Early Earth as Hell. Or not.
Geologists now almost universally agree that by 4.2 billion years ago, the Earth was a pretty placid place, with both land and oceans. Instead of hellishly hot, it may have frozen over. Because the young Sun put out 30 percent less energy than it does today, temperatures on Earth might have been cold enough for parts of the surface to have been covered by expanses of ice.
In a new analysis, published in the current issue of the journal Nature, the zircons, the only bits of earth older than 4 billion years definitively known to have survived, provide another tantalizing hint about the Hadean period. Dr. Harrison and two U.C.L.A. colleagues, Michelle Hopkins, a graduate student, and Craig Manning, a professor of geology and geochemistry, report that minerals trapped inside zircons offer evidence that the processes of plate tectonics — the forces that push around the planet’s outer crust, forming and shaping the continents and oceans — had already begun.
“The picture that’s emerging is a watery world with normal rock recycling processes,” said Stephen J. Mojzsis, a professor of geology at the University of Colorado who was not involved with the U.C.L.A. research. “And that’s a comforting thought for the origin of life.”