JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 97, NO. E8, PAGES 13,319-13,345, AUGUST 25, 1992
Aeolian Featureson Venus' Preliminary Magellan Results
RONALD
GREELEY,
• RAYMOND
E. ARVIDSON,
2 CHARLES
ELACHI,
3 MAUREEN
A. GERINGER,
• JEFFREY
J. PLAUT,
3
R. STEPHEN
SAUNDERS,
• GERALD
SCHUBERT,
4ELLENR. STOFAN,
• Emc J.P. THOUVENOT,
3's
STEPHEN
D. WALL)
• ANDCATHERINE
M. WEITZ
•
Magellan synthetic apertureradar data reveal numerous surface features that are attributed to
aeolian, or wind processes. Wind streaksare the most common aeolian feature. They consistof
radar backscatterpatternsthat are high, low, or mixed in relation to the surfaceon which they occur.
A data baseof more than 3400 wind streaksshowsthat low backscatterlinear forms (long, narrow
streaks)are the most common and that most streaksoccurbetween 17øSto 30øS and 5øN to 53øN on
smoothplains. Moreover, most streaksare associatedwith depositsfrom certain impact cratersand
some tectonicallydeformedterrains. We infer that both of these geological settingsprovide fine
particulatematerial that can be entrainedby the low-velocity winds on Venus. Turbulenceand wind
patternsgeneratedby the topographicfeatureswith which many streaksare associatedcan account
for differencesin particle distributionsand in the patternsof the wind streaks. Thus, some high
backscatterstreaksare consideredto be zonesthat are sweptfree of sedimentaryparticlesto expose
rough bedrock;otherhigh backscatterstreaksmay be lag depositsof densematerialsfrom which
low-density grains have been removed (densematerials such as ilmenite or pyrite have dielectric
propertiesthat would producehigh backscatterpatterns). Wind streaksgenerally occur on slopes
< 2 ø and tend to be oriented toward the equator,consistentwith the Hadley model of atmospheric
circulation. In additionto wind streaks,other aeolianfeatureson Venusincludg[
yardangs(?)and
dunefields. The Aglaonicedunefield, centeredat 25øS,340øE, covers~1290 km': and is locatedin
anejecta
flow
channel
•n•the
Aglaonice
impact
crater.
The
Meshkenet
dune
field,
located
at67øN,
90øE, covers ~17,120
in a valley betweenIshtar Terra and MeshkenetTessera. Wind streaks
associatedwith both dune fields suggestthat the dunesare of transverseforms in which the dune
crestsare perpendicularto the prevailingwinds. Duneson Venus signal the presenceof sand-size
(~60 to 2,000 gm) grains. The possibleyardangsare foundat 9øN, 60.5øE, about300 km southeast
of the crater Mead. Although most aeolian features are concentratedin smooth plains near the
equator,the occurrenceof wind streaksis widespread,and somehave been found at all latitudesand
elevations. They demonstratethat aeolian processesoperate widely on Venus. The intensity of
wind erosionand deposits,however,varies with locality and is dependenton the wind regime and
supply of particles.
1. INTRODUCTION
Aeolian,or wind-related,processes
on the surfaceof Venus
have been debatedfor more than two decades,and many
investigators
predictedthataeolianfeatureswouldeventually
be found (reviewed by Greeley and Arvidson [1990]).
Althoughimagesof the surfacereturnedfrom SovietVenera
landersand measurementsof near-surfacewinds suggested
localmodificationof the surfaceby wind,definitiveevidence
for more widespreadaeolianactivity was not observeduntil
the Magellan mission[Saunderset al., 1991]. Preliminary
analysesof Magellanradarimagesrevealedseveralregions
where wind-related features are abundant, as well as other
isolatedoccurrences[Arvidsonet al., 1991].
using (primarily) cycle 1 Magellan radar data (Figure 1).
Aeolianfeaturesincludepossible(1) dunefields,(2) yardangs
(wind-erodedhills), and (3)various typesof wind streaks
(surface patterns of contrastingradar backscattercross
sections). We describe these aeolian features and their
characteristics
as seenon Magellanradarimagesand assess
the geologicalsettingsandpropertiesof the surfacein which
they occur. We alsodiscussthe possiblemodesof formation
of the mostcommonaeolianfeatures(wind streaks),drawing
on terrestrialexamples,Martian analogs,and resultsfrom
wind tunnelsimulations.We thenconsiderthe relationships
between aeolian features and patterns of atmospheric
circulation on Venus.
For thisreport,about44% of the surfaceof Venushasbeen
searched in a reconnaissance mode for wind-related
features
1.1. Background
1Department
of Geology,
Arizona
State
University,
Tempe.
Wind-relatedfeaturesobserved
onplanetaryimagesprovide
2Department
of EarthandPlanetary
Sciences,
Washingtondirectevidencefor the interactionof theatmosphere
with the
University, St. Louis, Missouri.
3Jet Propulsion Laboratory, Pasadena, California.
surface. The presenceof depositionalaeolianfeatures,such
as
dunes,showsareaswhereparticlescapableof movement
4Department
of Earthand SpaceSciences,
Instituteof
by the wind occur and gives indicationsof weathering
Geophysicsand Planetary Physics, University of California,
processes.The identificationof the type andorientationof
Los Angeles.
5Centre
Nationalde la Recherche
Scientifique,
Toulouse, aeolianfeaturesprovidescluesto thephysicalpropertiesof
France.
surfacematerialswheretheyoccurand the wind directionat
the time of their formation. Assessmentof their age
Copyright 1992 by the American GeophysicalUnion.
providesinsightinto pastwind regimesandclimates.
Wind streaksare amongthe mostcommonaeolianfeature
Papernumber92JE00980.
observed
on planetarysurfaces.They occuron Earth,Mars,
014 8 -022 7/9 2/92 JE- 0098 0 $05.00
Triton, and Venus. On Earth, wind streaks are surface
13,319
13,320
GREELEY ET AL.: AEOLIAN FEATURESON VENUS
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..............
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•.•.:•:•.•,•
Fig. 1. Map of Venus showingorientationof wind streaksequal-areaby latitude and longitude"bin" and their
distribution. Symbols are given in the center of each bin, or positionedwithin the bin to maximize legibility.
Arrows indicate inferred downwind direction. Also shown are the location of dune (D) and yardang (Y) fields.
