The rocks beneath an ancient volcano on the far side of the moon remain surprisingly hot, scientists have revealed using data from orbiting Chinese spacecraft.
They point to a large slab of granite that solidified from magma in the geological aquifer beneath what is known as the Compton-Belkovich Volcanic Complex.
“I would say we’re putting the nail in the coffin of this really being a volcanic feature,” said Matthew Siegler, a scientist at the Planetary Science Institute, based in Tucson, Arizona, who led the research. “But then, what’s interesting, it’s very similar to Earth’s volcanic feature.”
The findings that came out 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 avoid obstacles to using 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 defunct Chinese spacecraft. Because Congress currently prohibits direct collaboration between NASA and China and the research was funded by a NASA grant, Dr. Siegler was unable to collaborate with scientists and engineers who collected the data.
“That was a limitation that we couldn’t just call up the engineers who built the instrument in China and say, ‘Hey, how do we interpret this data?'” he said. “It would be really great if we could just work on this with the Chinese scientists all the time. But we’re not allowed to. But, luckily, they’ve released some of their databases.”
He was able to tap into the expertise of a Chinese scientist, Jianqing Feng, who had met Dr. Siegler at a conference. Dr. Feng worked on a lunar research 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 the 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 Compton-Belkovich oddity.
Visually, the region looks unremarkable. (It doesn’t even have a name of its own; the hyphenated name is derived from two nearby impact craters, Compton and Belkovich.) The region has nevertheless fascinated scientists for several 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 bright spot of gamma rays shooting from this place 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 weird places that just stuck out like a sore thumb in terms of thorium abundance,” said Dr. Lawrence, now a planetary scientist at the Johns Hopkins Applied Physics Laboratory in Maryland. “I am a physicist. I am not an expert on lunar geology. But even as a physicist, I saw that stand out and said, ‘Okay, 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 in St. Louis, led a team that examined these high-resolution Compton-Belkovich images.
What they saw “looked suspiciously like a caldera,” Dr. Jolliff said, referring to the remnants 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 Shirleynow at the University of Oxford in England, estimated the age of the volcano at 3.5 billion years.
Because the lunar soil acts as a good insulator, dampening the temperature variations between day and night, the microwave emissions mostly reflect the flow of heat from the interior of the moon. “You only need to go about two meters below the surface to stop seeing the heat of the sun,” said Dr. Siegler.
At Compton-Belkovich, the heat flux was as high as 180 milliwatts per square meter, or about 20 times the average for the highlands of the far side of the moon. 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 stood out because it was just hot compared to anywhere else on the moon,” Dr. Siegler said.
To produce so 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 must be a lot of it.
“It seems to nail down more specifically what material is really underneath,” said Dr. Lawrence, who was one of the paper’s reviewers for Nature.
“It’s kind of a tip of the iceberg,” he said of the original gamma-ray emissions. “What you see at Compton-Belkovich is kind of a surface expression of something much bigger underneath.”
Volcanism is evident elsewhere on the moon. Plains of hardened lava – the mare, or seas, of basalt – cover vast areas of the surface, mostly on the near side. But Compton-Belkovich is different, similar to certain volcanoes on Earth, such as Mount Fuji 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 oceanic crust is pushed down under a continent by plate tectonics, the geological forces that push 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 suggest a granite formation more than 30 miles wide below Compton-Belkovich.
“Now we need the geologists to figure out how you can produce a feature like this on the moon without water, without plate tectonics,” Dr. Siegler said.
Dr. Jolliff, who was not involved with the research, said the paper was “a very nice new contribution.” He said he hopes NASA or another space agency will 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 crust, much like what happens beneath the Hawaiian islands.
For Dr. Feng, his current visa allowing him to work in the United States is about to expire. He is applying for a new one, navigating his scientific career amid geopolitical bickering between the United States and China.
“We are starting to study other possible granite systems now on the moon,” he said. “We will also expand our models to explore Jupiter’s icy moons. That’s why I’m trying to stay in the US as long as possible.”