Though they are older than any found on Earth, new research shows that rocks brought back to Earth by the Apollo missions contain water. The amounts are only trace, of course, but these 4.5 billion year old Moon rocks have been carefully assayed for their “wetness,” specifically in the mineral apatite.The findings support recent thinking that the Moon is wetter than previously thought, particularly in the denser minerals beneath the surface. The study went on to take a stab at the origin of this water, however, and came up with something even more interesting: the water seems to have come from Earth.Not Earth as we know it, however. The researchers looked into the hydrogen isotopic signature of the water, in other words what proportion of the hydrogen in these H2O molecules is which of hydrogen’s two stable isotopes, protium and deuterium. What they found was a “remarkable consistency” between the Lunar and terrestrial water, implying that “there is a common origin for water in the Earth-Moon system.” How that could be possible is related to the very origin of the Moon itself, during a time when the Earth had not yet fully formed.Theia vs proto-Earth.The prevailing theory goes that somewhere during the formation of the Solar System, a theoretical, Mars-sized proto-planet dubbed Theia impacted a still-forming Earth and created a large ring of debris. Much of this blew off into nothingness, much fell back down onto the planet, but some of it fell into a semi-stable orbit. How exactly this debris might have come together to form a single, monolithic moon is poorly understood at present, but we do know with some certainty that Luna is made of Earth stuff — thanks to precisely this sort of isotopic testing.It’s not really that surprising that water on the Moon comes from Earth, since we know that the rest of the Moon probably comes from Earth as well. The isotopic signature for oxygen and titanium have both been found to be impossibly close to Earth’s — impossible, that is, if they aren’t in fact from the same source. Each planet has an identifiable signature for these elements, distinct from every other, but our Moon’s are virtually identical to our own.On the one hand, this is further evidence for the Giant Impact Hypothesis of the Moon’ origin. On the other, it provides compelling evidence that water existed on Earth at the time of this great collision, over 4 billion years ago. Even more interesting, the researchers claim their evidence implies that Earth’s water itself was deposited by asteroids sent our way by a temporarily roaming Jupiter.All that from rocks we’ve had for decades.
Ancient water in Moon rocks may actually be from protoEarth