China’s Rover Finds Layers of Surprise Under Moon’s Far Side
The Chang’e-4 mission, the first to land on the lunar far side, is demonstrating the promise and peril of using ground-penetrating radar in planetary science.
China’s Yutu-2 rover on the moon’s surface. Ground-penetrating radar on the rover examined soil on the moon at depths of more than 100 feet.
China’s robotic Chang’e-4 spacecraft did something last year that had never been done before: It landed on the moon’s far side, and Yutu-2, a small rover it was carrying, began trundling through a crater there. One of the rover’s instruments, a ground-penetrating radar, is now revealing what lies beneath.
In a paper published Wednesday in the journal Science Advances, a team of Chinese and Italian researchers showed that the top layer of the lunar soil on that part of the moon is considerably thicker than some expected — about 130 feet of what scientists call regolith.
“It’s a fine, dusty, sandy environment,” said Elena Pettinelli, a professor of mathematics and physics at Rome Tre University who was one of the authors of the paper.
Based on what NASA astronauts observed during the Apollo moon landings, other scientists said they would have expected one-quarter as much soil.
Arrows at lower right show the Chang’e 4 landing site in Von Karman crater on the moon.
Although Von Karman crater lies within what is known as the South Pole-Aitken basin, an ancient 1,100-mile-wide impact crater, it is too far north for there to be ice in the soil.
The radar waves passed through the top 40 feet or so almost effortlessly, indicating a porous granular material. Below that, there were boulders, perhaps a couple of feet to a couple of yards in size. A third slice of soil, even lower, appeared to consist of alternating layers of fine and coarse particles but without boulders.
One surprise was that the researchers saw no signs of the radar bouncing off basalt — solidified lava — that would have pooled at the bottom of a crater as the rocks melted by a meteor impact cooled. Yutu-2’s radar signals would have bounced off that rock if the rover had visited Von Karman crater soon after it formed.
But several billion years later, the basalt surface has been buried by regolith that was subsequently tossed up by later impacts. The top layer of fine particles may have also once contained boulders, but those may have been broken apart in eons of subsequent cosmic pummeling.
The Chang’e-4 lander, photographed by the Yutu-2 rover.