Planetary Beginnings: Data reveal Earth’s quick gestation

Earth’s core formed in a hurry–during the first 30 million years after the birth of the solar system. That’s the consensus of two independent research teams that used the same radioactive-dating technique to estimate the age of Earth and some of its neighbors. A previous analysis of similar data suggested the core took 60 million years to take shape, but the new estimates are in accord with several other lines of evidence, as well as the widely accepted theoretical models of planetary formation. Both teams report their findings in the Aug. 29 Nature.

“Clearly, solid bodies were forming during the first few million years [of the solar system], as theorists have been saying for some time now,” comments Alan P. Boss of the Carnegie Institution of Washington (D.C.). “The previous work . . . was widely accepted and cited, so the new ages implied by these two papers should shake things up a bit.”

According to the current model of the sun’s formation, the densest part of an interstellar gas cloud collapsed to form the star. Dust grains that accumulated around the sun then collided and stuck together, eventually building planet cores and ultimately full-scale planets.

To determine the formation time of Earth’s core, the two teams of researchers reexamined a standard radioactive-dating technique that uses the decay of the isotope hafnium-182 into its stable daughter product, tungsten-182.

Hafnium and tungsten have distinctive locations in early Earth, says Thorsten Kleine of the University of Munster in Germany. Undecayed hafnium in Earth’s mantle, the region that accumulated around the growing core, would have remained locked in minerals there. In contrast, tungsten produced in the mantle would have sunk into the molten core during the time that the core was forming.

Any tungsten-182 now found in the mantle must have been produced by the decay of hafnium-182 after the core had finished forming and no longer interacted with the mantle. Therefore, the abundance of tungsten in rocks from Earth’s mantle provides a measure of the core’s age.

Kleine and his colleagues, as well as a team led by Qingzhu Yin of Harvard University, measured the concentration of tungsten-182 in a variety of Earth rocks originally from the mantle. To date the rocks, the teams had to compare these numbers with the amount of tungsten-182 found in objects in the solar system that had never formed a core. Meteorites, the fragments of asteroids that have fallen to Earth, fit the bill. The measurements reveal that Earth’s core–the bulk of the planet–formed in 30 million years. Using the abundance of tungsten-182 in meteorites as a benchmark, the researchers say that Mars’ core took only 13 million years to form. The asteroid called Vesta formed in only 3 to 4 million years, the scientists report.

Kleine suggests that in previous studies, researchers came up with formation times that were twice as long because they had less accurate determinations of the amount of tungsten-182 in meteorites.

The newly reported ages show that the bigger the body in the inner solar system, the longer it took to form. Although not surprising, this trend hadn’t been shown before, Kleine says.

Earth’s moon, however, doesn’t fit this pattern. The new data confirm that the moon is about as old as Earth. That’s consistent with the theory that the moon formed not from the accretion of smaller bodies but by the collision of a planet with the newly formed Earth.