The rocks beneath an ancient volcano on the moon’s far side remain surprisingly warm, scientists have revealed using data from orbiting Chinese spacecraft.
They point to a large slab of granite that solidified from magma in the geological plumbing beneath what is known as the Compton-Belkovich Volcanic Complex.
“I would say we’re putting the nail in the coffin of this really is a volcanic feature,” said Matthew Siegler, a scientist at the Planetary Science Institute, headquartered in Tucson, Ariz., and who led the research. “But then what’s interesting is, it’s a very Earth-like volcanic feature.”
The findings, which appeared last week in the journal Nature, help explain what happened long ago beneath an odd part of the moon. The study also highlights the scientific potential of data gathered by China’s space program, and how researchers in the United States have to circumvent obstacles to use that data.
For this study, Dr. Siegler and his colleagues analyzed data from microwave instruments on Chang’e-1, launched in 2007, and Chang’e-2, launched in 2010, two early Chinese spacecraft no longer in operation. Because Congress currently prohibits direct collaboration between NASA and China and the research was financed by a NASA grant, Dr. Siegler could not work with scientists and engineers who collected the data.
“That was a limitation, that we couldn’t just call up the engineers that had built the instrument in China and say, ‘Hey, how should we be interpreting this data?’” he said. “It would be really great if we could just have been working on this with the Chinese scientists the whole time. But we’re not allowed to. But, luckily, they made some of their databases public.”
He was able to tap into the expertise of a Chinese scientist, Jianqing Feng, who met Dr. Siegler at a conference. Dr. Feng was working 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 make exciting findings,” Dr. Feng said in an email. “Therefore, I quit my job in China, moved to the United States, and joined Planetary Science Institute.”
The Chinese orbiters both had microwave instruments, common on many Earth-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 the flow of heat up to 15 feet below the surface — and proved ideal for investigating the oddity of Compton-Belkovich.
Visually, the region looks unremarkable. (It does not even have a name of its own; the hyphenated designation is derived from two adjoining impact craters, Compton and Belkovich.) The region has nonetheless fascinated scientists for a couple of 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 noticed a bright spot of gamma-rays shooting from this location on the moon’s far side. The energy of the gamma-rays, the highest energy form of light, corresponded to thorium, a radioactive element.
“It was one of these oddball places that just stood out like a sore thumb in terms of the thorium abundance,” 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 saw that stand out and said, ‘OK, this is something worth further study.’”
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 that high-resolution images of Compton-Belkovich.
What they saw “looked suspiciously like a caldera,” Dr. Jolliff said referring to the remnants of a volcano’s rim. “If you consider 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 age of the volcano at 3.5 billion years old.
Because the lunar soil acts as a good insulator, dampening the temperature variations between day and night, the microwave emissions largely reflect the flow of heat from the moon’s interior. “You only need to go about two meters below the surface to stop seeing the heat from the sun,” Dr. Siegler said.
At Compton-Belkovich, the heat flow was as high as 180 milliwatts per square meter, or about 20 times the average for the highlands of the moon’s far side. That measure corresponds to a temperature of minus 10 degrees Fahrenheit about six feet below the surface, or about 90 degrees warmer than elsewhere.
“This one stuck out, as it was just glowing hot compared to anywhere else on the moon,” Dr. Siegler said.
To produce that much heat and the thorium gamma-rays, Dr. Siegler, Dr. Feng and the other researchers concluded that granite, which contains radioactive elements like thorium, was the most likely source and that there had to be a lot of it.
“It seems to nail down more particularly what kind of material is really underneath,” said Dr. Lawrence, who was one of the reviewers of the paper for Nature.
“It’s sort of a tip-of-the-iceberg type of thing,” he said of the original gamma-ray emissions. “What you see at Compton-Belkovich is sort of a surface expression of something a lot bigger underneath.”
Volcanism is evident elsewhere on the moon. Plains of hardened lava — the mare, or seas, of basalt — cover vast swaths of the surface, mostly on the near side. But Compton-Belkovich is different, resembling certain volcanoes on Earth, like Mount Fiji and Mount St. Helens, that spew more viscous lava.
Granite appears to be scarce 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 are pushing around pieces of the Earth’s outer crust. Water is also a key ingredient for granite.
But the moon is mostly dry and lacks plate tectonics. The moon rocks brought back by NASA astronauts more than 50 years ago contained only a few grains of granite. But the data from the Chinese orbiters suggests a formation of granite more than 30 miles wide below Compton-Belkovich.
“Now we need the geologists to figure out how you can produce that kind of feature on the moon without water, without plate tectonics,” Dr. Siegler said.
Dr. Jolliff, who was not involved with 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 up from the mantle beneath the crust, much as what occurs under the Hawaiian islands.
For Dr. Feng, his current visa allowing him to work in the United States is expiring soon. He is applying for a new one, navigating his scientific career amid U.S.-China geopolitical wrangling.
“We are starting to study other potential granitic systems on the moon now,” he said. “Also, we will expand our models to explore the icy moons of Jupiter. Therefore, I am trying to stay in the United States as long as possible.”