In January 2019, China landed its Chang’e 4 mission on the Moon’s far side. The Yutu-2 rover got busy exploring its surroundings. It’s still going, even though the rover’s nominal operating mission was only three months.
Among the mission’s findings was a strange material described as “gel-like.” Now an analysis of the material has revealed that it’s just rock: impact melt breccia.
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The context of the discovery is important. The rover was on the Moon’s far side, a place we’ve never visited before. Was it possible that China would find something new there? Something that simply doesn’t exist on the near side? After all, the near and far sides of the Moon are quite different from each other.
Rovers use spectral analysis to analyze the chemistry of things they find. Yutu-2 has a spectrometer, the Visible and Near-Infrared Imaging Spectrometer (VNIS). But the problem is that interpreting spectrometry properly also relies on knowing the background spectrometry of the area the instrument is in. And since nobody, or no rover, has ever been to the Moon’s far side, that background information doesn’t exist.
“We don’t have samples from this region that would help inform the model parameters. For this reason, the precise regolith composition results presented in this paper may not be completely accurate,” said NASA’s Dan Moriarty in an interview at Space.com.
But the new paper presents the results of a rigorous scientific analysis. The title of the paper is “Impact melt breccia and surrounding regolith measured by Chang’e-4 rover.” The lead author is Sheng Gou, from the Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Sciences. The paper is published in the journal Earth and Planetary Science Letters.
In their paper, the authors note that the spectrometry measurements were done in difficult conditions, with poor illumination. But they still say that the “gel-like” substance is in fact simple impact melt breccia. They also say that it resembles Moon rocks brought back by the Apollo missions.
After analyzing the rock, and the surrounding areas, the researchers think they have a best-fit explanation for the rock itself.
The “gel-like” rock is inside a small crater, which is inside a large crater, which is inside an enormous crater. The enormous crater is the South-Pole Aitken basin, which would have created an enormous melt pool at the time of impact. The sample location is right on the edge of the melt pool. According to the researchers, the rock is likely a mixture “from a differentiated melt pool or from a suite of igneous rocks,” resulting from nearby crater impacts.
They say the rock “was formed by impact-generated welding, cementing and agglutinating of lunar regolith and breccia.”
When the Yutu-2 rover found the strange rock, scientists were puzzled. The team piloting the rover even paused the normal driving activities. And the discovery made headlines around the world.
Now, it’s almost a let-down to discover that the “gel-like” substance is just rock. Even if it is a complex rock, created from the heat of an impact.