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Scientists have found water dissolved inside tiny, glassy rocks that astronauts on the Apollo missions brought back from the Moon about 40 years ago. Clues to the water content have turned up since the 1980's, yet technology is only now sufficiently advanced to detect such trace amounts.

The water originated from deep in the Moon's once-molten interior. Three billion years ago, fountains of melted rock erupted on the young Moon, much like those on today's active Kilauea volcano in Hawaii. The magma droplets solidified into glass beads as they surfaced on the airless Moon. Water and other volatiles—substances that evaporate at low temperatures—inside the magma actually propelled the eruptions, expanding as bubbles of gas just like a freshly opened bottle of fizzy soda. 

The study team used the properties of this fizzing process to calculate the original water content of the lunar magma before it erupted. The result? The Moon's mantle once contained as much as 745 parts per million of water—similar to estimates of the water concentration of Earth's mantle. The finding reinforces the idea that the Moon formed from a massive collision of the young Earth 4.5 billion years ago. The water, perhaps, came from Earth and was not vaporized in the collision.

"The Moon is not as dry as was once thought," says Denton Ebel, associate curator of astrophysics at AMNH. "In fact, wherever we go in the Solar System, we're finding that things are more volatile-rich than we thought."