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Your moon base here? Sun-washed crater rim has big vistas, but little water.

A team using observations from a lunar orbiter studied 'the living daylights' out of the Shackleton Crater, near the moon's South Pole. Their findings suggest scant water would be available to supply a lunar base there.

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The possibility was plausible. Like Earth, the moon would have been smacked by comets over the course of its history. Ices falling into the depths of polar craters with perpetually sun-deprived floors would have remained there as ice ever since.

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In June 2009, NASA launched LRO and a companion mission LCROSS to test the notion. Once LRO separated from the upper stage of the rocket that launched the duo, a small navigation and propulsion package guided the upper stage to a collision with the moon inside Cabeus Crater, another, smaller South Pole ding. Scientists analyzing the material the collision kicked up detected water ice in the debris, along with ices of methane and other so-called "volatiles." The amount was about 5 percent by mass ­– comparable to the amount Zuber's team estimates might be present in Shackleton.

Moreover, Japanese and Indian lunar orbiters, as well as LRO, have identified elevated levels of hydrogen all over the moon's surface – generated by the collision of particles in the sun's solar wind with the lunar surface. Modeling suggested that the moon has its own water cycle, in which the hydrogen combines with oxygen during the lunar night, separates again during the long lunar day, and over many of these cycles migrates toward the lunar poles, where the water molecules get trapped in the perpetual cold of polar-crater floors.

In Shackleton's case, with LRO orbiting the moon over its poles, Zuber's team "decided to study the living daylights out of this crater," she said in a prepared statement. The team had amassed some 5 million altimeter measurements over 5,000 orbital tracks to create a topographical map of the crater, as well as measure the reflectivity of its surface and walls.

Reflected light from the laser altimeter showed that Shackleton's interior was significantly brighter than other craters near the South Pole. The simplest explanation is water ice, Zuber says. But a closer look revealed that the walls were brighter than the floor. And portions of the walls receive some indirect light.

If brightness alone indicates water, the bright walls wouldn't square with water's tendency to build up in the coldest, darkest crater bottoms. "That doesn't mean there isn't water ice there," Zuber says. "But there has to be something else going on."

Using counts of craters in the floor, walls, and rim as clocks, the team found that the walls are 2 billion years younger than the floor – strongly suggesting that the walls' brightness was likely due to rocks freshly exposed as overlying layers slide to the crater floor during events such as moonquakes or asteroid hits. The shape of soil deposits at the base of the walls seems to favor this explanation as well – shapes similar to fans of soil seen at the base of mountains on Earth.

The reduced brightness of the crater floor could easily be attributed at least in part to the soil's sheltered position on the crater floor, which would reduce the weathering effects of the solar wind as well as to water ice. Those weathering effects tend to darken the lunar soil.

If water ice is present, the team's estimate is an upper limit, Zuber says – unless there are deposits deeper that any of LRO's instruments are capable of reaching.

An answer to the deeper-deposit question will come from NASA's recently extended GRAIL mission, orbiting the moon to measure its gravity field. This approach could uncover deep-ice deposits, if any are there.

The LRO results appear in Thursday's issue of the journal Nature.

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