A massive blob of granite has been discovered under the surface of the moon, upending scientists' understanding of our closest celestial neighbor.
The traces of granite, which would be common on Earth, is evidence of a type of volcanic activity not previously thought to be on the moon, and scientists aren’t sure yet how it got there.
Analysis of the site suggests the rock formed from solidified magma, also known as a batholith, that formed roughly 3.5 billion years ago.
Large bodies of granite on Earth are used to feed volcanoes, like the large system that feeds volcanoes in the Northwest region of the United States today.
But what puzzles scientists is that the conditions necessary to create granite do not exist on the moon. On Earth, granite formation requires a large amount of water and plate tectonics which help the material from Earth’s crust melt and be recycled since granite - unlike other volcanic materials - requires multiple re-meltings of basaltic rock.
But since the moon doesn’t have liquid water or plate tectonics, scientists are stumped on what created the granite.
The granite blob was discovered by two Chinese orbiters, Chang’e 1 and Chang'e 2. They detected a spot under a suspected volcano dubbed Compton-Belkovich, near the moon's north pole on the far side, using their microwave sensor. The data was made publicly available and researchers at the Southern Methodist University were able to analyze it.
"What we found was that one of these suspected volcanoes, known as Compton-Belkovich, was absolutely glowing at microwave wavelengths," planetary scientist Matthew Siegler says. "What this means is that it is hot, not necessarily at the surface, as you would see in the infrared, but under the surface.”
The area is roughly 20 times hotter than the average for the lunar highlands.
The Compton-Belkovich area is also notable because it has a large amount of thorium, which is a byproduct of radioactive decay, creating heat. Siegler and his team argue the heat anomaly can be explained by radioactive elements in the granite matrix.
The team say this is evidence of a magma plumbing system under the lunar surface that would be much larger than expected.
However, to create such a system would require one of three things - all of which are inconsistent with current models of the moon: a large mantle plume that feeds magma from inside the moon; an anomalous pocket of water in the area; or a patch of elements creating enough radioactive material to facilitate the heating and re-melting of basaltic rocks.
“Was there water on the moon – at least in this one spot?” Siegler wonders. “Or was it just especially hot?”
The team's findings were published in the journal Nature.