Several years ago, a team led by Kring detected fragments of asteroids in similar moon rocks, so looking for pieces of Earth was a logical next step.
Trace elements in the rock’s minerals, which are a granitelike mix of quartz, feldspar, and zircon crystals, provided clues to its origin. By measuring uranium and its decay products in the zircons, the team dated the formation of the rock, while titanium levels helped reveal the temperature and pressure at the time. Still other trace elements, such as cerium, pointed to the amount of water likely to have been present.
The results, Kring says, indicate that the rock formed in a water-rich environment at temperatures and pressures corresponding to either 19 kilometers beneath the surface of Earth, or about 170 kilometers deep in the moon. Craig O’Neill, a geodynamicist at Macquarie University in Sydney, Australia, favors an Earth origin because a depth of 170 kilometers would be “crazy”—way below the moon’s crust, where granitic rocks could have formed.
The rock isn’t Earth’s oldest relic: Zircon crystals from western Australia have been dated to as far back as 4.4 billion years, only 150 million years after Earth’s formation. But these zircons were stripped from their parent rocks and reworked into new materials. Here, Kring says, there’s no doubt that the rock and its zircons formed at the same time. “We’re sure it’s a complete rock,” he says. The rock is about as old as the oldest rocks found on Earth—metamorphic rocks from Canada and Greenland.
Bell says its preservation is not so surprising because the moon lacks the weather and geologic processes that erase ancient rocks on Earth. In fact, she says, the moon might be a better place to look for ancient Earth rocks than Earth itself. Norm Sleep, a geophysicist at Stanford University in Palo Alto, California, agrees. He says that although meteorites from Earth probably constitute a tiny fraction of the moon’s surface material, eons of subsequent asteroid impacts have churned them throughout the lunar soil, making it easier to find a small piece of Earth in a random sample of moon.
If the rock is truly terrestrial, it holds clues about an ancient time called the Hadean. For starters, it confirms Earth was being hit by asteroids big enough to blast rocks all the way to the moon. It also shows that the granitic rocks that make up Earth’s continents were already forming, Kring says. “That’s a big thing.”
Kring believes other scientists will soon be combing the Apollo moon rocks for bits of early Earth. Only a small fraction of the 382 kilograms of rocks brought back by the moonwalkers have been studied, he says, and analytical techniques are constantly improving. “I think we are going to get a little library of fragments of the early Earth emerging in the next few years,” he says.