of impacts about 3.9 billion years ago? According to the researchers, the current findings confirm assumptions that this intense phase did not actually exist. “It was previously suspected that the lunar rocks examined to date consist primarily of material from a single impact basin – the Mare Imbrium in the north center of the Earth’s side of the Moon,” Warsham explains.

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It is still assumed that the orbits of gas and ice giants changed at some point in the early history of the solar system and put many asteroids from the outer solar system on a collision course with Earth and the Moon. But current results now show that this happened much earlier: “Because there is no evidence of asteroids or comets from the outer regions of the Solar System colliding into lunar rocks, this event appears to have occurred earlier,” says Klein.

The orbits of the gas and ice giants could have changed during the major formation stage of Earth-like planets—that is, in the first 100 million years of the solar system’s history. This, in turn, is in good agreement with modern dynamic models of the history of the planetary system, scientists assert. “The results of our study also indicate that Earth-like planets incorporated water-rich bodies from the outer solar system relatively early in their formation, creating conditions for the development of life,” Klein says in conclusion.

Source: Westfälische Wilhelms-Universität Münster, specialist article: Science Advance, doi: 10.1126/sciadv.abh2837

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