Until recently, most scientists thought the moon was bone dry – it isn’t. But the way the moon got water may be different from the way Earth got water.

For a long time, experts believe that Earth got its water from comets and asteroids. But recent measurements indicate that Earth water and comet water are chemically different. Comet water has twice as much deuterium as Earth water. Likewise, the water in asteroids has a different ratio of deuterium to hydrogen than the water in earthly rivers, lakes and oceans. So it’s possible that Earth formed with the ingredients to make water four-and-a-half billion years ago.

It’s thought that a large object struck Earth billions of years ago – and created a molten disk of debris around our planet – which later became the moon. The heat of the impact and exposure to the vacuum of space baked water out of this disk.

Today, though, there’s evidence for water ice in permanently shadowed craters at the moon’s poles. So while scientists no longer believe that Earth’s water originated in asteroids and comets, they think this lunar ice just might have.

Scientists date the Earth’s formation to 4.5 billion years ago by measuring isotopes of radioactive elements in Earth’s rocks. They date the moon’s formation to 4.5 billion years ago by measuring isotopes in moon rocks.

In the 1990s, astronomers studying three comets – Hale-Bopp (1997), Hyakutake (1996), and Halley (1986) – first learned that Earth water and comet water are chemically different. The difference is in the ratio of deuterium to hydrogen. Deuterium is a form of hydrogen with a neutron in its atomic nucleus (normal hydrogen has only a proton in its nucleus). Comet water has twice as much deuterium as Earth water. Similarly, asteroid water has a different ratio of deuterium to hydrogen.

There are still many unanswered questions and contending theories about how Earth got its water. Some scientists believe, for example, that the giant impactor that formed the moon should have melted Earth’s surface, destroying our planet’s water. Others maintain that Earth’s water could have combined chemically with the magma and condensed to form oceans after Earth cooled. Others still argue that Earth formed dry. They suggest that a single 500-kilometer-diameter planetary “embryo” containing water with Earth’s deuterium/hydrogen ratio collided with our arid world, delivering all of Earth’s water at one time.

The Clementine (1994) and Lunar Prospector (1998) lunar polar orbiters detected evidence of water ice in permanently shadowed craters at the moon’s poles, though the existence of lunar polar ice was theorized in the 1960s. Scientists believe that water-rich comets and asteroids have smashed into the moon over the past 4.5 billion years. Most of their water vaporized and escaped into space, but a tiny fraction found its way to the poles.

Molecule by molecule, water ice built up in the shadowed craters. Now scientists estimate that the moon has at least 10 billion metric tons of water – about as much as Utah’s Great Salt Lake. A sample of lunar ice would help scientists confirm that it originated in asteroids and comets.

Also, the newest evidence suggests that large portions of the moon may be hydrated with trace amounts of water, which might somehow be resulting from solar radiation. Research is ongoing.