A pair of translucent glass spheres, each measuring over a half-inch thick, have been spotted in an impact crater near the lunar south pole. They’re the first of their kind to ever be discovered on the Moon.
New research in Science Bulletin describes the “the first discovery of macroscopic and translucent glass globules on the Moon.” The beads likely formed from the heat generated by a violent impact or possibly from early volcanic activity. The finding is significant, as glass spheres “record important information about the mantle composition and the history of lunar volcanism and impact cratering,” according to the research, lead by Zhiyong Xiao, associate professor at the Planetary Environmental and Astrobiological Research Laboratory at Sun Yat-sen University in Guangzhou, China.
The “translucent glass globules,” as the scientists describe them, were spotted by the eight-wheeled Yutu-2 rover. The rover, as part of China’s Chang’e 4 mission, is currently investigating selenological (the lunar equivalent of “geological”) and chemical differences between the near and far sides of the Moon. The tiny rover landed in the 115-mile-wide (186-kilometer) Von Kármán Crater on January 3, 2019. This large crater is situated within the much larger Aitken Basin—the Moon’s biggest impact basin—near the lunar south pole.
That Yutu-2 stumbled upon glass is hardly special, as the stuff is strewn across much of the Moon’s surface. What makes these particular objects unique is their large size and translucent nature. Lunar glass tends to be small, measuring less than 1 mm (0.04 inches), but Apollo astronauts did manage to spot some beads as large as these. The two globules in Von Kármán Crater are estimated to be between 0.5 and 1 inches (1.5 cm-2.5 cm) in diameter, hence their description as being “macroscopic” in size.
But whereas the Apollo samples were dark and dull, the newly discovered beads are “translucent or semi-transparent and they exhibit a vitreous luster,” the researchers write. Four other similar objects were spotted by Yutu-2’s panoramic camera, but the image resolution wasn’t clear enough for the scientists to identify them as being glass spheres.
The shape and location of these spherules suggest they’re impact glasses, rather than objects delivered from other planetary bodies or products of volcanic activity. Glass globules form from the intense heat produced by large impacts, in which silicates liquify and assume a spherical shape when airborne. Tossed into the sky, the liquid balls cool rapidly, returning to the surface as glass.
Volcanic eruptions can do the same, but volcanoes haven’t been active on the Moon in quite some time. What’s more, the characteristics of these beads aren’t really consistent with volcanism, the scientists say, arguing that they’re impact glasses produced by “anorthositic melt.” Lunar anorthosite is a type of igneous rock that’s common in the lunar highlands near the south pole. A limitation of the paper is that the exact composition of the beads could not be determined.
The scientists say the glass beads formed recently or were only recently exposed. The top inch of the lunar regolith gets covered in less than 100,000 years, “indicating the globules might be extremely young,” according to the paper. It’s likely that a recent impact event kicked up the beads, depositing them onto the surface.
Similar objects have been found on Earth, and they’re called tektites. The scientists suspect that these macroscopic objects are a common feature of the Moon and that other spherules should “be abundant across the lunar highland.”