Today it’s dusty, crater-ridden and largely inert, but our moon has a fascinating geological history. The generally accepted theory is that it formed from a catastrophic impact between a protoplanet and the Earth, leaving it covered by an ocean of magma for millennia. That ocean eventually cooled into the lunar crust, but volcanic activity persisted for many millions of years.
Exactly how long volcanism persisted has been something of a mystery. However, a new paper published September 5 in Science suggests that it continued for much longer than previous evidence indicated. The study examined beads of volcanic glass collected by the Chang’e-5 mission, which landed on the moon in December 2020, and found that they formed around 120 million years ago. The study demonstrates how painstaking such research can be: it involved carrying out a series of tests on 3000 tiny beads of glass, looking for minute differences between them to identify which of them were created by volcanism.
[ Related: The moon was once covered in an ocean of magma: new data supports theory ]
Glass formation is a complex phenomenon, but the key mechanism in this context is that it is created when a material is heated past its melting point and then solidifies into an amorphous solid that lacks a regular atomic structure. Volcanic magma can certainly provide sufficient heat to melt the lunar regolith, so glass can be a telltale sign of volcanism. However, volcanism isn’t the only process that can generate such temperatures. Objects like meteorites colliding with the moon’s surface can also do so, so to derive any information about lunar volcanism from the glass beads in the Chang’e-5, researchers first had to identify which (if any) of these beads were created by volcanic heat.
The first step was examining the beads’ levels of magnesium oxide (MgO) and ratio of MgO to aluminum oxide (Al2O3). Examination of samples taken by the Apollo-16 mission established that volcanic glass has relatively high MgO levels and a relatively high ratio of MgO to Al2O3. Only 13 beads met these criteria.
However, the paper also notes that this method is not infallible: “[Around] 3% of … previously identified impact glasses in the Apollo samples also have ratios consistent with a volcanic origin.” The authors then moved onto examining the glass’s nickel (Ni) content: glass created by impacts has relatively high levels of Ni “owing to the incorporation of impactor materials,” while the level of Ni volcanic glass is correlated with the level of MgO.
Once the beads with excessive Ni levels were eliminated, the researchers carried out more tests, this time examining the composition of sulfur isotopes in the glass. When they were done, they were left with precisely three humble pieces of glass, each less than a millimeter in diameter, from an initial sample of around 3,000.
Nevertheless, these three beads may well upend our understanding of the moon’s history of volcanism. Researchers dated them via a process called uranium-lead dating and found that they were around 120 million years old, implying that volcanoes were active on the moon 120 million years ago. Considering that previous evidence—including other studies of material from Chang’e-5—have confirmed only that volcanism on the moon persisted until some 2 billion years ago, this result raises as many questions as it answers.
The paper theorizes that the volcanism that generated the glass beads could have been caused by “radiogenic heating, generating local thermal anomalies that trigger[ed] partial melting” of the lunar mantle. However, they conclude that it is ultimately “unclear how the Moon could have remained volcanically active at such a late stage”. It seems that our closest neighbor will hang onto some of its secrets for a while longer.