Is the Moon Wet or Completely Dry?

We're in the midst of the 40th anniversaries of the six Apollo landings; the last, Apollo 17's, comes in late 2012. It's hard to imagine that we might still be making fundamental discoveries about the Moon from the study of the dusty rocks hauled back to Earth all those years ago, but consider the following.

In the past weeks, two new analyses of Apollo samples have appeared in competing but equally prestigious journals. The articles themselves were competing too: one claims that at least some of the lunar interior contains as much water as do the lavas now oozing out along Earth's mid-ocean ridges, while the other asserts that on the whole the Moon must be very, very, very dry. It's too early to know who's right — perhaps both are — but here's why the eventual correct answer has major implications for solar-system history and the Moon's formation in particular.

Most planetary scientists now accept that a Mars-size object whacked the just-formed Earth so hard that much of our young planet's mantle, and most of the impactor, spurted into orbit as white-hot rock vapor that eventually condensed and collected into that big, beautiful, solitary satellite of ours. The Moon ended up both very much different from Earth (it's nearly iron-free, for example) yet very much like it (the two bodies' ratios of oxygen and chromium isotopes are identical).…

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