Earth’s magnetism is older than we thought . .
New research suggests that evidence of Earth’s magnetic field has been pushed back about 250 million years.
It seems that Earth’s magnetic field was in place 3.45 billion years ago, according to a team from the Universities of Rochester, New York, and KwaZulu-Natal, South Africa - reporting in the latest issue of Science.
That date falls during life’s earliest stages of development, when the atmosphere was filling with oxygen. Earlier studies suggest that a magnetic field was necessary to shield against solar radiation that can strip away the atmosphere, evaporate water and snuff out life.
The researchers measured the magnetic strength of certain rocks found in Kaapvaal, South Africa - a geologic region known to date back more than 3 billion years. Iron minerals record the strength and direction of the magnetic field that was present during their formation, but when rocks are heated in subsequent geological processes, they can lose or overwrite that record.
So the team had to find a rock that had just enough iron to record a magnetic signature, but not so much that it would be affected by later chemical changes. The Greenstone Belt in South Africa had rocks that were just right - crystals of quartz less than two mm long with nanometer-sized bits of iron-containing magnetite embedded.
The team concluded that the magnetic field 3.45 billion years ago was 50-to-70% of the strength today.
The team also calculated that the point where the Earth’s magnetic field cancels out the solar wind would be only about 5 Earth-radii above the planet’s center - less than half of the 10.7 radii it is today.
As a result, the radiation regularly reaching Earth from the sun 3.45 billion years ago would be comparable to that during the most powerful solar storms today. And the aurora borealis, - caused by solar wind particles accelerating along Earth’s magnetic field - would have been visible as far south as 40º N.