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Earth's magnetic field was at least half as strong 3.2 billion years ago as it is today, researchers report.
That means the planet was pretty well protected way back then from solar output that could otherwise have stripped away the atmosphere and doused early living organisms with lethal radiation.
Tarduno had previously estimated that as far back as 2.5 billion years ago, Earth's magnetic field was just as intense as today.
The new estimate was made by using a laser to heat ancient crystals of feldspar and quartz and measuring their magnetic intensity.
The tiny grains were picked out of out of 3.2 billion-year-old granite outcroppings in South Africa.
"The data suggest that the ancient magnetic field strength was at least 50 percent of the present-day field," Tarduno said.
"This means that a magnetosphere was definitely present, sheltering the Earth 3.2 billion years ago."