The new theory suggests that these elements might have collected in the lower magma ocean, which could explain their relative paucity in upper layers of the mantle.
That would also mean that the buried magma ocean was relatively radioactive, producing heat that would have kept the magma liquid for billions of years and would also have delayed the core from cooling enough to generate Earth's magnetic field.
The magnetic field is thought to be the result of liquid iron circulating in Earth's outer core.
Under the new theory, such conditions in the core may not have existed until Earth was about a billion years old, leaving our world unprotected from solar wind and other cosmic effects.
"The solar wind will have shaped the Earth's atmosphere very early on if there was not a magnetic field in this period of time," Hernlund said.
(Read related story: "Earth's Magnetic Field Is Fading" [September 9, 2004].)
Remnants of the magma ocean exist, Hernlund said, in "ultraslow zones" seen by scientists who use seismic waves from earthquakes to probe the boundary between the core and the mantle.
These zones, where seismic waves bounce and bend oddly, appear to represent what Hernlund calls "mushy patches" at the base of the mantle.
These soft patches are easily identifiable because they reflect seismic waves in a manner similar to the way in which a thin sheen of oil reflects light.
Norm Sleep is a geophysicist at Stanford University who was not part of Labrosse's team.
"It's an attractive hypothesis," he said of the new model, but he added that it is still speculative.
"Their hypothesis could well be right," he said, "[but] it's something that needs to be fleshed out to where it's coupled with the overlying mantle throughout the Earth's history."
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