The proportion of deuterium ("heavy hydrogen") in water trapped inside lunar rocks suggests it came from the same primitve-asteroid feedstock that supplied most of Earth's water.
The title of this post sounds like the beginning of a joke you might hear on The Big Bang Theory, but instead it's a tipoff that the geochemical evidence linking Earth and Moon is more robust than ever.
The new insight comes from studies of primitive volcanic rocks returned from the Moon by the crews of Apollos 15 and 17. These contain tiny bits of glass trapped within olivine crystals, and those glassy bits (called melt inclusions) have a higher concentration of water — up to 1,200 parts per million (0.12%) — than any known lunar sample.
They found the proportion of deuterium ("heavy" hydrogen that includes a neutron) in water trapped inside lunar rocks is close match to the deuterium-to-hydrogen (D:H) ratio in primitive meteorites called carbonaceous chondrites. Almost all of Earth's water must likewise have come from carbonaceous chondrites and other water-rich asteroids.
Decades ago, astronomers thought comets were the source of Earth's water, but in most cases their D:H ratio is too high, so comets contributed at most just a small fraction of what fills our oceans. While certain "Jupiter-family" (short-period) comets have Earth-like (and, now, Moon-like) D:H ratios, they can't have been major players because their nitrogen-isotope ratios are quite different.
The new finding means that lunar water likewise came from carbonaceous chondrites rather than comets. And since the Moon apparently formed from superheated matter splashed out during an enormous impact on Earth, the thinking now is that whatever water ended up inside the Moon must have come from Earth. In other words, our young planet was already wet (or at least very damp) when it got clobbered.
"The simplest explanation for what we found is that there was water on the proto-Earth at the time of the giant impact," Saal explains in a Brown press release. "Some of that water survived the impact, and that’s what we see in the Moon."
These weren't easy measurements to make, notes co-author Erik Hauri (Carnegie Institution of Washington). That's because, once the lava flows reached the lunar surface, it started losing water molecules due to slow leaks from the rock itself and from molecular-scale damage caused by cosmic rays. These effects depleted hydrogen more readily than deuterium, artificially driving up the D:H ratio. So the researchers concentrated on water trapped in the melt inclusions, because it was protected from loss.
In candid moments, planet modelers will tell you that they need to figure out how the Moon managed to hang onto any water at all. But, that said, the realization that lunar water once resided on Earth strengthens the genetic link between the two worlds. In fact, evidence like this is forcing wholly new thinking about the type, mass, and velocity of the object that hit the young Earth. Specifically, it's becoming ever clearer that that the Moon assembled itself mostly at Earth's expense, not the impactor's.