Early in Earth's history, the sun was about 20% dimmer than it is today. Climate models suggest that our planet should have been frozen over at the time, yet there is geological evidence for liquid water aplenty—a disparity that planetary scientists have dubbed the faint young sun paradox. Scientists have long debated how the planet stayed warm, suggesting that a thicker atmosphere, increased concentrations of greenhouse gases, or both were responsible. But new analyses of the imprints left by raindrops in ancient, now-solidified volcanic ash bolsters the notion that greenhouse gases alone were responsible for the anomalous warmth at that time, researchers report online today in Nature. The researchers note that while atmospheric density doesn't affect the maximum size a raindrop can reach—about 6.8 millimeters—it does affect a droplet's terminal velocity and, therefore, strongly influences its maximum momentum and the size of the imprint it leaves when it strikes. By measuring the size of the largest raindrop imprints (inset) in ash that solidified soon after an eruption 2.7 billion years ago (pocked slab, main image) and comparing them to the imprints made by drops of various sizes and momentums in lab tests, the team estimates that the density of Earth's oxygen-free atmosphere 2.7 billion years ago most likely ranged between 50% and 108% of today's air and was certainly less than twice its modern density—a thickness insufficient to offset the dimness of the sun at the time. Within that range of atmospheric density, even higher concentrations of carbon dioxide wouldn't have been adequate to counteract the faint young sun, suggesting that methane, ethane or other strong greenhouse gases kept Earth from freezing.
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