After analysing rocks from China's Chang'e-5 spacecraft, researchers came to this conclusion after studying them.
These are the first lunar samples to be returned since 1976's Apollo missions. They contradict the earlier findings. Together with samples from the Soviet Union's Luna 24 missions, the Apollo rocks and some other samples indicate that the moon had cooled down and its volcanoes stopped erupting around 3 billion years ago.
Bradley Jolliff, a planetary scientist from Washington University in St. Louis, who assisted with the research, stated that "the general assumption was the moon is such a tiny body compared to Earth or Mars, for instance, so it cooled down more quickly and stopped producing volcanoes."
The rocks Chang'e-5 returned are volcanic and approximately 2 billion years old. This means that lava flows must have covered the area where the spacecraft landed nearly one billion years earlier than scientists believed possible.
This surprising discovery was published in Science earlier this month. On Tuesday, another study of the Chang'e-5 rock samples was published in Nature. It also revealed that those samples are 2 billion years old.
Two studies that were done this week also showed that new moon samples have surprisingly low levels of radioactive elements and water. This makes it easier for volcanic eruptions to occur. Water reduces rock's melting point, while radioactive potassium, radioactive uranium and thorium heat magma. They were found in greater concentrations in the Apollo moon and Luna moon samples.
According to QingZhu Yin (a geochemist at University of California, Davis), the new papers present "a real conundrum." Science's Yin said that the studies raise questions over how a body so small as the moon could support volcanic eruptions at the end of life.
It is because the moon does not have an atmosphere or magnetic field that can retain heat necessary for volcanic activity, which makes the findings quite puzzling. There are some theories about how heat stayed around for so long.
One possibility is that the moon's earth might have been thicker than initially thought, allowing it to retain heat for billions of years. Another possibility is that the moon was heated by tidal winds from Earth. This could be because the moon's inner structure may have been stretched or relaxed as it orbited the Earth.
Another possible explanation is an impact of a large comet or asteroid, which in certain cases can cause a volcanic eruption.
For decoding lunar history, moon samples are crucial
Chang'e-5 was launched to the Moon in November. It landed close to an inactive volcano in the Oceanus Procellarum region, which is paved with a black volcanic rock called Basalt. It then collected more than 4 lbs (2 kgs) lunar rock. Some material was taken from the surface, while others were taken from 6 feet (1.83m), deep within the lunar crust.
Chang'e-5 then packed the samples in a capsule and sent it back to Earth. In December, the capsule reached Mongolia.
Scientists expect to learn more from future samples such as the Chang'e-5 rocks, which are so small in comparison to previous missions. China's next lunar mission Chang'e-6 will launch in 2024 to the far side and return more samples.
"If the moon were a continent-sized object like the Apollo and Luna samples, it would be like only sampling one state. Jolliff stated that if this were all we knew about a continent, it would be a nightmare.
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