TRANSITIONAL DUNE FORMS ON MARS

Dune fields are common on Mars. They occupy the floor of large basins, craters, and many valleys. The northern polar region is surrounded by a vast sand sea. The outer edges of the dune fields transition from continuous sand cover to clusters of individual dunes. Individual dunes exhibit a variety of forms that reflect sediment supply and wind characteristics.

Martian dome dunes are found on the edge of sand seas in close proximity to or intermingled with barchans in regions in which the interdune areas are exposed bedrock. The typical martian dome dune field is characterized by circular mounds 40-100 meter diameter and less than 30 meters high. Spacing between dunes is on the order of 100-1000 meters. Where dome dunes and barchans are found together, dome dunes are smaller than barchans.

Mars dune fields exhibit transitional dune forms in which simple domes give way to slightly larger incipient barchans as an intermediary step in the progression to larger crescent-shaped barchans. These incipient forms are oval to circular in plan view with rudimentary slip faces on the leeward side of the dunes. Often called “fortune cookies”, these dunes represent mounds that have grown to sufficient height for the airflow over that top of the dune to develop flow separation and a lee vortex. Over steepening of the downwind portion of the sand dome leads to the development of an incipient slip face.

Another transitional dune-form exhibits a teardrop shaped consisting of opposing, incipient, lateral slip faces that merge to form a downwind tail. Presumably, vortices skirting the flanks of the dome remove material from the sides of the mound and develop opposing slip faces. Sand removed from the sides of the dune are deposited as a lee dune tail as lateral vortices merge. The orientation of the dune within fields of barchans suggests that they are formed by a unidirectional wind. These teardrop dunes are larger than dome dunes and are approximately the same size as the smallest barchans of the same dune field. They appear to represent a transition from dome dunes to longitudinal dunes.