From the outside, the moon seems to be as dry as a bone. But in addition to frozen water molecules, scientists have found that our satellite hides tiny bits of liquid water inside its volcanic rocks — enough to fill 4 billion Olympic-size swimming pools, by some estimates. A new study, published Tuesday in Nature Communications, suggests that asteroids may have delivered this water to the moon back when it was a wobbly ball of magma.
Comets — bodies made mostly of ice — were once tentatively credited with delivering the first water to Earth. But the Rosetta orbiter's analysis of the comet 67P/Churyumov-Gerasimenko revealed water of the wrong molecular composition, leaving asteroids — bodies that orbit closer to the heat of the sun, leaving them mostly composed of rock and metal — as the most likely water bearers.
Now scientists say that the molecular makeup of the moon's water suggests an asteroidal origin as well.
“We believe that asteroids delivered the majority of water to the moon and comets delivered very little — they weren’t major players in the first few hundred million years of inner solar system history,” co-author Jessica Barnes, a planetary scientist at the Open University, told the Guardian.
To puzzle out water's origins, scientists look for deuterium, a modified form of hydrogen that forms what we call "heavy water." The ratio of hydrogen to deuterium serves as a kind of barcode, showing scientists what kind of object the water probably came from. The water on comets, which orbit out in the far reaches of the solar system, tends to contain a high ratio of deuterium. But closer to the sun — where asteroids orbit — hydrogen takes center stage.
Based on their analysis, the researchers believe that the moon's water arrived during a window about 4.5 billion to 4.3 billion years ago, when asteroids and comets — but mostly just asteroids — came crashing down into the roiling magma on its young surface. The moon was formed when a Mars-size body called Theia crashed into Earth just 4.5 billion years ago, so our satellite would have been hot and bubbling during that period. Rocks that collided with the magma ocean could have been trapped as the moon cooled down, preventing any water molecules from escaping. The study suggests that just 20 percent of these watery impacts involved comets, with the other 80 percent implicating primitive, water-rich asteroids not unlike certain meteorites that reach Earth today.
The moon and the Earth were even closer to each other in those days – and the molecular composition of our water is pretty similar – so an asteroidal origin for the moon's water is pretty good evidence of the same on Earth.
"At some level that makes perfect sense," co-author David Kring, senior scientist at the Center for Lunar Science and Exploration at the Lunar and Planetary Institute in Houston, told the Christian Science Monitor. "The asteroid belt is closer to the Earth and the moon than comets, so it's a more proximal source of material."
But why don't scientists assume that the moon just got some of the Earth's water in the divorce? According to Barnes and her colleagues, our water signatures are actually too similar for that — it's unlikely that the water could maintain its molecular makeup in the chaotic aftermath of Theia's impact and the moon's formation. Their similarity would be more easily explained by later events that peppered them both with similar water bearers.
This isn't the last word on the moon's water. Experts not involved in the study have suggested that the water accumulation may be a result of much more recent meteorite impacts — ones that occurred long after the moon cooled down and solidified. Jeremy Boyce, a geochemist at the University of California at Los Angeles, told the BBC that the water might be trapped in volcanic rock not because it has been there for 4.5 billion years, but because traces of it from the surface got caught up in magma during more recent volcanic eruptions. Others argue that we don't know enough about what kind of water is "normal" on a comet to keep ruling them out, and it's true that our sample size is pretty limited. But a 2015 study using computer simulations also concluded that asteroids moistened the moon — because based on the researchers' models, water from these hunks of metal would be more likely to last than that from icy comets.
It is hoped that the new findings will make enough of a splash to inspire further research.