http://www.sciencemag.org/news/2018/07/ghost-dunes-spotted-mars

Newly spotted croissant-shape impressions (denoted with arrows, above) on Mars are giving scientists clues to the history of the Red Planet.

The shapes, known as “ghost dunes,” are the hardened casts of ancient sand dunes, created when lava or sediments seeped and solidified around the lower contours of the dunes. As winds eventually dispersed the sand on top, the negative space where the dune once stood was preserved as a pit on the martian surface. While giant sand dunes have been documented in many images of the Red Planet, these new geologic features were first discovered on Earth in 2016 and only observed on Mars earlier this year.