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Produced by the NASA Center for Aerospace Information (CASI)
Principal Investigator

e

r ^\

Final Technical Report

for

Grant NCC 2-60

WIND TUNNEL SIMULATIONS OF AEOLIAN PROCESSES

for the period March 1980 to November 1984

Submitted to

Ames Research Center

National Aeronautics and Space Administration

Moffett Field, California 94035

Attn: A. Margozzi, Code 245-5

and

NASA Scientific and Technical Information Facility

P.O. Box 8757

Baltimore Washington International Airport

Maryland 21240

Ronald Greeley

Physics Department

University of Santa Clara

Santa Clara, CA 95053

( NASA -CF-176382) WIND TUNNEL SI MULATIONS OF

AEOLIAN PROCESSES Final Technical Report,

Mar. 1980 - Nov. 1984 (Santa Clara Univ.)

19 p HC A02/MF A01

CSCL 01A

M86-13289

Unclas

G3/02 U2868
TABLE. O^ CONTENTS

Page

1.0 Objective .......................................................... 1

2.0 Background ......................................................... 1

3.0 Approach ............................................ 0.............. 4

4.0 Relevance to NASA-Ames ............................................. 5

5.0 Facilities ......................................................... 5

6 . 0 Res u It s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

6.1 Results applied to Earth and in the general

planetary context ............................................. 5

6.2 Results applied to Mars ....................................... 8

6.3 Results applied to Venus ...................................... 10

7.0 Publications for work wholly or partly supported by this

project ............................................................ 13
WIND TUNNEL SIMULATIONS OF AEOLIAN PROCESSES

1.0 Objective

The objective of this project was to determine the characteristics of

aeolian (wind) activity as a surface-modifying process on Earth, Mars, Venus,

and appropriate satellites. This objective was met through a combination of

spacecraft data-analysis, wind tunnel simulations, and terrestrial field

analog studies as part of a consortium of investigators from several

universities and NASA-Ames Research Center.

2.0 Background

Many physical and chemical processes modify planetary surfaces. Aeolian

is defined (Gary et al., 1972) as "Pertaining to the wind; esp. said of rocks,

soils, and deposits (such as loess, dune sand, and some volcanic tuffs) whose

constituents were transported (blown) and laid down by atmospheric currents,

or of landforms produced or eroded by the wind, or of sedimentary structures

(such as ripple marks) made by the wind, or of geologic processes (such as

erosion and deposition) accomplished by the wind." Thus, any planet or

satellite having a dynamic atmosphere and a solid surface is subject to

aeolian or wind processes. A survey of the Solar System shows that Earth,

Mars, Venus, and possibly Titan meet these criteria. These planets afford the

opportunity to study a basic geological process--aeolian activity--in a

comparative sense with each planet being a vast natural laboratory having

strikingly different environments. Because terrestrial processes and features

have been studied for many years, Earth is the primary data base. However,

because surface processes are much more complicated on Earth (primarily

because of the presence of liquid water and vegetation) many aspects of

aeolian processes that are difficult to assess on Earth are more easily

studied on other planets.
2

Wind blowing across a planetary surface has the potential for directly

eroding and redistributing material to other areas. Winds transport sediments

via three modes: suspension (mostly silt and clay particles, i.e., smaller

than about 60 pm), saltation (mostly sand size particles, 60 to 2000 pm in

diameter), and surface creep (particles larger than about 2000 pm in

diameter). Wind threshold curves define the minimum wind speeds required to

initiate movement of different particles for given planetary environments.

The ability of wind to attain threshold is a func-.ion primarily of atmospheric

density, viscosity, composition, and temperature. Thus, the very low density

atmosphere on Mars (surface pressure is about 1/200 that of Earth) requires

wind speeds that are about an order of magnitude stronger than on Earth.

