Aeolian Deposits

  • Henrik HargitaiEmail author
  • Ákos Kereszturi
Living reference work entry
DOI: https://doi.org/10.1007/978-1-4614-9213-9_457-1

Keywords

Sand Transport Aeolian Deposit Dune Field Planetary Surface Sand Sheet 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Definition

Wind-blown deposits on planetary surfaces, may be unconsolidated (loose) or indurated (cemented, lithified).

Subtypes

Description

Wind-transported and deposited particulate material on planetary surfaces that may form structures with varied morphology, including sand sheets, ripples and dunes, (aeolian sand deposit). Aeolian deposits in ice-cemented structures may form Polar Layered Deposits on Mars.

Grain Size

In the geological sciences, dust is defined as particles with diameters smaller than 62.5 μm. Sand is defined as particles (regardless of composition) in the range of 62.5–2,000 μm.

In the atmospheric sciences, dust is usually defined as the material that can be readily suspended by wind, whereas sand is rarely suspended and is predominantly transported by saltation (Kok et al. 2012).

Mean grain size of sand on Earth is 160–330 μm. Grain size on Mars is estimated between 60 and 600 μm depending on author, calculation, and method of observation. Some of the Martian dunes may be composed of cemented or uncemented dust aggregates (Bourke et al. 2010 and references therein).

Grain size on Earth is well sorted (limited to a narrow range) in sand deposits (Claudin and Andreotti 2006), but grain size of deposits may be bimodal if comprised of grains of different composition. On terrestrial active ergs, 90 % of grains in are sand sized (Thomas 1989). Finer grains of silt and clay (<0.05 mm) are cohesive and can resist wind erosion (Tsoar 2001).

Consequently, sand mobilization occurs at a lower threshold friction speed for sand than for very small grain sized dust (Iverson and White 1982). This explains why dune sand typically is well sorted and composed of fine particles between 0.125 and 0.250 mm (Fig. 2). Another consequence is that once dust is raised, sand must also be transported if there is available sand in displaceable form (Claudin and Andreotti 2006). Silt is removed from the deserts by suspension (Thomas 1989) (aeolian dust deposits).

Coarse sand is usually found in sand sheets and ripples. Gravel forms lag deposits.

Formation

Aeolian deposits occur where a supply of granular material is present and atmospheric currents have sufficient strength to transport grains (Kok et al. 2012).

Aeolian landforms are produced by the interaction between a fluid (shearing flow at the atmospheric boundary layer) and a granular material (sand) (Lancaster 1995:44). Aeolian sediments may accumulate at topographic obstacles or by self-accumulation on a surface with variable roughness. In places where there is a transition from rough (rocky) to smooth (sandy) surfaces, shear velocity drops, which leads to sand deposition (Wilson 1972). Alternatively, formations may be explained by the wave form theory. In this model, irregular surfaces initiate wavelike movement of the air, which cause variations in sand transport rate, alternating zones of erosion and deposition until equilibrium is reached (ripple) (Tsoar 2001).

Dunes tend to develop a form that is in dynamic equilibrium with the controlling parameters (Thomas 1989, p. 250 and references therein) and grow upwards into the atmospheric boundary layer which modifies airflow characteristics. Dune formation is controlled by available sediment supply and sediment grain size and long-term surface wind patterns (Greeley and Iversen 1985, p. 148). A peculiarity of Titanian winds is that pressure variations are driven by the tidal pull of Saturn instead of solar insolation (Tokano and Neubauer 2002). In Venus, the pristine morphology of almost all craters suggests limited aeolian activity over long time, but aeolian deposits may be mobilized and redistributed by strong winds produced by the ~1,000 impact events (Basilevsky and Head 2012).

Aeolian transport has different modes: traction or (impact) creep, rolling, saltation, reptation, suspension (Bagnold 1941; Sharp 1963; Greeley and Iversen 1985; Greeley and Arvidson 1990; Ungar and Haff 1987) (Fig. 1). Grains participating in these modes have several possible trajectory types depending on their actual energy (Andreotti 2004).
Fig. 1

Modes of aeolian transport: saltating, reptating, and rolling sand grains (After fig. 5 in Greeley and Arvidson 1990)

  1. 1.

