Did changes in the orbits of the large gas planets suddenly direct asteroids onto the earth and moon around 3.9 billion years ago? This assumption within the framework of the theory of “late heavy bombardment” is now contradicted by another study: Through isotope analyzes of lunar rocks, scientists found indications that the bombardment of the moon is due to rather continuous impacts from asteroids, which are still left over from the building material of the inner rock planets are. On the other hand, possibly water-rich asteroids from the outer solar system may have crashed into the Earth-Moon system earlier, the researchers suspect.
The planets of our solar system have literally clustered together: The material of the loose disc of matter around the young sun united to form ever larger structures in the course of the formation of the planetary system. This process of accretion then continued in the form of asteroid impacts that fell on planets and their moons. Even today, as is well known, smaller and larger celestial bodies can fall on earth. But in the early development of the earth-moon system, this was the case even more frequently. However, the earthly 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 clues to the history of both celestial bodies.
Questioning look at signs of impact
Investigations of lunar rocks that were brought to earth by the Apollo missions have led to the development of the theory of “Late Heavy Bombardments” (LHB). It is based on the dating results of certain traces of impact in the rocks, which seemed to indicate a special phase of the cosmic bombardment: The moon – and thus also the earth – were therefore exposed to a particularly intense hail of cosmic bombs about 3.9 billion years ago. Changes in the planetary system at that time were cited as a possible explanation for this apparently intense bombardment: instabilities in the orbits of the gas and ice giants could have led to a sudden sharp increase in impacts by asteroids and comets from the outer solar system. But meanwhile there are more and more indications that question the concept of the LHB – including the explanation of a planetary change in constellation as the cause. The results of Emily Worsham and Thorsten Kleine from the University of Münster are now contributing to this.
The results are based on the investigation of the previously unclear origin of the asteroids that once struck the moon. The researchers carried out isotope analyzes on moon rocks that were brought to Earth by the Apollo missions. As they explain, they contain tiny metal spheres that were formed during the asteroid impacts 3.9 billion years ago. The focus was on isotopes of the elements ruthenium and molybdenum. By comparing them with the isotope patterns of samples of known origin, it is possible to determine where material comes from in the solar system, the scientists explain.
Not a hail of bombs from afar
The rock was therefore not characterized by the characteristics of impacts that came from asteroids from the outer solar system: “Our investigations show that the bombardment of the moon was carried out by the same bodies from which the earth and the moon were formed,” reports Worsham. This result fits in with the explanation that the impact craters on the moon can be traced back to a rather continuous bombardment with remaining asteroids from the main phase of the Earth’s formation.
Apparently there was no sudden increase in the impact rate due to the bombardment with bodies from the outer solar system. But what about the apparent accumulation of impacts around 3.9 billion years ago? According to the researchers, current results confirm assumptions that this intense phase did not actually exist. “It was previously suspected that the lunar rocks examined so far consist mainly of material from a single impact basin – the Mare Imbrium in the northern center of the earth-facing side of the moon,” explains Worsham.
It is still assumed that the orbits of the 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 the earth and moon. But the current results now show that this happened quite early: “Since we find no evidence of impacts by asteroids or comets from the outer regions of the solar system in the lunar rocks, this event seems to have taken place earlier,” says Kleine.
The orbits of the gas and ice giants could have changed during the main formation phase of the Earth-like planets – that is, in the first 100 million years of the history of the solar system. This, in turn, agrees well with recent dynamic models of the history of the planetary system, the scientists emphasize. “Our study results also imply that the Earth-like planets incorporated water-rich bodies from the outer solar system relatively early during their formation, thus creating the conditions for life to develop,” says Kleine in conclusion.
Source: Westfälische Wilhelms-Universität Münster, specialist article: Science Advances, doi: 10.1126 / sciadv.abh2837