Aeolian processes are capable of redistributing enormous quantities of

sediment over planetary surfaces, resulting in the formation of landforms

large enough to be seen from orbit and deposition of windblown sediments that

can be hundreds of meters thick. Any process capable of effecting these

changes is relevant to understanding the geological environment of planets

involved. Furthermore, because aeolian processes involve the interaction of

the atmosphere and lithosphere, an understanding of aeolian activity sheds

light on meteorological problems. Aeolian activity can be considered in terms

of large—scale modifications, small—scale modifications, and as an observable

active process.

Large—scale modifications involve features that can be observed from

distances of orbiting spacecraft. One of the most useful types of features

for interpretation of surface processes is the dune, a depositional lardform.

Both the planimetric shape and cross sectional profile of dunes can reflect

the prevailing winds in a given area. Thus, if certain dune shapes and/or
It

3

an be determined from orbital data, local wind patterns can be

aetermined. Repetitive viewing of the same dunes as a function of season may

reveal seasonal wind patterns.

Small scale aeolian features include ventifacts (wind-shaped rocks) and

aeolian sedimentary structures--features that can be observed only directly on

the ground or inferred from remote sensing data. Ventifacts can provide

information about local wind directions and the lengths of time a surface has

been exposed. Identification of ventifacts has relevance to other aspects of

planetology. For example, rocks at the martian Viking landing sites that show

pitted surfaces have been interpreted as vesicular igneous rocks and are part

of the basis for identifying the surrounding plains as volcanic;

alternatively, the pitted rocks could be the result of aeolian erosion and are

not igneous.

Observations of active aeolian features provide direct information on the

atmosphere. For example, crater streaks on Mars are albedo surface patterns

that show surface wind direction; they occur in great number over much of

Mars. Repetitive imaging of these and other variable features has shown that

many of them disappear, reappear, or change their size, shape, or position

with time. Mapping the orientations of variable features has been used to

derive a near-surface atmospheric circulation model.

Impact crater frequency distributions are widely used in planetary geology

to obtain relative dates for different surfaces. On planets having active

aeolian processes, the erasure of craters by erosion or burial by aeolian

sediments can drastically alter the crater record and invalidate craterderived ages. Thus, knowledge of rates of aeolian erosion and deposition for

a wide range of planetary environments is required in order to assess the

possible effects on the impact crater record.
4

3.0 Approach for Investigating Aeolian Processes

Aeolian processes incorporate elements of geology, meteorology, physics

and chemistry. A unified study, therefore, was required using a

multidisciplinary approach in which aspects of the aeolian process were

isolated for detailed study. In the study of planetary aeolian processes

there is little opportunity to field-test extraterrestrial predictions. Thus,

we used a somewhat different method, as follows:

1. Identification of the general problem and isolation of specific

factors for study (e.g., wind threshold speeds for particles of

different sizes on Mars).

2. Investigate the problem under laboratory conditions where various

parameters can be controlled for the "Earth case" (e.g., wind tunnel

tests of threshold wind speeds).

3. Field test the laboratory results under natural conditions to verify

that the simulations were done correctly.

4. Correct, modify, and/or calibrate the laboratory simulations to take

into account the field results.

5. Carry out laboratory experiments for the extraterrestrial case

duplicating or simulating as nearly as possible the planetary

environment involved.

6. Extrapolate the results to the planetary case using the laboratory

results and theory (for parameters that cannot be duplicated).

7. Field test the extrapolation via spacecraft observations and apply the

results toward the solution of problems involving aeolian processes.

A benefit of this approach is not only to provide a logical means for

understanding extraterrestrial problems, but also contributes toward solving

aeolian problems on Earth.
5

4.0 Relevance to NASA-Ames

This research effort is part of the NASA-Ames Aeolian Consortium, formed

in the early 1970s to address general aeolian processes in the planetary

context. Members of the consortium included:

Ronald Greeley University of Santa Clara Geologist

James Iversen Iowa State University Aerodynamic engineer

Rodman Leach University of Santa Clara Engineer

John Marshall University of Santa Clara Geologist

James Pollack NASA-Ames Research Center Physicist

Haim Tsoar Ben Gurion University Geomorphologist

Joseph Veverka Cornell University Planetologist

Wes Ward U.S. Geological Survey Geologist

Bruce White University of California/Davis Mechanical engineer

Periodic workshops were held to discuss results and to outline areas of

future research.

