Researchers have discovered around 800 crescent-shaped pits on Mars. They are the imprints of long-vanished ancient sand dunes that once rose high above the Red Planet's surface, according to a study published in the American Geophysical Union's (AGU) Journal of Geophysical Research.
The pits were formed around two billion years ago. Back then Mars would have looked very different to how it appears today, with flowing water on its surface and active volcanoes.
It is thought that lava or sediments in water partially buried the dunes before hardening into a mold-like structure that preserved their contours. Over time, winds blew sand away from the tops and the inside leaving an empty shell known as a "ghost dune," according to the AGU blog.
Studying these ghost dunes can provide potential clues to Mars's past climate, and may even contain evidence of ancient life, the researchers said.
Ghost dunes were first uncovered on Earth in 2016 on the Snake River Plain in eastern Idaho. However, the new study is the first to report this phenomenon on the Red Planet.
Using satellite images, planetary geomorphologist Mackenzie Day and astrobiologist David Catling, both from the University of Washington, identified around 300 of the ghost dunes in the Hellas Basin—an enormous, 4-billion-year-old impact crater that stretches for more than 1,600 miles across the planet's southern hemisphere.
Meanwhile, the other 500 or so are located in Noctis Labyrinthus—a maze-like system of steep-walled valleys on the equator.
To find them, researchers looked for clusters of crescent-shaped pits which were all aligned in the same way.
"Any one of these pits is not enough to tell you that it's a dune, or from an ancient dune field, but when you put them all together, they have so many commonalities with dunes on Mars and on Earth that you have to employ some kind of fantastic explanation to explain them as anything other than dunes," Day told the AGU blog.
The pits appear very similar in shape to "barchan dunes," the most common type of dune on both Earth and Mars. These are formed by winds blowing mainly from a single direction.
"They are all going the same way, which you would expect for dunes because they are all migrating and forming in the same wind regime," Day said. "So just the shape and size tell us that these are features that are coming from an ancient dune system."
Because the "horns" of the crescent-shaped pits point in the direction of the prevailing wind at the time they were formed, the scientists were able to work out that, in both Hellas Basin and Noctis Labyrinthus, it was blowing from the north and pushing the dunes south. This is different from the winds that blow in these areas today, indicating that climatic conditions have changed on Mars over time.
The researchers were also able to estimate how big the "ghost dunes" once were by comparing them to existing dunes on Mars. They found that those in the Hellas Basin measured around 250 feet tall on average, while those in Noctis Labyrinthus came to about 130 feet.
Aside from helping scientists understand more about Mars's ancient environment, the ghost dunes may also prove to be good places to search for signs of life on the Red Planet.
The authors suggest winds did not completely clear out the shell of the vanished dunes, leaving sheltered pockets with ancient sand in them that could potentially contain traces of ancient microbes. These pockets date back to a more hospitable time in Mars's history and have been protected from the harsh environment of the surface.
"We know that dunes on Earth can support life, and dunes on Earth are very similar to dunes on Mars," Day said. "One problem Mars has that Earth doesn't is the surface radiation. If you are inside a dune, or at the bottom of a dune, and you are microbial life, the dune is protecting you from a lot of that radiation. There is probably nothing living there now. But if there ever was anything on Mars, this is a better place than average to look."
