NASA Technical Reports Server (NTRS) 19940030968: Windblown sand on Mars: The effect of saltation threshold on drift potentials derived from Mars GCM

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WINDBLOWN SAND ON MARS: THE EFFECT OF SALTATION
THRESHOLD ON DRIFT POTENTIALS DERIVED FROM MARS GCM;
P. Xu, R. Greeley and S. Williams, Dept. of Geology, Arizona State University, Tempe, AZ.
85287-1404, J. B. Pollack, Space Science Division, NASA Ames Research Center, Moffett Field,//
California
INTRODUCTION. The rate at which the wind can redistribute sedimentary material is an
important part of any planet's sedimentologic cycle, particularly for Mars, where the competing
effects of other gradational processes are less than on Earth. The aeolian drift potential (DP) is a
measure of the mount of material capable of being moved through a unit length by the wind for a
given period of time [1]. DP is a useful measure of the potential redistribution rate of windblown
material on regional scales. The martian aeolian DP was calculated from laboratory studies of sand
movement conducted at Martian atmospheric densities and from surface stress, temperature, and
pressure values for that region as determined from the Mars General (Atmospheric) Circulation
Model (GCM) developed at the NASA/Ames Research Center [2,3]. In our simulations for Mars,
DP changes in both magnitude (as expected) and direction if the saltation threshold is altered.
METHOD. The Ames GCM divides the surface of Mars into 7.5 ° by 9° bins, each with
predicted values of pressure, temperature, and surface shear stress. The surface shear vector was
used in the saltation flux equation of White [3] to derive DP magnitude and direction for each bin.
Seven different values for threshold were used to assess its effect on magnitude and direction of
the resultant sand drift. The variation in sand drift direction in a given GCM bin as a function of
saltation threshold was analyzed as was the correlation between the calculated sand drift direction
and the orientation of aeolian surface features, such as wind streaks.
RESULTS. Threshold shear stresses must be exceeded for a given GCM bin for sand drift
to occur. Many bins have no motion, and there is an inverse relationship between the threshold
stress and the number of bins reporting sand drift (Figure 1), as well as maximum drift potential
value for any given azimuth (Figure 2). Resultant drift directions also change with increasing
saltation threshold. The biggest change in direction averaged -13.25 °, which occurs when the
threshold increases from 0.008 to 0.016 N/m 2 (Figure 3). The smallest change averaged -3.6 ° for
an increase in threshold stress from 0.040 to 0.048 N/m 2 (Figure 4). The inverse relationship
between saltation threshold and shift in sand drift resultant azimuth reflects the greater variability of
gentle winds compared to strong winds.
There is no correlation between the axial orientation of dark (erosional) wind streaks onMars
and the resultant sand drift directions determined in this study (Figure 5). The correlation is much
better when bright (depositional) streaks are considered (Figure.6): the besf correlation is for a
threshold stress of 0.032 N/m 2. Thf likely explanatiori for the poor correlation in the case of the
dark streaks is that their formation isdor_'nated by local topographic and meteorological conditions
that are unaffected by the regional schle parameters utilized by the GCM [5]. Bright streaks form
in the aftermath of dust storms when the atmosphere is very stable and less responsive to local
conditions [6], hence, bright streak orientation correlate better with the regional sand drift.
CONCLUSIONS. The number of bins which have drift potentials and the maximum drift
potential on the planet at one compass direction over the whole year period decrease with
increasing threshold. The resultant drift directions also change with increasing threshold. The
larger the threshold, the less change in resultant drift direction. The best correlation of bright
streaks with resultant drift directions is at a threshold of 0.032 N/m 2.
REFERENCES. [1] Fryberger, S.G. (1979) U.S.G S. Prof. Paper 1052, 137-169. [2]
Pollack, J.B. et al. (1981) J. Atmos. Sci. 38, 3-29. [3] Pollack, J.B. et al. (1990) JGR. 95,
1447-1473. [4] White, B.R. (1979) JGR, 84, 4643-4651. [5] Greeley, et al. (1993) JGR, 98,
3183-3196. [6] Veverka, et al. (1981) ICARUS, 45, 154-166.
1522 WINDBLOWN SAND ON MARS: Xu, P., R.Greeley, S. Williams, and J.B. Pollack
1000 4.5
8o0 E'_ 43'.051
Jd
60o
_ 2.5
400,
200 i_ '-1.5°1
'_ 1,0
0
o"" o._1''' o._''' o._3''' o.64''' o.o5 0.5 .... , ..... , , ..... , ....
0 0.01 0.02 0.03 0.04 0.05
Threshold (N/m*'2) Threshold (N/m*'2)
Figure 1.The number of GCM bins which have drift Figure 2.The maximum drift potential on the
potenfinls vs. threshold planet atonecompassdirection over
whole year periodvs.threshold
700- 200-
MIn 0.0113 gln 0.0
600i Max 175.74 Max 35.16
Mean 13.25 Meen 3.62
[ 500 MSteddidaenviation 244..4377 _" 150 MSteddiDanevOtion 52..0001
300 _ 100
i i 50
0-: 0"
0 20 40 60 80 100 120 140 160 180 0 20 40 60 BO 100 120 140 160 180
Angle Oiff_ence (degree) Angle Dlflerence (degree)
Figure 3. Histogram of angle differences of resultant drift Figure 4. Histogram of angle differences of resultant drift
directions from two threshold 0.008 and directions from twothresholds 0.040 and
0.016 Him**2 0.048 N/m**2
40 40-
kiln O.48
Max 179.41 Max 17g.sg
Datapoints 222 Datapoints 151
30 M_In 09.18.37 Mean 51.89
Median 93.76 30 Median 29.75
Stddeviation 53.12
i Std deviation 56.77 i'°
lO
0' o
0 20 40 60 80 100 120 140 160 180 0 20 40 5o 80 lOO 12o 14o 16o 18o
Angle Difference (degree) Angle DIf_mnce (degree)
Figure 5. Histogram of angle differences of dark streaks Figure 6. Histogram of angle differences of bright streaks
and resultant drift directions at threshold and resultant drift directions at threshold
0.016 N/m**2 0.032 Him**2