A favorite theory is that water, either as water by itself or as its components of hydrogen and oxygen, was deposited on Earth during its early historymostly during a period of "late heavy bombardment" 3.9 billion years agoby the impacts of comets and asteroids. Because the Moon shares the same area of space as Earth, it should have received its share of water as well. However, since it has only a tiny fraction of Earth's gravity, most of the Moon's water supply should have evaporated and drifted off into space long ago. Most, but perhaps not all.
In ancient times, observers commonly thought the Moon had abundant waterin fact, the great lava plains like Mare Imbrium were called maria, or seas. But when Neil Armstrong and Buzz Aldrin landed on the Moon in 1969, they stepped out not into the water of the Sea of Tranquillity, but onto basaltic rock. No one was surprised by thatthe idea of lunar maria had been replaced by lava plains decades earlier.
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