From Crescents to Ripples

Another geologically unique feature on Mars is its sand dunes, which are found in many different areas of the planet. The most common types are crescent-shaped barchan dunes and steep ridges of sand known as transverse dunes. Photographs have also shown Martian dunes in unusual shapes that people have likened to sharks' teeth, fish scales, chocolate candy kisses, or horseshoe crabs. A particularly interesting photo taken by a spacecraft during the summer of 2003 showed what looked like entire fields of fortune cookies made of sand. These odd shapes are created by the direction and strength of the Martian winds, which also influence the size of the dunes. Some are small sand hills, while others stretch more than three hundred feet into the Martian sky. Their color can vary, but most dunes on Mars are dark because of the color of the minerals that make up Martian sand.

The most expansive area of dunes is found in the northern hemisphere of Mars. Sometimes referred to as a "sea of sand," the massive dune field surrounds the north polar cap and covers nearly 250,000 square miles. Photos taken by National Aeronautics and Space Administration (NASA) spacecraft have confirmed that many northern Martian dunes are active, which means they grow, shrink, and move based on the force and direction of the wind. However, a rare grouping of dunes discovered in an area known as the Herschel Basin have rough, deeply grooved surfaces. Unlike dunes that are composed of loose sand, these look as though they are cemented together.

Other sand formations on Mars are categorized as sand ripples rather than dunes. These ripples are usually found in low-lying areas and inside craters, and can reach heights of about twenty feet, which is in stark contrast to those on Earth, where sand ripples are usually no more than a few feet high.

Scientists believe that the main reason Martian sand ripples grow so tall is that the gravity on Mars is so weak that they are not as likely to collapse from gravitational pull.

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