A riddle written in sand
Across the surface of Mars, scientists have observed vast fields of enigmatic sandy ridges. These are not small, decimeter-scale ripples, nor are they massive dunes. They are an intermediate size, often called megaripples or Transverse Aeolian Ridges (TARs), with crests typically spaced 5 to 40 meters apart. For years, their existence was a puzzle. The Martian atmosphere is incredibly thin, with a surface pressure less than 1% of Earth's. According to conventional models of wind-driven (aeolian) transport, the winds on Mars, while fast, should not have enough density to consistently move the coarser sand grains that armor the crests of these large ripples. This led many to believe the megaripples were static, inactive relics from a past epoch when Mars had a thicker atmosphere.
The features are distinct from dunes. They are often brighter in color and have a more symmetrical profile, lacking the characteristic steep slip-face of a dune. Their lower thermal inertia suggests they are composed of smaller particles than large dunes. Observations from orbiters like the Mars Reconnaissance Orbiter and rovers like Perseverance show these features covering about 7% of the planet's surface, particularly common in crater floors and the southern hemisphere. While some megaripples appear ancient and immobile, recent studies tracking high-resolution images over several Martian years have shown that some fields are surprisingly active, migrating at an average rate of about 0.13 meters per Earth year.