On the Moon as it was on Earth

摘要

The onset of the terrestrial dynamo, resulting from slow cooling of the core and crystallization of its inner part, led to the development of the geomagnetic field that shields Earth's surface from extraterrestrial material and cosmic rays. The mechanism and timing of this event are unknown, but Ozima et al. (page 655 of this issue) argue that the problem can be tackled from a fresh perspective. Ozima et al. base their thinking on a new interpretation of geochemical data obtained from lunar soils recovered by the Apollo missions. They propose that, before the terrestrial dynamo kicked in, ions escaped from the top of Earths atmosphere, and were implanted in lunar soils together with ions emitted by the Sun in the so-called solar wind. The process ended with the emergence of the geomagnetic field, and ions implanted in lunar soils have since been dominated by input from the solar wind. According to this model, then, the composition of volatile elements trapped in lunar soils is a mix of ancient terrestrial atmosphere and solar gas. The relative proportions varied through geological time, offering the prospect of investigating the epoch when the geo-dynamo started, as well as the compositions of these two sources.