The Compton-Belkovich region of the moon hides a granite slab beneath the surface
NASA/GSFC/Arizona State University
The far side of the moon hides evidence of a massive ancient volcano. But while researchers are confident 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 a strange topography and the top meter of soil seemed to contain more thorium than the surrounding area.
Now, Matt Siegler at the Planetary Science Institute 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 to repeatedly melt the rock via volcanism.
“That little piece of thorium that we saw on the surface is the tip of the iceberg of a huge subsurface body that was the conduit 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 area’s topography suggests the volcano last erupted about 3.5 billion years ago, so all that molten rock will have cooled and solidified by now into a huge slab of granite called a batholith. There are a few similar regions on the near side of the moon, but they aren’t as large and none are as radioactive as Compton-Belkovich’s, probably because they haven’t gone through as many melting and melting cycles. 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 has neither,” says Siegler. “People really didn’t think volcanism could happen on the moon on this scale.”
This could mean that the moon formed with a strange wet pocket in its crust, which could allow the rock to melt at a lower temperature. “That’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 the violent formation of the moon, similar to that under Yellowstone in the US, that has caused widespread volcanism in the area. More detailed data from future lunar missions is needed to solve this lunar mystery.
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