Chinese rover spots a pair of glassy orbs on the Moon

A couple of glass marbles would be way down on the list of weird things I would expect to see on the Moon.

The Chinese Yutu-2 rover appears to have found a pair of glassy-looking spherules, just sitting in the open on the lunar dust.

It is not clear how they formed or what they are doing there. The lunar regolith is made from micrometeorites eroding rocks and thermal expansion and contraction breaking them apart as well as tiny glass beads, which are common in the lunar regolith. A human hair is about 100 microns wide.

The glass beads are thought to be volcanic in origin, created as a result of the cooling of ancient lunar volcanoes. If that material cools slowly, it can form crystals, but if it cools rapidly, there isn't enough time for them to form, and the result is an amorphous structure: glass. The story of the orange glass beads found on the surface of the moon is a cool one, and I imagine how the astronauts felt when they saw the bright orange material on the moon.

During a comet impact, glassy beads can be formed. The material surrounding the impact is blasted out in jets by the energy of the impact. These break up into smaller bits as they fly over the surface, which then cool rapidly to form glassy spheroidal beads as well. These can be up to 4 centimeters in size, and are usually darker than the smaller volcanic beads.

The new ones are so big that they appear translucent in the images taken by Yutu-2. It is not possible to prove they are glass from the images alone, but given the circumstantial evidence outlined below it is not a bad bet.

How did they form?

The first mission to land on the far side of the Moon was by the Chinese Chang'e 4 lander. The floor of the crater is flat and indicates that lava flooded the floor after the impact. There is a massive landslide to the north that covers much of the crater floor, and there is a viscous volcanic upwelling in the southwest corner. There are indications that the crater surface has younger impacts nearby.

The Yutu-2 rover has traveled over a kilometer of the lunar surface, and has seen a greenish blob of material that is likely glassy impact melt. The newly found spherules still stand out. They are sitting on the surface. If that is where they landed, they must be young, because over time smaller impacts overturn the top few centimeters of regolith, which is lovely. On the other hand, there are large chunks of rock around them that are likely pieces ejected from cratering and thrown out by smaller, later impacts.

There isn't a lot of evidence of eruptive volcanism in the area where Yutu-2 is landing. Basalt, a volcanic rock on the Moon, is hard to turn into glass because it tends to cool slowly and form crystals. The area of the crater where anorthosite is common has a complicated geologic history according to the authors of the paper. When heated by an impact and melted, anorthosites cool quickly enough that they can become glassy, so it is a decent candidate for the source material of the odd glass marbles. Yutu-2 is on top of a 40-meter layer of debris that was blown out by impacts some distance away.

Yutu-2 doesn't have the right instruments to determine the composition of the spherules It was not designed for that kind of analysis. It has a camera, a ground-penetrating radar, a spectrometer, and a neutral atom analyzer to see how the solar wind interacts with the lunar surface. The glassy spherules can't be analyzed by these.

The finding of two of them sitting on the surface suggests they could be common in this area, and possibly other places on the Moon as well. They could have gasses trapped in bubbles inside them, which can be analyzed on Earth to get a better idea of how and when they formed, providing insight on what the Moon was like in the past. Multiple countries are planning extensive exploration of the Moon in the near future, so we may soon know a lot more about these little lunar marbles.