Winner: Iraklis Giannakis

University of Aberdeen.

Title: Exploring the far side of the Moon using Ground-Penetrating Radar

Abstract:

In 2019, the Chang’E-4 lander and its rover Yutu-2 were the first human objects landed on the far side of the Moon. This marked a pivotal milestone in space exploration, an achievement of equal importance to the Apollo 8 mission in 1968, when the astronauts Frank Bornman, Jim Lovell and William Anders were the first humans to see the far side of the Moon from orbit. Chang’E-4 landed at the Von Kármán (VK) crater, a pre-Nectarian formation within the South Pole Aitken (SPA) basin. SPA is one of the largest impact craters in the Solar system, and it is believed that it has penetrated the Lunar’s crust and uplifted mantle materials. Mapping the subsurface of VK crater will shed a light into Lunar geology and evolution, especially for the basalt flooding events that took place across the Lunar surface during the Imbrian period. To that extent, Yutu-2 rover is equipped with Ground-Penetrating Radar (GPR), a mature geophysical technique capable of mapping the subsurface dielectric properties of an investigated medium. 

Chang’E-4 was the second space mission that used GPR as part of its scientific payload. Currently, there are four active/planned planetary rovers (Yutu-2, Perseverance, Rosalind Franklin, Chang’E-7) that include GPR as part of their scientific instrumentation. Rover-coupled GPR was also used on the –currently inactive– Yutu-1 (Moon) and Zhurong (Mars) rovers, and was also part of the payload of the Chang’E-5 lander. As well as rover-coupled GPR, satellite-borne GPR is being used in active orbiter missions (MRO, Mars Express, SELENE) to map sub-glacier lakes on Mars and Lunar ejecta layers. GPR is, therefore, one of the most widely used scientific instruments on planetary rovers and space missions in general, and one of the few in-situ geophysical methods applied in planetary exploration. GPR on board of Yutu-2 rover has three different channels; two high frequency antennas with 500 MHz central frequency, and one low frequency antenna with 70 MHz central frequency. The high frequency channels can provide a detailed imaging of the first 40-50 meters of the Lunar regolith, while the low frequency can reveal layered structures for down to 200-300 meters.  

We have developed novel GPR processing and interpretation tools, tuned for planetary environments, that allowed us to map the Lunar subsurface in great detail and infer its mineralogical composition. This process revealed multiple ejecta layers, and a series of basaltic lava floods with a thickness that decreases with  decreasing depth. This suggests that the Lunar volcanic activity cooled gradually with progressively smaller lava volumes through time, constraining the thermal evolution of the Lunar’s interior at the late stage of mare volcanism.