January 6, 2021
The universe has spent billions of years making things that shine. On your birthday, we celebrate one of its quieter masterpieces.
Striped Sand Dunes on Mars
Birthday · January 6, 2021
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APOD published January 6, 2021. Full image credits are on the original APOD entry below. A publication-date match, not a photo taken on your date. Independent of NASA.
Why are these sand dunes on Mars striped? No one is sure. The featured image shows striped dunes in Kunowsky Crater on Mars, photographed recently with the Mars Reconnaissance Orbiter’s HiRISE Camera. Many Martian dunes are known to be covered unevenly with carbon dioxide (dry ice) frost, creating patterns of light and dark areas. Carbon dioxide doesn’t melt, but sublimates, turning directly into a gas. Carbon dioxide is also a greenhouse material even as a solid, so it can trap heat under the ice and sublimate from the bottom up, causing geyser-like eruptions. During Martian spring, these eruptions can cause a pattern of dark defrosting spots, where the darker sand is exposed. The featured image, though, was taken during Martian autumn, when the weather is getting colder – making these stripes particularly puzzling. One hypothesis is that they are caused by cracks in the ice that form from weaker eruptions or thermal stress as part of the day-night cycle, but research continues. Watching these dunes and others through more Martian seasons may give us more clues to solve this mystery.