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.
Argentina's high desert solution
To solve the puzzle of how these megaripples form in such a thin atmosphere, scientists turn to terrestrial analog sites—locations on Earth that mimic Martian conditions. One the best analogs is the Puna plateau in the Catamarca Province of Argentina. This region is a high-altitude desert, with elevations over 4,000 meters, where the air is thin and the climate is hyper-arid. Here, researchers have found fields of gravel-mantled megaripples that are morphologically very similar to the TARs on Mars.
Studies in the Puna reveal a "perfect storm" of conditions that allow these giant ripples to form. The process begins with wind eroding weakly cemented volcanic rock (ignimbrite), leaving behind a mix of fine sand and coarser gravel. While the wind alone isn't strong enough to lift the heavier gravel, it can easily lift the lighter sand grains. These saltating (bouncing) sand grains impact the coarser pebbles, slowly pushing them forward in a process called surface creep. Over long periods, this action sorts the particles, with the coarse gravel accumulating on the crests of the growing ripples, creating a protective armor. This two-part mechanism—saltation of fine sand driving the creep of coarse gravel—allows for the construction of large-scale ripples even under lower atmospheric pressures. This Earth-based model helps explain how Martian winds, despite their low density, can and do build these impressive features.