The Telegraph
Thursday , April 3 , 2014
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New geological clock puts an age on Moon

New Delhi, April 2: Scientists have used a new geological clock to assign an age to the Moon that astronomers believed was born through a collision between the Earth and a Mars-like planet during the solar system’s infancy.

An international team of scientists has calculated that the collision that fully melted the Earth and threw off debris that accreted into the Moon occurred about 95 million years after the solar system's formation 4,567 million years ago.

Several earlier attempts to determine the age of the Moon through studies on radioactive elements have led to a range of ages — from 30 million years to 100 million years after the solar system’s formation.

In the latest study, the team from France, Germany, and the US used a geological clock that relies on the abundance of the so-called siderophile elements with an affinity for iron and likely to sink to the molten Earth's core.

“The collision that gave rise to the Moon was the last one that melted the entire Earth,” said Seth Jacobson, an American planetary scientist at the Cote d’Azur Observatory in France and first author of the study.

The findings of the study will be published tomorrow in the journal Nature.

SInce the formation of the Moon was the last time the Earth’s core was formed, siderophile elements present in the Earth’s mantle above the core, the scientists say, would represent material delivered by smaller impacts after the formation of the Moon. The abundance of these elements in the mantle would allow them to estimate the amount of mass accreted by the Earth after the Moon’s formation.

Jacobson and his colleagues simulated on computer the conditions of the infant solar system during which millions of proto-planetary bodies were colliding with each other.

These simulations showed that the age of the Moon may be determined by the amount of mass accreted after the collision. “If we estimate the mass acceted after the collision, we can get the date of the Moon-forming impact,” Jacbson told The Telegraph. “And we can get an estimate for the mass accretion by studying the highly siderophile elements in the mantle.”

Their calculations suggest the Moon formed about 95 million years after the solar system's formation. Scientists also view this as the age of the Earth because it was the last time the Earth gained a significant amount of mass in a collision.

“This is a new simulation-based approach indicating that earlier estimates that the collision occurred about 30 million years after the formation of the solar system needs to be revised,” said Jitendra Goswami, director of the Physical Research Laboratory, Ahmedabad, a planetary scientist who was not associated with the study.

The study used simulations of solar system formation and siderophile elements to determine the date of an event early in the solar system's history. Until now, scientists have relied almost exclusively on geological clocks using radioactive isotopes.