Balme, M. R.

Eddlemon, E. E.

Foley, D. J.

Greeley, R.

Iversen, J. D.

Neakrase, L. D. V

English

Dust devils have been proposed as effective mechanisms for lofting large quantities of dust into the martian atmosphere. Previous work showed that vortices lift dust more easily than simple boundary layer winds. The aim of this study is to determine experimentally the effects of non-erodable roughness elements on vortex particle threshold through laboratory simulations of natural surfaces. Additional information is included in the original extended abstract

Eddlemon, Eric E.

Balme, Matthew L.

Foley, Daniel J.

Neakrase, Lynn D.

Iversen, James D.

Greeley, Ronald

NASA Technical Reports Server

Dust Devils on Mars: Effects of Surface Roughness on Particle Threshold

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsDust devils on Mars: Effects of surface roughness onparticle thresholdConference or Workshop ItemHow to cite:Neakrase, L. D. V; Greeley, R.; Iversen, J. D.; Balme, M. R.; Foley, D. J. and Eddlemon, E. E. (2005). Dustdevils on Mars: Effects of surface roughness on particle threshold. In: 36th Lunar and Planetary Science Conference,14-18 Mar 2006, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2005/Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukDUST DEVILS ON MARS: EFFECTS OF SURFACE ROUGHNESS ON PARTICLE THRESHOLDLynn D. V. Neakrase1, Ronald Greeley1, James D. Iversen2, Matthew L. Balme1, Daniel J. Foley1, Eric E. Eddlemon31 Dept. of Geological Sciences, Planetary Geology Group, Arizona State University, Tempe, AZ 85287-14042 Dept. of Aerospace Engineering and Engineering Mechanics, Iowa State University, Ames, IA3 Mars Surface Wind Tunnel Facility, Ames Research Facility, Moffett Field, CAIntroduction.  Dust devils have been proposedas effective mechanisms for lofting largequantities of dust into the martian atmosphere.Previous work [1] showed that vortices lift dustmore easily than simple boundary layer winds.The aim of this study is to determineexperimentally the effects of non-erodableroughness elements on vortex particle thresholdthrough laboratory simulations of naturalsurfaces.Approach and Methods.  Building on previouswork [1], we investigated how dust devilsbehave when they pass over rough terrains. Weemployed similar techniques outlined in [1] fordetermining threshold using the Arizona StateUniversity Vortex Generator (ASUVG).Roughness element targets were added to the testbed and several sediment types were analyzed.In each experimental run, dust or sand wassieved over the roughness target and the vortexgenerator then was run at increasing speeds todetermine the point at which particle movementbegins.Sediments tested include silica sand (90-106µm, 106-125 µm, 125-212µm, 212-350µm, 500-700µm in diameter) and walnut shells (< 60 µm,60-250 µm, 425-600µm, 590-1410µm indiameter).  The lower-density walnut shells areused to simulate the lower gravity of Mars.The roughness targets consisted of three 1.22x 1.22 m boards with cylindrical, woodenroughness elements attached in regular arrayswith centers spaced 5.08 cm apart.  The threesizes of roughness elements used in this studyinclude diameters 0.64, 1.91, and 2.54 cm withrespective heights of 0.64, 1.59, and 2.54 cm.Each target had 529 roughness elements evenlyspaced over its surface.Roughness can be described by simple ratiosof element height to their spacing (H/D).  For thethree targets used in this study H/D ratios include0.13, 0.5, and 1.0 – showing an increase in theoverall surface roughness with increasing values.Preliminary Results.  Initial results show that forthe tested sediments, increasing surface roughnessincreases the velocity needed to lift particles fromthe test bed (Figure 1).  Little or no differenceexisted between the lowest roughness target and noroughness.  However, in some instances, the lowestroughness target allowed particles to be lifted atlower velocities than with no roughness.  This resultsuggests that there could be an optimal roughnessfor particle threshold (Figure 2).The potential existence of optimal roughnesscurves could be useful in determining the amount ofsediment in dust devils seen over terrains of knownroughnesses on both Earth and Mars.  This could beuseful from a remote sensing perspective to yieldbetter estimates of the dust devil dust concentrationin cases such as the dust devils seen at the MarsPathfinder site [4].Future Work.  Based on this preliminary work,threshold curves as a function of roughness will becompleted for a larger variety of sediment types andsizes.  Also, increasing the range in ambientatmospheric conditions will prove useful incomparing terrestrial and martian conditions, andultimately aid in the understanding of the role thatdust devils play in the global dust cycle on Mars.Combining surface roughness results with othervortex sediment flux studies such as [5] and [6]should yield estimates of the potential sedimentcapacity of a dust devil on Mars, if the surfaceroughness is known.  Addition of estimates fromstudies such as [7] (martian dust devil track densitiesand variability) could also lead to information aboutwhich regions on Mars are most likely to contributesignificantly to the global atmospheric dust cycle.References. [1] Greeley et al. (2003) J. Geophys.Res., 108(E5), 5041. [2] Pye (1983) Aeolian Dustand Dust Deposits, 30-43. [3] MacDonald et al.(1997) Atmos. Envir., 31(6), 783-795. [4] Metzger etal. (1999) Geophys. Res. Let., 26(18), 2781-2784.[5] Neakrase et al. (2004) LPSC XXXV, #1395(Abstract). [6] Neakrase et al. (2004) AGU FallMtg., #P31-0982 (Abstract). [7] Balme et al., (2003)Geophys. Res. Let., 30(16), 1830.Lunar and Planetary Science XXXVI (2005) 1857.pdfVortex Particle Threshold Curves01002003004005006000 1 2 3 4 5 6 7φ      Threshold Parameter (Particle Size)Vortex Generator RPMLow Roughness (H/D=0.13)Med Roughness (H/D=0.50)High Roughness (H/D=1.00)Greeley et al., 2003 (no roughness)Figure 1. A plot of the preliminary data showing vortex particle threshold curves for a range in particle sizes.  Particle sizes and densitiesare represented by a dimensionless term, φ, (Threshold Parameter) which relates the particle’s density and diameter to the surfacegravitational acceleration and mean air density (After [1]).  Vortex generator RPM is roughly analogous to tangential velocity of the vortex.Blue points show the low roughness target (H/D = 0.13), which is roughly similar to the “no roughness” curve of the Greeley et al., 2003data.  Red points show an elevated threshold curve for the medium roughness target (H/D = 0.50). Green points show an elevated curve forthe high roughness target (H/D = 1.00)Threshold Roughness Curves01002003004005006000.00 0.20 0.40 0.60 0.80 1.00 1.20Height / Spacing  (H/D)Vortex Generator RPMOK90 (silica) 90-106µmOK90 (silica) 106-125µmOK90 (silica) 125-212µmOK90 (silica) 212-350µmSS (silica) 500-700µm Figure 2. A plot of roughness curves for silica sands tested in this study.  The height/spacing ratio (H/D) is a measure of the surfaceroughness where increasing values correspond to increasing roughness.  Vortex generator RPM is roughly analogous to tangential velocityof the vortex.Lunar and Planetary Science XXXVI (2005) 1857.pdf

Dust devils on Mars: Effects of surface roughness on particle threshold

Dust devils on Mars: Effects of surface roughness on particle threshold

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