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When a small object forcefully impacts a larger object, it usually creates a crater with a circular outline. Such craters are common on the Moon, Mars, Mercury, and other bodies. But a few craters have a square outline.
Square craters are formed when the material at the impact site is already fractured along intersecting directions prior to the impact. Fractured material responds differently to stress applied in different directions. If the fractures are at right angles, square craters can form.

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