Images and caption contributed by Haim Tsoar.
Mariner 9, launched on May 30, 1971 conducted an intensive orbital reconnaissance of the red planet between November 14, 1971 and October 22, 1972. One of the astonishing discoveries of Mariner 9 was vast dune fields all over the Red Planet. Viking discovered many more dune fields in the late 1970s. However, the resolution of the images taken by Mariner 9 and Viking 1 and 2 was very poor and one dominant dune type of barchan and transverse dunes was chiefly discerned by this low resolution (Tsoar et al, 1979). The Mars Global Surveyor (MGS) was the next successful mission to Mars, launched 20 years after Viking, in November 1996, and operated for 10 years. The Mars Orbiter Camera (MOC) on board MGS acquired high resolution images of the sand dunes on Mars and revealed some other dune types that were not known before. The latest mission to Mars, the Mars Reconnaissance Orbiter not only reveals the variety of dunes but its high resolution camera (HiRISE) allows us to see the smaller ripples on the dunes.
Image 1: Barchan and linear dunes west of Hellas Planitia near 41.8°S, 315.5°W, formed on the floor of a crater and extending from a mesa. Note the breakdown of the rectilinear dune into barchans with distance from the flow obstruction. HiRISE Image PSP_007676_1385. [Full resolution image]
The tremendous variety of sand dunes in the world deserts makes their classification a difficult task. The dominant process of sand dune formation is by self accumulation of sand into dunes where the variety of wind directions determines the dune shape. Sand can further accumulate into dunes where there are obstructions to the wind flow such as cliffs, mesas, boulders or shrubs (Pye and Tsoar, 1990; Tsoar, 1983).
One prevailing dune type, shown in many of the MOC and HiRISE mission images, is a rectilinear dune similar to that shown in Image 1. Such straight linear dunes are uncommon on Earth but are known to be formed as lee (shadow) dunes emanating from cliffs (Image 2). The barchan dunes in the area indicate that the wind is unidirectional and the linear dunes elongate parallel to this wind direction.
Image 2: Lee dunes formed at the lee side of a cliff in NE Sinai, Egypt (30° 28' 13.50"N; 33° 51' 15.85"E). The lee dune breaks up into barchan dunes similar to the lee dunes on Mars shown by Image 1. "A" indicates the location Image 3 was taken from. Image captured from Google Earth Google Earth. [Full resolution image]
The characteristic flow-field that forms lee dunes is described by the convergence of airflow diverted around an obstruction to form a horseshoe vortex (Greeley, 1986; Greeley et al, 1974; Hesp, 1981). The shape of the obstacle causes the flow to spiral and sweep sand along the obstacle and deposit sand in the lee side wake ("shadow" zone) where opposed horizontal flow is met. This vortex induces the formation of linear lee dunes pointing from the base of the obstacle's lee side (Image 3).
Image 3: Ground view of the lee dune in Sinai shown in Image 2. The picture was taken from the cliff top marked by "A" in Image 2. [Full resolution image]
Lee dunes, formed beneath cliffs (Images 1 and 2), are located where the brink of the cliff juts out (Images 2 and 3) or is breached to set off a convergence of streamlines. Lee dunes are formed by wind from one direction. They thereby tend to break up downwind into individual barchans at a distance where the topographic obstacle is no longer effective as a vortex generator. These barchans are the preferred dune form in an open area with a predominantly unidirectional wind regime (Images 1, 2, and 3).
The lee linear dunes shown in these images are considered by some as seif dunes (Smith, 1978). Their resemblance to seif dunes (the Arabic word for a sword) is reflected in the sharp crest. A Lee dune that resembles a seif in their sharp crest is the only dune with a linear shape that runs parallel to the wind. Since little field and simulation work has been done on lee dunes, their formation and dynamics are not well understood.
In Chasma Boreale, in the North Polar Region of Mars, rectilinear and barchan dunes occur in close proximity (Warner and Farmer, 2008). These dunes were identified as seifs (Edgett and Malin, 2000). In this location, there are no topographic obstacles to explain the presence of the rectilinear dune. Seif dunes that are not formed in the lee of an obstacle are shaped under bi-directional wind regimes that flow across the dune obliquely and are characterized by a sinuous crestline (in planform) which is related to the dynamics of elongation (Tsoar, 1983a). However, a rectilinear dune form, not formed in the lee of an obstacle, in unidirectional wind regimes is unstable and tends to disintegrate into a chain of barchan dunes (Schatz et al, 2006; Tsoar et al, 2008). This has led to a suggestion that the rectilinear dune forms that are found in barchan dune fields may be stabilized by induration and that these then act as a solid linear obstacle to the wind that will continue to elongate downwind.
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