

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. 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 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."
This work was funded by NASA Cosmochemistry and LASER programs, and by the Brown/MIT NASA Lunar Science Institute.