Four billion years ago, it was Hell on Earth—or so many scientists believe. Direct evidence is hard to come by, because our world conceals its distant past well. The oldest known outcrop of terrestrial rock, the Acasta gneiss 350 kilometers north of Yellowknife in Canada's Northwest Territories, dates back slightly less than four billion years, leaving the first half billion years of Earth's history without a rock record at all.* To make matters worse, even those fragments of ancient crust that remain, like the Acasta gneiss, have been so melted and distorted over the ages that they carry only a cryptic record of their infancy.
Fortunately, to get some idea of what the young Earth might have been like, geologists aren't confined to studying the ground beneath their feet. Other clues come from space—from our neighboring worlds, in particular the Moon.
Untouched by weathering or the relentless shifting of continents, the lunar surface bears witness to a ferocious battering it took during the very time the geological record is missing or hard to read on Earth. Those dark, round patches that suggest a human face are in reality wide basins, now lavafilled, that were excavated by colliding asteroids many tens of kilometers across. Elsewhere in the solar system, every other planet and moon, including the Earth, is thought to have been bombarded during this holocaust age by debris left over from the initial bout of world-making.
It was the most inhospitable time imaginable, fittingly christened the Hadean or "hellish" era by American paleontologist Preston Cloud. A number of big asteroids (no one knows how many) are reckoned to have slammed____________________