    Creep and roll: “tractons” are the first few grains set into motion and dragged by the fluid. Tracton grains roll at the surface until aerodynamic lift force or a bump makes them take off and “jump.” Entrained sand grains are accelerated to velocities similar to that of the wind. Upon impact with the ground surface, the grains impart energy and momentum into other sand grains on the ground and eject them. This leads to the exponential increase of sand flux. The ejected grains may have high or low energy. Displacement by creep or traction (Greeley and Iversen 1985) (“impact creep” – Sharp 1963) can be initiated by the impact of a saltating particle. Rolling, sliding, and pushing of particles along the surface typically involve coarser grains (>500 μm) (Kok et al. 2012). Rolling has a more important role on Venus than on Earth. On Venus, wobbling the grains quiver but do not move out of place (Greeley and Arvidson 1990).

     
  2. 2.

    Saltation (Table 1): “saltons” are high-energy grains ejected by the impact of another salton. Saltons have enough energy to eject other grains upon impact. Saltation (hopping) typically involves sand-sized grains (typically 70–500 μm) (Kok et al. 2012). Saltation refers to the motion of sand grains in a sequence of ballistic trajectories close to the ground (Almeida et al. 2008). Wind velocities are practically undisturbed (only slightly reduced) in the saltation layer, above the reptation layer (Andreotti 2004). A characteristic height of saltation is below which 50 % of the mass transport occurs. This height is predicted to increase with increasing wind speed; however, recent measurements show that this height stays approximately constant as the wind speed increases. This discrepancy may be resolved with the inclusion of sand electrification in the models (Kok and Renno 2008). Maximum kinetic energy of impacting windblown grains on Earth occurs 15–20 cm above the surface. On Venus where the atmosphere is denser, saltation height is lower than on Earth (Greeley and Arvidson 1990), which may be more similar to saltation in water than saltation on Earth or Mars (Kok et al. 2012).

     
  3. 3.

    Reptation: “reptons” are low-energy grains ejected by the impact of a salton, a fast moving saltating grain. Reptons are relatively low-velocity grains that fall back onto the surface without ejecting other grains. They have a short path length (in the order of few grain diameters) (Anderson 1987) and thus a very short lifetime, but they are reproduced continuously by impacting saltons. Reptons move in the reptation layer, at few millimeters from the sand bed, where wind velocities are strongly reduced.

    If wind accelerates some of the reptons, they can become saltons. An increasing number of saltons cause the number of reptons to increase. Accelerating grains exert a further stress on the air. This leads to an equilibrium sand flux value, the saturation flux that can be transported by a given wind. Saturation flux value increases with the wind strength. Saturation occurs when the number of newly produced saltons is similar to the number of “lost” saltons, in other words, when the number of reptons promoted to saltation becomes similar to the number of saltons that remain trapped in the collisions (e.g., due to deceasing wind velocity). Thus, the small loss from trapped high-energy saltons is balanced by the few accelerated, originally low-energy reptons. This way, feedback of the grains on the wind is localized in the reptation layer, whereas sand transport essentially occurs in the saltation layer (Andreotti 2004).

     
  4. 4.

    Suspension: Fine grains (dust) are typically transported by suspension. Short-term suspension move 20–70 micron grains (on Earth) sporadically. Smaller grains (<20 micron) participate in long-term suspension (Tsoar and Pye 1987) (on Earth, up to several weeks) (Kok et al. 2012). On Earth, dust is primarily emitted by the impact of saltating grains on the soil bed (Greeley and Arvidson 1990; Kok and Renno 2006).

     
Table 1

Quantities governing saltation on Earth, Mars, Venus, and Titan (Parteli 2007)

 

Earth

Mars

Venus

Titan

Gravity g (m s−2)

9.81

3.71

8.87

1.35

Atmospheric density ρ fluid (kg m−3)

1.225

0.014

66.5

5.44

Particle density ρ grain (kg m−3)

2,650

3,200

3,200

1,000

Particle diameter d (mm)

0.25

0.50

0.125

0.30

Threshold friction speed u∗t (m s−1)

0.22

2.12

0.022

0.028

Mean saltation height zm (mm)

16.3

1,132

0.56

3.7

ℓ drag (m)

0.54

115.4

0.006

0.055

Entrainment rate γ

0.2

2.24

0.03

0.06

Grain velocity vs (m s−1)