5.0 Facilities

The principal facility in this research effort was the Planetary Geology

Aeolian Laboratory at Ames Research Center. This laboratory consists of a

space environment chamber (a 4000 m3 chamber capable of being evacuated to 3

mb pressure) plus the control room and offices. The Mars tunnel, Venus

tunnel, and a new Venus simulation apparatus are all housed in the chamber.

These wind tunnels and associated apparatus were conceived by members of the

Consortium, designed by engineers of the Facilities Services organization of

NASA-Ames, and fabricated either through the shops at Ames or on contract from

Ames to private concerns.

6.0 Results

Following the approach outlined above, the following provides a summary of

results; full citation to published papers and reports is given in section

7.0.
6

6.1 Results applied to Earth and in theeg neral planetary context

A book entitled Wind as a Geological Process: Earth, Mars, Venus, and

Titan was published as part of the Cambridge University Press, Planetary

Science Series (Greeley and Iversen, 1984), which synthesizes and summarizes

much of the work carried out under this project. A review paper, based on the

same theme, was presented at a NATO meeting and published as a chapter in a

book on comparative planetology as part of the NATO Advanced Study Institute

Series (Greeley, 1982).

Aeolian Abrasion

Because wind is an important aspect of erosion on Earth and Mars, a

complex series of experiments and numerical analyses was carried out to assess

rates of aeolian abrasion (Greeley et al., 1982,1984). Estimation of the rate

of aeolian abrasion of rocks requires knowledge of (1) particle flux, (2)

susceptibilities to abrasion of various rocks, and (3) wind frequencies.

Fluxes and susceptibilities for a wide range of conditions were obtained in

the laboratory and combined with wind frequency data for Earth and Mars. On

Mars, assuming an abundant supply of sand-sized particles, estimated rates

range up to 2.1 x 10-2 cm of abrasion per year in the vicinity of Viking

Lander 1. This rate is orders of magnitude too great to be in agreement with

the inferred age of the surface based on models of impact crater flux. The

discrepancy in the estimated rate of abrasion and the presumed old age of the

surface cannot be explained easily by changes in climate or exhumation of

ancient surfaces. We considered the primary reason to be related to the

agents of abrasion; either windblown grains are in very short suppl y , or the

grains are ineffective as agents of abrasion. At least some sand-sized (-100

um) grains appear to be present, as inferred from both lander and orbiter

observations. High rates of abrasion occur for all experimental cases
7

involving sands of quartz, basalt, or ash. However, previous studies have

shown that sand is quickly comminuted to silt- and clay-sized grains in the

martian aeolian regime. Experiments also show that these fine grains are

electrostatically charged and bond together as sand-sized aggregates.

Laboratory simulations of wind abrasion involving aggregates show that at

impact velocities capable of destroying sand, aggregates form a protective

veneer on the target surface and can give rise to extremely low abrasion

rates.

Particle Velocities

Because both rates of wind erosion and rates of sediment transport require

knowledge of the velocities of particles related to the wind, a series of

experiments was carried out for various planetary conditions (Greeley et al.,

1983). Velocities of windblown particles were determined as functions of wind

speed, height above the ground, and particle diameter for conditions

simulating Earth, Mars, and Venus in environmental wind tunnels. Similar

data, although of limited range, were obtained from a field experiment for

comparison with the wind tunnel results simulating the terrestrial

environment. In general, the results showed that particles travel at higher

speeds with increased height above the ground, and that smaller particles

travel faster than large ones. However, for a given height above the ground,

particle speed does not increase with higher freestream wind speeds. It must

be remembered that this is with reference to modal values and not to maximum

values; a few particles do increase in velocity with freestream wind speeds.

