Where on Earth Did the Moon’s Water Come From?

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Water is perhaps the most important molecule in our Solar System. Figuring out where it came from and how it was distributed within and among the planets can help scientists understand how planets formed and evolved. New research from a team including Carnegie’s Erik Hauri demonstrates that water from the interiors of the Earth and Moon has a common origin. Their work is published online in Science Express.

Water on a planet, in this case on Earth (Pippa, Brazil). Image Credit: Rex Rhemrev, my husband 

The Moon is thought to have formed from a disk of debris left when a Mars-sized impactor hit the Earth 4.5 billion years ago, the Giant Impact. Scientists have estimated that the heat from an impact of that size would cause hydrogen and other volatile elements to boil off into space, meaning the Moon must have started off completely dry. But recently, NASA spacecraft and new research on samples from the Apollo missions have shown that the Moon actually has water, both on its surface and beneath. The Giant Impact theory also requires that most of the material that formed the Moon came from the impactor.

Hauri and his teammates—author Alberto Saal and co-author Malcolm Rutherford of Brown University and James Van Orman, of Case Western Reserve University—looked for evidence of this water’s origin by honing in on the element hydrogen, and its heavier isotope deuterium. (Isotopes are atoms of the same element with different numbers of neutrons). The ratio of these isotopes can tell scientists about the water’s origin.

Hauri used the Carnegie NanoSIMS 50L ion probe to determine the deuterium-hydrogen ratio for water trapped in extremely primitive volcanic glass samples from the Moon, which were brought back to Earth by the Apollo 15 and 17 missions. The measurements revealed that the hydrogen isotopes matched those from a type of meteorite called carbonaceous chondrites.

This is a slice from the 4.5-billion-year-old Allende meteorite which is an example of a carbonaceous chondrite meteorite. This rock was formed along with the Solar System. Image Credit: Matteo Chinellato

Because Earth’s water is also matched by carbonaceous chondrites, the two bodies likely have obtained their water from the same source. The evidence suggests that the Earth was already wet at the time of the Moon-forming collision and that the water within the Moon was inherited from the Earth.

“The measurements themselves were very difficult, but the new data provide the best evidence yet that the carbonaceous chondrite meteorites were a common source for the water in the Earth and Moon, and perhaps the entire inner Solar System,” Hauri said.

“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, a geochemist at Brown University and the study’s lead author. “Some of that water survived the impact, and that’s what we see in the Moon.”

Co-author James Van Orman states: “Our work suggests that even highly volatile elements may not be lost completely during a giant impact. We need to go back to the drawing board and discover more about what giant impacts do, and we also need a better handle on volatile inventories in the Moon.”

Source: The Carnegie Institution for Science

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