China’s Yutu-2 mission has made another fascinating discovery on the far side of the Moon. The rover’s panoramic camera found two small, pristine translucent glass spheres glowing amidst the dry, gray dust.
Such spheres can provide information about the Moon’s history, including the composition of its mantle and impact events. Yutu-2 was unable to obtain compositional data, but these natural lunar spheres could be important research targets in the future.
Actually, glass is not uncommon on the Moon. But glass is formed when the silicate material is exposed to high temperatures, and both of these components are readily available on the Moon.
There was extensive volcanism in the lunar past that led to the formation of volcanic glass, and impacts from smaller objects such as meteorites can also generate intense heat, causing glass to form. This latter reason appears to be behind the globules observed by Yutu-2, according to a team of scientists led by planetary geologist Zhiyong Xiao of Sun Yat-sen University and the Chinese Academy of Sciences.
It’s hard to say for sure though, because most of the glass found on the Moon to date looks different from the spheres discovered by Yutu-2. There are many globules on the Moon, but they are usually less than a millimeter in size.
Yutu-2’s spheres are much larger, with a diameter of 15 to 25 millimeters. But this size alone does not make them unique; Glass spheres 40 millimeters in diameter were found near the Moon during the Apollo 16 mission. These have been traced to a nearby crater and are also thought to be impact spheres.
Also on Earth, many small glass globules can be formed due to the enormous heat generated during an impact and the crust that melts into the air. The molten material hardens and falls back into tiny glass beads.
But there are differences between the two discoveries. As Xiao and colleagues explain, the far-side orbs appear translucent or transparent and have a vitreous luster. In addition to the two spheres that appeared translucent, they found four more spheres of similar luminosity, but their translucency could not be confirmed.
These globules were found near new impact craters, suggesting that they were formed during lunar asteroid impacts, although they already exist, are buried below the surface, and were only excavated by impacts.
However, the team believes that the most likely explanation is volcanic glass called anorthosite, which melts again on impact and turns into translucent round spheres again. “Collectively, the distinctive morphology, geometry, and local context of the glass beads are consistent with being anorthocytic impact glasses,” the researchers write in their paper.
This could make objects the Moon equivalent of formations called tektites, a larger version of pebble-sized glassy spheres that form when Earth material melts, blasts into the air and hardens and falls back into a ball.
We can’t know for sure without examining their composition, but if they are lunar tektites, they may be quite common on the lunar surface. The team says this offers some encouraging possibilities for future research…
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