PTYS 554 Evolution of Planetary Surfaces Aeolian Processes

PTYS 554 Evolution of Planetary Surfaces Aeolian Processes I

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

PYTS 554 – Aeolian Processes I l Suspension vs saltation 3

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

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

PYTS 554 – Aeolian Processes I l Turbulent eddies have speeds ~0. 2 the mean windspeed l For suspension: n n For dust sized particles: Mars, Venus and Titan are effective at suspending particles …but Venus (and Titan? ) probably doesn’t have high near-surface winds 6

PYTS 554 – Aeolian Processes I l In a planetary boundary layer n Drag of wind on surface produces a shear stress Measured with drag plates or by measuring windspeed at two heights n We define a ‘shear velocity’ u* n w Just another way to quantify the shear stress n For laminar flow of a Newtonian fluid: n In a thin laminar sub-layer η is constant and a property of the fluid (and temperature) n Above this layer, turbulence dominates, viscosity is a property of the flow (not just the fluid) and increases with height, fluid density and u* w Empirically – law of the wall… (κ is Von Karman’s constant ~ 0. 41) 7

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

PYTS 554 – Aeolian Processes I l 9 Two regimes Transition at: δ ~ 5 d i. e. frictional-Re ~ 1 Neither approach works well in the transition zone Anderson and Anderson 2010 n Small particles hide within the laminar zone, larger particles stick up into the turbulent zone Balance shear stresses with weight – buoyancy of particles n At the threshold velocity, some component of drag force balances the particle weight n or A 2 often called θ A~0. 1

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

PYTS 554 – Aeolian Processes I l 11 ‘A’ should be constant in the fully-turbulent case n n Instead is depends on the fluid/particle density ratio A cautionary tale in using ‘dimensionless’ scaling from one planet to another… Basalt on Venus Quartz in water Ice on Titan Quartz on Earth Iversen et al. 1987 Basalt on Mars

PYTS 554 – Aeolian Processes I l 12 Define the frictional Reynolds number n A varies with this value where n >>>1 Recall: A Turbulent zone: ~1 Small particles in laminar zone Laminar zone: Re* Large particles in turbulent zone U*T ? d

PYTS 554 – Aeolian Processes I l 13 Minimum exists when Frictional-Re ~ 1 u. T ? 1 d 1 1 l Easiest particles to move depends on n n ~100 microns for Earth Atm. viscosity Atm. density Particle weight (density and gravity) Buoyancy effects minor (until we get to the fluvial processes lectures)

PYTS 554 – Aeolian Processes I l 14 Particles classified by Udden-Wentworth scale Easiest particles to move are sand-sized Dust Sand-sized 0. 1 mm 1 mm Gravel 1 cm Greeley, 1985

PYTS 554 – Aeolian Processes I l Necessary wind speed depends on atmospheric density, but most-easilymoved particle size doesn’t change much 15

PYTS 554 – Aeolian Processes I 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 16

PYTS 554 – Aeolian Processes I l Grains travel by saltation n l 17 Impacting grains can dislodge new particles (reptation) Impacting grains can push larger particles (creep) Impacting grains knock finer particles into suspension Impact vs fluid threshold n n n It’s easier to keep saltation going than start it Impact threshold is ~0. 8 times the fluid threshold for Earth …but ~0. 1 times the fluid threshold for Mars w This is what makes martian saltation possible rs Fluid Ma ars M ct a Imp arth E t c a Imp Kok, 2010 Kansas State University

PYTS 554 – Aeolian Processes I l 18 Bagnold’s description of momentum loss n n n Mass flux per unit length – q Momentum change of grains mass x (u 2 -u 1) over a distance L, with u 2>>u 1 Stress is: Avg. horizontal velocity ~ 0. 5 u 2 Time of flight is 2 w 1/g L = u 2 w 1/g so: u 2/L = g/w 1 n Stress is also And w 1 ~ u* (ignoring factors ~1) Sand flux per unit length is proportional to shear velocity cubed Bagnold’s experimental work showed particle size is also a factor v 1 u 1 v 2 v 1 L n w 1

PYTS 554 – Aeolian Processes I l There are many variations fit to empirical data Greeley, 1985 19

PYTS 554 – Aeolian Processes I Titan 95% Dune Potential (All else being equal) Zero Venus Zero 5% methane Density Kg m-3 71. 92 1. 27 0. 027 5. 3 Gravity (m s-2) 8. 9 9. 8 3. 7 1. 35 Dune material Basalt Quartz Basalt Organics (lower density) Titan Earth Mars 20

PYTS 554 – Aeolian Processes I l As usual – all else is not equal l Venus has very few dunes (two fields known) n n n Lack of weathering into small particles Detectability of dunes ? Low surface winds 21 Dune Potential (All else being equal) Venus Titan Earth Fortuna-Meshkenet field Weitz et al. 1994 l Mars has extensive dunefields n n Very high wind speeds Lots of active weathering breaking up rocks Mars

PYTS 554 – Aeolian Processes I l Aeolian Processes I n n l Entrainment of particles – settling timescales Threshold friction speeds Suspension vs. saltation vs. reptation vs. creep Dependences on gravity, densities of particle/air Aeolian Processes II n n n Migration rates Dune types Dunefield pattern formation Ripples vs. dunes Ventifact, yardang erosion Dust-devils and wind streaks 22

PYTS 554 – Aeolian Processes I l 23 Saltation length scales ~cm Greeley, 1985