China’s Yutu-2 mission has made another great discovery on the other side of the moon. The rover’s panoramic camera, shimmering amid dry and gray dust, captured two small, intact circles of clear glass.
Such balls can record information about the moon’s history, including the composition of its atmosphere and impact events. Yutu-2 has not been able to obtain compositional data, but these natural lunar globes could be important research targets in the future.
Glass is not rare on the Moon, as it happens. The material forms when silicate material is exposed to high temperatures, and both components are readily available on the moon’s surface.
In the lunar past, there was widespread volcanic activity, which led to the formation of volcanic glass. Impacts from smaller objects such as meteorites also generate intense heat, which leads to the formation of glass.
The latter is what could be behind the spheres 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 know for sure, because most of the glass on the Moon so far looks different from the spheres discovered by Yutu-2. There are many globules out there, but they tend to be less than a millimeter in size.
Here on Earth, such tiny glass balls are created during the impact, generating heat so intense that the crust melts and sprays into the air. The molten material solidifies and retracts as small glass beads.
Yutu-2 pellets are much larger, with a diameter of 15-25 mm. This alone does not make it unique; Glass spheres up to 40 mm in diameter were found from the side of the near moon during the Apollo 16 mission. These were traced to a nearby crater, and are believed to be artifacts as well.
But there are differences between the two discoveries. As explained by Xiao and colleagues, the distal lateral balls appear translucent or semi-transparent, and have a vitreous luster. In addition to the two that appeared to be semi-transparent, they found four other spheres that had a similar luster, but their transparency was not confirmed.
These balls were found near new impact craters, which may indicate that they formed during the lunar meteorite impact, although it is likely that they were already there, buried beneath the surface and only excavated by the impacts.
However, the team believes the most likely explanation is that it formed from a volcanic glass called anorthosite that melted again upon impact, reshaping into transparent, circular spheres.
“Collectively, the peculiar shape, geometry, and local context of the glass spheres are consistent with their being anorthosite impingement glass,” The researchers write in their paper.
This can make the lunar equivalent objects to the terrestrial formations called taktet Pebble-sized glass objects that form when earth material melts, volatilizes in the air, hardening and shaped like a ball when dropped again, like a larger version of those little balls.
We can’t know for sure without studying its composition, but if it’s a lunar tectonic, it could be very common on the moon. The team says this offers some exciting possibilities for future research.
“As the first detection of macroscopic and transparent glass spheres on the Moon, this study predicts that such globules should be abundant across the lunar highlands, providing promising sampling targets for revealing the early impact history of the Moon,” they write.
The paper detailing the discovery was published in Science Bulletin.
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