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A popular theory about the early Solar System comes under fire.
Early in Earth's history, roughly half a billion years after the planet formed, all hell broke loose in the inner Solar System. A barrage of asteroids - some the size of Hong Kong - pummelled the globe intensely enough to melt large parts of its surface. This incendiary spree around 4 billion years ago vaporized most of Earth's water and perhaps even sterilized its exterior, killing offany life that might have started to emerge. Only after this storm of impacts passed did the planet become safe enough for hardy organisms to take firm root and eventually give rise to all later life.
That horrific episode, known as the Late Heavy Bombardment (LHB), has been an integral part of Earth's origin story for decades, ever since geologists did a systematic study of samples brought back from the Moon by NASA Apollo missions. But now, the once-popular theory has come under attack, and mounting evidence is causing many researchers to abandon it. A growing community of planetary scientists thinks that things quietened down relatively quickly, with a steadily decreasing rain of asteroids that ended a few hundred million years after Earth and the Moon formed.
Settling the debate could have major ramifications for some of the biggest questions in geoscience: when did life emerge and what were conditions like on early Earth? But some researchers think that fresh samples will be needed to finally put this conundrum to rest. They are looking with hope at the United States' recent pledge to send astronauts back to the Moon - although no timeline has yet been set. In the meantime, the community is grappling with the fact that a key chapter of Solar System history might be vanishing before their eyes.
"The Late Heavy Bombardment was seen as one of the great triumphs of the Apollo era," says geochemist Mark Harrison of the University of California, Los Angeles. "There's no question that something has happened in the past few years that has profoundly upset the apple cart."
The Solar System formed some 4.6 billion years ago, after the centre of a massive cloud of gas and dust collapsed into a dense sphere that became our Sun. Pebbles...