Regionsnot yet analyzedon F-BIDRs includelongitude-30 ø through50ø (superiorconjunction)and longitudes
-160 ø through330ø (digital basemap from U.S. GeologicalSurvey).
patternsin whichloosematerialscommonly<1 m thick are
distributed
by sediment-moving
winds. Typically,theyare
associated
with wind patternsand turbulencegeneratedby
topographicfeaturessuchas smallhills. On Earth andMars,
windstreaksare visibleon opticalimagesbecauseof albedo
contrastsrelated to particle size or compositionand to
exposuresof bedrock. Wind streaksare alsovisibleon Earth
on radarimages[Greeleyet al., 1989;Saunderset al., 1990],
wherefactorssuchasbedrockexposures
andsedimentcover
resultin contrastingradarbackscatter
crosssections.Wind
streakson Earth rangein lengthfrom a few centimetersfor
2h). Wind streaksof several types also occur on Mars
(Figures 2e and 2f), as reviewedby Greeley et al. [ 1992].
Thomaset al. [ 1981] derived a classificationof Martian wind
streaksbasedon (1) theirupwindsources(sedimentdepositor
topographicobstacle),(2) their albedo contrast(bright or
dark) in relation to the backgroundsurface,and (3) special
morphologic or compositionalfeatures. Some, termed
variable features,appear,disappear,or changetheir size,
shape,and orientation on time scalesof weeks to years
[Thomas and Veverka, 1979, etc.]. Martian wind streaks
rangein lengthfrom a few centimetersat the Viking landing
small sand drifts behind rocks to more than 15 km for
sites to 115 km for a dark, plume-shapedstreak in the
patternsdevelopedin the lee of hills and small mountains Mesogaea
region[Veverkaet al., 1976].
(Figures2a-2d)andin association
withimpactcraters(Figure Regardless
of type,modeof formation,or planetwherethey
Fig. 2. (Opposite)
Typicalwindstreaks
on Earth,Mars,andVenus;arrowsindicate
prevailing
winddirection.
(a) Amboy,California,
opticalimageshowing
cindercone(460m in diameter)
anddarkstreak.Prevailing
windis
fromthe west(left); generalbackground
consists
of pahoehoe
basalticlavaflowsandis mantledwith windblown
sand(whimareas).Areaof streakis darkbasaltsweptfreeof sanddueto windturbulence
shedfromflow aroundthe
cone(fromGreeley
andIversen
[1987];airphotoAXL-26K-36
takenJanuary
10,1953). (b) Seasat
radarimage
(revolution
882)of Amboy,California,
showing
radar-bright
streakcorresponding
to darkareasonFigure2a thatis
relatively
freeof windblown
sand.Darkareas
correspond
toconcentrations
of sandand(extending
toward
thetopof
thepicture
fromthecone)basalt
pebbles
andcinders.
Radarillumination
fromthebottom.(c)SIR-Aradarimage
of
theAltiplano,Bolivia,showing
radar-dark
streaks
as longas 15 km formedin association
with hills (bright
features).Contrasts
in radarbackscatter
crosssections
resultfromdifferences
in sandmantles,duneforms,and
vegetation,
all relatedto erosion
anddeposition
patterns
generated
by windflowaround
thehills. Prevailing
winds
arefromthePacificOceanto thewest(left)(SIR-A: DT-31). (d) SIR-Aradarimageof linearstreaks
southeast
of
LaskarGan,Afghanistan,
formedpredominantly
in sedimentary
deposits,
including
playasiltsandclays.
Prevailing
windsarefromthenortheast
(marked
withanarrow),
whichfunnelthrough
a gapin a lowridgeandthen
spreadout to the southwest.Contrasts
in radarbackscatter
crosssectionevidentlyresultfromdifferences
in the
distribution
of sediments
(SIR-A: DT-35/36). (e) Darkwindstreaks
on Marsin thePhoenicis
Lacusregion;
prevailing wind is from the southeast(left). These and similar dark wind streaksare consideredto result from
GREELEYET AL.: AEOIJANFEATURESON VENUS
13,321
.
.
erosion.Area sho• is 30 by 43 •
(Viking•biter 459A79). • •right wi•d s•e•s i• •e •es•ria Pla•um,
M•s; prevail•g wi•ds •e •om •e not.west (up•r left). Bright wi•d s•e•s •e •ought to be dustdepsited i•
ß e lee of topgraphic fea•es, •rhaps under stableatmosphericcondition. •ea show• is a•ut 5• km across
(Viking •bier 453•5).
(g) Rad•-bright wi•d see• o• Venusat 23.9øS,345.1øE. S•e• is a•ut 10 • Io•g,
' is associatedwi• a small hill, •d occurswi•
•e "p•a•lic ½oI1•" associatedwi• the impactcraterC•son (see
Eigure 19); w•d flow at the time of streak fo•atio• is i•fe•ed to have been from •e south (left). Radar
illumi•afio• •om •e top (Magell• F-MIDR 25S345). (h) Wolf Creek•pact crater,Austria, showingassociated
s•d depsi• (•ight •eas) •d erosion• •eas (d•k zones);ridges•e li•e• dunesp•allel to •e wi•d; prevailing
winds •e from •e east (left); •ea shownis -6 • by 9 • (Commonwealthof Aus•alia photograph,•illiluna
13,322
GREELEYET AL.: AEOLIANFEA7URF3ON VENUS
occur,there is near-unanimous
agreementthat wind streaks
representthe prevailingwind directionat the time of their
formation. As such,they can be usedas local "wind vanes"
to mapnear-surface
windsandhavebeenusedto assess
local,
regional,and global patternsof atmosphericcirculationon
Mars [Sagan et al., 1972; Thomas and Veverka, 1979;
Greeleyet al., 1992].
The discoveryof wind streakson Venus (Figure 2g) as
describedby Arvidsonet al. [ 1991]affordstheopportunityto
learn about the interactionof the atmosphereand surface,
both for the identificationof sedimentscapableof being
moved by the wind and in mapping near-surfacewinds.
Mapping winds is especially important becauseof the
paucityof observationalconstraintson the circulationin the
lower atmosphere[Schubert,1983]. For example,Doppler
trackingof Venera probesand landers[Marov et al., 1973;
Antsiboret al., 1976; Keldysh, 1977; Kerzhanovichet al.,
!N
HADLEY
CELL
i
ALLEY
1979; Moroz, 1981; Kerzhanovich and Marov, 1983] and
trackingof the Pioneer Venus probes[Counselmanet al.,
1979, 1980]providedestimates
of zonalandmeridionalwind
speedsas high as severalmetersper.secondto an altitudeof
~10 km. Surface winds measuredby Veneras 9 and 10
HALLEY
i
CELL
i
et al., 1980; Rossow, 1983; Schubert, 1983]. This direct
meridional circulationis symmetricabout the equatorand
involvesequatorward
surfacewinds,upflowovertheequator,
polewardwindsaloft, and downflow at high latitudes. The
long-term, zonally averaged circulation of the deep
atmosphereof Venus may resemblea Hadley circulation.
