Scientists have revealed that rocks beneath an ancient volcano on the far side of the moon stay surprisingly warm, using data from orbiting Chinese spacecraft.
They point to a large slab of granite that solidified from magma in the geological plumbing below what is known as the Compton-Belkovitch Volcanic Complex.
“I would say we’re putting the nail in the coffin of this really volcanic feature,” said Matthew Siegler, a planetary science institute scientist based in Tucson, Arizona, who led the research. “But what’s interesting is that it’s an Earth-like volcanic feature.”
Results that came out last week In the journal Nature, he helped explain what happened long ago under a strange part of the moon. The study also highlights the scientific potential of the data collected by the Chinese space program, and how researchers in the United States have to circumvent obstacles to using that data.
In this study, Dr. Siegler and colleagues analyzed data from microwave instruments on Chang’e-1, launched in 2007, and Chang’e-2, launched in 2010, two Chinese spacecraft that are no longer operational. Because direct collaboration between NASA and China is currently prohibited by Congress and the research was funded by a NASA grant, Dr. Siegler was unable to work with the scientists and engineers who collected the data.
“That was a limitation,” he said, “in that we couldn’t call up the engineers who built the device in China and say, ‘Hey, how should we interpret this data?'” “It would be really nice if we worked on this with Chinese scientists all the time. But we are not allowed to do that. But fortunately, they’ve made some of their databases public.”
He was able to draw on the expertise of a Chinese scholar, Jianqing Feng, who had met Dr. Siegler at a conference. Dr. Feng was working on the 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 lead to exciting results,” Dr. Feng said in an email. “So, I quit my job in China, moved to the United States, and joined the Planetary Science Institute.”
Both Chinese orbiters had microwave instruments, common on many Earth-orbiting weather satellites but rare on interplanetary spacecraft.
And so the data from Chang’e-1 and Chang’e-2 provided a different view of the Moon, measuring heat flow up to 15 feet below the surface—and proving ideal for investigating the Compton-Pelkovitch weirdness.
Visually, the area looks unremarkable. (It doesn’t even have a name of its own; the hyphenated designation derives from two nearby impact craters, Compton and Belkowitz.) Still, the area has fascinated scientists for a few decades.
In the late 1990s, David Lawrence, then a scientist at Los Alamos National Laboratory, was working on data collected by the NASA Lunar Prospector mission and noted A bright spot of gamma rays Shooting from this site on the far side of the Moon. The energy of gamma rays, the highest energy form of light, corresponds to thorium, which is a radioactive element.
“It was one of these strange places that stood out like a sore thumb in terms of thorium abundance,” said Dr. Lawrence, a planetary scientist now at the Johns Hopkins Applied Physics Laboratory in Maryland. “I’m a physicist. I’m not an expert on the geology of the moon. But even as a physicist, I saw that prominently and said, ‘Well, this is something that deserves further study.'”
The next discoveries came after the arrival of NASA’s Lunar Reconnaissance Orbiter in 2009. Bradley L. Jolliffe, professor of earth and planetary sciences at St. Louis University in Washington, led a team I checked out those high-resolution photos of Compton Belkowitz.
Dr. Jolliffe said what they saw “looked suspiciously like a caldera,” referring to the remnants of a volcano’s rim. “If you consider that these features are billions of years old, they are remarkably well preserved.”
More recent analysis led by Catherine ShirleyNow at the University of Oxford in England, the volcano is estimated to be 3.5 billion years old.
Since lunar soil acts as a good insulator, reducing temperature changes between day and night, microwave emissions largely reflect heat flux from the moon’s interior. “You only need to walk about two meters below the surface to stop seeing the sun’s heat,” said Dr. Siegler.
At Compton-Belkovitch, heat flux was as high as 180 milliwatts per square metre, or about 20 times the average heights on the far side of the moon. This scale corresponds to a temperature of less than 10 degrees Fahrenheit about six feet below the surface, or about 90 degrees warmer than anywhere else.
“This stuck out, as it was hot compared to anywhere else on the moon,” Dr. Siegler said.
To produce this much heat and thorium gamma rays, Dr. Siegler, Dr. Feng, and other researchers concluded that granite, which contains radioactive elements such as thorium, was the most likely source and that there must be plenty of it.
“It seems to specifically identify what kind of material is underneath,” said Dr. Lawrence, who was one of the reviewers on the Nature article.
“It’s kind of the tip of the iceberg,” he said of the original gamma-ray emissions. “What you see in Compton-Belkovich is a kind of superficial expression of something much larger underneath.”
Volcanoes are visible elsewhere on the Moon. Solidified lava plains – mare or seas of basalt – cover large areas of the surface, mostly on the near side. But Compton-Bilkowitz is different, similar to some of Earth’s volcanoes, like Mount Fiji and Mount St. Helens, that spew more viscous lava.
Granite is rare elsewhere in the solar system. On Earth, granite forms in volcanic regions where oceanic crust is pushed down beneath a continent by plate tectonics, the geological forces that push around parts of the Earth’s outer crust. Water is also a major component of granite.
But the Moon is mostly dry and lacks plate tectonics. Moon rocks brought back by NASA astronauts more than 50 years ago contain only a few granite grains. But data from the Chinese orbiters point to a granite formation more than 30 miles wide below Compton-Belkovitch.
“Now we need geologists to figure out how you can produce this kind of feature on the Moon without water, without plate tectonics,” said Dr. Siegler.
Dr. Jolliffe, who was not involved in the research, said the paper was a “very nice new contribution.” He said he hoped NASA or another space agency would send a spacecraft to Compton-Belkovitch to perform seismic and mineralogical measurements.
Such a mission could help test ideas about how the volcano formed there in the first place. One hypothesis is that a plume of hot material rose from the mantle beneath the crust, just as it does under the Hawaiian Islands.
As for Dr. Feng, his current visa that allows him to work in the United States will soon expire. He’s applying for a new one, navigating his scientific career amid the geopolitical wrangling between the US and China.
“We are beginning to study other possible granitic systems on the moon now,” he said. “Also, we will extend our models to explore the icy moons of Jupiter. Therefore, I am trying to stay in the United States for as long as possible.”