Although it sounds like the title of a John Carter of Mars novel, ghost dunes were first discovered on Earth in 2016. They are formed when lava or other sediments seep into the base region of a dune and solidify there. Over time, wind can blow the upper layers of the sand dune away, leaving a “ghost” remnant of the ancient dune. The formation of ghost dunes requires rather strong prevailing winds, so it’s rather surprising to find them on Mars.1
The current atmosphere of Mars is far too thin to cause serious wind erosion, so these ghost dunes must have been formed long ago when Mars had a much thicker atmosphere, probably about 2 billion years ago. Since the orientation of erosion patterns depend upon the direction of prevailing winds, the dunes provide clues about the weather patterns of early Mars. But these ghost dunes could also hold a record of life on the red planet.
Early Mars not only had a thicker atmosphere, it also was warmer and had liquid water on its surface, giving it the perfect conditions for life. If life did appear on young Mars, it could be found in old sediment deposits, such as the ones forming these ghost dunes.
Day, Mackenzie D., and David C. Catling. Dune Casts Preserved by Partial Burial: The first identification of “ghost dune” pits on Mars. Journal of Geophysical Research: Planets (2018). ↩︎