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Water in Ice on the Moon

Water ice probably exists at both the Moon's north and south poles, according to data sent back to Earth by NASA's Lunar Prospector spacecraft.
The spacecraft is over the Moon in the NASA artist rendering above.
The robot probe's neutron spectrometer spotted the lunar water ice, allowing scientists on Earth to estimate its volume and location. The Moon's water ice is not concentrated in polar ice sheets. Rather, it seems to be in very low amounts distributed across a significant number of craters in the polar regions. The water probably is only 0.3 percent to 1 percent of the Moon's rocky soil.

How Much Lunar Water Ice Is There? Assuming a water ice depth of about a foot and a half — the depth to which the neutron spectrometer's signal was able to penetrate — there may be as much as 11 million to 330 million tons of lunar water ice dispersed over 3,600 to 18,000 square miles around the north pole, and 1,800 to 7,200 square miles around the south pole. Twice as much water ice was detected in the north as in the south.

Scientists assume most water ice on the Moon must be a result of meteors and comets striking the lunar surface. The amount of soil that could have been "gardened" by all meteor and comet impacts over the last 2 billion or so years extends down to a depth of about 6.5 feet. Researchers caution that their estimates of the amount of water could be off by a factor of ten in either direction.

An earlier Defense Department-NASA mission to the Moon known as Clementine used a radar-based technique to detect ice deposits in permanently shadowed regions of the lunar south pole. Unfortunately, those results can't be compared directly with the Lunar Prospector results because of the different sensors, measurement "footprints," and analysis techniques. The Clementine science team concluded that its radar signal detected from 110 million to 1.1 billion tons of water ice, over 5,500 square miles of south pole terrain.

How Valuable Is This Water? NASA is calculating the economic value of the lunar water as a resource for human exploration of the Moon. For instance, the space agency has estimated the cost of transporting the same volume of water from Earth to orbit. Today, it costs about $10,000 to put one pound of material into orbit, but NASA is working to reduce that to only $1,000 per pound. It would cost $60 trillion to transport 33 million tons of water to space, plus an unknown additional cost to transport it to the Moon's surface.

From another perspective, a typical person on Earth consumes an estimated 100 gallons of water per day for drinking, food preparation, bathing and washing. At that rate, 33 million tons of water — which is 7.2 billion gallons of water — could support a community of 1,000 two-person households for well over a century on the lunar surface, without recycling. Of course, a cheap way to mine the water crystals from the lunar soil would have to be developed for the water to become a useful resource for drinking or rocket fuel to support human explorers.

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