1.97 billion years ago, lava was still flowing on the surface of the moon. This is evident from stones brought back by the Chinese lunar mission Chang’e-5. But that creates a new mystery: The moon should have been cold for two billion years by then. What explains the later than expected volcanic activity? For now, that’s a mystery.
Chang’e-5 was an unmanned mission that landed on the near side of the Moon (the Earth-facing side) in December 2020. The mission returned 1.7 kilograms of moon rock — the first samples collected from the moon since the Soviet Union’s Luna 24 mission in 1976.
One of the goals of the Chang’e-5 mission was to find evidence of some of the moon’s most recent volcanic eruptions. While scientists have previously been able to predict volcanic rocks of this age on the moon by studying the number of impact craters on the lunar surface, it was impossible to confirm without having samples to examine.
That was now possible. Chinese scientists manually picked out several small fragments of basalt (a volcanic rock), about 2 millimeters in size, for research. Which showed effectively an eruption time of 1.97 billion years ago, a billion years younger than any previously dated basaltic lava from the moon.
A new mystery
Many volcanic eruptions occurred on the moon’s surface during its geological history, forming large layers of basalt rock. These can be seen as dark spots on the moon. But most of the volcanic activity took place between 3 and 4 billion years ago. Planetary scientists have confirmed this by dating basalts from the Apollo and Luna rock collections, as well as meteorites from the moon. Until now, however, younger volcanic rocks predicted by crater count studies had remained elusive.
For volcanic eruptions to occur, heat is needed inside a planet to generate the molten material. For a moon-sized planet, this heat is believed to have been lost long before these eruptions 2 billion years ago.
Some theories
So the question now is how a small rocky planetary body like the moon could retain enough internal heat to continue producing volcanic eruptions 2.5 billion years after it formed 4.5 billion years ago.
Although scientists have previously suggested that high concentrations of radioactive elements in the moon’s interior could melt rocky material in the moon, the compositions of these samples indicate that this was not the driving force in this case.
It remains to be seen whether so-called tidal heating could have played a role, in which heat was generated in the moon’s interior as a result of the gravitational pull between the moon, Earth and sun.
Another explanation is that a unique aspect of the moon’s mantle composition may have resulted in a lower melting temperature, which explains how the molten material formed.
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