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New Moon-Formation Theory Also Raises Questions About Early Earth

A new model of the impact that created the moon might upend theories about earth, too

Visualization of the giant impact that formed the moon (William Hartmann)
smithsonian.com

A new theory about how the moon formed might also tweak our understanding of early life on Earth.

The presence of gold and platinum in Earth's mantle has previously been assumed to be the result of a heavy shower of meteors raining down on early Earth, but new research suggests another source—one enormous impact with the object that crashed into the planet to create the moon.

Around 4 billion years ago the Earth was under constant attack, according to geophysicists. Asteroids and meteors continuously smashed into the planet for about 100 million years, a period known as the Late Heavy Bombardment. Any life on the planet at that time would be in constant peril.

We know about these impacts not because of the craters they left—erosion and plate tectonics have long spirited those away—but because of the presence of certain metals in the Earth's mantle. The pockmarked surface of the moon, which is not tectonically active, also helps bolster this theory.

But new research suggests that the bombardment may have been milder than expected, because the metals found in Earth's mantle could instead be from the moon-forming impact, about 500 million years earlier.

Early in the life of the solar system, a growing world known to scientists as Theia collided with the young Earth. The violent impact liquefied Earth's outer layers and pulverized Theia, creating a ring of debris that swirled around the scarred world. Iron from Theia's core drew together to form the heart of the moon. The remaining heavy material rained back down on Earth, and gravity drew the lighter components together to create the moon.

But new research suggests not all of Theia's iron built the lunar core. Instead, some may have settled on Earth's crust, and was later drawn into the mantle through plate tectonics. Elements such as gold and platinum, which are drawn to iron, may have been pulled into the mantle along with it. Such elements are sparse in the lunar mantle, presumably because all of the iron delivered to the moon created its core while Earth's original core remained intact after the collision.

That could mean good news for life on the early Earth. If Theia's core brought in traces of iron that attracted scarcer, iron-loving elements, the rain of asteroids and meteors couldn't have been as heavy as previously estimated.

"The Earth is not going to be completely unhabitable for a long period of time because the bombardment is relatively benign," says Norman Sleep, a geophysicist at Stanford University. Sleep investigated the idea that Theia could have brought platinum and similar elements to Earth's mantle, comparing it with previous suggestions that meteors delivered the material. In a recent paper published in the journal Geochemistry, Geophysics, Geosystems, he found that Theia could have brought in enough iron-loving elements to suggest later bombardment was milder than previously considered.

"It was certainly not anything we would survive, but we're dealing with microbes," he says.

However, without a heavy bombardment of meteorites, a new problem arises. The collision between Theia and the young Earth would have vaporized any water on the planet. The leading theory for how Earth got its water back is via collisions with water-carrying meteorites, but meteorites would also have delivered more iron-loving elements along with iron, leaving behind too much gold and platinum than measured. That means Sleep's calculations would require another method of bringing water to the planet.

That doesn't make the theory a deal-breaker. "There's no guarantee that there's one event that solves every problem," says Tim Swindle, who studies planetary materials at the University of Arizona. Water could have come from another source unrelated to Theia.

Figuring out exactly what happened in the early life of Earth and its moon may require a return to our satellite. "We've got to go back to the moon and get a better handle on the age of the basins," Swindle says, especially those on the back side of the moon. "We might be able to get an age with a rover that could answer the questions, but I think we'd do better to bring the samples back." That doesn’t necessarily mean humans have to be onboard the lunar mission, but, as Swindle points out, people do a great job.

Sleep agrees, calling for a visit to the South Pole Aiken basin, the largest and oldest of those on the moon. That basin has never been sampled, and should provide insight into the timing of the bombardment, which would give clues into how much material rained down on Earth.

According to Edward Young, a planetary scientist at the University of California at Los Angeles, the biggest result of Sleep's research is the mental shift it requires for the scientists studying Earth and the moon. "I think what he's doing is exposing the soft underbelly of what we do," Young says, adding that geochemical arguments are filled with basic assumptions of the processes that go into building the Earth and moon. "He's challenging some of those assumptions."

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