It is still unclear whether the hydrogen represents the release of water by the heat of the impact, or whether it simply sheds light on the way that water is formed on the Moon from its chemical constituents.

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That is, the moon pulls hardest on the Earth's oceans on the side facing the moon, making a bulge of water (high tide).

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Crops need a sheltered environment, protected from the extreme temperatures and frightening levels of radiation found on the moon's surface, with water, carbon dioxide and light.

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The idea that there might be water or, rather, ice on the moon goes back to 1961.

ECONOMIST: Creative destruction

But what's novel about the CEAC design is that it incorporates a bio-regenerative life-support system -- the kind of technology that could one day not only feed an outpost on the moon or Mars, but also provide it with oxygen and recycle its water.

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Tide: periodic rising and falling of large bodies of water resulting from the gravitational attraction of the moon and sun acting on the rotating earth.

UNESCO: L O S S A R Y

And water would be all the more exciting on Ganymede because it is the largest moon in the solar system (larger even than the planets Mercury and Pluto) and, like earth, it has a permanent magnetic field that protects part of its surface from bombardment by certain sorts of life-destroying radiation.

ECONOMIST: Blue moon