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       Just three small glass spheres contain evidence that the Moon was volcanically active as far back as 120 million years ago.
       Three tiny glass beads show that volcanic activity on the Moon occurred much earlier than geologists thought.
       In samples of lunar dust collected by China’s Chang’e 5 lander, a team of geologists recently discovered volcanic glass from an eruption about 120 million years ago — much later than the last known eruption of a lunar volcano. The discovery suggests that the moon’s recent history is far more complex and colorful than we thought.
       Geologist Wang Biben from the Chinese Academy of Sciences and several of his colleagues published their research results in the journal Science.
       This photograph taken by the Apollo 15 crew in 1971 shows the area near what would later become the Chang’e 5 landing site.
       When China’s Chang’e 5 lander touched down on the moon in 2020, it kicked up nearly 4 pounds of regolith: the rocks and dust that cover the lunar surface. Recently, Wang and his colleagues carefully examined the regolith and found more than 3,000 tiny, round particles of natural glass. Based on their texture and chemical composition, most of these tiny glass particles were formed by the heat and pressure of meteorite impacts, but three of them are cooled, smooth blobs of magma that once erupted from lunar volcanoes. (That’s 3 glass particles in nearly 4 pounds of material!)
       The chemical composition of the glass also revealed something else: the rapidly cooling magma droplets that erupted near the Chang’e 5 landing site were recent. If Wang and his colleagues are right, that could change our understanding of the moon’s recent geological history.
       The mixture of uranium and lead in the glass (uranium decays into lead over time, so the ratio of the two can give a clue to the age of the rock) suggests that the volcanic eruption that formed the glass occurred about 120 million years ago. From our perspective, that seems like an unimaginably long time ago, just around the time when the first placental mammals appeared on Earth. But geology moves much more slowly than evolution, so for the moon, 120 million years ago was essentially last week.
       This is somewhat surprising to geologists who study the history of the Moon, since the last evidence of volcanic eruptions on the Moon dates back to about 2 billion years ago (around the time life on Earth began storing its DNA in the nuclei of cells).
       ”It is unclear why the Moon remains volcanically active at such a late stage,” Wang and colleagues wrote in a recent paper. “As the Moon’s interior cools and the lithosphere (the part of the Moon made of rock) thickens, the likelihood of volcanic activity decreases.”
       Lunar soil simply does not exist because it contains organic matter (such as decaying plant matter and microorganisms). The fine-grained material that covers the surface of the Moon is called lunar regolith.
       The Moon formed about 4.5 billion years ago (from a giant collision between the newborn Earth and a slightly smaller planet), and for the next 2.5 billion years it was a volcanic hell. Lava flooded the lunar surface, covering such a huge area that after it cooled, the black basalt plains looked like oceans to astronomers. But about 2 billion years ago, these giant eruptions slowed and stopped—or so we thought.
       Some basaltic plains on the moon may be as young as 800 million years old because they have fewer craters than older plains that have been bombarded by meteorites over a longer period of time. But until geologists get actual rock samples from these areas of the moon, they won’t know for sure. That essentially leaves a 2-billion-year gap in the moon’s history, meaning there’s still a lot we don’t know about how the moon cooled and turned into the soft, rocky mass we know today. Because understanding how the moon cooled could reveal more about its interior, geologists are eager to fill that gap, especially as countries around the world show interest in exploring the moon.
       The Chang’e 5 probe found 2-billion-year-old volcanic rocks at its landing site near the extinct volcano Mons Rumke on the northwestern lunar front. They were much younger than geologists had thought at the time. But they were the youngest evidence of volcanoes ever found on the moon.
       Wang and his colleagues wondered whether the regolith returned by the lander might contain tiny traces of volcanic eruptions far from the landing site. When magma doesn’t contain much gas, it tends to ooze out of the ground. On the other hand, gas-bearing magma can erupt upward like a fountain, like a soda bottle that’s been shaken and then opened too quickly. Tiny droplets of magma could land far from the eruption site. And on a planet like the Moon, where meteorite impacts constantly reshape the surface, these tiny beads of volcanic glass would likely travel much farther from their point of origin.
       It turns out that Wang and his colleagues were right: Three pieces of volcanic glass, formed by distant volcanic eruptions, made it to the Chang’e 5 landing site. Geologists don’t have enough information to trace the glass shards to their source, but their presence alone suggests that the eruption was relatively recent in geological terms: about 120 million years ago.
       Wang and his colleagues compared the chemical composition of the three tiny glass fragments they found with other volcanic glass samples brought back by the Apollo missions in the 1970s. Wherever the Chang’e 5 volcanic glass fragments came from, they did not come from the same magma reservoir in the basaltic plains near the Apollo landing site.
       Instead, geologists believe that the moon’s recent eruptions may be driven by the last pockets of liquid magma remaining in the lunar mantle, kept warm by radiation emitted by the slow decay of potassium, thorium, and rare earth elements (all of which were detected in the glass by Wang and his colleagues).