Marvelous moons

Lunar Prospector measured neutrons coming from the small ovals at the moon's poles. These regions can't be seen from Earth.

Inside Prospector's "footprint," ice was found only in areas shaded by the crater rim.
Courtesy NASA.

  Wherezit from?
If it's true that the moon's surface has water, we can rule out a number of sources for it. It didn't come from a toddler's "unspillable" cup (see, it's not just craters that distinguish the lunar surface from the kitchen floors of certain Whyfilers).
there's ice in there
Seriously, the water can't be a relic of the moon's distant past, since the satellite probably was formed when a hot chunk of Earth was blasted into space several billion years back, and that junk was just too hot to hold water. Furthermore, the moon shifted its position about 2 billion years ago, exposing the poles to sunlight that would have boiled water away. "Any water has to be after that fact," concludes William Feldman of Los Alamos National Laboratory, who designed the instrument that discovered the water. (Want to read the top 10 lunar discoveries from the Apollo missions?)
fi, fi, fo, fum
So... the best guess is comets. These interplanetary wanderers are major carriers of water. In the 2 billion years since its orbit stabilized, comets could have delivered 10 billion to 100 billion tons of water to the moon, according to an estimate by James Arnold of the University of California at San Diego.

How much water in the lunar canteen?
Most of that water fell in areas that were sunny, and it boiled away into space. Feldman calculates that only about 1 percent of that quantity is still present on the moon -- meaning 300 million to 500 million metric tons. The larger figure translates to a lake 10 kilometers on a side and 5 meters deep.

But the water is spread out all over the place. Even in the shaded parts of the craters, Feldman says the maximum concentration of water is 1 percent of the soil by weight.

So is the water a resource -- or just a curiosity?

As anybody who's traveled the desert knows, water is handy stuff. You can use it to rejuvenate revolting dehydrated rations. Shoot water pistols. Even slake your thirst.

Scientists have not missed this potential. "The presence of water repositories would be a fantastic resource" for future lunar explorers, says James Garvin of NASA Goddard Space Flight Center.

Simply by decomposing the water into hydrogen and oxygen, it could be made into rocket fuel (this highly combustible mixture already powers rockets). The idea would be to send rockets to the moon for refueling before they left for elsewhere in the solar system. Here's information on how to use that water.

Clearly, water is heavy stuff, and it would be better to find it in space than haul it from Earth. But is the moon water actually usable? Since the most obvious place for a lunar base is at the equator, which is bathed in warm sunshine for half the time, The Why Files got to wondering: Is this small amount of ice at the remote poles really worth worrying about??

Not according to Bruce Murray of the California Institute of Technology. Murray told The New York Times, "Water at 1 percent by volume is not a resource," (see "Craft Sees Signs... " in the bibliography). "For something to be a resource, it has to be, by definition, economically viable, and this would not be."

Think about it. The miners might be able to skip that pesky environmental-impact statement. But they would have to work in -280 Fahrenheit conditions and transport the water to the moon base -- both daunting requirements.

It turns out that water is key to another planetary debate. Does Jupiter's moon, Europa, harbor a hidden ocean? Is Europa's surf breaking left today?

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