Comparisons of results for Earth, Mars, and Venus reveal some remarkable

differences. Most particles achieve speeds nearly equal to freestream wind

speed on Venus, but seldom achieve half the wind speed on Mars; Earth cases
8

are of intermediate values. This is attributed to the differences in

atmospheric density and to the threshold wind speeds among the three planetary

environments. Particles are more easily moved in the dense venusian

atmosphere than on Mars; consequently, threshold speeds are very low, and for

the range of wind speeds in which most movement is presumed to occur (just

above threshold speeds), the grains need not be moving very fast to achieve

100% of the wind speed. Conversely:, particles on Mars must accelerate very

rapidly to achieve the speed of the high winds required for threshold, and

despite the fact that saltation path lengths are long on Mars (White, 1979),

most grains fall to the surface before achieving even 50-60% of freestream

wind speed.

Yardangs

An important erosion landform seen on both Earth and Mars are elongate,

sculpted hills termed yardangs. A study was completed (Ward and Greeley,

1984) involving study of "classic" yardangs in California and development of a

formational model through wind tunnel simulation and numerical analyses.

Results showed that yardangs at Rogers Lake, Mojave Desert, California, have

streamlined forms characteristic of objects eroded by moving fluids, a

teardrop shape that approaches an ideal 1:4 width-to-length ratio. In wind

tunnel simulations, miniature forms of various shapes changed sequentially by

(1) erosion of the windward corner, (2) erosion of the windward slope, (3)

erosion of the leeward corners and flanks, and (4) erosion of the leeward

slope. Prominent mechanisms in yardang evolution apparently are abrasion at

the windward end and deflation and reverse air flow near the middle and at

the downstream end. Width-to-length ratios of yardangs are grossly similar to

those of some fluvial and glacial streamlined landforms. The low kinetic

energy of wind relative to ice and water, the erosional resistance to wind of
9

most rocks, the rarity of long-term, unidirectional winds, and the presence of

running water, topographic roughness, and vegetation all limit the abundance

of yardangs.

6.2 Results applied to Mara

As reviewed by Greeley at al. (1981), Earth-based observations .,d

spacecraft results show that aeolian processes are currently active on Mars.

Analyses of various landforms, including dunes, yardangs, and mantling

sediments of probable aeolian origin, suggest that aeolian processes have been

important in the geological past. Dust storms originate in specific areas of

Mars and are most vigorous during the martian summer in the southern

hemisphere. In order tc understand aeolian processes in the low surface

pressure (-7 mb), carbon dioxide atmosphere of Mars, a special wind-tunnel was

fabricated to carry out investigations of the physics of windblown particles

under martian conditions. Martian threshold wind speeds have been derived for

a range of particle diameters and densities; the threshold curve parallels

that for Earth but is offset toward higher wind velocities by about an order

of magnitude. The "optimum" size particle (the size most easily moved by

minimum wind) is about 100 um in diameter; minimum freestream winds to

generate particle motion are about 40 m/s. Grains smaller than 100 um

("dust") require increasingly higher winds to initiate threshold; yet,

estimates of grain sizes in the dust clouds are in the size range of a few

microns and smaller. Because the Viking Lander has recorded winds no stronger

than those for minimum threshold, it is suggested that some other mechanism

than uniform strong winds is required for "dust" threshold. Experiments and

theoretical considerations suggest that such mechanisms could be cyclonic

("dust devil") winds, a saltation cascading effect by larger (more easily
in

moved) particles, and injection of fine grains into the wind stream by

outgassing volatiles absorbed on the grains.

