By KENNETH CHANG

The first 700 million years of Earth’s 4.5-billion-year existence are known as the Hadean period, after Hades, or, to shed the ancient Greek name, Hell.

That name seemed to fit with the common perception that the young Earth was a hot, dry, desolate landscape interspersed with seas of magma and inhospitable for life. Even if some organism had somehow popped into existence, the old story went, surely it would soon have been extinguished in the firestorm of one of the giant meteorites that slammed into the Earth when the young solar system was still crowded with debris.

Scars on the surface of the Moon record a hail of impacts during what is called the Late Heavy Bombardment. The Earth would have received an even more intense bombardment, and the common thinking until recently was that life could not have emerged on Earth until the bombardment eased about 3.85 billion years ago.

Norman H.Sleep, a professor of geophysics at Stanford University, recalled that in 1986 he submitted a paper that calculated the probability of life surviving one of the giant, early impacts. It was summarily rejected because a reviewer said that obviously nothing could have lived then.

That is no longer thought to be true.“We thought we knew something we didn’t,”said T.Mark Harrison, a professor of geochemistry at the University of California, Los Angeles. In hindsight the evidence was just not there. And new evidence has suggested a new view of the early Earth.

Over the last decade, the mineralogical analysis of small hardy crystals known as zircons embedded in old Australian rocks has painted a picture of the Hadean period“completely inconsistent with this myth we made up,”Dr.Harrison said.

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.

In a new analysis, published in a recent 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 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.”

The revolution in early Earth studies comes largely from rocks in western Australia. The rocks are three billion years old, but they contain zircons that are older. Zircons, made primarily of the elements zirconium, oxygen and silicon, are extremely hard and durable and can survive conditions that erode, melt or otherwise transform the rock around them.

Dr.Sleep said his calculations suggested that during the 700 million years of the Hadean period about 15 objects 160 kilometers wide or wider hit the Earth. About four of the objects were wider than 320 kilometers, and those collisions would have been violent enough to boil off most of the oceans. (By contrast, the more recent object that hit the Earth 65 million years ago and helped kill off the dinosaurs was about 10 kilometers wide.)

But in numerical simulations presented this month at a meeting of the American Geophysical Union in San Francisco, Dr.Mojzsis showed that the Late Heavy Bombardment impacts were not quite as lethal as had been thought.

“Things are hurt really bad,’’Dr.Mojzsis said. But the computer calculations indicated that even rocks up to 482 kilometers wide would not kill everything, that pockets would exist where organisms that thrive in high-temperature environments like hydrothermal vents could survive.

Dr.Mojzsis said“Hadean’’might not be a misleading name for the earliest eon of Earth’s history, after all. The ancient Greek concept of hell was not one of fire and brimstone.“In Greek mythology, Hades was a dark, cold, mysterious place,’’he said.“It seems to me the Hadean is living up to that moniker.’’