Terrestrial atmospheric ion implantation occurred in the nearside lunar regolith during the history of Earth's dynamo

TL;DR

This study uses 3-D MHD simulations to explore how Earth's magnetic field influenced terrestrial ion implantation in lunar soil, revealing implications for understanding Earth's paleoatmosphere and its record in lunar samples.

Contribution

The paper demonstrates that Earth's magnetic field significantly affected terrestrial ion transfer to the Moon, providing new insights into the preservation of Earth's atmospheric history in lunar soils.

Findings

Ion implantation was efficient only within Earth's magnetotail.

Terrestrial contribution to lunar soil is from the long geodynamo history.

Earth's exobase altitude during implantation was never below 190 km.

Abstract

Light volatile elements in lunar regolith are thought to be a mixture of the solar wind and Earth's atmosphere, the latter sourced in the absence of geomagnetic field. However, the extent to which both the current and primitive geodynamo influence the transport of terrestrial ions still remains unclear, and this uncertainty is further complicated by the enigmatic composition and poorly constrained location of the Eoarchean exosphere. Here we use 3-D MHD numerical simulations with present-day magnetized and Archean unmagnetized atmospheres to investigate how Earth's intrinsic magnetic field affects this transfer, aiming to constrain how and when the lunar isotopic signature provides a record of Earth's paleoatmosphere. We find that atmospheric transfer is efficient only when the Moon is within Earth's magnetotail. The non-solar contribution to the lunar soil is best explained by implantation during the long history of the geodynamo, rather than any short, putatively unmagnetized epoch of early Earth. This further suggests the history of the terrestrial atmosphere, spanning billions of years, could be preserved in buried lunar soils. Our results indicate that the elemental abundances of Apollo samples are very sensitive to Earth's exobase altitude, which, at the time of ion implantation, was never smaller than 190 km.