Chang’E-4 LPR unveiled the Moon’s farside shallow subsurface structure

What lurks below the surface?

China’s spacecraft Chang’E-4 (CE-4) landed on the eastern floor of the Van Kármán crater, near the Moon’s south pole, on Jan. 3, 2019. After landing, the probe immediately deployed its Yutu-2 rover, which uses Lunar Penetrating Radar (LPR) to investigate the underground it roams.

A year after landing, the Chang’E-4 spacecraft continuing to unveil secrets from the far side of the Moon. In a recently published study, scientists revealed the observations made by the Lunar Penetrating Radar (LPR) onboard the Yutu-2 rover during the first two lunar days.

Paper author Li Chunlai, a research professor and deputy director-general of the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) said, “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. The subsurface at the CE-4 landing site is much more transparent to radio waves, and this qualitative observation suggests a different geological context for the two landing sites.”

Scientists used LPR to send radio signals deep into the surface of the moon, arriving at a depth of 40 meters by the high-frequency channel of 500 MHz. This data allowed the specialists to build up a rough picture of the subsurface stratigraphy.

Su Yan, a corresponding author who is also affiliated with NAOC, said, “Despite the good quality of the radar image along the rover route at the distance of about 106 meters, the complexity of the spatial distribution and shape of the radar features make identification of the geological structures and events that generated such features quite difficult.”

Scientists combined the radar image with tomographic data and quantitative analysis of the subsurface. They inferred that the subsurface is mainly made by highly porous granular materials embedding boulders of various sizes. The substance is likely the result of turbulent early galaxy when meteors and other space debris frequently struck the Moon. The effect-site would eject material to different areas, making a cratered surface on a subsurface with changing layers.

The results of the radar data provide the first electromagnetic image of the far side subsurface structure and the first ‘ground truth’ of the stratigraphic architecture of an ejecta deposit.

Li said, “The results illustrate, in an unprecedented way, the spatial distribution of the different products that contribute to the ejecta sequence and their geometrical characteristics. 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.”

The study is published on Feb. 26 in Science Advances.

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