By Andrew Grant
Water trapped deep within the moon’s interior came from the same source as water on Earth, a new study reveals. The research suggests that the moon seized a healthy supply of water from Earth when the satellite formed in the aftermath of a cataclysmic collision 4.5 billion years ago.
“This is an important result and a surprising result,” says David Stevenson, a planetary scientist at Caltech.
The findings come from the laboratory of Brown University geochemist Alberto Saal, who has spent the last five years trying to overturn the conventional wisdom that the moon was born dry. In the new study, published May 9 in Science, Saal and his team analyzed the water in two moon rocks returned by Apollo astronauts in the 1970s. The rocks probably formed from buried magma that was forced to the surface during volcanic eruptions early in the moon’s lifetime. They contain small globules of hardened lava embedded within crystals that prevented the water within from venting into space.
The team analyzed the rocks’ water by measuring the concentrations of hydrogen and deuterium, a form of hydrogen with an extra neutron. The ratio of these two isotopes reflects the origin of water within the solar system. The water on most comets that formed in the outer solar system has a high deuterium-to-hydrogen ratio, while Earth’s water has a lower ratio.
To Saal’s surprise, the deuterium-to-hydrogen ratio of his lunar samples is very similar to that of water on Earth and in meteorites, suggesting that water on Earth and the moon originated from the same meteorite impacts billions of years ago. “The reservoir of water for Earth and the moon is the same,” he says.
Not everyone agrees. Francis Albarede, a geochemist at École Normale Supérieure in Lyon, France, notes that the rocks Saal analyzed are far richer in water and other volatile molecules than the thousands of other rocks returned by the Apollo astronauts. He says that there is no way to prove they are representative of the infant moon’s composition. “They are rogue samples,” Albarede says. “I don’t think they represent the interior of the moon, so I don’t think we can say anything about the moon’s water content.”
If Saal’s interpretation is valid, then it introduces a twist in the already-complicated quest to understand how the moon formed. The leading theory is that a giant object, perhaps the size of Mars, slammed into an infant Earth about 4.5 billion years ago. Simulations suggest that the heat from such an impact would have created an orbiting ring of molten rock around Earth that eventually coalesced into the moon.
The problem is that those extreme temperatures, estimated to be in excess of 5,000° Celsius, should have vaporized any water that existed on Earth and the impacting object, leaving the newly formed moon bone dry. (Earth could have reacquired water later, through meteorite impacts, and held on to it due to the planet’s thick atmosphere.) But Saal’s new results have him convinced that the water fossilized in moon rocks came from Earth and somehow survived the moon-forming impact.
Stevenson agrees that this scenario makes the most sense for now, but he points out that there are a lot of strange measurements that still need to be explained. For example, compared with Earth, the moon has a very small amount of potassium, an element that, like water, should have vaporized after the impact. Why would potassium disappear but water, which is lighter and more volatile, survive unscathed?
“There is no story for the formation of the moon that satisfies everything we know,” Stevenson says. “But that’s fine. That’s what drives science.”
Editor’s Note: This story was corrected on June 3, 2013, to delete an incorrect statement about gas giants’ deuterium-to-hydrogen ratios.
