Moon soils store Earth's early breath
Mark Peplow
Traces of ancient nitrogen could
date our magnetic field.
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The Moon was pumelled by energetic nitrogen
atoms whizzing out of the Earth's early atmosphere.
© NASA |
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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.
Earthly origins
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.
References
- Ozima M., et al. Nature, 436. 655
- 659 (2005). | Article |
- Hashizume K., Chaussidon M., Marty B. & Robert F.
Science, 290. 1142 - 1145 (2000). | Article | PubMed | ChemPort |
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