Moon and Earth Share Common Water Source: Ancient Meteorites

What does Earth's and the Moon's water have in common? Apparently they came from the same place. New research has revealed that the Moon's water did not come from colliding comets billions of years ago, but was instead present on Earth before it was sent shooting into space.

The Moon is thought to have formed 4.5 billion years ago. At that time, a giant object slammed into Earth very early in our planet's history. Debris from this collision was hurled into space, eventually forming our moon. The impact itself would have probably caused hydrogen and other volatile elements to boil off into space, which means that the Moon must have started off completely dry. Yet this new finding seems to point to the fact that the Moon's water was present all along, giving rise to questions as to how exactly it formed.

"The simplest explanation for what we found is that there was water on the proto-Earth at the time of the giant impact," said Alberto Saal, the study's lead author, in a news release. "Some of that water survived the impact, and that's what we see in the Moon."

Yet how did they discover that the Moon's water came from Earth? The researchers looked at melt inclusions found in samples brought back from the Apollo missions. These melt inclusions are actually tiny dots of volcanic glass trapped within crystals called olivine. The crystals themselves actually prevent water from escaping during an eruption, and allow researchers to understand what the inside of the Moon is like.

In fact, the hydrogen trapped within the inclusions was what allowed the researchers to get a better understanding of where the water came from. The isotopic composition of the hydrogen is a bit like a fingerprint--it has unique characteristics that allow researchers to trace it back to its source.

By examining these compositions, the researchers were able to determine that they matched meteorites that originated in the asteroid belt near Jupiter; these meteorites are thought to be among the oldest objects in the solar system. This, in turn, meant that the source of water on the Moon is primitive meteorites rather than comets.

"The measurements themselves were very difficult," said Hauri. "But the new data provides the best evidence yet that the carbon-bearing chrondrites were a common source for the volatiles in the Earth and Moon, and perhaps the entire inner solar system."

Since as much as 98 percent of the water on Earth comes from primitive meteorites, it's very possible that both the Moon and our planet share a common source. Researchers believe that water was already present on early Earth before it was transferred to the moon after the collision.

The findings are published in the journal Science.