The existenceof a Hadley cell in the lower atmosphereof
Venus can be evaluatedusing Magellan imagesof wind
streaksin thisstudy.Shoulda Hadleycell exist,wind streaks
might also provideinformationon the latitudinalextentof
WESTWARD
MEAN ZONAL
SUPERROTATION
I
!
I
[Avduevskii
et al., 1976]were0.3to 1 rns-1. Theobserved
motionsin the lowestscaleheightare sluggishbut appearto
be neither mainly meridional nor zonal. However, the
available measurements are inadequate for even
approximatingthe patternsof lower atmospheric
circulation
[Schubert,1983]. Above 10 km, zonalwind speedsincrease
monotonicallywith altitude,and the dominantcirculationis
a westwardzonalsuperrotation
[Schubertet al., 1980].
Althoughthe lower atmosphericcirculationof Venus may
be temporallyand spatiallycomplex,it is usefulto consider
two end-membermodelsof the circulation(Figure3). One is
a Hadley circulationthat redistfibutessolarenergyabsorbed
in the lower atmosphereand at the groundnearthe equator
[Stone,1974, 1975; Kdlney de Rivas, 1973, 1975; Schubert
i
is
Fig. 3.
Sketch of possible circulation patterns in the
atmosphereof Venus. The mean zonal velocity h is a westward
superrotation. The magnitude of h increaseswith height above
the surface. The meridionalHadley circulationmay not extendto
the poles. Centersof convergenceand divergencein the diurnal
Halley and anti-Halley circulationsmay not occur at noon. The
diurnal circulationshave other flow componentsnot shownhere.
is conceivablethat such changescould be detectedwith
multipleMagellanobservations
over time, as areplannedin
futuremappingcycles. Moreover,as notedby Saunderset
al. [1991] strongest,winds were predictedto flow to the
west,away from the solarnoonlongitude,and to flow down
hill. Thesepredictions
canbe addressed
with Magellandata.
1.2. Methodology
Observations for Venusian aeolian features included searches
of syntheticaperatureradar (SAR) images,assessment
of
surfacepropertiesand elevationswherefeatureswere found,
andcorrelations
of aeolianfeatureswith localgeology.The
searchfor wind-related
featureswasconducted
usingF-BIDRs
(full resolutionbasic image data record), F-MIDRs (full
the circulation.
resolution mosaiked image data record), and C1-MIDR
A secondpossiblemodelof theloweratmosphere
on Venus (mosaikedimagedatarecord,compressed
once)(seeSaunders
involvesdiurnalcirculation. In this model (Figure3), there et al. [1990, this issue] and Pettengill et al. [1991] for
is downflowover the subsolarregionjust abovethe surface, explanationof dataproducts).The bestspatialresolution
on
flow towardtheantisolarregionnearthesurface,
upflowover F-BIDR and MIDR imagesis 150 m/line pair (represented
theantisolarregionjust abovethe surface,andflow toward by 75 m/pixel). F-BIDR printsthroughorbit 1319 were
the subsolar
pointaloft [Dobrovolskis
and Ingersoll,1980; examinedfor small(<10 km) features(Figure1). Aeolian
Covey et al., 1986]. This is an "anti-Halley cell" above features>10 km wereassessed
on F-MIDRs andC1-MIDRs;
which(in theloweratmosphere)
is a simple,thermallydirect consequently,
becausenot all data havebeenassembled
as
subsolar_to_antisolar
circulationpattern,with upflowin the mosaics,somelargerfeaturesmay not yet be recognized
in
subsolar
region(Halleycell, Figure3). Wind streaksin this theareasanalyzedfor smallerfeaturesusingonlyF-BIDRs.
modelwouldbe orientedawayfrom the warm,daysideof The distribution
of aeolianfeatureswasplottedon a global
Venusandtowardthe coolernightregion. Thiscirculation scaleandcorrelated
withlocalandregionalgeologicsettings
patternwouldbe moredifficultto detectusingwindstreaks to placeconstraints
on the possiblesources
for thedeposits
than a Hadley cell becausethe movementof the subsolar associated
with thefeatures.
pointover the surface(1 Venussolarday = 117 Earthdays) In identifyingaeolianfeaturessuchas wind streaks,it is
wouldchangewind streakdirectionswith time. However,it importantto note that BIDRs and MIDRs havebrighiness
GREEI F•Y ET AL.: AEOLIAN FEAT[IRKS ON VENUS
13,323
valuesthatareproportionalto specificradarbackscatter
cross modes,respectively[Pettengillet el., 1991]. Emissivityand
section(o0 whichis crosssectionper unit areadividedby the Fresnelreflectivity of geologicmaterialsare closelytied to
averagevaluefor therelevantincidenceangleandconvenedto the dielectric constant and thus allow model-dependent
decibels). An understandingof the behavior of o0 as a separation
of theeffectsof roughness
anddielectricproperties
function of incidenceangle and look azimuth is critical to on the SAR signatures [Tyler et el., 1991]. These
interpretationsof the radar appearanceof aeolianfeatures. observations
provide informationon the physicalproperties
The incidenceanglefor Magellan datavariessystematically of surface materials.
with the latitude,from a valueof 43ø at the periapsislatitude Magellanaltimetricdataenabledassessment
of topographic
(10øN) to a value of 18ø at the northpole. In general,in this control of aeolian features. Pettengill et el. [this issue]
rangeof incidenceangles,radarwavelength-scale
roughness derived a global topographicmap of about 5 km spatial
appearsto dominatethe cross-section
valuesof the aeolian resolutionand <100 m vertical resolution. For our analyses
featuresdiscussed
here,with secondarycontrolby topography of wind streaks,the elevation of each streak was determined
anddielectricproperties.However,the identificationof wind and the local slopes were assessed,using a bilinear
streaksin the SAR images at high northern and southern interpolationof the topographicdata pointswithin a 10-km
latitudesmay be moredifficult,becausesubtledifferencesin radius of the streak origin. However, these statisticsare
small-scaleroughnessare more difficult to detect at small based on a preliminary global topographic map which
incidenceangles.Biota and Elachi [ 1981, 1987] haveshown containssomeerrors. As shownin Figure 4, the azimuthof
that the azimuth of the radar viewing geometry may also the maximumslope ([3, in the downslopedirection,i.e.,
substantially
affect the visibilityof aeolianbedformssuchas "dip" direction)and the amount(magnitude)of the slopein
sand dunes. Continuing analysis of data from future degreesfrom the horizontalwere determined. Finally, the
mappingcyclesof Magellan with different incidenceangle angle0') betweenstreakandslopeazimuthwasdeterminedin
and look azimuthprofileswill allow a completeassessment order to assesswhetherstreakstend to be orientedupslope,
of the streakpopulationon Venus.
downslope,or randomlywith respectto slope.
