People who write about water like to say that the liquid in our drinking glasses and toilet bowls has been with us since the age of dinosaurs, continually recycled through plants, animals (including us) and the atmosphere. But really, some of the water on (and in) Earth was here billions of years before T. rex terrorized Triceratops. And now, research released today shows that, nearly 4.5 billion years ago, Earth shared some of that water with the Moon.
It was around that time that a Mars-size object collided with Earth, or so the Giant Impact theory goes. Debris released from the impact object became locked in orbit around the planet, eventually coming together to form our Moon.
The theory also used to go that the Moon was originally dry, any water present during the collision wouldn't have survived impact. Water found on the moon would have had to come from subsequent meteorite and comet strikes. But a study published today from the Carnegie Institution for Science in D.C., Brown University and Case Western Reserve University provides evidence otherwise — that any water found on the Moon could have, and probably did, come from our planet and has been there all along.
In an effort to track the source of the Moon's water — first found in 2008 trapped in volcanic glass brought back from NASA’s Apollo 15 and 17 missions — researchers identified the unique isotopic, or chemical, signature of the water’s hydrogen. Specifically, they were looking for the amount of deuterium, a hydrogen isotope with an extra neutron, in the water molecules. Identifying the amount of deuterium helped them to pinpoint where in the universe the water originated. Water molecules from different parts of the solar system have different amounts of deuterium. It turns out that the chemical signature of the Moon’s water matched that of Earth’s water.
And the unique isotopic composition of water on Earth matches that of a certain type of meteorite coming from the asteroid belt near Jupiter. In fact, up to 98 percent of the water on Earth is suspected to come from past collisions with these so-called carbonaceous chondrite meteorites, which formed as the solar system first began to emerge from a mere cloud of gas and dust.
The isotopic links among all three — the meteorites, Earth and the Moon — indicate that the Moon inherited some of Earth’s water, and was formed from pieces of Earth and not another impact source. It also means that the Moon never had a dry phase, as previously thought. Its water has likely been there all along.
“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,” says Erik Hauri, a geochemist who performed the study's measurements at Carnegie’s Department of Terrestrial Magnetism.
Now they just need to figure out how water from Earth survived the Giant Impact. "Our work suggest that even highly volatile elements may not be lost completely during a giant impact," says study co-author James Van Orman at Case Western Reserve. "But we don't know what that process was," adds co-author Alberto Saal at Brown.
Their research appears in Science Express.