Metal on the Moon
Sounds like the opening scene of an episode of Star Gate (he said, revealing probably more than he needed to). But yes, that’s exactly what NASA has discovered.
NASA cleverly combined two separate sources of information to figure this all out. The first source was an experiment called GRAIL. In this, two spacecraft (cutely called EBB and FLOW) measured gravitational fields and how they differed over the various parts of the moon. The second source was a pile of new, high-accuracy maps of the lunar surface, including heights. This was gathered by the Lunar Reconnaissance Orbiter.
Combining these sources, NASA found that the South Pole-Aitken Basin, which is a depression on the far side of the Moon, down towards the south pole, had higher gravity than other areas. This indicated something very large and very dense underneath it. So large and dense, in fact, that it’s pulling the entire South Pole-Aitken Basin in towards the centre of the Moon.
You know what, this isn’t Star Gate – it’s 2001!
The boffins at NASA went scurrying to their computers and ran some simulations in order to guess what the dense object might be. Their best guess is that it’s a large lump of metal, probably nickel and iron.
How large? They’ve said it’s about 95 kilometres across.
Somewhat helpfully, they then said this was larger than the Big Island in Hawaii, which would be useful if we had any concept of how big that was. How many MCGs are we talking?
So now the question is how did it get there? One theory (there are others, of course, because more than one scientist was asked) is that it was a metallic asteroid that collided with the Moon back in the day when the Moon was less solid than now. It can’t have been too soft though, or the metal would have sunk to the centre.
Perhaps the Chinese lunar rover that's currently in the area could make some inquiries...?