Mapping the Lunar Volcanic Stratigraphy in the Chang'E-4 Landing Site: Evidence for Episodic Ilmenite-Rich Magmatism

The Chang'E-4 mission, the first soft-landing on the lunar far side, provides an unprecedented opportunity to probe the subsurface structure of the Von Kármán crater within the South Pole-Aitken basin. The Yutu-2 rover carries a ground-penetrating radar (GPR) instrument, operating at central frequencies of 60 MHz (Channel-1) and 500 MHz (Channels-2A & 2B). The GPR antennas actively probe the shallow lunar stratigraphy, retrieving information on the dielectric properties of subsurface materials. These properties are intrinsically linked to lithology and geochemical composition, offering a direct window into lunar geological evolution.

A key to interpreting these radar reflections is the influence of ilmenite (FeTiO₃), a prevalent mineral in both lunar soils and mare basalts. Unlike most lunar minerals, ilmenite exhibits strong frequency-dependent dielectric behaviour, acting as an electromagnetic dispersive medium. Laboratory measurements demonstrate that signals propagating through ilmenite-bearing materials experience a frequency-downshift effect, where the central frequency of the radar pulse is attenuated in proportion to the ilmenite abundance. This physical relationship transforms the GPR’s frequency attributes into a quantitative proxy for ilmenite content.

In this study we analyse data from both GPR antennas. By applying time-frequency analysis to the radargrams, we extract the spectral characteristics of reflected signals to construct a vertical profile of inferred ilmenite content. Our results reveal a clear stratigraphic sequence of multiple basalt layers, delineated by distinct shifts in central frequency.

We identify three principal volcanic episodes within the probed stratigraphy. The deepest detected units correspond to an initial phase of high-ilmenite magmatism. This is overlain by a substantial sequence characterized by formations with significantly lower ilmenite content, indicative of a distinct geochemical source or evolving magmatic conditions. The uppermost major unit marks a return to higher ilmenite content, signifying a final phase of volcanism enriched in titanium and iron.

This resolved stratigraphy i.e. high, followed by low, followed by high ilmenite abundance, provides critical in-situ constraints for models of lunar volcanic history. The Chang'E-4 GPR data thus offer the first continuous, deep subsurface geochemical profile from the lunar far side, directly linking geophysical remote sensing with the magmatic evolution of the Moon.