Excess 40Ar in Chang'e-5 lunar soils suggests a possible origin from Earth wind

Abstract

The abundance of ⁴⁰Ar in lunar soils is significantly higher than the expected values from solar wind implantation and ⁴⁰K decay, a phenomenon known as ⁴⁰Ar excess. Traditionally, this excess is attributed to ⁴⁰Ar generated by the decay of ⁴⁰K within the Moon. This radiogenic ⁴⁰Ar degasses to the lunar surface, where it is ionized by solar radiation and subsequently captured by lunar soils. However, stepwise heating (200 °C–1300 °C) and degassing analyses of noble gas isotopes in Chang'e-5 lunar soils samples reveal the presence of two types of ⁴⁰Ar: one unrelated to ³⁶Ar, likely originating from in situ ⁴⁰K decay in the soils, and another correlated with ³⁶Ar, which may primarily derive from Earth wind. Earth wind, an ion flux formed by the escape of Earth's atmosphere, is thought to be injected onto the lunar surface under the regulation of Earth's magnetosphere. The study proposes that the excess ⁴⁰Ar in lunar soils may primarily stem from the continuous escape of Earth's atmosphere and be injected onto the lunar surface through both the inner and outer regions of Earth's magnetosphere, offering a new perspective for understanding volatile exchange between the Earth-Moon system.