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Earth and Venus are sometimes described as twins, as they have similar masses and densities. That’s about as far as the family resemblance goes, though—as our readers know, the two planets have hugely different surface conditions. Venus has a tremendously inhospitable surface climate, with temperatures hotter than a pizza oven and surface pressures that would only be experienced on Earth at ocean depths beyond the design limit of many conventional submarines. The climates of the two planets have obviously evolved very differently, and the likely culprit is the mechanism known as runaway greenhouse. In a nutshell, the greenhouse effect works