Polar endeavours

Almost exactly 85 years ago, Sir Ernest Shackleton set out to cross the Antarctic continent. After his ship was frozen into the ice pack, the expedition waited and worked through the long polar nights, then the long polar days, before Shackleton managed to get them all rescued, a story that continues to generate popular books¹. In honour of his exploits, Shackleton had a lunar crater (just to the right of centre in this image of the lunar south pole) named after him. That crater is now getting attention as a part of another story that involves ice, long days and nights — and perhaps, ultimately, human exploration.

Lunar scientists have long known that because the Moon's axis of rotation is almost perpendicular to its path around the Sun, the polar regions could have high points that are permanently sunlit, and crater floors that are permanently in shadow. This false-colour image (inside the yellow line, and overlaid on a radar image of the region) shows the percentage of the lunar day for which a given location is illuminated (white, orange and red receive the most illumination, the clear regions receive none), based on a newly reported analysis of images taken in 1994 by the Clementine spacecraft². A separate radar study³ has confirmed that many regions, including the bottom of Shackleton Crater, are likely to be permanently shadowed. Conversely, some regions are sunlit most of the time and would be prime locations for lunar bases, or at least for solar arrays to support a polar base. For example, the white region at the left (poleward) edge of Shackleton Crater is sunlit more than 80% of the time, and there is a ridge 10 km away that is sunlit 90% of the time that the first spot is not.

A polar base would be more attractive if the permanently shadowed crater floors have managed to cold-trap water ice, which could be mined for life support or fuel. Data from the neutron spectrometer aboard the Lunar Prospector spacecraft⁴ apparently showed abundant hydrogen at the poles, possibly, though not definitely, in the form of water ice; similarly, a Clementine radar experiment may⁵ or may not⁶ have provided evidence for ice.

At the end of July, with its funding expired and its fuel nearly gone, Lunar Prospector will be crashed into a crater near the pole (the larger crater above and to the right of Shackleton) and Earth-bound telescopes will search for evidence of water released by the impact⁷. The chances of the experiment detecting water are no better than 10%, but this is just the next step in a search for the conditions that might make exploration of the lunar poles possible.