The difference in crater crowding tells us that the

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The difference in crater crowding tells us that the southern highlands are a much older surface than the northern plains Volcanism erased crater impacts The frequency of volcanic eruptions on Mars has decreased steadily since at least about 3.5 billion years ago, just as we would expect for a planet small enough to have lost much of its internal heat by now What geological evidence tells us that water once flowed on Mars? Photos that showed features that looked much like dry riverbeds on Earth seen from above The branching nature of the channels suggests that they were carved gradually, os liquid water must have been stable at or just below the surface at that time Rainfall Rocks at the Opportunity landing site contain tiny spheres – nicknamed “blueberries” although they’re neither blue nor as large as the berries we find in stores – and odd indentations suggesting that they formed in standing water, or possibly groundwater percolating through rocks Past existence of underground pockets of liquid water near volcanoes Water still exists in the form of ice Why was Mars warmer and wetter in the past? Greenhouse effect Today Mars has such a thin atmosphere that it has only a weak greenhouse effect, despite the fact that 95% of its atmosphere is composed of the greenhouse gas CO2
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