Earth Deemed Older, Calling Moon Formation Theory into Question.
A pair of new studies has helped pin down how long it took Earth to form, breaking down the final barrier of disagreement over the precise timing but creating a problem for the leading theory of the Moon's formation.
Earth reached mature size 30 million years after the Sun's birth, the two independent results show. This is in line with the leading theoretical model and most other indicators.
However, this is about 70 million years quicker than what was expected by Moon formation theorists. These researchers' computer models have the satellite being carved from a nearly mature Earth by a large impact about 100 million years after the origin of the solar system.
Earth is thought to have formed within a large, fairly flat and rotating disk of gas and dust that circled the nascent Sun. Dust coalesced to form rocks, which banged into each other. Some stuck and grew into asteroids and larger objects called proto-planets, a few of which survived to become planets.
Nearly all scientists agree on this scenario for building the four inner planets.
Yet the method considered to be best suited to determine how long the process took -- it looks at ratios of radioactive hafnium and tungsten in ancient materials -- has in the past resulted in an answer of 60 million years. The two new studies determined that previous tests were in error.
The results will be published in the Aug. 29 issue of the journal Nature.
One team was led by led by Qingzhu Yin at Harvard University, the other by Thorsten Kleine at the University of Muenster in Germany.
"Earth is older than previously thought," Kleine told SPACE.com. "Our data indicate that these collisions caused almost complete melting of Earth resulting in a scenario called magma ocean, in which the Earth was covered by a layer of magma."
Both groups reanalyzed the ratios of hafnium and tungsten in meteorites thought to be about as old as the solar system itself. They studied meteorites thought to have come from the asteroid Vesta, along with rock from Earth.
Both studies concluded that Earth formed in the first 30 million years of the solar system's existence. Mars, because it is smaller, took about 13 million years to develop, they say. Vesta, meanwhile, seems to have formed within 4 million years, compared to previous estimates of 16 million years.
Kleine said the work suggests that the time it takes a planet's core to develop is correlated to the planet's ultimate size. That might seem intuitive, but it hadn't been shown before.
That doesn't mean, however, that planet formation is simple.
"The situation for the Earth and Mars is complicated because their growth involves absorbing bodies similar to Vesta and then reprocessing the cores within them," says Alastair Cameron of Harvard University and the Lunar and Planetary Laboratory at the University of Arizona.
Cameron was not involved in the studies, but he wrote an analysis of them for Nature.
Cameron said the new dates might call into question the leading theory of the Moon's formation. That theory holds that about 100 million years after the Sun's birth, the Earth was about 90 percent of its full size and was hit by a single Mars-sized object. The impact kicked up material that went into orbit around the planet and gathered together to become the Moon.
More computer simulations of Moon formation might now be required, given the newly suggested time frames, Cameron says. He ought to know. He was on one of two research teams back in the mid-1970s that developed the original idea of the Moon being formed by an impact.