Moon soils store Earth's early breath
Traces of ancient nitrogen could date our magnetic field.
The Moon's soil preserves gases from the ancient Earth's atmosphere, say scientists who have studied results from the Apollo missions.
The discovery hints that our planet's magnetic field switched on about 3.9 billion years ago. This in turn points to when life began on Earth, as the magnetic field protects us from a hail of DNA-damaging particles from space.
Although the Earth formed some 4.5 billion years ago, current theories suggest that its magnetic field only kicked in after its core cooled. But no one knows exactly when this was, and researchers have been short on evidence.
Minoru Ozima, a geochemist at the University of Tokyo, Japan, and his colleagues came up with a new approach to the question after looking at the data from soil samples brought back by astronauts in the 1970s.
Moon soils contain traces of volatile elements such as nitrogen and argon. Scientists have long assumed that blasts of solar wind, flowing from the Sun's upper atmosphere, drilled these atoms into the soil.
But there's a problem with this theory. Moon dust contains a very different ratio of two types of nitrogen, called nitrogen-14 and nitrogen-15, compared with the solar wind, explains Ozima. The ratio also varies from one grain of lunar soil to the next. Some of the Moon's nitrogen must have come from elsewhere, he says.
Ozima's team now argues that some of the nitrogen came from the Earth before it got its magnetic shield. Energetic cosmic particles would have whacked into the atmosphere, kicking some charged nitrogen atoms into space. Some of this nitrogen, which is richer in nitrogen-15 than the solar wind, would have wound up on the Moon.
This source would have been more sporadic that the flood of nitrogen in the solar wind. That could explain why the isotopes are spread so unevenly across the Moon, they report in this week's Nature1.
The team used computer models to work out how much nitrogen would have flown from the pre-magnetic Earth to the Moon. Then, by calculating how long the soil must have been sucking up nitrogen to attain its current isotopic ratio, they speculate that the Earth's magnetic field must have been either very weak or non-existent before about 3.9 billion years ago.
Life is thought to have begun on Earth at least 3.5 billion years ago, according to microorganism fossils found in Australia. This would mean that life probably established itself quite quickly after the magnetic shield was in place.
Far side of the Moon
But pinning down exactly when the magnetic field turned on is tricky. "It's very difficult to date lunar soil," points out Bernard Marty, a geochemist at the Petrographic and Geochemical Research Centre in Vandoeuvre-lès-Nancy, France. This makes it difficult to determine exactly when the flow of nitrogen from the Earth stopped.
To verify Ozima's idea, lunar soil samples would have to be collected from the far side of the Moon for comparison. This hemisphere has probably always faced away from the Earth and so would never have been exposed to Earthly nitrogen.
The Moon is thought by some to be a chunk of the early Earth chipped off in a collision with another body, but lunar soils should carry no trace of the earthly atmosphere. Any volatile gases on the soil grains would have evaporated long ago.
Marty, who discovered the unusual distribution of nitrogen isotopes in lunar soil2, says he thinks the nitrogen came from a mixture of the solar wind, the Earth, and interplanetary dust.
Untangling the three sources will be hard. But, he says, it could be helped by results from the Genesis spacecraft, which recently brought back samples of the solar wind.
- Ozima M., et al. Nature, 436. 655 - 659 (2005).
- Hashizume K., Chaussidon M., Marty B. & Robert F. Science, 290. 1142 - 1145 (2000).