In additionto the high-resolutionspecificbackscattercross
sectiondata obtainedby the Magellan SAR, estimatesof
2. WtND STREAKS
surface emissivity, Fresnel reflectivity, and rms slope (at
greaterthanwavelengthscales)with ~10 km resolutionwere Wind streaks of several forms have been found on Venus.
obtainedby the radarsystemin its radiometricand altimetric Although it is tempting to derive a formal classification,
ridgeor
fracture
•
strea
,•.•-•'•
Transverse
../•....r•. .•
:'-:'•:'/,,'7 ragged
'".'••)
,,,,•....,,,2"
..••"
Transverse
both
smooth
are
fan
streaks
•
ridge
or
DOWN-WIND
fracture
DIRECTION
N
slo
,
**-,,,
'
a
Fig. 4. (a) Pl•fom sha•s •d termsappliedto Venusi• wind stre•s; a is •e az•u• measuredfrom north to the
s•e• in •e infe•ed downw•d direction. (b) P•meters usedin •alysis: a is stre• length (m•imum) p•allel to
infe•ed w•d direction,b is s•e• wid• (maximum)nomal to leng•, c is dimeter of l•dfom with which stre•
is associated(or •e averagewid• of the ridge or •ench for tr•sverse stre•s), • is azimuthof stre• in the infe•ed
downwinddirection,• is azimuthof te•ain in the downslopedirection,•d T is minimum•gle betweenstreak
az•u•
obtained
•d slo• az•u•
for some stre•s.
(5180ø), A-A' •dicates the l•e along which rad• backscattercrosssectionprofiles were
13,324
TABLE
GREELEYET AL.: AEOLIAN FEATURESON VENUS
1. Parameters used in Data Base to Describe Wind Streaks
on Venus
Principalparameter
Sub-type
Planimetric shape
Fan
Linear
Transverse
Wispy
Radar reflectivity
Bright
Dark
Mixed
Landform origin
Cone
Hill
Crater
Ridge
Trench
Planimetric
measurements
Length
Width
Azimuth
Landform diameter(or width)
Terrain
consideration
Geologic setting
Elevation
Local slope direction
Local slope magnitude
crater deposit. However, for most wispy streaksit is not
possibleto determinewhichendis theapparentsource,noris
it feasibleto determinea meaningfulorientation(azimuth)
because of their meanderous character.
Contrastsin radarspecificbackscatter
crosssectionbetween
the streak and the backgroundenable wind streaksto be
identified.Brightstreaksarebrighterthanthebackground
on
whichtheyoccur(Figures5a and5e), darkstreaksaredarker
thanthe background
(Figures5b, 5d, 5g and5h), andmixed
streaks(Figure 5c) have both bright and dark components
(generally a bright interior and a dark "halo", set on a
background
of intermediate
specificcrosssection).All wispy
streaksareradar-dark(Figure5h). Nearlyall linearstreaksare
radar-dark,most fan-shapedstreaksare radar-bright,and
wansverse
raggedstreaks
arenearlyequallyradar-dark,
-bright,
and -mixed (Figure 6a). However, somelinear streaksand
transverse-ragged
streaksoccurin multiple setsand createa
brightand dark patternin whichit is impossibleto separate
darkstreakson a brightbackground
from brightstreakson a
darkbackground
(the "zebra"effect,Figure5f).
The landform with which the wind streak is associated is
alsoconsidered
in thedescriptions
whereappropriate.Cones
areconicalin topographic
crosssectionandcommonlyhave
summit craters; hills are more rounded, typically lack
summitcratersand includedomical crosssections;cratersare
circulardepressions
andmay showevidenceof modestraised
See text for explanations.
rims;trenches
arelinearfeatures
of negativerelief;andridges
sucha derivationwouldbe prematureuntil the full rangeof are linear featuresof positiverelief. The small widthsof
possibilitiesis known upon completion of mappingby trenches and ridges make it difficult in most cases to
Magellan. Table 1 showsthe parametersthat appearto be distinguishbetweenthe two on SAR images. Somewind
importantin describingVenusianwind streaks.A database streaksoccur on otherwisefeaturelessplains and are not
associated with obvious landforms.
is being compiled which includes these parametersand
descriptions
of theterrain,topography,
andsurfaceproperties The parameters shown in Table 1 were used in the
in which the streaksoccur. Each streakis "tagged"in the assessment for each wind streak found. Measurements
databaseby thelatitudeandlongitudeof itsinferred(upwind) (Figure4b) weremadefor themaximumlengthandwidthof
point of origin, usingbestavailablepositionalinformation. the streak (length was taken to be the axis of the streak
Aeolianfeaturesin someareasoccuras multiplestreaks.For oriented parallel with the inferred wind direction), the
arbitraryfor somehills
suchareas,oneentryis madein the database,alongwith an diameterof thelandform(somewhat
andconesthatmergewith the surrounding
plain) or widthof
estimate of the total number of individual streaks. The
the
trench
or
ridge
where
determination
was
possible.The
valuesof length,width,etc.,givenareestimatedto represent
streak
azimuth
(i.e.,
degrees
from
north)
in
the
downwind
the set, but the estimatesare not based on a rigorous
direction was also measured. The terrain in which streaksand
statisticalanalysis.
otheraeolianfeatures
occurwasassessed
forgeneralgeology,
2.1. Wind StreakDescription
usingpreliminaryunitsdefinedby Saunderset al. [1991].
The shapeof the streakin planformis consideredto be a Backscatter cross sections were obtained in lines across
to theiraxes.
primarydescriptivecharacteristic.
Five shapesarecommonly selectedstreaks,perpendicular
found: fans, linear streaks,wispy streaks,transverse-ragged Following the method outlined above, more than 3400
havebeenidentifiedon Venusthusfar in theanalysis
streaks,and transverse-smooth
streaks(Figures 4 and 5). streaks
Fan-shapedstreakshave a variety of outlinesand are often of Magellandam.
associated
with landformssuchassmallhills (Figures5a-5c).
