China's Yutu-2 mission has made another fascinating discovery on the far side of the Moon. Glistening amid the dry, gray dust, the rover's panoramic camera picked out two small intact spheres of translucent glass.
Such spherules can record 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 marbles could be important research targets in the future.
Glass isn't uncommon on the Moon, as it happens. The material forms when silicate material is subjected to high temperature, and both of those ingredients are readily available on the Moon.
In the lunar past, there was extensive volcanism, leading to the formation of volcanic glass; and impacts from smaller objects such as meteorites also generate intense heat, resulting in the formation of glass.
The latter is what could be behind the spherules 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, though, because most of the glass found on the Moon to date looks different from the spherules discovered by Yutu-2. There are spherules up there aplenty, but they tend to be less than a millimeter in size.
Here on Earth, such tiny glass spherules are created during an impact, generating such intense heat that the crust melts and sprays into the air. The molten material hardens and falls back down as tiny glass beads.
Yutu-2's spherules are much larger, at 15 to 25 millimeters across. That alone doesn't make them unique; glass balls up to 40 millimeters across were recovered from the Moon's near side during the Apollo 16 mission. These were traced to a nearby crater, and are thought to be impact spherules also.
But there are differences between the two discoveries. As Xiao and his colleagues explain, the far side spherules seem to be translucent or semi-transparent, and have a vitreous luster. In addition to the two that seem to be translucent, they found four more spherules that have a similar luster, but their translucency could not be confirmed.
These spherules were found near fresh impact craters, which could suggest that they formed during lunar meteorite impacts, although it's possible that they were already present, buried below the surface and were merely excavated by impacts.
However, the team believes that the most likely explanation is that they formed from volcanic glass called anorthosite that melted again on impact, re-forming into translucent round globs.
"Collectively, the peculiar morphology, geometry, and local context of the glass globules are consistent with being anorthositic impact glasses," the researchers write in their paper.
This could make the objects the lunar equivalent of terrestrial formations called tektites – pebble-sized glassy objects that form when Earth material melts, sprays into the air, and hardens and forms into a ball as it falls back down, like a larger version of those tiny spherules.
We can't know for sure without studying their composition, but if they are lunar tektites, they might be quite common on the lunar surface. This offers some tantalizing possibilities for future research, the team says.
"As the first discovery of macroscopic and translucent glass globules on the Moon, this study predicts that such globules should be abundant across the lunar highland, providing promising sampling targets to reveal the early impact history of the Moon," they write.
The paper detailing the discovery has been published in Science Bulletin.