The question of when and how the Moon was formed has long been a topic of research for astronomers. The giant impact hypothesis has become somewhat of a prevailing theory regarding the formation of the Moon. The hypothesis suggests that the debris from a collision between early Earth and a primordial planet the size of Mars coalesced to form the Moon we see today.
There is still much debate regarding the age of the Moon. Various hypotheses clash over when the early Earth collided with the primordial planet. With the increase of exploration missions to the Moon and the collection of rock and dust samples, diverse research is being conducted, and studies on the timing of the Moon's formation through dating analysis are also active.
When estimating the formation period of the Moon, researchers measure the age of zircon fragments contained in rock and dust samples brought back from the Moon. These zircon fragments contain radioactive isotopes, such as uranium, which makes age measurement relatively straightforward. The ages of many zircon fragments that have been collected so far indicate they were formed approximately 4.35 billion years ago. This has led to many estimates suggesting the Moon is about 4.35 billion years old.
However, new research has indicated that the Moon is at least 100 million years older than 4.35 billion years. Professors Francis Nimmo from the University of California, Santa Cruz, Thorsten Kleine from the University of Münster, and Alessandro Morbidelli from the Côte d'Azur Observatory stated on the 19th that the Moon's formation period should be traced back not to the known 4.35 billion years ago, but to 4.53 billion years ago. The findings were published in the international journal Nature on the same day.
The Moon, formed from a cataclysmic collision between early Earth and a primordial planet, initially had a surface consisting of a vast magma ocean. Due to the tremendous energy generated from the collision, all the surface rocks melted into magma. As time progressed, this magma cooled and evolved into the rocks we now see on the Moon. Until now, it was believed that the process of the Moon's magma ocean solidifying into rock occurred only once.
However, the research team believes that the zircon fragments used to estimate the age of the Moon were formed through remelting. They propose that the Moon was already formed 4.35 billion years ago, underwent a process of a magma ocean solidifying into rock, and then, 4.35 billion years ago, the Moon's surface reheated, causing the rocks to melt back into magma.
The researchers hypothesized that the cause of the Moon's remelting was tidal heating. They noted that tidal heating, caused by the satellite's response to the strong gravity of a protoplanet, occurred on the Moon 4.35 billion years ago. They explained that the orbit of the Moon around Earth changed at that time, triggering the tidal heating.
Through new modeling to estimate the age of the Moon, the researchers suggested that the Moon likely formed between 4.43 billion and 4.53 billion years ago. This means the Moon is estimated to be 80 million to 180 million years older than the current estimate of 4.35 billion years.
Professor Nimmo noted, "The hypothesis that the Moon formed earlier than previously known and underwent remelting due to tidal heating is much more consistent with theories of Earth-like planet formation, such as the noble metal content of the Earth's mantle," adding, "It also makes sense that the reason there are fewer early collision traces on the Moon's surface is that the initial collision marks were erased due to remelting."
Reference materials
Nature(2024), DOI : https://doi.org/10.1038/s41586-024-08231-0