Transversestreakstypically occurin setsalongfracturesor 2.2. Wind Streak Distribution
ridgesorientedperpendicularto the inferredwind direction,
and may be either ragged (serrated)or smoothin outline The distribution of wind streaks was assessedin relation to
(Figures 5d and 5e). Linear streaks(Figures 5f and 5g) type, streak length, latitude, elevation on Venus, and the
typicallyare morethan20 timeslongerthantheir width and slopeand slopedirection(downslope)of the surfaceson
often occur in sets of a half dozen or more similar streaks.
which they occur. Distributionsof the azimuthsof wind
Wispy streaksare wavy, meanderouspatternsthat vary in streakswerealsodeterminedfor comparisons
with modelsof
widthalongtheirlength(Figure5h). Wispy streaksareoften atmosphericcirculation. These distributions include the
associated
with ridgesandimpactcraters.Many of thewispy estimatedtotal numberof streaksin areaswheremultiple
streaksassociatedwith ridges are parallel to (and have the features
occurexceptfor thedistributions
by elevation,
slope,
same length as) the ridge. Wispy streaksassociatedwith andazimuth. As shownin Figure6a, darklinearstreaksare
impactcratersoccurin setsof a half dozenor more and form the most common, whereas dark transversesmooth streaks
a meanderouspatternapproximatelyradial to the crater or are the least common,with only one havingbeenfound.
5km
, !.-O'.km
'"
ß
-..
..
Fig. 5.
Venuswind streaks(arrowsindicateinferreddownwinddirection).(a) Radar-bright
fan-shaped
wind streak 10.5 km long associatedwith a small hill in easternNiobe Planitia, centeredat 36.5øN, 174.6øE
(MagellanF-BIDR 1194). (b) Radar-darkfan-shaped
wind streakabout10 km long associated
with a smallhill
centered
at 29.4øN,57øE(MagellanMRPS40983). (c) Radar-bright
and-dark(mixed)fan-shaped
wind streakin the
Carsoncraterarea,centeredat 23øS,344.9øE. Area shownis about25 by 36 km (MagellanF-MIDR 23S345). (d)
Transverseraggedwind streak(radar-dark)associated
with a ridge systemin southernLeda Planitia,centeredat
37.5øN,65.5øE;area shownis about44 by 64 km (MagellanMRPS 38883). (e) Transversesmoothwind streak
(radar-bright)
associated
with a ridgein GuineverePlanitia,centeredat 26.2øN,331.4øE;areashownis about39 by
57 km (MagellanF-MIDR 25N333). (f) Multiple linear streaksin the vicinity of Mead crater,centeredat 15øN,
65øE;areashownis about44 by 64 km (MagellanMRPS 37877). (g) Multiplelinearstreaks(radar-dark)in western
A.phrodite,
centered
at 0.9øS,71.1øE;areashownis 82 by 120km (Magellan
F-MIDR 00N070). (h) Radar-dark
wispy streakin easternSednaPlanitia, centeredat 37øN, 2øE; area shownis about 87 by 128 km (Magellan
C1-MIDR 30N009).
13,326
GREELEY ET AL.: AEOLIAN FEATURESON VENUS
12oo
14oo
•
Dark
•
I--I Bright
12oo
1000
-:'•
Mixed
•
i•i lOOO
n-
800
o
600
Dark
I--I Bright
:.,:...:•
Mixed
0• 800
o
a
n- 600
n-
b
rn 400
•
400
z
200
200
o
Transv. Tmnsv.
ragged
smooth
STREAK
Fan
,I ,
o
Wispy
Ridge
Hill
TYPE
Cone
STREAK
Trench
,law[_
Crater
ORIGIN
Fig. 6. (a) Histograms
of wind streakson Venusby shapeandradarbrighiness.(b) Histograms
of the type of'
landforms
with which streaks are associated.
Figure 6b showsthe distributionof radar-dark,-bright,and location of the altimetry data acquisition(nadir) and the
-mixed backscatter cross section and the landform with which
location of SAR and radiometry data acquisition.
they are associated.Most radar-darkstreaks(mostlylinear Consequently,
wheregapsin Magellandataoccur,theyaffect
forms)occurin associationwith ridges,whereasmostradar- dataat differentlocationson the planet. Where no altimetry
bright streaks are found with small hills and cones. data are presentfor a streak,the streakwas not includedin
Assessment
of strengthlengthsshowsthatdarklinearstreaks elevationdistributions.Moreover,wherealtimetrydatawere
are the longest(>100 km), whereasmost bright fan-shaped missing in the slope calculation region, the streak was
streaksare <10 km long.
omitted in both slope magnitude and slope azimuth
Because of these considerations, the
Figure 7 showsthat streaksoccur over a broadrange of distributions.
latitudeswith peaksin the latitude bands23øS to 30øSand distributionsshown in Figure 8 contain considerablyless
23øN to 30øN. Many streaksoccur in clustersassociated than the total number of streaksin the data base. Finally,
with ejecta depositsfrom impact cratersin plains east of wherecomparativeglobal distributionsare shown,the two
Alpha Regio, southernGuinevere Planitia, and in eastern curvesare normalizedto haveequalareas.
AphroditeTerrae(Figure 1). No assessment
wasmadeof the Figure 8a showsthe distributionof elevationsfor streaks
distributionof streakswith longitudebecausethe Magellan relative to 6051 km (the referenceelevationon Venus, taken
coverageby longitude was incompleteat the time of this from the center of the planet) and the distribution of all
elevationsdeterminedby the Magellanaltimetryexperiment
study.
Figure 8 shows distributionsof streaks with elevation, [Pettengillet al., this issue]. Resultsshowthat streaksform
slope magnitude, and slope-streakangle. Becausemany at nearlyall elevations.Figure 8b showsthat moststreaks
streaksare >10 km long, the elevation,slope,and anglemay form on surfacesof low slope, equal to or only slightly
changeas a function of distancealong the streak. For a steeperthan averageslopeson Venus. The distributionof
given spacecraftorbit thereis a variableoffsetbetweenthe the smallestangle betweendownslopedirectionand streak
0.25
•Z
-
0.2
",
,
•
all elevation
.......
streaks
data
,
,
o
o o.15
ß
,
,
ß
o
LU
•
o.1
0.05
o
z
-90-64-53-44-36-30-23-17-11
South
-5
0
5
11 17 23 30
LATITUDE (degrees)
36 44
53
64 90
North
o
-o.o5
'
-4
I
I
-2
0
'
I
2
'
I
4
'
I
6
ELEVATION (kin above 6051 )
Fig. 7. Distributionof streaks(of all types)by equal-area
bands Fig. 8a. Distributionof streaksby elevationon Venus; curves
of latitude.
are normalized
to unit area under each.
