China Focus: Chinese scientists propose excess 40Ar in lunar soil possibly from Earth wind

LANZHOU -- Chinese scientists have newly proposed that surplus 40Ar in lunar soil possibly originates from the Earth wind, according to the Northwest Institute of Eco-environment and Resources (NIEER) under the Chinese Academy of Sciences.

Conducted by researchers at the NIEER, this latest study offers a new perspective for understanding volatile exchanges within the Earth-Moon system. The study results have been published in the journal Icarus, said the NIEER.

Ar, the chemical element argon, is a pivotal noble gas present within lunar soil. It contains vital information crucial for deciphering the Moon's internal architecture and surface evolution processes, as well as the material exchange processes occurring between the Earth and the Moon, said Li Liwu, a researcher at the NIEER.

"Analysis of the isotopic composition of Ar in lunar soil can provide new clues for understanding multiple key issues regarding the Moon and the Earth-Moon system," Li said.

Li also noted that, alongside advancement of lunar exploration in recent years, excess 40Ar in lunar soil remains a significant topic in planetary science, drawing the attention of the scientific community. Lunar soil samples collected by China's Chang'e 5 mission provide an opportunity for scientists to tackle this issue.

Researchers with the Oil and Gas Research Center of the NIEER have conducted stepwise heating and degassing analyses of noble gas isotopes in Chang'e 5 lunar soil samples.

They further conducted fine analysis of the argon isotope composition within these lunar soil samples. The study results revealed the presence of two types of 40Ar — one unrelated to 36Ar, likely originating from in situ 40K which decays into 40Ar in the soils, and the other correlated with 36Ar — which may primarily derive from the Earth wind.

The study proposes that the excess 40Ar in lunar soil could primarily originate from continuous escape from Earth's atmosphere — with it injected onto the lunar surface via both the inner and outer regions of Earth's magnetosphere.

The findings of this new study provide an innovative framework for understanding the atmospheric escape process of terrestrial planets and their impact on neighboring celestial bodies, Li noted.

He added that the study also offers a new perspective for analysis of samples from the far side of the Moon as provided by the Chang'e 6 mission, and for the analysis of lunar soil retrieved by subsequent exploration projects. The outcomes of this study are expected to help further verify the spatial influence range of the Earth wind.

The Chang'e 5 probe, which returned to Earth on Dec 17, 2020, retrieved 1,731 grams of lunar samples, consisting primarily of rocks and soil from the lunar surface.