Excuse the blemishes, but Mars has been beaten and battered by comets, asteroids, and other cosmic buckshot for billions of years. Now, using data gathered by probes orbiting the Red Planet, including those that have delivered landers and rovers, researchers have tallied the impacts—at least those that gouged holes 500 meters wide or larger—for a grand total of nearly 635,000 craters  (each a red dot on the image). That's likely a major underestimate of how much Mars has been battered, the team reports online this month in the Journal of Geophysical Research-Planets, because of the limited resolution of the images and the fact that many areas of Mars have been resurfaced by erosion and possibly by hydrothermal activity, and smothered by lava or volcanic ash (including gray, lightly cratered areas surrounding the planet's large volcanoes, left part of image). While there are only about 200 known impact craters on Earth, both the moon and Mercury—two of the most heavily pocked orbs in the solar system—are peppered with craters due to their lack of atmosphere and plate tectonic activity.
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