GRF•LEY ET AL.: AEOLIAN FEATURES ON VENUS
13,327
70
0.25
globalslopes
streak slopes
••Z
0.2
oO0.15
• 0.1
z0.05
0
0.5
I
DOWNSLOPE
0
1.5
15
30
ANGLE
45
60
75
90 105 120135
SLOPE-STREAK
(degrees)
150 165 180
ANGLE
(gamma, in degrees)
Fig. 8b. Distribution of streaksby slope magnitude at 10 km
scale; curves are normalized to unit area under each.
direction(Figure8c) revealsthat streaksform at nearlyall
anglesto the local slope,with only a slightpreferencefor
downslopeorientation.However,in someplacesthereis a
correlationwith local slope. For example, streaksin the
Ovdaregiontendto be orientedupslope,
asdiscussed
later.
Figure9 showsthe distributionof streakazimuthsin the
northernand southernhemispheres.Streaksin the northern
hemisphere
suggestformativewindspredominandy
from the
northtowardtheequator(azimuthsmainlybetween-120ø and
250ø). Very few wind streaksin the southernhemisphere
have azimuths between -90 ø and 270 ø, indicating a
preponderance
of inferredwinddirections
towardtheequator.
2.3. TemporalChanges
During
thesecond
cycleof Magellan
radarmappin
g, 51
orbitsof data with the sameleft-lookinggeometryas those
in cycle 1 wereacquired.Usingdatafromtheseorbits,the
northernNavkaRegionwasstudiedto determineif anyof the
streaksfoundin cycle 1 hadchanged.The regioncovers10ø
to 30øN, and 329ø to 336øElongitude. Most of the streaks
foundin thisregionduringcycle1 areradar-bright
and-mixed
Fig. 8c. Distributionof streaksin relationto slopedirection
= 0ø = downslope,7 = 180ø = upslope).
fan-shapedstreaksassociatedwith small cones, although
there are also some transverse-smoothbright streaks
associatedwith ridges. Comparisonsof F-BIDRs and FMIDRs from both cycle 1 and 2 showedno apparentchanges
in the streaksduringthe eight monthsbetweenacquisitionof
spacecraft
data.
2.4.
Wind Streak Formation
Studyof wind streaksin the planetarycontextbeganwith
their discoveryon Mars in the early 1970s [Saganet al.,
1972, 1973]. Various investigationshave been made of
Martian features[Veverkaet al., 1977], terrestrialanalogs
[GreeleyandIversen,1986], wind tunnelsimulationsliversen
and Greeley, 1984], and atmosphericconditions[Veverkaet
al., 1981] in an attempt to understandthe formation and
evolution of wind streaks.
Most wind streaks on Earth and Mars are associated with
topographic
obstacles
andform in response
to wind patterns
and turbulencedevelopedaroundthe obstacles.Dark (low
opticalalbedo)Martianstreaksrepresent
eitherbedrockareas
sweptfree of looseparticlesor lag depositsof coarse•rains
15
:
rr 15
•
:i:!:
lO
:.:.:
.:.:,
.:.:.
:•:•:
:• i½ii•::i:i:':
i:i:i
:.:.ii•!ii:i!
!i:i:
•: •Ji:
::.:i:
:•::.:jii
:i:i',
:i•::i:!!
:i:i
LU
.....
m5
.:.:
:3
z
5
iii:'•••?:
'•'"'
•
'•i•
:!:i
•:•:
!i:.•
.:!:i!•
i•ii• i:::•-
.....
>::•
......
>.•.:•'•• •i::
•::•i •::::::::
• • • ;:•
• ::•
• •?:•
........
o
o
60
120
180
240
300
360
AZIMUTH (degrees)
(0ø = North)
Fig. 9a. Distributionof streaksby azimuth (inferred downwind
direction)in the northernhemisphere.
0
•
120
180
240
•
360
AZIMUTH (degrees)
(0ø=North)
Fig. 9b. Distributionof streaksby azimuth(inferreddownwind
direction)in the southernhemisphere.
13,328
GREELEYET AL.: AEOLIANFEATURESON VENUS
terrestrial wind streaks,such as those shown in Figures
2a-2d, aredueprimarilyto roughness
differences.On Venus,
regionalcontextoften providesinsightinto the responsible
mechanism. We considerroughnessdifferencesrelatedto
sediment cover to explain the radar contrast for most
Venusian wind streaks, although in some cases such
interpretationis ambiguousand differencesin dielectric
constantmay be involved.
Figure 10 showsa regionof Venuswhereinsightmay be
the estimates of sediment thickness for the Martian streaks
gainedinto the thicknessof windblownmaterial forming
are only lower limits,andtheycouldbe muchthicker.
radar-darkstreaks. It showsa radar-brightcrater outflow
What are the requirementsfor wind streaksto be visibleon deposit(inferredto be rough)overlainwith severalradar-dark
radarimages?The backscatter
signatures
of aeolianfeatures wind streaks. The most plausibleexplanationis that the
layer
may indicateone of severalpossiblemodesof origin, some windblownmaterial forms a smooth,homogeneous
someof theradarenergy,leading
of which can be assessedquantitativelyor by analogy to overtheflowsandabsorbs
in backscatter
fromtheunderlyingflow. Using
featureson other planets(Table 2). At full resolution,the to a decrease
layer overa roughsurface
ability to detectfeaturesby theircontrastin radarbackscatter a simplemodelof a homogeneous
is limited primarily by coherentnoise ("speckle"). The (neglectingsurfacerefractioneffectsand assumingthat the
minimumdetectablecontrastratio in Magellan SAR images layerhasa low dielectricconstan0,thechangein backscatter
allows identificationof streaksin which radarspecificcross (Ao) dueto the overlyinglayeris givenby
sectiondiffers from the surroundingmaterialby ~1 dB or
AO- 8.7 H
more,with betterthan67% confidence.This corresponds
to
cos0 L
~10% changein returnedpower. In general,for Magellan
radar,energyreturnedfrom a surfacedependson (1) surface in which0 is the incidenceangle,L is the penetrationdepth
sloperelative to the incomingradiationat the scaleof the of the material, and H is the thickness[Elachi et al., 1984].
SAR resolution(for Magellan,--150to 300 m, dependingon For the area shown in Figure 10, 0 = 43ø and AO for the
latitude), (2) surfaceroughnessat the scale of the SAR streaksrangesfrom 2 to 6 dB. Assuminga losstangentof
wavelength (for Magellan, 12.5 cm) averaged over a 0.005 to 0.01 [Campbelland Ulrichs, 1969], this leadsto a
resolutionelement,and(3) thecomplexdielectricconstantof penetrationdepth of ~0.6 to 1.2 m and a corresponding
the material.
