A nebular winter could have doomed the dinosaurs. The clue is a thick layer of an extraterrestrial element on the ocean floor, now claimed to be the result of Earth colliding with a galactic cloud. The most-heard explanation for the dinosaurs' demise is an asteroid impact, which left a crater off the coast of Mexico and a worldwide 30-centimetre-thick layer of iridium, an element otherwise rare on Earth. But every so often, astronomers suspect, the solar system ploughs into a giant nebula of molecular gas and dust much denser than typical interstellar space. The resulting galactic fog would have darkened skies and cooled the ground until a harsh winter set in. It would also have destroyed the ozone layer and halted photosynthesis.
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