More than a year after landing on the far side of the moon, China’s Chang’E-4 (CE-4) spacecraft continues to unravel lunar secrets, this time allowing astronomers a glimpse 40 meters (130 feet) below the surface of the moon in an attempt to understand its geological architecture.
On January 3, 2019, CE-4 landed on the eastern floor of the Van Kármán crater, near the Moon's south pole, making it the first spacecraft to successfully touch down on the far side of the moon. Just two days after landing, its rover Yutu-2 captured the first high-resolution images of what the moon looks like below its surface, providing direct measurements of thickness and the lunar internal architecture. Those images are now published in the journal Science Advances.
Yutu-2 uses Lunar Penetrating Radar to investigate underground while it roams, sending radio signals deep into the surface of the moon at a frequency of 500 MHz – three times stronger than previous attempts reached by CE-4’s predecessor, CE-3. Radar images combined with tomographic data and quantitative analysis of the subsurface paint an image of how the moon is made up. (Spoiler alert: it's not cheese.)
"We found that the signal penetration at the CE-4 site is much greater than that measured by the previous spacecraft, Chang'E-3, at its near-side landing site," said study author LI Chunlai, a research professor and deputy director-general of the National Astronomical Observatories of the Chinese Academy of Sciences, in a statement. “The subsurface at the CE-4 landing site is much more transparent to radio waves, and this qualitative observation suggests a totally different geological context for the two landing sites."
Though it is difficult to identify specific geological structures and construct a timeline of events, the researchers are able to determine that the moon is made of highly porous granular materials with boulders of varying sizes embedded within. Such makeup is the result of a turbulent early galaxy when meteors and other objects struck the moon, ejecting impact materials to other areas to create different levels of material within. Last fall, the rover discovered a “gel-like” substance with “mysterious luster” on the far side of the moon that turned out to be impact glass created when meteors or other space objects impacted the moon’s surface.
"This work shows the extensive use of the LPR could greatly improve our understanding of the history of lunar impact and volcanism and could shed new light on the comprehension of the geological evolution of the Moon's far side,” said Li.