KeywordsSand Ridge Ripple Sand Ripple Pattern Slip Face Dune Migration
Small (centimeter to meter wavelength) aeolian bedforms, commonly occurring in trains, created by the mobilization and accumulation of sand-sized grains into roughly parallel ridges that are typically oriented transverse to the direction of the wind.
Ripples are small, regularly repeated depositional bedforms consisting of sand-sized grains that develop almost anywhere that sand and wind occur together (Greeley and Iversen 1985). Ripples generally form transverse to the wind direction and their straight to slightly sinuous crests generally end or bifurcate within a few meters (Greeley and Iversen 1985, p. 149). Aeolian ripples have wavelengths from 0.01 to 20 m, heights from a few mm to 1 m, and indices between 12 and 50 (Leeder 1982). Unlike dunes, ripples lack slip faces (Thomas 1989; Greeley and Iversen 1985). The coarsest and finest grains are concentrated at the crests and troughs, respectively (Bagnold 1941, p. 145), and they are typically less than tens of saltation path length long (Bagnold 1941; White 1979). Ripples can form on free-standing surfaces as well as on larger aeolian bedforms such as dunes.
Normal ripples on Mars have been observed by landed missions, but megaripples, termed transverse aeolian ridges, are large enough to study with orbital data. While ripples and dunes on Earth are morphologically distinct, the increased saltation path length of sand-sized material on Mars due to the thinner atmosphere (White 1979) may cause overlap between the dimensions of large ripples and small dunes despite their unique formation mechanisms (Wilson et al. 2003).
It is the horizontal distance between the deepest points of two troughs or the horizontal distance between two crests in a rippled sand surface (Glenn 1979).
It is the vertical distance from the base of the ripple to the crest.
It is the ratio of ripple wavelength to ripple height, which generally varies inversely with grain size and directly with wind velocity (Greeley and Iversen 1985).
Normal ripples (also known as sand ripples, ballistic ripples, and impact ripples) have wavelengths of 1–25 cm and heights of 0.5–1.0 cm (Sharp 1963) and are asymmetric in profile with windward and lee slopes of 10° and 30°, respectively (Mabbutt 1977). Crests are generally straight or slightly sinuous and form transverse to wind direction in well-sorted sand (0.3–2.55 mm).
Megaripples (also known as giant ripples, granule ripples, erosion ripples, sand ridges, and pebble ridges) have wavelengths of ~3 m up to 25 m (Greeley and Iversen 1985) and form in bimodal sands transverse to the wind direction. The wind is sufficient to mobilize the fine material but coarser grains become concentrated on the ripple crest, resulting in a more symmetric profile relative to normal ripples.
Fluid drag ripples (Bagnold 1941), also known as aerodynamic ripples (Wilson 1972), can form either longitudinally or transversely (Thomas 1989) to the high velocity wind direction in well-sorted fine sand and are characterized by very long, flat ripple forms.
Ripples form when unidirectional winds are strong enough to initiate saltation of sand grains that exploit chance irregularities in the surface (Bagnold 1941). The windward (stoss) side is bombarded by wind-transported grains, each moving downwind at a distance defined by the average saltation path length of the grain. The subsequent erosion on the stoss side of the ripple creates a zone of accumulation on the leeward side, moving the entire system forward.
In general, ripple wavelength increases with ripple height (Thomas 1989, p. 238). Sharp (1963) observed that wavelength increases with time and suggested ripple wavelength is a function of ripple height and the angle that saltating grains impact the bed. Therefore, Sharp (1963) proposed that ripple wavelength is controlled by wind velocity and the length of the impact-shadowed zone.
Bagnold argued that ripple wavelength is a function of the saltation and reptation path length of the sand grains, which is controlled by wind speed (Bagnold 1941). However, recent studies show that saltation trajectories are many times longer than ripple wavelength. Ripple spacing is about six times the mean reptation path length (Thomas 1989). In the new model, ripples are controlled by reptation flux. Particles are entrained into reptation not directly by the wind, but by the nearly unidirectional bombardment of saltating particles (Prigozhin 1999). Saltating grains that impact coarser material cause reptation or creep or low-velocity movement along a short path length (Anderson 1987). These low-energy particles outnumber high-energy trajectories by about 9:1 (Thomas 1989). Coarse grains that move by creep processes accumulate on the lee side of the ripple and limit the height of the ripple (Bagnold 1941) until equilibrium is reached and the bedform migrates in the direction of sand transport (Thomas 1989, p. 238).
Reticulate ridge. In Hellespontus, Mars, transverse ridges show straight, bifurcating, sinuous, and catenary-to-lunate ripple patterns, oriented parallel and also orthogonal to dune migration direction, with a primary component (parallel) and a secondary component (orthogonal), indicating bimodal winds (Fig. 4). A reticulate or network arrangement is formed as interference patterns. Different ripple sets have different albedos that may result from grain size sorting or mineral separation (Bishop and Wheeler 2010).
Ripples show evidence of migration in several locations on Mars including the North Polar Erg, Nili Patera, Kaiser, Herschel, Rabe, and Matara craters but are immobile on several intracrater dunes in the southern highlands (Bridges et al. 2011).
History of Investigation
Misconceptions regarding the formation of aeolian ripples are as follows: (1) they form as a result of a special wave motion, analogous to water waves, generated by the shear (different speed) of two adjacent fluids (sand and wind), and (2) they are microdunes that failed to grow into regular dunes. Modern conceptions were developed by Bagnold (1941) and Wilson (1972). Wilson established the hierarchic system of aeolian features realizing that ripples and dunes are not gradational stages, but rather represent different mechanisms of formation (Greeley and Iversen 1985, p. 148 and references therein).
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