The rocks beneath an ancient volcano on the far side of the moon stay surprisingly warm, scientists have revealed using data from orbiting Chinese spacecraft.
They point to a large slab of granite that has solidified from magma in the geological conduits beneath what is known as the Compton-Belkovich volcanic complex.
“I’d say we’re hitting the nail in the coffin: This is really a volcanic feature,” said Matthew Siegler, a scientist at the Planetary Science Institute, headquartered in Tucson, Arizona, who led the study. “But what’s interesting is it’s a very Earth-like volcanic feature.”
The findings, published last week in the journal Nature, help explain what happened long ago under a strange part of the moon. The study also highlights the scientific potential of data collected by China’s space program, and how researchers in the United States must overcome obstacles to use that data.
For this study, Dr. Siegler and colleagues collected data from microwave instruments on Chang’e-1, launched in 2007, and Chang’e-2, launched in 2010, two early Chinese spacecraft that are no longer in service. Because Congress currently prohibits direct collaboration between NASA and China and the research was funded by a NASA grant, Dr. Siegler did not collaborate with scientists and engineers who collected the data.
“That was a limitation, that we couldn’t just call the engineers who built the instrument in China and say, ‘Hey, how are we supposed to interpret this data?'” he said. “It would be really great if we could have been working on this with the Chinese scientists all the time. But we are not allowed. Fortunately, they have made some of their databases public.”
He was able to tap into the expertise of a Chinese scientist, Jianqing Feng, who Dr. Siegler at a conference. Dr. Feng worked on a lunar exploration project at the Chinese Academy of Sciences.
“I realized that combining the lunar exploration data from different countries would deepen our understanding of lunar geology and yield exciting findings,” said Dr. Feng in an email. “That’s why I quit my job in China, moved to the United States and joined the Planetary Science Institute.”
The Chinese orbiters both had microwave instruments, common on many orbiting weather satellites, but rare on interplanetary spacecraft.
The data from Chang’e-1 and Chang’e-2 thus provided a different view of the moon, measuring heat flux down to 4.5 meters below the surface – and proved ideal for investigating the Compton-Belkovich oddity .
Visually, the region looks unremarkable. (It doesn’t even have its own name; the hyphenated designation derives from two adjacent impact craters, Compton and Belkovich.) Nevertheless, the region 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 NASA’s Lunar Prospector mission and saw a bright spot of gamma rays shooting from this location on the far side of the moon. The energy of the gamma rays, the highest energy form of light, corresponded to thorium, a radioactive element.
“It was one of these odd spots that stood out like a sore thumb in terms of the abundance of thorium,” said Dr. Lawrence, now a planetary scientist at the Johns Hopkins Applied Physics Laboratory in Maryland. ‘I’m a physicist. I’m not an expert in lunar geology. But even as a physicist, I noticed that and said, ‘OK, this is something worth studying further.’”
The next revelations came after the arrival of NASA’s Lunar Reconnaissance Orbiter in 2009. Bradley L. Jolliff, a professor of earth and planetary sciences at Washington University of St. Louis, led a team that examined those high-resolution images from Compton-Belkovich .
What they saw “looked suspiciously like a caldera,” said Dr. Jolliff, referring to the remains of a volcano’s rim. “When you consider that these features are billions of years old, they are remarkably well preserved.”
A more recent analysis led by Katherine Shirley, now at the University of Oxford in England, estimated the volcano’s age to be 3.5 billion years old.
Because the lunar soil acts as a good insulator, dampening temperature fluctuations between day and night, microwave emissions largely reflect heat flow from the moon’s interior. “You only have to go about two meters below the surface to stop seeing the heat from the sun,” said Dr. Siegler.
At Compton-Belkovich, heat flow was as high as 180 milliwatts per square meter, or about 20 times the average for the highlands on the far side of the moon. That measure equates to a temperature of minus 10 degrees Fahrenheit about six feet below the surface, or about 90 degrees warmer than elsewhere.
“This one stood out because it was just blazing hot compared to anywhere else on the moon,” said Dr. Siegler.
To produce so much heat and the thorium gamma rays, Dr. Siegler, Dr. Feng and the other researchers found that granite, which contains radioactive elements such as thorium, was the most likely source and that there must be a lot of it.
“More specifically, it seems to establish what kind of material is actually underneath,” said Dr. Lawrence, one of the reviewers of the article for Nature.
“It’s kind of the tip of the iceberg,” he said of the original gamma-ray emission. “What you see with Compton-Belkovich is kind of a superficial expression of something much bigger down there.”
Volcanism is visible elsewhere on the moon. Plains of hardened lava – the mare, or seas, of basalt – cover vast areas of the surface, usually on the near side. But Compton-Belkovich is different and resembles certain volcanoes on Earth, such as Mount Fiji and Mount St. Helens, which spew more viscous lava.
Granite appears to be scarce elsewhere in the solar system. On Earth, granite forms in volcanic regions where the oceanic crust is pushed under a continent by plate tectonics, the geological forces that push around pieces of the Earth’s outer crust. Water is also an important ingredient for granite.
But the moon is mostly dry and has no plate tectonics. The moon rocks brought back by NASA astronauts more than 50 years ago contain only a few grains of granite. But the data from the Chinese orbiters suggests a granite formation more than 30 miles wide below Compton-Belkovich.
“Now we need the geologists to figure out how to produce that kind of feature on the moon without water, without plate tectonics,” said Dr. Siegler.
Dr. Jolliff, who was not involved in any 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-Belkovich for seismic and mineralogical measurements.
Such a mission could help test ideas about how a volcano formed there in the first place. One hypothesis is that a plume of hot material rose from the mantle beneath the Earth’s crust, much like what happens under the Hawaiian Islands.
For dr. Feng’s current visa that allows him to work in the United States will soon expire. He is applying for a new one as he navigates his scientific career amid the geopolitical bickering between the US and China.
“We are now beginning to study other potential granite systems on the moon,” he said. “We will also extend our models to explore Jupiter’s icy moons. That is why I try to stay in the United States as long as possible.”
