ESP_017741_1745_1.0xA piece of Mars: These C and S-shaped things were once dunes that marched across the scene (from upper left to lower right), formed from sand deposited by the wind. Then that sand somehow became cemented, locking the dunes in place. But the wind didn’t stop — it continued to blow from the same direction, eroding grooves and flutes into the cemented dunes. (ESP_017741_1745, NASA/JPL/Univ. of Arizona)

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