1.41

17.8

0.17

0.21

Saturated flux qs (kg m−1 s−1)

0.008

0.146

0.0008

0.0008

Trajectory length ℓs (m)

0.80

15.43

0.11

0.53

Minimal dune width Wmin (m)

10.4

200.4

1.5

7.0

Entrainment rate of grains into saltation determines how fast the system reaches saturation

Wmin = 13ℓs

Rates of Sediment Transport

Once initiated, sand transport is sustained by wind speeds almost an order of magnitude less (the “zone of hysteresis”) due to collision-induced ejections (Claudin and Andreotti 2006) (Fig. 2).
Fig. 2

Threshold friction speeds for Venus, Mars, and Earth. Dashed line shows dynamical threshold (impact threshold wind velocity, the minimum wind velocity required to sustain the sand in saltation or the wind velocity at which the sand movement ceases (Bagnold 1941, p. 32, Watson 1989, p. 214). Dotted line shows static threshold (the minimum wind speed for initiating mobilization/entrainment (Bagnold 1941, p. 88). Between the dotted and dashed lines is the zone of hysteresis where transport can be sustained due to collision-induced ejections. Thin dashed line shows the highest wind speed at Venera landing site (Greeley and Arvidson 1990). This graph explains why sorting of sediments occurs at a given wind speed. For Mars and Venus, it is assumed that interparticle cohesion is similar to that on Earth. In the absence of cohesion (and hiding effects), the graphs would be straight lines (running from bottom-left to top-right) (fig 2 from Lorenz et al. 1995) (Data for Mars, Earth is from Claudin and Andreotti (2006 Fig. 4 and A2). Data for Venus is from Greeley and Arvidson (1990))

Characteristic path (saltation trajectory) may reach 1–2 m above the surface and extend several meters downwind. The sand grain reaches maximum velocity (0.5–0.66 × of wind speed) at the top of its trajectory (Anderson and Hallet 1986).

After the grain impacted the bed, other grains may be ejected in a splash (grain-bed collision) from the surface. This initiates a cascade process in which the number of grains in saltation increases exponentially. The number of ejected grains is proportional to the velocity of the impacting grain (Parteli et al. 2007).

As momentum is transferred to the grains from the air, wind is decelerated due to the acceleration of the grains (“feedback effect”), which leads to flux saturation. The characteristic distance to reach flux saturation is called saturation length (Parteli et al. 2007).

When the saturation length of the flux is exceeded, the air is saturated with grains and is not able to carry any more particles and erode the surface (Parteli 2007).

Dune wavelength (at which the bed destabilizes) scales with the drag length (the length needed for a grain in saltation to be accelerated to the fluid [e.g., wind] velocity; in other words, the length over which the saltation flux becomes saturated) (Elbelrhiti et al. 2005) and is proportional to the grain size (d) times the sand grain to fluid density ratio (ρs/ρf).

The most uncertain element of the calculations for extraterrestrial dunes is the grain size (Claudin and Andreotti 2006). Martian dunes seen today may have been formed under atmospheric conditions different from today’s (Parteli 2007) which makes both element of the equation uncertain.

The highest rates of sand transport occur across sand sheets, often leading into dune fields. Barchans are highly mobile dune type (Thomas 1989 and references therein).

On Venus, wind tunnel simulations show that threshold velocity is in the order of few mm s−1; even large particles are easily transported with a slight breeze. Despite this, only two dune fields have been identified that may be attributed to the lack of sand-sized particles due to slow weathering or the limited capabilities of radar imaging in identifying aeolian features (Craddock 2011 and references therein).

On Mars, grains enter saltation at a rate one order of magnitude higher than on Earth (Parteli et al. 2007) that may occur only a few times a decade during gusts of extreme aeolian activity (Almeida et al. 2008). With the low atmospheric density, friction velocities to initiate saltation must be higher. As a consequence, saltation path is longer and dune migration rates, once initiated, are rapid (Zimbelman 2000; Bourke et al. 2008). Wind accelerates particles more than on Earth. High velocity and low gravity yields large impact velocities; particles fly higher and remain longer in the atmosphere (Almeida et al. 2008). From observation of changes of Martian dunes, Bourke et al. (2008) concluded that rates of sediment transport are comparable with terrestrial values.

