Three tiny beads of glass reveal that the Moon was volcanically active much more recently than geologists thought.
Amid samples of Moon dust from China’s Chang’e-5 lander, a team of geologists recently found volcanic glass that formed in an eruption about 120 million years ago — long after the last known volcanic eruptions on the Moon. The finding suggests that the Moon’s recent history is a lot more complicated, and a lot more eventful, than we realized.
Geologist Bi-Ben Wang of the Chinese Academy of Sciences, with several colleagues, published their work in the journal Science.
An Extremely Lucky Find: Just 3 Glass Beads
When it visited the Moon in 2020, China’s Chang’e-5 lander scooped up almost 4 pounds of regolith: the crushed rock and dust that covers the surface of the Moon. Wang and colleagues recently sifted through that regolith to find more than 3,000 tiny, round grains of natural glass. Based on their texture and chemical makeup, most of those teeny glass bits had formed in the tremendous heat and pressure of the meteor impacts, but three of them are the cooled, smooth droplets of magma that once erupted from lunar volcanoes. (That’s 3 grains of glass in almost 4 pounds of material!)
The chemical makeup of the glass revealed something else, too: the eruptions that splattered droplets of rapidly-cooling magma near Chang’e-5’s landing site happened fairly recently. If Wang and colleagues are right, that could change what we think we know about 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 elements can tell you how old a chunk of rock is) suggests that the eruptions that formed the glass happened about 120 million years ago. That’s around the time that, here on Earth, the first placental mammals were just arriving on the scene, which feels like an unfathomably long time ago from our perspective. But geology moves much more slowly than evolution, so for the Moon, 120 million years ago is basically last week.
That’s a bit of a surprise for geologists who study the Moon’s history, because the most recent evidence of volcanoes erupting on the Moon dates back to about 2 billion years ago (about the time that life on Earth started keeping its DNA in a cell nucleus).
“It’s unclear how the Moon could have remained volcanically active at such a late stage,” write Wang and colleagues in their recent paper. “As the interior cooled and the lithosphere (the part of the Moon made of solid rock) thickened, volcanic activity would have become less likely.”
The Moon’s Eventful Past
The Moon formed (in a giant crash between newborn Earth and a slightly smaller planet) around 4.5 billion years ago, and it spent the next 2.5 billion years as a volcanic hellscape. Lava flooded the surface, covering areas so vast that, once cool, the plains of black basalt looked like oceans to astronomers. But sometime around 2 billion years ago, those tremendous eruptions slowed and stopped — at least, we think so.
Some of the Moon’s basalt plains may be as young as 800 million years, based on how few craters they bear compared to older plains (which have suffered meteor bombardments for much longer). But until geologists get their hands on actual samples of the rock from those areas of the Moon, they won’t know for sure. That leaves basically a 2 billion year gap in the Moon’s history, which means there’s a lot we don’t yet know about how the Moon cooled and settled into the laid-back chunk of rock we know today. Since learning how the Moon cooled could reveal more about the structure of its interior, that’s a blank space geologists would like to fill in, especially with countries around the world ramping up for a frenzy of lunar exploration.
Chang’e-5 found volcanic rock dating back to 2 billion years ago, which was much younger than geologists expected to find at the time, at its landing site near the extinct volcano Mons Rümker, in the northwestern part of the Moon’s near side. But that’s the youngest evidence of volcanoes on the Moon anyone has found so far.
Wang and colleagues wondered if the regolith the lander had brought home might contain tiny traces of eruptions that had happened far from the landing site. When magma doesn’t contain much gas, it tends to ooze out of the ground. Gassy magma, on the otherhand, bursts upward in a fountain, like a soda bottle that’s been shaken and then opened too quickly. Tiny droplets of magma could land far away from the eruption. And on a world like the Moon, where meteor impacts are constantly rearranging the surface, it’s likely that those tiny beads of volcanic glass could get shuffled around even farther from where they started.
It turns out that Wang and colleagues were right: Three bits of volcanic glass from far-off eruptions had found their way to the Chang’e-5 landing site. Geologists don’t have enough information to trace those bits of glass to the eruptions that spawned them, but just the fact that they exist shows that volcanoes were erupting on the Moon pretty recently in geological time: around 120 million years ago.
Last Gasp of the Moon Volcanoes?
Wang and colleagues compared the chemical makeup of their three tiny pieces of glass to other samples of volcanic glass brought home aboard the Apollo missions in the 1970s. Wherever Change’e-5’s stray bits of volcanic glass came from, it wasn’t the same reservoir of magma that filled the basalt plains near where the Apollo missions landed.
Instead, the geologists suggest, the most recent eruptions on the Moon could have come from the last few pockets of liquid magma left in the Moon’s mantle — kept hot by radiation from the slow decay of potassium, thorium, and rare-earth elements (all of which Wang and colleagues found in the glass).
