The moon has been volcanically active for longer than expected
A year ago, a probe brought moon rocks to Earth. Analyzes show that the stormy sea, visible from Earth, was a lava lake two billion years ago – and the moon has been volcanically active for much longer than expected.
nTwo billion years ago, Oceanus Procellarum – an ocean of storms – was a huge lake of lava on the moon’s surface. This is evidenced by the investigation of rock samples that the Chinese probe Chang’e-5 returned to Earth in December 2020. Thus there was volcanic activity on the Earth satellite that was a billion years longer than previously assumed based on the Apollo lunar samples.
The reason why the lava lake has been preserved for so long is still unclear, an international team of researchers wrote in “Science” magazine.
“There is no evidence of any radioactive elements in the lunar mantle that could produce enough heat to generate lava,” wrote Xiao Zhaoqi of the Chinese Academy of Geosciences in Beijing and colleagues. “So we have to look for alternative explanations for the longevity of lunar lava.”
It’s true that a larger asteroid impact could also produce a lava lake — but in the Oceanus Procellarum other impact basin characteristics such as ballistics or gravitational anomalies were missing, according to the researchers.
As a possible explanation, Chi and colleagues favor heating through tidal friction. Because the Moon was closer to Earth two billion years ago than it is today — about a third or a quarter, co-author Alexander Nemchin of Curtin University in Australia estimates.
“Of course, tidal friction will work everywhere on the moon,” says the researcher. “But there are areas where the melting temperature is lower due to the chemical composition.” The perimeter of the storms may be such an area.
Chang’e-5 took off on November 20, 2020 and landed on Earth’s satellite on December 1. Just two days later, the probe began returning to Earth with 1,731 grams of soil samples – this was the first mission of its kind since the Soviet Luna-24 probe brought 170 grams of lunar rock to Earth in 1976. The ocean of storms, visible from Earth, It is the largest of the dark lava plains on the moon’s surface, which is 2,500 kilometers long. Until now it is not clear how old this region is – assumptions fluctuated between 1.2 and 3.2 billion years.
However, the exact age of Oceanus Procellarum is of great interest to lunar and planetary researchers – because it enables calibration of age determination by crater counts. “The more craters there are on a surface, the older it is,” explains co-author Brad Jolev of the University of Washington, USA. “But to determine the absolute age, we need rock samples.”
And yet there has been a huge gap: all of the previous igneous rock samples were older than three billion years. The rock samples from the impact craters were less than a billion years old.
Researchers have now been able to fill that gap with the age of Oceanus procellarum, which has been determined to be 2 billion years old. In the future, it will be possible to determine the age of the landscape not only on the Moon, but also, for example, on Mercury and Mars more accurately than before.
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