Science Puzzle
Why Is the Moon Covered in Craters but Earth Isn’t?
Look at the Moon through binoculars and you'll see craters everywhere, overlapping one another, from tiny pits to great basins hundreds of kilometres across. Earth, by contrast, has only about 200 known impact craters.
Earth and the Moon sit side by side in space, so they must have been hit by the same kinds of rocks from space for billions of years. Earth, the bigger target with the stronger pull, must have been hit even more. So where are Earth's craters?
The Answer
Earth was hit just as hard, but it has been wiping its craters away. The Moon can't.
Earth's surface is always being remade. Wind, rain, rivers and ice wear craters down and fill them with mud and sand, just as the tide smooths away footprints on a beach. Plants and soil cover what's left. Volcanoes bury old ground under new lava. Plate tectonics slowly drags the ocean floor back down into the mantle; hardly any of today's ocean floor is older than about 200 million years. Over hundreds of millions of years, almost every crater gets erased. Earth's atmosphere also protects it: most small space rocks burn up as shooting stars before they reach the ground.
The Moon has no air, no water, no weather and no moving plates, and its volcanoes went quiet billions of years ago. Once a crater forms there, almost nothing disturbs it except the next impact. Many of its craters are billions of years old. The footprints left by the Apollo astronauts could last for millions of years.
Earth's craters that survive are mostly young or well hidden. The Chicxulub crater in Mexico, from the impact linked to the end of the dinosaurs 66 million years ago, lies buried under later rock. The Vredefort crater in South Africa, around 2 billion years old, has been worn down so far that only the roots of it remain.
The principle: Surfaces record their history. A surface keeps its craters only if nothing wears it away; the fewer craters it has, the more actively it has been reshaped.