About 4 billion years ago, the Solar System was an extraordinarily violent place. Remnants of the recent formation of the Sun collided with each other everywhere, merged and grew to give rise to the planets, and everyone disputed the orbital positions that are today occupied by the few survivors of those turbulent times.
The Earth, of course, was not spared from that era of continuous collisions. The largest of all was with a planet candidate that was already the size of Mars and which we know as Theia. The impact must have been tremendous. The Earth’s nascent crust became an ocean of lava, and huge chunks of Theia sank deep into the mantle, where they still lie today.
After the titanic collision, the rest of the shattered protoplanet ‘bounced’ back into space, in the form of countless debris of all sizes that, captured by Earth’s gravity, fused together and gave rise to the Moon. That is, today, the scenario that scientists consider most likely to explain the existence of our satellite. But the exact moment of this gigantic impact is not easy to determine.
A wrong age
So far, scientists have attempted this by dating samples of lunar rocks that are presumed to have crystallized in the lunar magma ocean that existed just after the impact, giving the Moon an approximate age of 4.35 billion years. But these calculations do not take into account the existing discrepancies with the thermal model, nor other evidence, such as the number of craters on the Moon (less than it should be), or the age of some zircon minerals obtained from the Moon itself. lunar surface, which at 4.53 billion years suggests that the Moon could be considerably older than previously thought.
Now, a team of researchers led by Francis Nimmo, from the University of California, Santa Cruz, has discovered that the Moon is ‘disguising’ its true age, appearing younger to scientists’ eyes than it really is. The work has just been published in ‘Nature‘.
Two mergers instead of one
According to Nimmo and his colleagues, in fact, the frequent appearance of younger lunar rocks, 4.35 billion years old, could be indicative of a second fusion event, driven by the orbital evolution of the Moon and several hundred million of years after the one that occurred just after the impact with Theia. That is to say, these ‘young’ rocks do not come from the first solidification of the lunar magma ocean, but from a later one.
To reach that conclusion, the authors used computer models that show that the Moon may have experienced enough tidal heating to cause this new ‘remelting’ of its surface about 4.35 billion years ago, which ‘reset’ the counter. of his age.
Additionally, this second melting of the Moon’s surface would explain why there are fewer impact basins and craters than one would expect, since they would have been obliterated during the warming event. The authors, therefore, maintain that the Moon formed earlier than previously thought, between 4.43 and 4.53 billion years ago, very shortly after the formation of the Sun itself. A finding that will help understand better the formation process of the terrestrial planets in our system.
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