Scientists say they have used computer simulations combined with studies of the Earth's interior to put an age to our moon. Estimating it formed almost 100 million years later than the beginning of the formation of our solar system around 4.4 billion years ago.

Researchers from the United States, France and Germany generated the estimate by creating computer models of the formation and subsequent growth of the inner planets -- Mercury and Venus, our Earth and Mars -- which suggested a link between the moment the Earth was hit by a Mars-sized cosmic object leading to the Moon's creation and the quantity of material the Earth has added following that impact.

Together, the simulations and the Earth studies provide a "clock" to date the cosmic collision that formed the moon.

"We were excited to find a 'clock' for the formation time of the Moon that didn't rely on radiometric dating methods," says Seth Jacobson of the Observatory de la Cote d'Azur in Nice, France.

Writing in the journal Nature, the researchers noted previous studies comparing the amount of siderophile elements -- atomic elements with a strong chemical affinity with iron -- in the mantle of the Earth found a direct proportional relationship to the amount of mass added to the Earth after the impact that created the Moon.

The final phase of the formation of the Earth's core is considered to have been a melting caused by the giant impact that tore off large amounts of matter that eventually coalesced into the moon.

"When the moon-forming event occurs, this melts the entire surface of the Earth," Jacobson says. "Causing iron at the surface to sink toward the Earth's core, taking the siderophile elements with it."

Any iron-elements now present at the surface of the Earth have arrived after the event of objects colliding with our planet, the researchers say. It is a geochemical result that can help date when the collision forming the moon occurred.

These new geochemical results provide a "clock" date for the formation of the moon of around 95 to 100 million years after our solar system began to form.

The finding represents the first use of a "geologic clock" for dating the early history of our solar system instead of relying on measuring and interpreting it radioactive decay in atomic nuclei.

"This result is exciting because in the same simulations that can successfully form Mars in only 2 to 5 million years, we can also form the Moon at 100 million years," researcher Kevin Walsh of the Southwest Research Institute says. "These vastly different timescales have been very hard to capture in simulations."