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.