Scientists date Moon at 4.470 billion years


(AFP) – The Moon was formed about 95 million years after the birth of our Solar System, in a collision that also settled the structure of Earth as we know it, according to the latest attempt at dating that impact.

A study in the journal Nature said the crash between an early, proto-Earth  and a Mars-sized object that dislodged what would become the Moon, happened  some 4.470 billion years ago — give or take 32 million years.
Apart from creating our satellite, the event is also believed to have  marked the final phase of Earth’s core formation from molten metals sinking to  the centre from a superhot surface.
Previous estimates had ranged from an “early” impact about 30 million years  after the start of the Solar System, to a later one as much as 200 million  years after.
The Solar System itself is known to be 4.567 billion years old thanks to  accurate dating of some components of meteorites — the oldest materials to be  found on our planet.
Earth is believed to have formed at some time during the first 150 million  years.
Many earlier age estimates had been based on measuring the rate of  radioactive decay of atomic nuclei found in rocks.
For the new study published in the journal Nature, a team of planetary  scientists from France, Germany and the United States created a computer model  of how dust and rock accumulated in the early Solar System to form tiny planets  called planetesimals.
These grew into “planetary embryos” that ended up as the rocky planets we  know today — Mercury, Venus, Earth and Mars — through a succession of giant  impacts, according to the new model.
Each massive collision allowed the planets to “accrete” or accumulate  matter. In Earth’s case, the lunar impact would have marked its final major  growth event.
The team also looked at the chemical composition of the Earth’s mantle to  trace the amount of material the planet accumulated after the impact — only  about 0.5 percent of its total mass.
If the impact had happened early in the Solar System’s history, there would  still have been many free-floating planetesimals for the Earth to sweep up, and  if it was later, fewer.
The evidence suggested Earth took 95 million years to form, “which confirms  it as the planet in our Solar System that took the longest to form,” study  co-author Alessandro Morbidelli told AFP by email.