The marks left by collisions on the Moon’s surface help not only understand the formation of our natural satellite, but also the Earth’s. However, new research proposes that some of these impacts left almost invisible marks, as they would have crashed into the Moon while it was still cooling in its first thousands of years.
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Around 4 billion years ago, the Moon was a global ocean of magma. The impacts that took place over the next millions of years, as it cooled, may have left marks quite different from those seen today. According to Katarina Miljkovic, the main author of the article and an associate professor at Curtin University, when asteroids or other objects hit the soft lunar surface, they would not have left any revealing marks.
There is still no consensus on the exact time it took the Moon to become completely solid, but it is believed to have been enough for it to be bombed while still in its magma phase. Some studies suggest the Moon cooled about 10 million years ago after it formed, while other studies suggest 200 million years, but there are also those indicating that some lunar regions took up to 500 million years to solidify.
For the researchers, the partially solidified Moon would have a “soft” layer between the crust and the mantle. Upon impact, any trace would be almost completely erased from the surface. For Miljkovic and his team, this information fits the evidence showing that the lunar surface was much harder hit early in its formation.
The study shows that ancient impact basins would be almost unrecognizable on the lunar surface, but finding them is critical to understanding the history of the Moon and Earth — as well as the Solar System’s youth. The Reviews also indicates that many basins, including the South Pole-Aitken Basin, formed when the surface was not yet fully solidified.
It is still difficult to estimate the number of these almost invisible craters. The researchers have a lot to analyze, but they believe the Reviews is consistent with recent predictions of fluxes that have a greater impact on the Moon’s youth than just indicated by its current craters. “Translating this discovery will help future researchers to understand the impact that the early Earth might have experienced and how it would have affected our planet’s evolution,” says Miljkovic.
The research was fully published on September 14 this year, in the journal Nature.