Earth’s magnetic field was born by 3.45 billion years ago, a team including researchers from the University of Rochester in New York and the University of KwaZulu-Natal in South Africa report in the March 5 issue of Science.
That date falls during life’s earliest stages of development, between the period when the Earth was pummeled by interplanetary debris and when the atmosphere filled with oxygen. Several earlier studies had suggested that a magnetic field is a necessary shield against deadly solar radiation that can strip away a planet’s atmosphere, evaporate water and snuff out life on its surface.
The researchers measured the magnetic strength of certain rocks found in the Kaapvaal craton of South Africa, a geologic region known to date back more than 3 billion years.
John Tarduno of the University of Rochesterand his colleagues had studied similar rocks in 2007 and found that a magnetic field half as strong as today’s was present 3.2 billion years ago. Using a specially designed magnetometer and improved lab techniques, the team detected a magnetic signal in 3.45-billion-year-old rocks that was between 50 and 70 percent the strength of the present-day field, Tarduno says.
To determine if the early magnetic field was enough to hold back the rain of radiation, the team needed to know what the sun was doing. Tarduno and Eric Mamajek, an astronomer at the University of Rochester, used observations of young sunlike stars to infer how strong a solar wind the Earth was up against.
The young sun probably rotated more quickly than it does today, Tarduno says. This quick rotation powered a strong magnetic field, which heated the sun’s atmosphere and carried away mass and angular momentum in a strong solar wind of charged particles. The team calculated that the point where the Earth’s magnetic field cancels out the solar wind would be only about five Earth radii above the planet’s center, less than half of the 10.7 radii it is today.
The amount of radiation regularly reaching Earth from the sun 3.45 billion years ago would be comparable to what rains down on the planet during the most powerful solar storms today, Tarduno says. The aurora borealis, caused by solar wind particles accelerating along Earth’s magnetic field, would have been visible as far south as present-day New York City.
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