Earth should have been frozen in its youth, when the sun was cooler. Yet the young Earth was warm enough for liquid water. This paradox has puzzled us for decades, but now a look at one of Saturn's moons suggests a new explanation. For the first 2 billion years of Earth's existence, the sun was up to 25 per cent dimmer than it is today. Our planet's average surface temperature should have been around -10 ℃, but the geological evidence is that Earth had liquid water at this time.
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