How old is the moon? New dating method offers clues.

Using a new "geologic clock," scientists have determined that the moon came into existence about 100 million years after the solar system formed, some 4.56 billion years ago.

The new study, titled "Highly siderophile elements in Earth’s mantle as a clock for the Moon-forming impact” and appearing in the April 3, 2014, issue of journal Nature, combines computer simulations with measurements of siderophile, or "iron loving," elements found in the Earth's mantle.

The team's finding is based on the Giant Impact hypothesis of the moon's formation. According to this hypothesis, a body roughly the size of Mars smashed into the Earth about 4.5 billion years ago, tearing free a large piece of our planet, which ultimately became the moon.

This impact was so intense that it melted the Earth. As a result, the Earth's mantle and core had to re-solidify, during which siderophile elements were drawn into the Earth's core. Siderophile elements, such as, osmium, radium and iridium, have an affinity for iron, so it is natural for them to move toward the core of the Earth, says Kevin Walsh, astronomer with the Southwest Research Institute (SwRI) Space Science and Engineering Division, and one of the study's authors.

But siderophile elements are also present within the Earth's mantle, carried there by asteroids that struck Earth after the Giant Impact, say scientists. 

Therefore, it's possible to estimate the number of asteroid strikes following the Giant Impact by measuring siderophile concentrations in the Earth's mantle. A higher concentration of siderophiles would indicate a large number of strikes after the Giant Impact, and a smaller concentration would indicate a lower number of strikes. 

The team ran hundreds of models of planetary formations to develop computer simulations of "asteroids impacting the Earth with respect to time," says Dr. Walsh. The results of these simulations, when compared with the proportion of siderophile elements, serves as a 'clock' that can help determine Moon's age.

“We were excited to find a clock for the formation time of the Moon that didn’t rely on radiometric dating methods. This correlation just jumped out of the simulations and held in each set of old simulations we looked at,” says lead author of the paper Seth Jacobson of the Observatory de la Cote d’Azur in Nice, France.

The team found out that the "siderophile elements were equal to one percent of Earth's mass in asteroids," says Walsh, suggesting a larger number of strikes.