Water On The Moon? Yes, No, and Maybe.

Judging by the headlines, the weather on the moon is as capricious as the weather in Boston. First, it was wet. Then, in August, it was bone dry. This week, it's drenched again.

What's going on here? There's no real weather on the moon, so the true tempest must be in the headlines. Anthony Colaprete, principal investigator on NASA's moon-smashing LCROSS mission, helped me sort through the moon's moist mixed messages.

As Colaprete and his team reported last week in Science, water ice may make up more than 5% of the dust inside a shadowed lunar crater called Cabeus, which they excavated kamikaze-style last year by smacking it with an empty rocket and analyzing the dust and gas that they kicked up. Five percent might not seem like much, but it's twice what you'd find in the Sahara--pretty wet for a place with no atmosphere.

It's also decisively damp compared to most of the rest of the moon's surface, where observations from the orbiting Moon Mineralogy Mapper (M3) showed that water occupies only 0.1% of the dust mass, and then only in a vanishingly thin veneer. As Colaprete tells it, "To extract a teaspoon [of water] from the types of concentrations M3 saw, one would need to dig up about a football field's worth of dirt, whereas in a single ton of Cabeus dirt (between 6-8 wheelbarrow loads) one could extract around 11-13 gallons of water."

Dry enough for you? It's still damp compared to the moon's interior, which is probably far drier. Estimates of water ice in the interior are typically based on chemical analysis of lunar rocks brought back to Earth, though, and Colaprete points out that it takes a few logical leaps to turns these measurements into water fractions. The result: "We really don't know how much water was or now is in the interior."

If you're scouting lunar real estate for your moon base, though, there might be even better spots than Cabeus. "There may be some areas that are wetter," says Colaprete, "For example, areas that show high radar reflection may contain deposits of slab ice 2-3 meters thick."

In other words, pack your skates, but leave the umbrella at home.

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