There’s a huge radioactive slab of volcanic granite buried on the moon

The far side of the moon is hiding evidence of an enormous ancient volcano. But even though researchers are sure the volcano was there, they remain mystified about how it could have formed.

For more than 20 years, we have known that an area on the far side of the moon called Compton–Belkovich was a bit strange. It had some odd topography, and the upper metre of soil seemed to have more thorium than its surroundings.

Now, Matt Siegler at the Planetary Science Institute in Arizona and his colleagues have used data from China’s Chang’e 1 and Chang’e 2 orbiters to determine that there is an area 50 kilometres across and several kilometres thick that is unexpectedly hot. The only way to produce all this heat on the moon is through the decay of radioactive elements such as thorium and uranium, and the best way to form such a concentration of those elements is through repeated melting of the rock via volcanism.

“That little bit of thorium we saw at the surface is the tip of the iceberg of a huge body below the surface that was the plumbing system for this volcano,” says Siegler. “It pushes the boundaries of what we know about how volcanoes form and specifically how they form on the moon.”

The topography of the area suggests the volcano last erupted about 3.5 billion years ago, so all that molten rock will have cooled and solidified by now into an enormous slab of granite called a batholith. There are a few similar areas on the near side of the moon, but they aren’t as large and none of them are quite as radioactive as the one at Compton-Belkovich, probably because they did not go through as many cycles of melting and cooling – each melting cycle concentrates the radioactive elements in the resulting magma.

Similar batholiths underlie many major volcanic systems on Earth, but we didn’t expect to find them on the moon. “On Earth this kind of volcanism is driven by plate tectonics and water, but the moon doesn’t have either of those,” says Siegler. “People really hadn’t thought that volcanism at this scale could happen on the moon.”

This may mean that the moon formed with a strange wet pocket in its crust, which would have allowed the rock to melt at a lower temperature. “That’s a kind of weird thing to have occurred, but it could have happened,” says Siegler. The other option is that there was a hot spot caused by the moon’s violent formation, similar to the one underneath Yellowstone in the US that has caused widespread volcanism in the area. It will take more detailed data from future lunar missions to solve this moon mystery.