On Titan, 1.5 atm surface pressure and low gravity results in threshold velocity as low as ~10 cm s (Craddock 2011 and references therein).

Composition

The terms dust and sand usually refer to solid inorganic, rock-derived particles (Kok et al. 2012). Aeolian particles on Earth are typically composed of quartz and feldspar; carbonate or gypsum sands are locally important. Some sands are also composed of volcanic materials (basaltic or felsic). Basaltic sand is common on Mars and Venus, while Titan displays complex aggregates of organic hydrocarbons. In theory, aeolian particles can be composed of any kind of solid material in the presence of a planetary atmosphere in which near-surface winds occur.

Studied Locations

Venus, Earth (Fig. 3), Mars (Fig. 4), Titan.
Fig. 3

Kelso dunes at Mojave Desert. Largest dune is ~1 km wide, 200 m high; dune field is ~10 km in diameter. Detail of cross-cut dune (Photo by Jarmo Korteniemi 2010)

Fig. 4

Dark sand dunes and brownish dust blown across the hard cemented surface of the northern plains of Mars. Scale bar 100 m. HiRISE PSP_008839_2575_MRGB. (NASA/JPL/University of Arizona)

Distribution

The dimensions of aeolian deposits vary widely. Sand deposits cover an area of ~904,000 km2 on Mars (0,6 % of the surface); 18,410 km2 on Venus (0,004 %); ~7 million km2 on Earth (about 5 % of the land area); and 10–18 million km2 of Titan (12–20 %) (Bourke et al. 2010). Loess deposits cover an area of ~14 million km (about 10 % of the land area) on Earth (Pécsi 1968).

Venus: widely distributed wind streaks; two dune fields: Menat Undae and Al-Uzza Undae.

Mars: widely distributed dust mantles and sand streaks; generally unconfined sand seas that surround the North Polar ice cap, on the floors of impact craters, in the interbasin plains of the southern hemisphere, and in low latitude topographic traps such as troughs and channels.

Titan: equatorial linear dune fields/sand seas; larger dunes found in the southern latitudes.

Significance

The presence of dust and sand on a planetary surface indicates that weathering occurs there (Thomas et al. 2005). The aeolian deposits with their large internal void spaces could accommodate other materials, e.g., volatiles and ices, and could store or release them according to the actual climatic conditions (Mizser and Kereszturi 2007).

Terrestrial Analog

Aeolian deposits are found in deserts, on beaches, and in other sparsely vegetated areas, such as dry lake beds (Kok et al. 2012).

History of Investigation of Planetary Sand Deposits

During the last few hundred years of telescopic observations, low albedo spots on Mars have been recorded to change their overall shape (Dark deposits (Mars)). Early observations of cyclical changes in the shape of these spots were interpreted to be a result of seasonally changing vegetation cover (Lowell 1910) or aeolian sand transport, or, in the case of uniform contrast enhancement, a process following dust storms. Thus, the biological model was not completely ruled out even in 1972 (Sagan et al. 1972).

Carpenter (1948) hypothesized that the winds of the dust-laden atmosphere of Mars wore down its original roughness, which was similar to that of the Moon. This produced craters with polished edges and a surface worn into lanes of wind travel. He also proposed that canals may grow darker when the wind is moisture laden and lighter when it becomes dry.

Today albedo changes are thought to be a result of the seasonal dust covering of dark albedo markings (Craddock 2011). Classic dark albedo features, such as Syrtis Major, Terra Meridiani, and Coprates (the latter corresponding to the topographic feature Valles Marineris) are covered by low albedo rock and dust (dark deposits, Mars) and typically contain dune fields (Chojnacki et al. 2010).

Note

Plume deposit (types)) of geyser-like eruptions on Triton, Seasonal Polar Fan-Shaped Deposits (Mars), and Radar-dark parabolas on Venus, all different types of particle falls oriented by local winds, are not considered aeolian deposits as their surfaces are not shaped by winds.

See Also

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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  1. 1.Planetary Science Research Group, Institute of Geography and Earth SciencesEötvös Loránd UniversityBudapestHungary
  2. 2.Konkoly Thege Miklos Astronomical InstituteResearch Center for Astronomy and Earth SciencesBudapestHungary