Credit: NASA

With the regular dispatch of new probes and spacecraft to exotic far-away planets and moons, Earth's own Moon isn't the locus of our fascination that it once was. Yet, as a recent report of substantial water in the Moon's mantle suggests, Earth's nearest neighbour hasn't revealed all its secrets. It is, after all, a body that perpetually hides one of its halves from our view.

Because of the way the Moon is tilted on its axis, its south polar region receives sunlight at a low angle, which makes it difficult to obtain information about this heavily cratered terrain. However, data returned by the recent SMART-1 mission as well as radar data from Earth-based antennae offer a fresh perspective. Paul Spudis from the Lunar and Planetary Institute, Houston, and colleagues used these data to re-evaluate the geology and age of the Shackleton Crater (Geophys. Res. Lett, 35, L14201; 2008).

The 20-km-wide crater straddles the lunar south pole, and as a result it has received a fair bit of attention. Because is almost always in the shadows, it is also likely to be much cooler than other regions on the Moon's surface. It may therefore act as a potential trap for volatiles, such as water carried by meteorites. But a key to the crater's potential for gathering water is its age — a relatively young crater would not be expected to trap a significant quantity of volatiles.

Previous work on the morphological attributes of Shackleton Crater suggested that it formed ∼1 billion years ago and is thus a young feature (in the eyes of planetary scientists). Using their high-resolution data, Spudis and colleagues tested this assessment by making a fresh estimate of the crater's age: they looked at the density of secondary craters that had been dug into material ejected when Shackleton Crater formed. The higher the number of these craters, the older the Shackleton Crater must be.

Comparing the density of secondary craters around Shackleton Crater with that on the Moon's surface where the absolute ages are known, they came up with a crater formation date a little more than 3.5 billion years ago, much older than previously thought. The relatively fresh appearance of Shackleton Crater may have more to do with the low angle at which sunlight illuminates it rather than its age.

Shackleton Crater appears to have been around for long enough to accumulate volatiles, which might prove to be an asset if long-term human settlement on the Moon is ever planned.