For Venusian wind streaks to be visible in
minimumlayer of particlescomposingthe dark streakof 0.1
Magellanimagesas a consequence
solelyof slopesrequiresa to 0.7 m thick, well within the range for wind streaks
physicallyimprobable(in somecases,unrealistic)surface. observed on Earth but thicker than the minimums inferred for
Thus,theradarcontrastbetweenstreaksandthe surrounding Mars. Consequently,the Venusianfeaturesmay form over
terraincouldresulteitherfrom differencesin roughness
or in much longer time scalesthan Martian features,which can
dielectric
constant.
Radar discrimination
of observed
changein as little as ~38 days.
from which smaller, brighter particleshave been deflated.
BrightMartianstreaksareconsidered
to be dustdeposited
in
the lee of topographicobstaclesto the wind, perhapsunder
conditionsof atmosphericstability [Veverkaet al., 1981].
The albedocontrastof Martian streakscan be explainedby
depositsof sedimentas thin as a few microns[Thomaset al.,
1981, 1984;Lee, 1984]. Under mostcircumstances,
deposits
thisthin wouldbe penetratedby radarenergy,andthe streaks
would not be seenon Magellanimages. On the otherhand,
TABLE 2. Wind Streaks: HypothesesandTests
Possible Origin
StreakType
Bright
High reflectivity
1.
Mineralogy
2.
Bulk density(welding)
Roughness
1.
Rough deposit
2.
Microdunes
3.
4.
Scouredsurface(bedrockor lag)
Nondeposition(assumingsmoothdeposit
off streak)
Radarpenetrationwith enhancedscatterfrom
subsurface
Tests
Emissivity, reflectivity anomaly.
RMS slopevalue (if "fractal").
RMS slopevalue (if "fractal").
Association with obstacles; "window" effect.
"Shadow"effect possiblein streaksetting?
interface
1.
Dark
Smoothdepositsoverlying rough substrate Consistent with necessaryconditions? (inclination
angle, dielectric, etc.)
Low reflectivity
1.
Low densitydeposit(soil, pumice)
Emissivity, reflectivity anomaly.
2.
Lossy material
Emissivity, reflectivity anomaly.
Smoothness
1.
Soil filling in or covering roughness
Possible sources,gradational contacts.
elements
2.
3.
Smooth sheet of rock
Exhumation of smooth surface
Superposition,morphology (margins).
4.
Nondeposition(assumingrough depositoff
"Shadow"effect possiblein streaksetting?
streak)
Association with obstacles; "window" effect.
GREEI•EYET AL.: AEOLIAN FEATURESON VENUS
13,329
Fig. 10. Part of westernNiobe Planitiashowingradar-darkstreaks(arrows)superposed
on radar-brightcrater
outflowdeposits.The outflowsare associated
with the 42-km-diarneter
craterManzolini. Radar-darkmaterial
formingthe streaksis estimated
to be 0.1 to 0.7 m thick;areashownis centeredat 26.5øN,93.5øE(MagellanC1MIDR 30N099).
.
Terrestrialanalogs. Wind streakson Earth can provide
insight into wind streak formation. Most wind streaks
VORTEX
associatedwith landforms such as hills, raised-tim craters,
andothertopographic
obstacles
canbe relatexlto a turbulent
wind patternknown as a horseshoevortex [Greeleyet al.,
1974]. As shownin Figure 11, wind flow separationand
reattachmentresults in distinctive patterns of sediment
erosion and deposition related to the geometry of the
landform and the shear stressexerted on the surfaceby
turbulent winds. Wolf Creek impact crater in Australia
[McCall, 1965] exemplifieswind erosionand deposition
patterns around a raised-rim crater (Figure 2h); sand
depositionoccursin a zoneupwindfrom the rim andas two
trailinglineardunesdownwindfrom the crater. The lee zone
of the craterlies beneaththe merging"cores"of the trailing
vorticesand is sweptfree of sandto exposebedrock. This
samebasicpatternis seenat theAmboylavafield, California
[GreeleyandIversen,1986],wherewindflow arounda cinder
conecreatespatternsof sanddepositionand sand-freezones;
this pattern is visible on both optical and radar images
(Figures2a and2b).
Wind streaksof severalformsandin a varietyof seuingson
Earth are visibleon Seasatand shutfieradarimages[Greeley
CORE
"SHADOV•
ZONE
POINT
OF
ATTACHMENT
:'
HORSESHOE
REVERSE FLOW
VORTEX
Fig. 11. Diagram of wind flow aroundtopographicobstaclesuch
as a small hill. A "horseshoevortex" wraps around the hill,
creatinga zone of turbulenceandhigh surfaceshearstressin the
wake of the hill. Material is preferentially erodedin this zone.
Rising turbulentwind componentsalong the outer edgesof the
vortex coresmay causepreferentialdeposition,creatinga "halo"
of particulate material around the eroded zone [from Greeley,
19861.
13,330
GREELEYETAL.: AEOLIANFEATURES
ONVENUS
%--,-.'-.;
-'-:..
.......
.:•:•.;•...-.
'"'"
ß
.•..,.........::::
........................
:.....................
]:;•:•::.,....
.....................
.•;:;;
......
4E:?..::?,:-:;•
•;;•:•
ß"½½:•t;i:•:,;•'•:,•;•!(:•':'•i½:•½•:•:.:;:;
.:.::;
.:..
'f,:..',•:;•.4•:•:•:::
:::
-...•.:½;•".
::•,':;;:.
:•!•,•;,.;•-•,:•..:';::&..
•:;•
::;:,%
';;?•.....-'½'.z
• •::,:;;4::.,,
•:•.:.....
."':'.•:::.;
;•:
;::•,,
...•;:
•.'..
3...:.3,•:;;•:•;:•::.•,%:.•:.:•4s•;:•,•R:;:½::;•:.:;•;2•:•%
Fig. 13. Radar-brightfan-shapedstreaksresultingfrom erosion.
Dark haloes around bright streaks are inferred to be
concentrationsof particulate material swept from the bright
zonesby wind. Area shownis centeredat 22.3øN,332.1øEandis
~47 by 60 km (Magellan F-MIDR 20N334).
run. Patterns of erosion are defined as zones where sand has
beenremoved,exposingthe bare (dark) wind tunnelfloor.
Thesezonescorrespond
to highwindshearrelativeto therest
of the floor and arepointsof flow reattachment
asrelatedto
the horseshoe
vortexflow field (Figure 11).
