Clare Kenyon
Moon samples from China’s Chang’e-5 mission were delivered to Earth in December, 2020. These are the first lunar samples returned since the Soviet Union’s Luna mission in 1976. Gathered from Oceanus Procellarum (the Ocean of Storms), a huge basin with a mysterious history, these samples surprised scientists as they indicated active lunar volcanism in the area as late as 2 billion years ago.
This is more than a billion years later than volcanic activity found in Apollo and Luna samples and, according to scientists’ calculations, the Moon should have cooled too much for volcanic activity to occur.
The two leading theories to explain this late volcanism suggest that the interior of the Moon contained either some heat-producing elements or had more water content than originally expected.
A new study published in Nature of the Chang’e-5 samples has turned these ideas on their head, suggesting that in fact the local chemistry of the mantle (the layer between the core and the thin crust) could be to blame.
By simulating how the moon’s mantle undergoes fractional crystallisation (a process by which crystals form and drop out of the host material) and mantle melting, the researchers from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS) were able to compare the chemical contents of the Chang’e-5 basalts to the Apollo samples.
The Chang’e-5 samples showed high levels of calcium oxide and titanium dioxide. This type of chemistry leads to rocks with a lower melting point compared to the rocks of the Apollo samples and showed that although the Chang’e-5 magmas were produced at the same depth below the surface (and therefore pressure) below the lunar surface, they were about 80 degrees Celsius cooler.
Read more: New insights into the Moon’s evolution
Early on in the Moon’s history, lunar oceans of magma covered its surface. The crystals precipitating out of the magma – the cumulates – are responsible for the lower melting point of the Chang’e-5 mantle.
“This is a fascinating result, indicating a significant contribution of late-stage lunar magma ocean cumulates to the Chang’E-5 volcanic formation,” said Dr. Su Bin, first author of the study.
The researchers suggest that these calcium-titanium-rich cumulates were added back into to the mantle by a process known as ‘gravitationally driven mantle overturn’, a process by which denser material sinks deeper into the mantle. This allowed volcanism within Oceanus Procellarum to occur, despite the mantle having cooled by 80 degrees Celsius by this time.