Fifty years ago, NASA and the Soviet space program conducted the first sample-return missions from the Moon. This included lunar rocks brought back to Earth by the Apollo astronauts and those obtained by robotic missions that were part of the Soviet Luna Program.
The analysis of these rocks revealed a great deal about the Moon's composition, formation, and geological history. In particular, scientists concluded that the rocks were formed from volcanic eruptions more than 3 billion years ago.
In recent years, there has been a resurgence in lunar exploration as NASA and other space agencies have sent robotic missions to the Moon (in preparation for crewed missions).
For instance, China has sent multiple orbiters, landers, and rovers to the Moon as part of the Chang'e program, including sample-return missions.
Their research could provide valuable insight into how young volcanism shaped the lunar surface.
The research was conducted by a team from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS), led by Su Bin, Yuan Jiangyan, and Chen Yi – members of the IGGCAS Laboratory of LIthospheric Evolution and Earth and Planetary Physics.
They were joined by researchers from the Lunar and Planetary Science Institute (LPSI) at Nanjing University and the CAS Center for Excellence in Comparative Planetology. A paper that describes their findings appeared in the journal Science Advances on 21 October.
Based on samples returned from the Apollo and Luna missions, scientists theorized that the Moon has been geologically dead for the past 3 billion years.
However, the new samples of lunar rock obtained by the Chang'e-5 mission (and returned to Earth in 2021) were only 2 billion years old, indicating that volcanic activity occurred at least a billion years longer than previously expected.
As a small rocky body, the heat that fueled volcanism on the Moon should have been lost long before these eruptions occurred.
Previously, scientists speculated that late-stage volcanism might have been driven by elevated water content or the decay of radioactive elements in the lunar mantle. However, the many analyses performed on the samples obtained by the Chang'e-5 rover have ruled out this consensus.
Based on their analysis, the CAS researchers found that minerals with low-melting points in the mantle could have allowed for compression, leading to young volcanism. Prof. Chen explained in a recent CAS statement:
"Recent melting of the lunar mantle can be achieved by either raising the temperature or lowering the melting point," he said. "To better understand this problem, we should estimate the temperature and pressure in which the young volcanism was created."
For their analysis, the CAS team conducted a series of fractional crystallization and lunar mantle melting simulations that compared 27 basalt clasts obtained by the Chang'e-5 mission to those returned by the Apollo missions.
They found that the young magma samples had higher calcium oxide and titanium oxide concentrations than older Apollo magma samples.
The presence of these minerals, which are more easily melted than earlier mineral cumulates in the lunar mantle, means that volcanism would have been gravitationally-driven and caused material in the mantle to overturn.
Their analysis revealed that the mantle's compression could have occurred at similar depths but under cooler temperatures that would have still produced volcanoes.
This research is not dissimilar to what planetary scientists have learned about Mars in recent years. Billions of years ago, the red planet had thousands of eruptions on its surface, some of which resulted in the largest volcanoes in the Solar System (like Olympus Mons).
Scientists suspected that Mars became geologically dead as its interior cooled. But recent findings indicate that it may still experience limited volcanic activity.
This study presents the first viable explanation for young volcanism on the Moon that is compatible with the samples returned by the Chang'e-5 rover.
This study could inform future planetary studies of the Moon's thermal and geological evolution.
As Dr. Su indicated:
"This is a fascinating result, indicating a significant contribution of late-stage lunar magma ocean cumulates to the Chang'e-5 volcanic formation. We discovered that the Chang'e-5 magma was produced at similar depths but 80 degrees Celsius cooler than older Apollo magmas. That means the lunar mantle experienced a sustained, slow cooling of 80 degrees Celsius from some 3 billion years to 2 billion years ago."