Of all the things we can say about the Moon, I would never have thought we could call it rusty.
But new results show that it literally is. Planetary scientists investigating observations made using India's Chandrayaan-1 Moon orbiting spacecraft have found evidence of hematite, a form of iron oxide… or, more commonly, rust.
This really surprised me. For one thing, iron binds with free (that is, not bonded to some other element) oxygen to form hematite in the presence of water, of which the Moon is not overly abundant. Also, what's the source of the oxygen?
Erlanger crater, near the Moon's north pole, with just its rim lit by low sunlight. Credit: NASA/GSFC/Arizona State University
But then I saw that the hematite was found mainly at the Moon's poles, which makes sense. At the extreme latitudes near the poles, craters can be deep enough to get very little or even no sunlight; their floors are permanently shadowed. In these very cold regions, any water that happens to fall there — say, from an asteroid or comet impacting the Moon, both of which can have lots of water ice in them — will stay there. It's predicted there may be billions ...