1. PTYS 554 Evolution of Planetary Surfaces Aeolian Processes I

2. PYTS 554 – Aeolian Processes I 2 l Aeolian Processes I n Entrainment of particles – settling timescales n Threshold friction speeds n Suspension vs. saltation vs. reptation vs. creep n Dependences on gravity, densities of particle/air l Aeolian Processes II n Migration rates n Dune types n Dunefield pattern formation n Ripples vs. dunes n Ventifact, yardang erosion n Dust-devils and wind streaks

3. PYTS 554 – Aeolian Processes I 3 l Suspension vs saltation

4. PYTS 554 – Aeolian Processes I 4 l Suspension n All particles eventually settle out of a quiescent atmosphere n Reynolds number quantifies whether an atmosphere is quiescent wRe > 10s means turbulent flow (viscosity doesn’t damp eddies) wHigh velocity flows are more turbulent wLow viscosity fluids are more turbulent n Consider laminar flow around a falling sphere n Drag from sphere affects air within a cylinder ~2d wide n Downward force from weight – buoyancy n Upward force from viscous drag wStress ~ viscosity x strain rate wArea affected is curved wall of cylinder w…and ignoring some numerical factors n Equating the two gives the terminal velocity n Stokes’ law v d 3d d

5. PYTS 554 – Aeolian Processes I 5 l Turbulent flow n As before downward force from weight – buoyancy n Falling particle is opposed by ram pressure n Equating these to find the settling velocity – not very sensitive to particle size v d Low pressure High pressure

6. PYTS 554 – Aeolian Processes I 6 l Turbulent eddies have speeds ~0.2 the mean windspeed l For suspension: n For dust sized particles: Mars, Venus and Titan are effective at suspending particles n …but Venus (and Titan?) probably doesn’t have high near-surface winds

7. PYTS 554 – Aeolian Processes I 7 l In a planetary boundary layer n Drag of wind on surface produces a shear stress n Measured with drag plates n We define a ‘shear velocity’ u * wJust another way to quantify the shear stress n For a Newtonian fluid (like air): n In a thin laminar sub-layer η is constant and a property of the fluid (and temperature) n Above this layer, turbulence dominates, η is a property of the flow and varies with height and u wEmpirically – law of the wall… (κ is Von Karman’s constant ~ 0.41)

8. PYTS 554 – Aeolian Processes I 8 l Z 0 is the equivalent roughness height n 1/30 th of the grain size for quiescent situations n Otherwise it’s empirically determined from several wind measurements at different heights Medium sand Greeley, 1985

9. PYTS 554 – Aeolian Processes I 9 l Two regimes n Small particles hide within the laminar zone, larger particles stick up into the turbulent zone n Balance shear stresses with weight – buoyancy of particles n At the threshold velocity, some component of drag force balances the particle weight Transition at: D ~ 0.7 δ Neither approach works well in the transition zone Anderson and Anderson 2010 or A 2 often called θ A~0.1

10. PYTS 554 – Aeolian Processes I 10 l More detailed, gets you within a factor of 2 of deriving A Anderson and Anderson 2010

11. PYTS 554 – Aeolian Processes I 11 l Define the frictional Reynolds number n A varies with this value A Re * ~3.5 where n >>>1 Small particles in laminar zone Large particles in turbulent zone Recall: Turbulent zone: Laminar zone: uTuT d ?

12. PYTS 554 – Aeolian Processes I 12 l ‘A’ should be constant in the fully-turbulent case n Instead is depends on the fluid/particle density ratio n A cautionary tale in using ‘dimensionless’ scaling from one planet to another… Quartz in water Quartz on Earth Iversen et al.1987 Ice on Titan Basalt on Venus Basalt on Mars

13. PYTS 554 – Aeolian Processes I 13 l Minimum exists when Re ~ 3.5 l Easiest particles to move depends on n Atm. viscosity n Atm. density n Particle weight (density and gravity) n Buoyancy effects minor (until we get to the fluvial processes lectures) uTuT d ? ~225 microns for Earth

14. PYTS 554 – Aeolian Processes I 14 Easiest particles to move are sand-sized 1mm 1cm 0.1 mm Sand-sizedDustGravel l Saltation threshold increases with particle size l Particles classified by Udden-Wentworth scale Greeley, 1985

15. PYTS 554 – Aeolian Processes I 15 l Necessary wind speed depends on atmospheric density

16. PYTS 554 – Aeolian Processes I 16 l Easy to move but not easy to suspend n Particles are launched off the surface, but re-impact a short time later – saltation! Greeley, 1985

17. PYTS 554 – Aeolian Processes I 17 l Impact vs fluid threshold n It’s easier to keep saltation going than start it n Impact threshold is ~0.8 times the fluid threshold for Earth n …but ~0.1 times the fluid threshold for Mars wThis is what makes martian saltation possible Kok, 2010 Kansas State University l Grains travel by saltation n Impacting grains can dislodge new particles (reptation) n Impacting grains can push larger particles (creep) n Impacting grains knock finer particles into suspension Fluid Mars Impact Mars Impact Earth

18. PYTS 554 – Aeolian Processes I 18 l Saltation length scales ~cm Greeley, 1985

19. PYTS 554 – Aeolian Processes I 19 l Bagnold’s description of momentum loss n Mass flux per unit length – q n Momentum change of grains mass x (u 2 -u 1 ) over a distance L, with u 2 >>u 1 n Stress is: Avg. horizontal velocity ~ 0.5 u 2 Time of flight is 2w 1 /g L = u 2 w 1 /g so: u 2 /L = g/w 1 n Stress is also n And w 1 ~ u * (ignoring factors ~1) L v1v1 v2v2 Sand flux per unit length is proportional to shear velocity cubed Bagnold’s experimental work showed particle size is also a factor v1v1 w1w1 u1u1

20. PYTS 554 – Aeolian Processes I 20 l There are many variations fit to empirical data Greeley, 1985

21. PYTS 554 – Aeolian Processes I 21 Titan 95% Zero 5% methane Density Kg m -3 71.921.270.0275.3 Gravity (m s -2 )8.99.83.71.35 Dune materialBasaltQuartzBasaltOrganics (lower density) Dune Potential (All else being equal) Venus Titan Earth Mars

22. PYTS 554 – Aeolian Processes I 22 l As usual – all else is not equal l Venus has very few dunes (two fields known) n Lack of weathering into small particles n Detectability of dunes ? n Low surface winds l Mars has extensive dunefields n Very high wind speeds n Lots of active weathering breaking up rocks Dune Potential (All else being equal) Venus Titan Earth Mars Fortuna-Meshkenet field Weitz et al. 1994

23. PYTS 554 – Aeolian Processes I 23 l Aeolian Processes I n Entrainment of particles – settling timescales n Threshold friction speeds n Suspension vs. saltation vs. reptation vs. creep n Dependences on gravity, densities of particle/air l Aeolian Processes II n Migration rates n Dune types n Dunefield pattern formation n Ripples vs. dunes n Ventifact, yardang erosion n Dust-devils and wind streaks