If you want to set up a base on the moon, and if plenty of local water-ice is a must, you may want to scratch Shackleton Crater from the list of possible locations.
As a site for a lunar base, the rim of Shackleton has a lot going for it. Positioned at the moon's South Pole and just off center of the moon's slightly tilted axis of rotation, the crater's rim receives sunlight for virtually an entire lunar "day," 27.32 Earth days.
That's important for minimizing exposure to the frigid temperatures of the moon's nearly 14-day "night" at lower latitudes – think 243 degrees below zero Fahrenheit. The sunlight also provides a near-continuous source of energy to power a moon base. Indeed, some researchers have proposed building a large infrared telescope – best served chilled – on the crater's shadowed floor and powering it with solar arrays on the rim.
For its part, water is desirable not only for human survival, but also as a source of hydrogen and oxygen for rocket fuel. Observations over the past 15 years, however, have proved inconclusive regarding the presence of water ice at Shackleton.
The latest LRO data indicate "that water is not there ... in a way that would facilitate human exploration," says planetary scientist Maria Zuber, who led the team analyzing the data.
If the signatures the team saw in the soils on the crater floor do indicate water, how much water might there be? Roughly 100 gallons – enough to fill two or three residential rain barrels – spread over a surface of about 133 square miles. Leave the swim-suit at home.
"This is not like Mars," says Dr. Zuber, a professor at the Massachusetts Institute of Technology in Cambridge, in an interview. On the red planet, explorers would find thick layers of icy soil in many locations just by turning over a shovelful or two of topsoil.
Using the lunar orbiter's laser altimeter, which measures the intensity of laser light reflected from the surface as well as the topography of the surface, Zuber’s team found that what some had interpreted as evidence for possible water-ice deposits is far more likely to signal mere rock and soil.
And while water ice remains a possible explanation for evidence seen in the soil on the crater floor, at best it would make up only about 22 percent of the top few hundred-thousandths of an inch of the soil. Data from the LRO's radar yielded no evidence of thick near-surface ice layers, which the device is capable of detecting.
Useful quantities of water or no, Shackleton is a remarkable feature in its own right. Estimated at about 3.6 billion to 3.7 billion years old, the ding in the moon's crust is 13 miles in diameter and 2.6 miles deep. It sits in a broader depression known as the South Pole-Aitken Basin – a region of keen interest to planetary scientists working to understand how such a feature could form and what its effect is on the structure of the moon's interior.
Hints that Shackleton, as well as other polar craters, might harbor water ice first emerged in 1998, when an instrument aboard NASA's Lunar Prospector mission detected signatures from the surface indicating higher-than-expected levels of hydrogen in polar regions, including in Shackleton Crater.
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.
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.