Although Von Karman crater lies within what is known as the South Pole-Aitken basin, an ancient 1, 288 – mile-wide impact crater, it is too far north for there to be ice in the soil. the radar waves passed through the top 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.
“It’s an old area,” Dr. Pettinelli said.
On Earth, very little of the surface is shaped by asteroid impacts . On the moon, the effects of space rock strikes can be seen almost everywhere.