Volcanic Hotspot Found on the Moon

A recent study published in Nature examines a volcanic hotspot that potentially exists beneath a feature on the Moon’s farside (the side facing away from the Earth) called the Compton-Belkovich Thorium Anomaly. Researchers led by the Planetary Science Institute collected data from the hotspot region using microwave instruments onboard the China National Space Administration’s Chang’e-1 and Chang’e-2 orbiters and holds the potential to help scientists better understand the past volcanic processes on our nearest celestial neighbor, as surface evidence indicates lunar volcanic activity ceased between 3 to 4 billion years ago.

What makes this finding unique is the source of the hotspot isn’t active volcanism, such as molten lava, but from radioactive elements within the now-solidified rock that was once molten lava billions of years ago. Since the only rock type capable of possessing a large enough number of radioactive elements is granite, this indicates a large granite magma chamber once existed beneath the lunar surface that fed a smaller surface volcano, much like how Earth’s volcanoes operate.

“What this means is that it is hot, not necessarily at the surface, as you would see in infrared, but under the surface,” said Dr. Matthew Siegler, who is a Senior Scientist at the Planetary Science Institute (PSI), and lead author of the study. “The only way to explain this is from extra heat coming from somewhere below the feature within the deeper lunar crust. So, Compton-Belkovich, thought to be a volcano, is also hiding a large heat source below it.”

The study refers to the object at Compton-Belkovich as a lunar granitic batholith, which exists beneath the lunar surface and consists of a much larger system estimated to be approximately 50 kilometers (31 miles) in diameter that cooled before erupting. When molten lava cools, it becomes granite, and examples of batholiths systems that exist beneath volcanic chains on Earth, including the Cascade Range or Andes Mountains, but have rarely been discovered throughout the solar system outside of Earth.

“It was a neat project in that China made their data public – as does NASA – and we were able to work with this unique data set to figure out something really interesting about the Moon,” said Dr. Siegler. “Following the rules, we could not collaborate with Chinese researchers directly and all funding came only from NASA, so we had to follow the breadcrumbs to crack this dataset open.”

Arrow displaying position of the batholith on the farside of the Moon (left) with the heat gradient from the granite within the Compton-Belkovich Thorium Anomaly (center and right). (Credit: Matthew Siegler, PSI)

Dr. Siegler credits study co-author, Dr. Jianqing Fang, who is a Research Scientist at PSI, for his ability to “navigate the data and existing literature on the topic” after arriving in the United States with a J visa and noted this was “a great example of what can be done if science and politics can work together.” Like Earth, our Moon has a violent history of volcanism, though Dr. Siegler notes the Apollo missions showed this volcanic activity was primarily from impacts as opposed to the traditional volcanism from interior planetary processes, as seen on Earth.

Aside from the study’s finding that this granite hotspot was not the result of active volcanism, finding it on the farside of the Moon is also unique. This is because the nearside and farside are in stark contrast to each other, as evidence for most of the Moon’s volcanism has been observed to be on the nearside (the side facing toward the Earth) in dark patches known as lunar maria, also known as lunar mare, which is Latin for sea. The much lighter regions are called the lunar highlands, comprised of the elevated, mountainous regions of the lunar surface.

The Moon’s near side (left) and far side (right). The near side is comprised of 97 percent of dark volcanic regions known as lunar maria, while the far side is comprised predominantly of lunar highlands. (Credit: NASA)

While this combination offers skywatchers a spectacular view during a Full Moon, the opposite can be said about the farside of the Moon, which is dominated by lunar highlands with sparse regions of maria, most notably Mare Orientale, which is just outside observable view from Earth, and Mare Moscoviense, which is entirely on the farside and completely outside our observable view.

What new discoveries will scientists make about past volcanism on the Moon in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!