Moon-Forming Impact Dated
April 3, 2014

New Estimates Suggest The Moon Is 60 Million Years Younger

redOrbit Staff & Wire Reports - Your Universe Online

Using a new method for estimating the moon’s age, an international team of planetary scientists has determined that the satellite formed nearly 100 million years after the formation of the solar system, according to research published Thursday in the journal Nature.

The study authors came to that conclusion based on measurements from the Earth’s interior, as well as computer simulations of the protoplanetary disk from which the our planet and other Earth-like worlds formed. The group of French, German and American researchers simulated the growth of Mercury, Venus, Earth and Mars from a disk of thousands of planetary building blocks in orbit around the sun.

By analyzing the growth history of those terrestrial worlds by using 259 different simulations, the scientists were able to uncover a relationship between the time that a Mars-sized object collided with the Earth to create the moon and the amount of material that was added to our planet in the wake of that impact.

Those computer models, combined with details on the mass of material added to the Earth by accretion following the moon’s formation, allowed them to uncover a relationship that functions much like a clock. This relationship allowed them to date the moon-forming event, making it the first so-called “geologic clock” in early solar system history that is not reliant upon measurements and the decay of radioactive elements to determine how old an object is.

According to Jane J. Lee of National Geographic News, the study authors discovered that the moon is roughly 4.47 billion years old and was formed approximately 95 million years after the solar system. The revised figure means that the satellite is about 60 million years younger than experts had previously estimated, she added.

“The new study uses a clever way of providing an estimate, based on the fact that the last major collision in the Earth's history didn't mean that the Earth escaped further bombardment from space,” explained Ars Technica’s John Timmer. “The logic is pretty clever. In the wake of the Moon-forming collision, the entire Earth was molten, which allowed the iron to sink to the core. A number of heavy elements that have an affinity for iron sunk to the core with it.”

As a result, the Earth’s surface would have been completely devoid of these metals, which include gold and platinum and are also known as siderophiles, Timmer continued. However, it is possible to mine these elements from the planet’s crust. Their presence indicates that the world’s supply of siderophiles was partially restocked by the arrival of new materials via collisions with other planetoids and smaller asteroids.

Based on the amount of those elements in the crust, as well as the composition of asteroids in our solar system, scientists can come up with an estimate of how much mass had to have collided with our planet following the moon-forming impact, he noted. Since the amount of space objects capable of delivering materials to Earth dwindled over time, the researchers used models of solar system formation in order to determine the moon formation date.

“Published literature provided the estimate for the mass accreted by Earth after the Moon-forming impact,” officials from the Southwest Research Institute (SWRI), one of the institutions involved in the study, explained in a statement. “Other scientists previously demonstrated that the abundance in the Earth's mantle of highly siderophile elements, which are atomic elements that prefer to be chemically associated with iron, is directly proportional to the mass accreted by the Earth after the Moon-forming impact.”

“From these geochemical measurements, the newly established clock dates the Moon to 95 ±32 million years after the beginning of the solar system,” they added. “This estimate for the Moon-formation agrees with some interpretations of radioactive dating measurements, but not others. Because the new dating method is an independent and direct measurement of the age of the Moon, it helps to guide which radioactive dating measurements are the most useful for this longstanding problem.”