Martian Fleets - August 2003
Now that all our eyes are trained on Mars for the arrival of Beagle II as well as the NASA and Japanese missions, it seems appropriate to discuss a landform which can be found not only on Earth, but occurs extensively on Mars as well.
Since the disappearance of liquid water and volcanic activity at least 2 Ga ago, aeolian erosion is now the dominant mechanism of geomorphic change on Mars. One prominent landform which results from this aeolian acivity are yardangs.
Yardangs are streamlined hills or ridges carved from any consolidated or semiconsolidated material – or, occasionally, bedrock – by wind erosion (aided by airborne dust and sand grains) and deflation (the removal of particles by wind).
Yardangs range in size from a few centimetres to several kilometres long, and can be up to 30 metres high. Viewed from above, their shape resembles inverted boat hulls. They are highest and broadest at the end which faces the direction of the incoming wind, and become lower and narrower toward the lee end.
Abrasion is the pimary process which carves the windward side of a yardang, while deflation shapes the leeward side. Along the base of the flank of the yardang, especially the larger ones, a wind-scoured trough may occur, which indicates the densest part of the airborne sediment load.
The formation of yardangs usually begins with the downcutting of low areas, producing a series of parallel ridges which are gradually further eroded into separate hills. These hills are then shaped into the typical aerodynamic shape of a yardang, while the troughs separating the yardangs usually have a U-shaped cross section.
Thus most yardangs occur in fields — often called fleets, because of their resemblance to inverted ship hulls —, although occasionally they may be found in isolation. Within any one yardang fleet, the individual sizes tend to be fairly consistent.
The development of yardangs requires strong, persistent, almost unidirectional winds with an airborne sediment load of dust and sand grains. Wind speeds are highest along the flanks and top of the yardang where its cross-section is largest, while the flow towards the lee end tends to become turbulent.
Most yardang fields are found in arid, relatively sand-poor areas. On Earth, large yardangs occur in the arid regions of the Ica Valley of Peru, the Lut desert in Iran, southwestern Egypt and the Taklimakan Desert of northwest China. They have been observed in evaporites, siltstone, sandstone, shale, limestone and very occasionally even in crystalline rocks such as schist and gneiss. Yardangs may also be formed from the cemented core of a large dune which becomes exposed as the dune migrates.
Although the name yardang certainly conjures up images of visitors from a distant planet in a Star Wars movie, it is actually derived from the Turkish ‘yar’, meaning ‘steep bank’ and the Turkish-Mongol ‘yardang’ which means ‘dividing’.
Paul De Schutter