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Water, or the past presence of water is one of the primary things scientists look for on other planets to determine if it is possible for that planet to support life as we know it. Without water there would be no life on earth. The weather on earth is driven by the cycle of evaporating water, so without it there would be no rain, no rivers and none of the erosion caused by water. The surface of the earth would be very different. Mountains would be far bigger than they are on earth now, places like the Grand Canyon wouldn't exist.