Water source (Image: NASA)

The notion that all Earth’s water was delivered by comets or asteroids has just taken a hit. Chemical analysis of lunar rocks suggests that Earth was born wet, and it held on to its water long enough to donate some to the moon.

The moon is thought to have formed after a massive collision between the infant Earth and another proto-planet around 4.5 billion years ago. Until a few years ago, most astronomers thought that the moon must be bone dry – any water would have been vaporised in the impact.

That changed when Alberto Saal of Brown University in Providence, Rhode Island, and colleagues studied rocks brought back from the Apollo 17 mission in the 1970s. In 2008 they found that crystallised magmas in the rocks do in fact contain water – and not just a little. The minerals hold as much as those in Earth’s upper mantle.


“The question became, where did the water come from?” says Saal. He and his colleagues now think the answer lies close to home.

Water level

Several research teams had already tackled the question by measuring the ratios of hydrogen and its heavier isotope, deuterium, in primitive rocks from Earth and the moon. Objects that formed closer to the sun tend to have less deuterium than objects that formed further out, because the two isotopes condense at different rates depending on temperature.

“It becomes a fingerprint for where the water condensed,” says Saal. “If I can measure this deuterium-to-hydrogen ratio on Earth, I can say where the water came from.”

Earlier work had suggested that the moon’s water arrived on incoming comets. However, the balance of hydrogen and deuterium may have shifted in the minerals studied: they could have lost some hydrogen while sitting on the moon’s surface for billions of years before the astronauts picked them up in the 1970s.

To get a more reliable measure, Saal and colleagues measured the ratio in volcanic glasses that were trapped inside the Apollo 17 rocks and so protected from surface weathering. They found that the deuterium ratios were almost identical to those found in Earthly rocks, suggesting the two worlds got their water from the same source.

Impact rate

Water could have been delivered by primitive asteroids called carbonaceous chondrites, which also have the same deuterium ratio as Earth rocks. However, that water could have become mixed into lunar rock only if the asteroids hit while the moon was still molten after the impact that birthed it. Based on known impact rates, there wouldn’t have been time for enough asteroids to hit the infant moon before it cooled and solidified: current models of the moon’s formation suggest that debris from that original impact would have congealed after only about a month.

“[The water] had to come from the only thing that we know had water: the Earth,” says Saal.

The explanation fits with the most recent moon-formation models, says Erik Asphaug of Arizona State University in Tempe, who was not involved in the new study. It no longer looks likely that all the water in the material that formed the moon evaporated instantly in the giant initial impact. Instead, it now seems more probable that water migrated over a period of centuries out of the cloud of debris that coalesced into the moon.

“That material is going to evolve, but it’s not going to lead to wholesale loss of the moon’s water,” says Asphaug. “So [the new work by Saal and his team] is highly consistent with the notion that the moon retained a lot of the water that was there to begin with.”

Migrating Earth

So if the moon’s water came from Earth, where did Earth’s water come from? The impact that formed the moon happened within about 100 million years after solid bodies began to form in the solar system – an eye-blink in astronomical timescales. Saal doubts that Earth could have accumulated enough water from asteroid strikes in that time.

“The implication, although I cannot absolutely prove it, is that probably the Earth formed with water,” he says. The trouble is that our planet is currently too close to the sun for it to have retained water as it coalesced from the swirling disc of material that was to become the solar system.

Saal thinks that Earth may have formed near where the asteroid belt is now, which is far enough from the sun for water to condense. The planet would then have migrated inward. It’ll be a tough theory to prove, because Earth’s geologic activity has been recycling rocks, and thus erasing the evidence, for billions of years. That’s what makes moon rocks so valuable.

“All that we know now is because we have a fossilised record of what happened 4.5 billion years ago on the moon,” says Saal. “We couldn’t get that conclusion from looking at the Earth.”

Journal reference: Science, DOI: 10.1126/science.1235142