To refine the general results described above, a series of wind tunnel

experiments was run in which the "working" fluid was carbon dioxide to

simulare more closely the atmospheric conditions on Mars (Greeley et al.,

1980). Wind friction threshold speeds ( u*t ) for particle movement

(saltation) were determined in a wt.nd tunnel operating at martian surface

pressure with a 95 percent CO2 and S percent air atmosphere. The relationship

between friction speed (u* ) and freestream velocity (%) is extended to the

critical case for Mars of momentum thickness Reynolds numbers (Re 6 ) between

425 and 2000. It is determined that the dynamic pressure required to initiate

saltation is nearly constant for pressures between 1 bar (Earth) and 4 mb

(Mars) for atmospheres of both air and CO2; however, the threshold friction

speed (u*t ) is about 10 times higher at low pressures than on Rarth. For

example, the u*t (Earth) for particles 210 Pm in diameter is 0.22 m/e and

the u*t (Mars, 5 mb, 200 K) is 2.2 m/s.

Wind streaks constitute the most abundant aeolian feature on Mars and

numerous explanations have been offered to explain their origin(s). As a

means of relating the size, shape, and geometry to rates of particle erosion/

deposits, a model was developed and refined (Iversen and Greeley, 084), based

on wind tunnel simulations. This model enables the estimation of the total

volume of material eroded to produce dark streaks, utilizing sequential

orbital images.

6.3 Results applied to Venus

Small particles and winds of sufficient strength to move them have been

detected from Venera and Pioneer-Venus data and suggest the existence of
11

aeolian processes on Venus. The Venus Wind Tunnel (VWT) was fabricated in

order to investigate the behavior of windblown particles in a simulated

venusian environment (Greeley et al., 1984). Preliminary results show that

sand-size material is readily entrained at the wind speeds detected on Venus

and that saltatiog grains achieve velocities closely matching those of the

wind. Measurements of saltation threshold and particle flux for various

particle sizes have been compared with theoretical models which were developed

by extrapolation of findings from martian and terrestrial simulations.

Results are in general agreement with theory, although certain discrepancies

are apparent which may be attributed to experimental and/or theoretical

modeling procedures. These findings enable a better understanding of venusian

surface processes and suggest that aeolian processes are important in the

geological evolution of Venus.

To assess the types of small-scale aeolian features on Venus, a series of

tests were run to develop bedforms in the Venus Wind Tunnel (VWT) simulating

the average venusian environment (Greeley et al., 19b-4). Even at the low wind

speeds measured on Venus, dunelike structures form in fine-grained quartz

sands (particles 50-200 um in diameter). The dunelike structures, referred to

as microdunes, are considered to be true dunes analogous to those on Earth

because they have (1) slip faces, (2) a lack of particle-size sorting, (3) a

low rotio of saltation path length to dune length, and (4) internal crossbedding. The microdunes typically produced in the VWT are 9 cm long and 0.75

cm high. It is proposed that there may be fields of microdunes on Venus that

are capable of very fast rates of migration and that they may grow into

features much larger than those observed in the VWT. However, neither dunes

nor other types of features develop above a wind speed of -1.5 m/sec; at this

wind speed, the bed is flat and featureless. Thus, it is predicted that
relatively short periods of higher winds may destroy microdunes and other

small bedforms which could account for the sparsity of definitive aeolian

12

features observed in Venera images. Some apparent cross-bedding observed in

Venera images, however, could represent preserved aeolian structures.

