Lunar Penetrating Radar Reveals Three Phases of Volcanism at Von Kármán Crater

The Chinese Lunar mission Chang'E-4 soft-landed on the far side of the Moon on January 2019 marking a significant milestone in space exploration. The mission's landing site is on the eastern floor of Von Kármán (VK) crater (45.4446°S, 177.5991°E), within the South Pole–Aitken (SPA) basin, one of the oldest and largest impact craters in the solar system. Yutu-2 is the rover of the Chang;E-4 mission. Similar to its twin rover Yutu-1, amongst its scientific payloads Yutu-2 carries a set of ground-penetrating radar (GPR) systems. GPR is a well-established geophysical method and has been instrumental in the new era of planetary exploration. Chang’E-3 was the first mission incorporating in-situ planetary GPR, a trend continued by subsequent Lunar and Martian missions, including Chang'E-4, Perseverance, Chang'E-5 and Tianwen-1; with plans for future missions such as Chang'E-7 and ExoMars [1]. Existing Lunar GPR studies often assume that the dielectric properties of Lunar materials can be modelled via a constant electric permittivity and a conductive term. However, treating the electric permittivity as non-dispersive overlook the frequency-dependent complex electric permittivity of ilmenite. Ilmenite is a titanium mineral, particularly abundant in Lunar mare basalts and soils. Recent investigations [1] using a complex Cole-Cole function have shown that ilmenite-mixtures act as low-pass filters, causing a decrease in the pulse's central frequency as the wave propagates through an ilmenite formation. This frequency shift, proportional to the ilmenite content, serves as a basis for inferring the presence of basalts and approximating their ilmenite content. In this study, we explore the frequency shift of signals received both from Channel-2B and Channel-1. Our analysis reveals a sequence of basaltic layers extending to approximately 300 m depth, displaying varying thickness and ilmenite content. Based on the estimated ilmenite content, the GPR data indicates three distinct phases of Lunar volcanism: an early phase with high-Ti basalts, followed by a low-Ti volcanic activity, and a final phase with high-Ti basalts. These findings align with generic models of Lunar lava emplacement [1]. According to these models, Lunar volcanic history includes an early "blue" titanium-rich volcanic event (~ 3.8-3.5 Ga), followed by low-Ti "red" basalts (~ 3.5-3 Ga), and a subsequent phase of "blue" high-Ti basalts (~3 Ga) [2]. References [1]   Giannakis, I., Martin-Torres, J., Su, Y., Feng, J., Zhou, F., Zorzano, M-P., Warren, C., Giannopoulos, A., (2024). Evidence of Shallow Basaltic Lava Layers in Von Kármán Crater from Yutu-2 Lunar Penetrating Radar, Icarus, 2024.   [2] Cattermole, P. J., (1996). Planetary Volcanism: A Study of Volcanic Activity in the Solar System, Wiley, Chichester, 2ndEdition, 1996.