Chang’e-5, China’s lunar mission, recovered rocks from the moon that reveal lava flowed across its surface 2 billion years ago.
Researchers report online October 7 in Science that a chemical analysis of volcanic rocks has verified that the moon was volcanically active far longer than its size suggests.
First time in over 40 years that rocks from the moon are to be returned to Earth via Chang’e-5 (SN: 12/1/20). Scientists estimated that the rocks formed 2 billion years ago, shortly after the first multicellular organisms appeared on Earth. Carolyn Crow, a planetary scientist at the University of Colorado Boulder and the study’s coauthor, says the rocks are the youngest ever found on the moon.
4.5 billion years ago, the moon formed. Lunar rocks from the Apollo and Soviet missions of the late 1960s and 1970s revealed that volcanism on the moon was common for about a billion years during which massive flows occurred.
Considering the moon’s size, scientists believe it began cooling off about 3 billion years ago, becoming the quiet, inactive neighbor it is today. Researchers have been puzzled by a lack of craters in certain areas. Devoid of volcanism, celestial bodies accumulate craters more and more over time, in part because lava flows don’t deposit material that hardens into smooth stretches. Moon spots with smoother surfaces indicated that volcanism persisted for a long time.
“Young volcanism on a small body like the moon is challenging to explain, because usually small bodies cool fast,” says Juliane Gross, a planetary scientist at Rutgers University in Piscataway, N.J., not involved in the study.
Radioactive elements have been suggested as a possible cause of later volcanism. Nuclear reactors keep cool by keeping their reactors submerged in water, since radioactive decay produces a lot of heat. The moon’s mantle, a layer just beneath the visible crust, contains enough radioactive materials to have provided a heat source for ancient lava flows.
As part of this experiment, Chang’e-5 collected chunks of basalt – a type of rock formed by volcanic activity – from a previously unexplored part of the moon thought to be younger than 3 billion years old. Scientists determined that the rocks formed 2 billion years ago from lava flows, but chemical analysis showed no evidence of radioactive decay, as one might expect if the volcanism was caused by radioactive decay.
Researchers must consider what other forces maintained volcanic activity on the moon in light of this finding.
Scientist Alexander Nemchin from the Beijing SHRIMP Center and Curtin University in Bentley, Australia, says gravity from the Earth could have liquefied the lunar interior, allowing it to flow for billions of years past when it should have stopped.
“The moon was a lot closer 2 billion years ago,” Nemchin explains. These forces would have become less and less powerful until volcanism eventually petered out as the moon slowly slipped away from the Earth – a process still in progress today.
Similarly, asteroids and comets could have kept the moon’s volcanic juices flowing, but Jessica Barnes, an independent planetary scientist at the University of Arizona, says that any guess is a good one at this point.
“This is a good example of why we need to get to know our closest neighbor,” Barnes says. “A lot people think we already know what’s going on with the moon, but it’s actually quite mysterious.”