The wind tunnel results simulating Venus reveal a hitherto unknown mode of

transport by the wind (Greeley and Marshall, in press). Simulations of

venusian wind processes show that particles are moved by rolling at wind

speeds as much as 30% lower than those required for saltation threshold. This

mode of wind transport is only observed for sustained periods in water on

Earth; thus, there are similarities between aqueous fluid transport on Earth

and atmospheric transport on Venus. The formation of small sand ridges and

grooves oriented parallel to the wind direction are associated with the

rolling of grains in venusian simulations and may be unique aeolian features

on Venus. The rolling mode of wind transport on Venus may be important from

several considerations:

a) Frequency of transport: Because threshold wind speeds for rolling are

significantly lower t ►► an for continuous saltation, the occurrence of

aeolian processes may be more frequent than if based solely on

saltation threshold, despite the relatively gentle winds recorded on

Venus.

b) Interpretation of venusian surface: If some bedforms are the

consequence of a mode of fluid transport that is unique to the

venusian envirorment, then interoretations of the surface should not

rely solely on cerrestrLal aeoltan (or water) analogs.

c) Movement of large particles: Particles 4-5 mm may be easily moved by

rolling (Fig. 1) by winds within the range expected on Venus.
Lt

13

d) Flux of windblown material: Previous estimates of the amount of

material moved by the wind on Venus (15) do not take into account the

rolling mode, but are based solely on saltation. Thus, substantial

material may be moved by surface creep involving rolling (especially

for L:+rZ-,e Srains) under relatively low (sub-saltation) wind speeds.

However, t.hl! 1:,A.al flux may still be far less than that of sediments

transported by water on Earth (16).

e) Small-scale surface features: Small features transverse to the wind

such as ripples and microdunes did not develop in experiments in which

transport- was solely by rolling. However, grooves and ridges parallel

to the wind formers and might be expected on Venus. The length of

these features was goverried by the size of the wind tunnel and it is

possible that on Venus they could extend for tens or even hundreds of

meters. Depending upon the characteristics of radar imaging systems,

such as wavelength, and the geometry of the radar beam, radar

signatures from the surface of Venus could be influenced by the

presence of these grooves and ridges. In these preliminary results,

we note that while there is an association of rolling with

longitudinal bedforms, these features are not necessarily a function

of this mode of transport.

In conclusion, wind tunnel experiments simulating venusian surface

conditions demonstrate that rolling of particles may be an important mode of

transport by winds on Venus and that aeolian processes in the dense atmosphere

may share attributes of both aeolian and aqueous environments on Earth.
7. 1 Books

1984, Wind as a Geological Process: Earth, Mars, Venus, and Titan, Cambridge

University Press, Cambridge (in press), Greeley, R. and J.D. Iversen.

1984, Planetary Landscapes, George Allen and Unwin, London (in press),

Greeley, R.

7.2 Journal Papers

1980, Threshold wind speeds for sand on Mars: Wind tunnel simulations

Geophys. Res. Lett., 7(2), 121-124, Greeley, R., R. Leach, B. White, J.

Iversen, and J. Pollack.

1980, Dust storms on Mars: Considerations and simulations, Geol. Soc. Amer.

Spec. Paper, 186, 101-121, Greeley, R., B.R. White, J.B. Pollack, J.D.

Iversen, and R.N. Leach.

1981, Aeolian activity as a planetary process, Mem. Soc. Ast., Italy, 409-418,

Greeley, R.

1982, Aeolian modification of planetary surfaces, in Coradini, A. and M.

Fulchignoni, eds., The Comparative Study of the Planets, NATO Advanced Study

Institute Series, D. Reidel Publ., Dordrecht, 419-434, Greeley, R.

1982, Rate of wind abrasion on Mars, J. Geophys. Res., 87, 10009-10029,

Greeley, R., R. Leach, S.H. Williams, B.R. White, J. Pollack, D.H. Krinsley,

and J.R. Marshall.

1983, Velocities of windblown particles in saltation: Preliminary laboratory

and field measurements, in Eolian Sediments and Processes, Elsevier,

Amsterdam, 133-148, Greeley, R., S.H. Williams, and J.R. Marshall.

1984, Evolution of yardangs at Rogers Lake, California, Geol. Soc. Amer. Bull.

95, 829-837, Ward, A.W. and R. Greeley.

1984, Windblown sand on Venus: Preliminary results of laboratory simulatons,

Icarus, 57, 112-124, Greeley, R., J. Iversen, R. Leach, J. Marshall, B.

