Lyon, France: An orbiting satellite discovers a mysterious, heat-emitting mass of granite buried under the surface of the far side of the Moon. It seems like the plot of 2001: a Space Odyssey, but it's real science – this is a natural feature, discovered by a group of international planetary scientists working for NASA, including a husband-and-wife team. The data comes from NASA and from Chinese orbital spacecraft.
Lead researcher, Dr Matt Siegler (of the Planetary Science Institute, Tucson, Az) said:
"We have discovered extra heat coming out of the ground at a location on the Moon believed to be a long dead volcano which last erupted over 3.5 billion years ago. It's around 50km across, and the only solution that we can think of which produces that much heat is a large body of granite, a rock which forms when a magma body – the unerupted lava - below a volcano cools. Granite has high concentrations of radioactive elements like uranium and thorium compared to other rocks in the lunar crust, causing the heating we can sense at the lunar surface".
Dr Siegler will present the work for the first time at the Goldschmidt Conference in Lyon, France on 12th July. The paper on which this work is based is available online in the peer-reviewed journal Nature (See below).
Dr Siegler continued, "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. This data showed a high geothermal gradient exactly coincident with a large 20 km wide silicon-rich surface feature believed to be an extinct volcanic caldera, which is between the craters Compton and Belkovich on the far side of the Moon. This is around 10C warmer than its surroundings. We interpret this heat flux as resulting from a radiogenic-rich granite body below the caldera. 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".
The find is unexpected. Granites are nearly absent in the Solar System outside of Earth. Until now, only small grains of granitic material have been found on the Moon in samples brought back on the Apollo missions.
Dr Economos said "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".
Finding such a large amount of granite opens the possibility of similar finds elsewhere under the Moon's surface. Matt Siegler said "This is more Earth-like than we had imagined can be produced on the Moon, which lacks the water and plate tectonics that help granites form on Earth. What this also does is show that remote sensing can pick up hidden features, and this will be useful in the exploration of other planetary bodies in the Solar System".
Commenting, Professor Stephen M. Elardo (NASA Early Career Fellow; Assistant Professor, Dept. of Geological Sciences, University of Florida) said:
"This new finding of a large mass of granite on the Moon is incredibly interesting. We have tons of granite of different flavors all over Earth. 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. So 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."
This is an independent comment, Professor Elardo was not involved in this work.
Funding: National Aeronautics and Space Administration (NASA)
The Goldschmidt Conference is the world's main geochemistry conference. It is a joint congress of the European Association of Geochemistry and the Geochemical Society (US). It takes place in Lyon, France, from 9-14 July. Almost 5000 delegates are expected to attend. https://conf.goldschmidt.info/goldschmidt/2023/goldschmidt/2023/meetingapp.cgi
Publication details: The paper "Remote Detection of a Lunar Granitic Batholith at Compton-Belkovich", Siegler et al, is available via advance online publication in the journal Nature at https://www.nature.com/articles/s41586-023-06183-5 This press release is based on the conference presentation and contains additional material and comments, not present in the above paper.
