Scientists have found a hot spot on the far side of the Moon – 07/23/2023 – Science

Rocks under an ancient volcano on the moon’s far side remain surprisingly hot, scientists have revealed, using data from orbiting Chinese spacecraft.

The data point to a large slab of granite that solidified from magma in the geological channels underneath what is known as the Compton-Belkovich Volcanic Complex.

“I would say we’re confirming that this is indeed a volcanic element,” said Matthew Siegler, a scientist at the Planetary Science Institute, based in Tucson, Arizona, who led the research. “But the interesting thing is that it’s a very Earth-like volcanic element.”

Published earlier this month in the journal Nature, the findings help explain what happened a long time ago under a weird part of the Moon. The study also highlights the scientific potential of data collected by the Chinese space program and how researchers in the United States had to overcome obstacles to use this data.

For this study, Siegler and his colleagues analyzed data from microwave instruments on the Chang’e-1, launched in 2007, and Chang’e-2, launched in 2010, two early Chinese spacecraft that are no longer in operation. Because Congress currently prohibits any direct collaboration between NASA and China, and because the research was funded by NASA, Siegler was unable to work with the scientists and engineers who collected the data.

“It was a limitation. We couldn’t just call the engineers who built the instrument in China and ask ‘hey, how should we interpret this data?'” he said. “It would have been great if we could have worked on this with the Chinese scientists all the time. But it’s not allowed. Luckily the Chinese have opened up some of their databases to the public.”

Siegler was able to draw on the expertise of a Chinese scientist, Jianqing Feng, whom he had met at a conference. Feng was working on a lunar exploration project at the Chinese Academy of Sciences.

“I realized that combining lunar exploration data from different countries would deepen our understanding of lunar geology and make exciting discoveries possible,” Feng said in an email. “So I left my job in China, moved to the United States and joined the Planetary Science Institute.”

Both Chinese orbiters had microwave instruments, which are common on many Earth-orbiting weather satellites but rare on interplanetary spacecraft.

So the Chang’e-1 and Chang’e-2 data opened up a different view of the Moon, measuring heat flux as far as 4.5 meters below the surface — and proved ideal for investigating the Compton-Belkovich abnormality.

Visually speaking, the region does not look exceptional. (It doesn’t even have a proper name; the designation by which it’s known is derived from two adjacent impact craters, Compton and Belkovich.) Yet it has fascinated scientists for more than two decades.

In the late 1990s, David Lawrence, then a scientist at Los Alamos National Laboratory, was working with data collected by NASA’s Lunar Prospector mission and noticed a bright spot of gamma rays coming from that location on the far side of the Moon. The energy of gamma rays, the form of light that has the highest energy, corresponded to thorium, a radioactive element.

“It was one of those unusual places that stands out tremendously for its abundance of thorium,” said Lawrence, now a planetary scientist at the Johns Hopkins Laboratory for Applied Physics in Maryland. “I’m a physicist. I’m not an expert on lunar geology. But even as a physicist, I saw that this was out of the ordinary and I thought ‘OK, this is something worth studying further.’

The next revelations came after the arrival of NASA’s Lunar Reconnaissance Orbiter in 2009. Brad Jolliff, professor of Earth and planetary studies at Washington University of St. Louis. Louis, led a team that examined the high-resolution Compton-Belkovich images.

What they saw “looked a lot like a volcanic caldea,” Jolliff said, alluding to the remnants of a volcano rim. “Considering these elements are billions of years old, they are surprisingly well preserved.”

A more recent analysis led by Katherine Shirley, now of Oxford University in England, estimated the age of the volcano at 3.5 billion years.

Because the lunar soil acts as a good thermal insulator, reducing temperature variations between day and night, microwave emissions largely reflect the flow of heat coming from the interior of the Moon. “You only have to go two meters below the surface to stop seeing the heat coming from the Sun,” Siegler said.

At the Compton-Belkovich Complex, the heat flux had reached 180 milliwatts per square meter—about 20 times the average for highlands on the far side of the Moon. That measurement corresponds to a temperature of minus 23 degrees Celsius 1.80 meters below the surface — about 32 degrees hotter than anywhere else.

“That spot stood out, it was very hot compared to anywhere else on the Moon,” Siegler said.

To produce so much heat and gamma rays from thorium, Siegler, Feng, and the other scientists concluded that the most likely source was granite, which contains radioactive elements like thorium, and that there must be a lot of it.

Granite appears to be scarce elsewhere in the solar system. On Earth, granite forms in volcanic regions where oceanic crust is pushed under a continent by plate tectonics, the geological forces that push pieces of Earth’s outer crust. Water is also a key ingredient for granite formation.

But the Moon is almost entirely dry and has no plate tectonics. Moon rock samples brought back by NASA astronauts more than 50 years ago contained only a few grains of granite. However, data from Chinese orbiters suggest a granite feature more than 50 kilometers wide below Compton-Belkovich.

“Now we need geologists to calculate how this type of element can be produced on the Moon without water and without plate tectonics,” said Siegler.

^{Translated by Clara Allain}