White, and S. Williams.

1984, Abrasion by aeolian particles: Earth and Mars, NASA CR 3788, 50 p.

Greeley, R., S. Williams, B.R. White, J. Pollack, J. Marshall, and D.

Krinsley.

1984, Wind abrasion on Earth and Mars, in Models in Geomorphology,

Geomorphology Symposium, Buffalo, Allen and Unwin Publ., London (in press),

Greeley, R., S.11. Williams, B.R. White, J.B. Pollack, and J.R. Marshall.

1984, Martian crater dark streaks - explanation from wind tunnel experiments,

Icarus, 58, 358-362, Iversen, J.L. and R. Greeley.

- ^^^'..^= is n
15

1984, Microdunes and other aeolian bedforms on Venus: Wind tunnel

simulations, Icaru,, 60, 152-160, Greeley, R., J.R. Marshall, and R.N.

Leach.

1984, Rolling "Stones" on Venus: A possible mode of wind transport, Nature

(in press), Greeley, R. and J.R. Marshall.

7.3 Published abstracts of oral presentations

1980, Venus: Consideration of aeolian (windblown) processes, in Lunar Planet.

Sci, XI, 360-361, Greeley, R., J. Iversen, R. Leach, and J. Pollack.

1980, Threshold windspeeds for sand on Mars: Wind tunnel simulations, in

Repts. P-lanetary Geology Program, NASA TM-81776, 226-227, Greeley, R., R.

Leach, B.R. White, J.D. Iversen, and J.B. Pollack.

1980, Mars: Simulation of surface wind abrasion, in Repts. Planetary

Geology Program, NASA TM-81776, 241-243, Greeley, R. and S.H. Williams.

1980, Dunes related to obstacles on Earth and Mars: Observation and

simulation, in Repts. Planetary Geology Program, NASA TM-81766, 257-29,

Tsoar, H. and R. Greeley.

1980, Estimate of characteristic grain sizes for martian dunes, in Lunar

Planet. Sci. XI, 1169-1171, Tsoar, H. and R. Greeley.

1980, Wind erosion on Mars: an estimate of the rate of abrasion, in Lunar

Planet. Sci. XI, 1254-1256, Williams, S.H. and R. Greeley.

1981, Venusian surface wind tunnel, NASA TM-84211, 200, Greeley, R., J.

Iversen, B. White, R. Leach, and S. Williams.

1981, Wind abrasion on Mars: Considerations, simulations, and implications,

Third Inter. Colloq. on Mars, 98-99, Greeley, R., R.N. Leach, S.H. Williams,

B.R. White, J.C. Pollack, D.H. Krinsley, and J.R. Marshall.

1981, Aeolian processes on Venus, Inter. Conf. Venus Environ., 10, Greeley,

R., S. Williams, R. Leach, B. White, J. Iversen, and J. Pollack.

1981, A method for modelling of small particle transport, NASA TM-84211_,

203-204, Iversen, J., R. Greeley, and J. Pollack.

1981, An experimental study of the behavior of elecrostatically-charged fine

particles in atmospheric suspension, NASA TM-84211, 208-210, Marshall, J.R.,

D. Krinsley, and R. Greeley.

1981, Sand on Mars, Third Inter. Colloq. on Mars, 188-190, Peterfreund, A.R.,

R. Greeley, and D. Krinsley.

1981, Surface roughness effects on aeolian processes: Wind tunnel

experiments, NASA TM-84211, 195-196, Reding, L.M., S. Williams, R. Leach,

B.R. White, and R. Greeley.
16

1981, Soil transport by winds on Venus, NASA TM-84211, 210-202, White, B.R.

and R. Greeley.

1981, Formation and evolution of playa ventifacts, Amboy, California, NASA

TM-84211, 197-199, Williams, S. and R. Greeley.

