Earth and Moon Are Younger Than Thought

Earth is not exactly getting its youth back, but a new study has determined that the collision from which the Earth and moon were formed may have occurred much later than previously thought, making our planet and moon younger than scientists had commonly believed.

The Earth and the moon were created as a result of a giant collision between two planets the size of Mars and Venus. Until now, it was believed that the collision occurred when the solar system was 30 million years old ? roughly 4.5 billion years ago.

But, according to a new study that was recently published in the scientific journal Earth and Planetary Science Letters, the true age of the Earth and moon can be determined by examining the presence of certain radioactive elements in the Earth's mantle.

The results of the research show that the Earth and moon must have formed much later than previously thought ? perhaps up to 150 million years after the formation of the solar system. That makes our home planet a bit younger than scientists thought.

"We have determined the ages of the Earth and the moon using tungsten isotopes, which can reveal whether the iron cores and their stone surfaces have been mixed together during the collision," said Tais Dahl, who conducted the research as his thesis project in geophysics at the Neils Bohr Institute at the University of Copenhagen, in collaboration with David Stevenson from the California Institute of Technology.

The theory surrounding the formation of the planets in our solar system is largely a tale of cosmic bumper cars. The planets as we see them today were created as the result of a series of collisions between small dwarf planets orbiting the newborn sun. By slamming into each other, the small planets melted together and formed larger planets.

The last of these giant impact events was thought to be the one from which the moon formed, said Dahl.

The planetary bodies that collided in this impact smashed into each other at a time when both had a core of metal (iron) and a surrounding mantle of silicates (rock).

It is believed that the collision took place in less than 24 hours, and since the temperature of the Earth was a scorching 12,600 degrees Fahrenheit (7,000 degrees Celsius), it was thought that both the rock and metal melted and emulsified together in the turbulent impact.

"The big question is: how do we date and determine the age of this event? There has been a lot of discussion about that," Dahl told SPACE.com.

Researchers previously believed that the rock and iron mixed completely during the collision and subsequent planet formation, and it was therefore concluded that the moon was formed when the solar system was 30 million years old.

This new study, however, paints a different picture.

The researchers were able to determine the age of the Earth and moon by examining the presence of certain radioactive elements in the Earth's mantle.

Hafnium-182 is one such radioactive substance, and it decays and is converted into the isotope tungsten-182. Hafnium and tungsten have markedly different chemical properties, and while the tungsten isotopes prefer to bond with metal, hafnium is prone to bonding to silicates, such as rock.

It takes 50-60 million years for all hafnium to decay and be converted into tungsten, and during the moon-forming collision, nearly all the metal sank into the Earth's core. Yet, researchers were unsure if the tungsten had fallen into the core as well.

"We have studied to what degree metal and rock mix together during the planet forming collisions," Dahl said. "Using dynamic model calculations of the turbulent mixing of the liquid rock and iron masses we have found that tungsten isotopes from the Earth's early formation remain in the rocky mantle."

From these models, the researchers claim that the moon-forming collision occurred after all of the hafnium had decayed completely into tungsten. As such, the Earth and moon must have formed much later than scientists had thought.

"Our results show that metal core and rock are unable to emulsify in these collisions between planets that are greater than 10 kilometers (6.2 miles) in diameter and therefore that most of the Earth's iron core (80-99 percent) did not remove tungsten from the rocky material in the mantle during formation."

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