How have spacecraft images shown that sand dunes occur on Mars?

Spacecraft images have provided clear evidence that sand dunes are widely distributed across Mars. This discovery was first made possible by data returned from the U.S. Mariner 9 and Viking spacecraft missions.1

These images revealed that Martian dunes are found in various locations, including within impact craters and across a vast sand sea that encircles the planet's north polar ice cap. Observations indicate that these dunes share many geometric similarities with those found on Earth, such as the crescent-shaped barchan dunes. Furthermore, the dunes in the north polar region exhibit evidence of seasonal activity, as seen in the interlayering of sand and seasonal carbon dioxide snow.123