Researchers used a new way to calculate the birthday of the planet's only natural satellite. Astronomer John Chambers, with the Carnegie Institution for Science in Washington DC, said the mega-asteroid that smashed into Earth, launching debris that later became the moon, occurred about 95 million years after the birth of the solar system.
"We think that the thing that hit Earth and ended up forming the moon, the lion's share of it stayed on Earth," he explained. "A small fraction of its mass and some material from Earth was pushed off into space to form the moon. That was probably the last big event," he added.
The study, published in the journal Nature, is based on 259 computer simulations of how the solar system evolved.
The programs simulate the crashes and mergers of the small bodies until they meld into the rocky planets that exist today. Earth's last big chuck came from a Mars-sized body that hit about 95 million years after the solar system's formation when measured by that geologic clock, the study showed.
At 99.9 per cent accurate, the study disputes some previous estimates that the moon-forming impact occurred as early as 30 million to 40 million years after the solar system's formation.
The results also open another even bigger mystery about why some planets, like Mars, form relatively quickly, while others, like Earth and possibly Venus, take far longer. Analysis of Martian meteorites and the computer simulations indicate Mars was finished in just a few million years.