Catching the solar wind. An astronaut sets up a foil to catch particles of the solar wind, a possible source of nitrogen on the moon.

The Moon Soaked Up Some Earth

The moon may have bathed in Earth's atmosphere during the early days of the solar system. That's the best explanation so far for the mysterious presence of an isotope of nitrogen common to our planet's atmosphere in the lunar soil, according to a new study. If true, the age of Earth's magnetic field and other secrets of its interior could be locked in alien dust.

The moon formed when a Mars-sized object from the nascent solar system smashed into Earth, and debris from the collision coalesced and went into orbit. Most gases boiled away, leaving the moon poor in elements such as hydrogen and nitrogen. Although these elements are present in soil samples taken from the moon, they tend to be found mostly on the surface of the soil grains, and their amounts differ tremendously from sample to sample, suggesting they were imported by the solar wind, meteorites, or some other route.

In the 4 August issue of Nature, Minoru Ozima of the University of Tokyo in Japan and colleagues suggest that at least some of the gaseous elements in the moon dust came from Earth. The team looked at several previous analyses of lunar soil samples and noticed that the ratio of nitrogen isotopes and a hydrogen isotope known as deuterium varied widely: Some samples contained lots of nitrogen-15 and deuterium, while others had more nitrogen-14 and zero deuterium. While the second type of sample could be explained by gasses floating to the moon on the solar wind, which contains almost no deuterium, the first type matched isotope ratios commonly found in Earth's atmosphere.

This similarity suggests that some of the moon's gases really did come from Earth, say the researchers. Such a journey would be impossible today because of Earth's magnetic field, but it could have happened before the field formed, they say.

"If the theory is correct, then we might have a way to look to the past of the Earth from the surface of the moon," says Bernard Marty, a geochemist at the National Center for Scientific Research in Nancy, France. The age of the lunar soils touched by Earth gases could reveal how long ago the planet's magnetic field formed, and because a magnetic field requires a cool, solid core, this could in turn reveal how fast Earth cooled. Marty cautions, however, that some of the samples contain gas combinations that fit neither Earth nor the solar wind, and he is not convinced the Earth-atmosphere idea can explain this.

Related site
NASA's moon page