In a rare find, huge granite mass discovered beneath Moon's far side

Until now, only small grains of granitic material have been brought back from the Apollo missions- finding such a large amount is unprecedented.
Sade Agard
The lunar farside as never seen before! LROC WAC orthographic projection centered at 180° longitude, 0° latitude.
The lunar farside as never seen before! LROC WAC orthographic projection centered at 180° longitude, 0° latitude.

NASA/Goddard/Arizona State University 

A 31-mile (50km) mass of heat-emitting granite hidden beneath the surface on the far side of the Moon has been discovered, according to a study published in Nature on July 7.

Finding such a large amount of granite is unexpected, given that they are nearly absent in the solar system outside Earth. Until now, only small grains of granitic material have been found on the Moon in samples from the Apollo missions.

The discovery raises the prospect of uncovering similar formations elsewhere on the Moon and challenges our perception of its Earth-like qualities. In other words, it could represent a new paradigm of lunar geology and its resemblance to our planet.

NASA and Chinese orbital spacecraft data

"People don't think twice about having a granite countertop in their kitchen. But geologically speaking, it's quite hard to make granite without water and plate tectonics, which is why we really don't see that type of rock on other planets," highlighted Professor Stephen M. Elardo, a NASA Early Career Fellow and Assistant Professor of Geological Sciences, University of Florida (who was not involved in the study). 

In a rare find, huge granite mass discovered beneath Moon's far side
On the left the arrow shows the position of the basolith on the far side of the Moon. Centre and right shows the heat gradient from the granite within the Compton-Belkovich area

According to the study's lead author, Dr. Matt Siegler of the Planetary Science Institute, Tucson, Az, the team detected an unusual release of heat from the ground at a specific location on the Moon. This location is thought to be an ancient volcano that ceased erupting over 3.5 billion years ago.

"We have been developing a method to use microwaves to remotely measure geothermal heat gradients on the Moon. These measurements come from the Chinese Chang'E 1 and 2 lunar orbiters with context from NASA's Lunar Prospector and Lunar Reconnaissance Orbiters," Siegler said. 

Revolutionizing our understanding of rocky bodies' internal dynamics

"To tell the truth, we were a bit puzzled when we found it: fortunately, my wife, Dr. Rita Economos, is the geochemist in the family, so with her guidance, we were able to piece together the probable geologic cause of the heat anomaly," he added. 

"This find is a 50km wide batholith; a batholith is a type of volcanic rock that forms when lava rises into the Earth's crust but does not erupt onto the surface. El Capitan and Half Dome, in Yosemite in California, are examples of similar granite rocks which have risen to the surface," said Dr. Rita Economos.

Granite's high heat production results from its uranium and thorium content. The large amount is likely to have formed when a magma body – the unerupted lava - below a volcano cools. 

Nestled between the craters Compton and Belkovich on the far side of the Moon, it's around 10 degrees Celsius warmer than its surroundings. 

 "If this finding by Siegler and colleagues holds up, it's going to be massively important for how we think about the internal workings of other rocky bodies in the Solar System," said Elardo. 

Furthermore, this discovery demonstrates the capability of remote sensing to detect concealed features, which holds promise for exploring other celestial bodies in the solar system.

The full study was published in Nature on July 6 and can be found here.

Study abstract

Granites are nearly absent in the Solar System outside of Earth. Achieving granitic compositions in magmatic systems requires multi-stage melting and fractionation, which also increases the concentration of radiogenic elements1. Abundant water and plate tectonics facilitate these processes on Earth, aiding in remelting. Although these drivers are absent on the Moon, small granite samples have been found, but details of their origin and the scale of systems they represent are unknown2. Here we report microwave-wavelength measurements of an anomalously hot geothermal source that is best explained by the presence of an approximately 50-kilometre-diameter granitic system below the thorium-rich farside feature known as Compton–Belkovich. Passive microwave radiometry is sensitive to the integrated thermal gradient to several wavelengths depth. The 3–37-gigahertz antenna temperatures of the Chang’e-1 and Chang’e-2 microwave instruments allow us to measure a peak heat flux of about 180 milliwatts per square metre, which is about 20 times higher than that of the average lunar highlands3,4. The surprising magnitude and geographic extent of this feature imply an Earth-like, evolved granitic system larger than believed possible on the Moon, especially outside of the Procellarum region5. Furthermore, these methods are generalizable: similar uses of passive radiometric data could vastly expand our knowledge of geothermal processes on the Moon and other planetary bodies.

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