You may remember way back at the start of January 2019 (yes, a lifetime ago), the historic landing of China’s Chang’e-4 mission that deposited its Yutu-2 rover on the far side of the Moon. Since then it has sent us back spectacular views from the lesser-known side of our satellite. Six months into its pottering across the lunar surface, however, it came across something curious in a crater: what appeared to be a “gel-like substance” with a “mysterious luster”.
Well, now we know what it is. If you had aliens on your 2020 bingo card, we’re sorry to disappoint. It turns out it’s… rocks. Cool, green, glassy melted rocks, but still rocks.
In a new paper published in Earth and Planetary Sciences Letters, Chinese scientists have confirmed what many lunar researchers suspected: the substance spotted that appeared so different to the surrounding soil was a “dark greenish and glistening impact melt breccia.”
A breccia is a rock made up of fragments from different types of rock cemented together by a finer material, in this case lunar regolith and melted rock in the form of a shiny, dark green, glassy mass, measuring 52 by 16 centimeters (20 by 6 inches). Impact melt breccias, as they sound, are the result of an object – a meteorite for example – impacting on a rocky surface with such force and heat that it melts the rocks, incorporating this melt into the breccia.
The substance was first noticed in July 2019 by a mission member surveying a panoramic image of the crater the rover had just been exploring. The substance was a noticeably different texture and color to the surrounding regolith (the layer of loose soil, dust, and gravel that covers the solid bedrock) and warranted a closer look.
Unfortunately, the crater it was spotted in was too deep for Yutu-2, so it scanned the crater with its Visible and Near-Infrared Spectrometer (VNIS), and both its panoramic camera and hazard avoidance cameras.
The researchers analyzed these images, including the light reflecting off the substance, to determine its chemical composition, as well as that of the surrounding regolith. According to the study, the material resembles two lunar samples, 15466 and 70019, that were brought back by the Apollo missions and later confirmed as impact melt breccia.
The researchers conclude the breccia was most likely caused by a meteorite impact, though it's unclear if this actually happened in the crater it was found in. This crater measures 2 meters (6.5 meters) across, and the researchers calculated that the estimated size of an impact object that would create a crater this size as just 2 centimeters, not big enough to create a breccia that measures over 50 centimeters. It's more likely it was formed elsewhere and blasted into this crater in a cosmic hole-in-one.
While the researchers note it's difficult to confirm the analysis without actual samples to analyze, Yutu-2 has already spotted another crater with a similar-looking shiny, reflective substance inside that has been identified for exploration, so hopefully, more details will be forthcoming.