On the far side of the Moon, China's Yutu 2 mission has made yet another remarkable finding. The rover's panoramic camera picked out two little intact orbs of clear glass glistening among the dry, gray dust.
As it turns out, glass isn't rare on the Moon. When silicate material is heated to a high temperature, the substance develops, and both of these elements are easily available on the Moon.
These spherules potentially store data about the Moon's past, such as the makeup of its mantle and impact occurrences. Although Yutu 2 was unable to gather compositional data, these natural lunar marbles could be interesting future research targets.
There was widespread volcanism in the lunar past, which resulted in the development of volcanic glass; and collisions from smaller objects, such as meteorites, also resulted in the formation of glass.
According to a team of scientists led by planetary geologist Zhiyong Xiao of Sun Yat-sen University and the Chinese Academy of Sciences, the latter could be responsible for the spherules discovered by Yutu 2.
But it's difficult to say for sure because much of the glass discovered on the Moon so far looks nothing like the spherules identified by Yutu 2. There are plenty of spherules up there, although they are typically smaller than a millimeter in size.
On Earth, such tiny glass spherules are formed during an impact, generating such intense heat that the crust melts and sprays into the air. The molten substance cools and condenses into tiny glass beads.
The spherules on Yutu 2 are substantially larger, measuring 15 to 25 millimeters across. Glass balls up to 40 millimeters across were found from the Moon's near side during the Apollo 16 mission, but that doesn't make them unusual. These have been linked to a nearby crater and are likewise likely to be impact spherules.
However, there are distinctions between the two discoveries. According to Xiao and his colleagues, the far side spherules seem translucent or semi-transparent and have a vitreous sheen. They discovered four more spherules with a similar sheen to the two that appear to be translucent, but their translucency could not be confirmed.
These spherules were discovered near new impact craters, which suggests that they developed as a result of lunar meteorite impacts, but it's also possible that they were previously existent, buried beneath the surface, and were merely dug by impacts.
The team believes that the most likely explanation is that they formed from anorthosite, a volcanic glass that melted on contact and reformed as translucent spherical globs.
The paper describing the amazing find has been published in Science Bulletin.