1982, Wind as a geological process: Earth, Mars, Venus and Titan, 11th Intl.

Congress on Sedimentologists, 63, Greeley, R.

1982, Velocities of windblown particles in saltation: Earth and Mars, 11th

Intl. Congress on Sedimentologists, 63, Greeley, R.

1982, Flux of windblown particles on Venus: Preliminary laboratory results,

NASA TM-85127, 170-172, Williams, S.H. and R. Greeley.

1982, Particle motion of venusian saltation, NASA TM-85127, 173-174, White,

B.R. and R. Greeley.

1982, Wind abrasion of rocks: Computer simulation, NASA TM-85127, 175-176,

Greeley, R.

1982, Wind tunnel modelling of bright and dark crater-associated streaks, NASA

TM-85127, 185-187, Iversen, J.D., R. Greeley, and J.B. Pollack.

1982, Rolling "stones" on Venus: A mode of wind transport, Geol. Soc. Am.

Abs. with Pro&., 502, Greeley, R. and S. Williams.

1982, Windblown sand on Venus: Preliminary laboratory simulations, EOS,

63(45), 1021, Greeley, R., S. Williams, J.D. Iversen, R. Leach, B.R. White,

and J. Pollack.

1982, Simulating aeolian processes on Venus with a high-pressure N2

atmosphere, NASA TM-85127, 165-166, Marshall, J., R. Leach, C. Treat, and R.

Greeley.

1982, Windblown sand on Venus: Preliminary laboratory simulations, NASA

TM-85127, 167-169, Greeley, R. and S. Williams.

1983, Wind abrasion of rocks: Computer simulation, Geol. Soc. Amer., Abst.,

15, no. 5, 401, Greeley, R.

1984, Aeolian processes on Venus, in Repts. Planetary Geology Program, NASA

TM-86246, 69-70, Greeley, R.

1984, Velocities of windblown particles in saltation: Venus, Earth, and Mars,

in Repts. Planetary Geology Program, NASA TM-86246, 166-168, Greeley, R.

1984, Wind abrasion on Venus: A means for experimental investigations, in

Repts. Planetary Geology Program, NASA TM-86246, 67-68, Greeley, R. and J.

Marshall.
17

1984, Radar-visible wind streaks in the Altiplano of Bolivia, in ^^Rept^ts^^.

Planetary Geology Program, NASA TM-86246, 271-272, Greeley, R.,FChristensen, and R. Carrasco.

1984, Desert pavement study at Amboy, California, in Repts. Planetary Geology

Program, NASA TM-86246, 169-170, Williams, S. and R. Greeley.

1984, Flux and bedforms of windblown material on Venus, Lunar Planet. Sci.,

XV, 80-81, Bougan, S., R. Greeley, and J. Marshall.

A space station wind tunnel for studying sediment transport, submitted to XVI

Intern. Cong. Theor. Apple Mech., Iversen, J . D. and R. Greeley.

1984, The 1950 sulfur flow of Mauna Loa: considerations for Io, in Re ts.

Planetary Geology Program, NASA TM-86246, 133-134, Greeley, R., E. Theilig,

and P. Christensen.

1984, Aeolian bedforms and sorting in mixed sands on Venus, EOS, 65, 982,

Bougan, S., J.R. Marshall, and R. Greeley.

1984, Venus: simulations of aeolian bedforms and comparisons with terrestrial

aeolian and subaqueous forms, EOS, 65, 982.

1984, The effect of particle density on aeolian transport, Geol. Soc. Amer.

abst. 16, 695, Williams, S.H. and R. Greeley.

1984, Parameters controlling aeolian bedform development on Venus, Geol. Soc.

Amer. abst. 16, 451, Bougan, S.J., J.R. Marshall, and R. Greeley.

1984, Planetary science experiments aboard Space Station: Workshop report

(abs)., Bull. Amer. Astron. Soc. 16, 651.