The far side of the moon hides evidence of a huge ancient volcano. But even though researchers are certain the volcano was there, they remain baffled as to how it could have formed.
For more than 20 years, we’ve known that an area on the far side of the moon called Compton-Belkovich was a bit strange. It had an odd topography, and the top meter of soil seemed to have more thorium than its surroundings.
Now, matt siegler at the Institute of Planetary Sciences in Arizona and colleagues have used data from China’s Chang’e 1 and Chang’e 2 orbiters to determine that there is an area 50 kilometers wide and several kilometers 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 rock through volcanism.
“That small amount of thorium that we saw on the surface is the tip of the iceberg of a huge body below the surface that was the plumbing system for this volcano,” Siegler says. “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 that the volcano last erupted around 3.5 billion years ago, so all that molten rock will have already cooled and solidified into a huge slab of granite called a batholith. There are a few similar areas on the near side of the moon, but they’re not as large and none of them are as radioactive as Compton-Belkovich, probably because they didn’t go through as many melting and melting cycles. Cooling: each fusion cycle concentrates the radioactive elements in the resulting magma.
Similar batholiths underlie many of Earth’s major volcanic systems, but we didn’t expect to find them on the Moon. “On Earth, this type of volcanism is driven by plate tectonics and water, but the Moon has neither of those,” Siegler says. “People hadn’t really thought that volcanism on 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. “It’s kind of weird that it happened, but it could have happened,” says Siegler. The other option is that there was a hot spot caused by violent formation of the moon, similar to the one below Yellowstone in the US that has caused widespread volcanism in the area. More detailed data from future lunar missions will be needed to solve this lunar mystery.