Summary
for Venusianwindstreaks. Complexities
in the
Fig. 12. Venuswind tunnelresultsfor flow over dome-shapedinterpretation
of backscatter
signatures
makeit difficultto
hill; flow is from left to right; area shownis about30 by 100 assess
theerosionalvs. alepositional
originof aeolianfeatures
cm: (a) beforetherun,surfacewascoveredwith fine quar• sand,
on Venus (Table 2). Results from studiesof terrestrial
(b) during the run, erodedzonesshow as dark areaswhere the
wind tunnel floor (black) becomes exposed; note the dark analogsand wind tunnelsimulationsshow that erosional
erosional "collar" that reflects turbulence associated with the
zonesare expectedin the wake of topographic
obstacles
to
horseshoevortex shown in Figure 11, and (c) after the run, the wind as a consequenceof flow reattachmentand
showingcontinuederosion and th6 merging of trailing cores acceleration of the wind. With time, on surfaces manfled
from the horseshoe vortex to form an eroded streak downwind
with loose sediments, these zones would be scouredfree of
from the hill. By analogy with wind streaksvisible on radar
to exposebarebedrock.In termsof radar-visible
images,the areascoveredwith sandwould be relativelyradar- sediments
dark, and eroded areas would be radar-bright, assumingthat
underlyingbedrockwere rougherthansediment-covered
areas.
streakson Venus,thesezonesgenerallywouldhavebrighter
radar backscatter cross sections than the surrounding,
sediment-manfled
terrain,resultingin radar-brightstreaks
et al., 1983, 1989; Elachi et al., 1982; Saunders et al., similarto the Amboy streakin the Mojave Desert.
It shouldbe notedthatradarbrightstreaks,as erosional
1990]. In most cases,contrastsin radar backscattercross
areconsidered
to beequivalent
to opticallydarkwind
sectionresultfrom depositsof windblownsand(radar-dark) features,
streaks
on
Mars.
For
example,
the
radar-bright
fan-shaped
and exposuresof bedrock(radar-bright)that form patterns
streaksshownin Figure13 areconsidered
to be theresultof
erosion. The terrain surroundingthe streak is relatively
smooth and radar-dark, suggestinga mantling deposit,
probably
part of the parabolicejectacollar from Aurelia
providecluesto thecomplexflow of windsoverandaround
crater.
Outside
this area, the surfaceis characterizedby a
topographic
featuresandindicatewherezonesof erosionand
pattern.We observe
thatthesamereticulate
pattern
deposition
occur[Greeley,1986]. The VenusWind Tunnel reticulate
streak,suggesting
thatthe
(VWT) was designed to study the physics of particle is foundwithinthe fan-shaped
movement in the Venusian environment and to model
manflingdeposithasbeenremovedby winderosionenhanced
erosionand depositionaroundlandforms[Greeleyet al., by turbulentflow aroundthesmallcone.
Someradar-brightstreakscouldrepresentdeposits
of high
1984]. To supportthe analysisof Venusianwind streaks,a
series of tests was conducted in which wind flow was
reflectivitymaterial. For example,the long, radar-bright
assessed
over stylizedhills. The modelhills wereplacedon streaks originating from small bright hills shown in
the floor of the wind tunnel, the floor was mantled with a Figures14a and 14bcouldbe materialerodedfromthehills
downwind.Candidatematerialsareillmenite,
0.5-cmlayer of loosesand,and the wind speedwassetjust anddeposited
abovethreshold
forthesandentrainment
(-0.5 cms-l). pyrite, or other high-densityminerals that have high
Runs were continued until most of the sand was removed.
dielectricconstants,
as suggested
by Pettengillet al. [1982,
Figure 12 showsVWT resultsbefore,during,andafterone 1983] and Garvin et al. [1985] for someregionson Venus.
related to the local wind flow field and which are consistent
with regional-scale
windregimes.
Wind tunnel simulations. Laboratory experimentsalso
GREELEYET AL.: AFDLIAN FEA•
•-" J
,••.•
t
'..'\"
......
•::::'::-:
..,
t
.:) '
..,
•,
.•
,
:(•, ,
•f•. 14•. •e
13,331
•.:•
/'
.
ON VENUS
,....... ....
:
,
•.
,•:.•":i,,
•o•n•-Mes•enet
dunefield, centered•t 67.7•N, 9O.•oE;•e• shown•s •7 b• •7 km (M•ell•
M•PS •9•4).
Becauseof theirrelativelyhighdensity,theymay form "lag"
deposits from which lower-density particles have been
removedby the wind. Wind tunnelexperiments
to simulate
Venus show that lag depositscould form under Venusian
conditions[Greeley et al., 1991]. Suchpreferentialwind
winnowingwould be expectedon Venusin the wake of the
hills seenin Figures14a and 14b, andthehills couldalsobe
the source of the radar-reflective material.
3. DUNES
Two possibledune fields have been identified on Venus,
one centeredat 25øS, 340øE and the other centeredat 67øN,
90øE. Bright wind streaksare associatedwith both dune
fields and indicate that the dunes are oriented transverse to the
prevailingwinds. The firstdunefield,initiallydescribed
by
Arvidsonet al. [1991], is about100 km northof the impact
Aglaonice
andcovers
anareaof-1290 km2. We
Radar-darkstreaksare moredifficultto explainin termsof crater
wind flow patterns.In general,we considerradar-darkstreaks designatethis the Aglaonicedunefield (Figure 16). The
to representdeposits of sedimentsthat have low radar Aglaonicedunesrangein lengthfrom 0.5 to 5 km; however,
backscatter cross sections. Most radar-dark streaks on Venus
becausethe dunesare dominatedby specularreturnson the
are associated
with ridgesand trenchesand thusprobably radarimages,theirspacing
cannotbe determined
accurately.
represent places where sedimentsare concentrated;for The orientation
of thedunesandnearbywindstreakssuggest
example,"gaps"in ridgescould funnelwindblownmaterial winds toward the west.
into narrow corridors. Radar-dark streaks on Venus associated
Duneson Earthresultfrom the accumulation
of saltating
with "point"featuressuchashillscouldalsobe explainedas particles;asdiscussed
by Bagnold[1941], "sand"sizegrains
depositsof sediments.Althoughrunshavenot beenmadein (-60 to 2000 gm in diameter)are commonlytransported
by
VWT, previousrunsmadeat 1 atm underlow wind speeds the windin saltation.Smaller(i.e., "dust")grainsaremoved
and long durationshave resultedin long depositsin the predominantly
in suspension,
andlargergrains(i.e. granules
obstacle wake (Figure 15). Under these conditions, a and gravels) move in "creep." Despite the differencein
"shadow zone" protected from the wind extended far atmospheric
densitybetweenEarthandVenus,approximately
downwind, whereasthe entire surfaceof the model was eroded the same mode of transportby size distributionoccurs
of loose sediments. Such featuresproducedin the wind [IversenandWhite,1982]. Consequently,
onewouldexpect
tunnelareconsidered
to be analogous
principallyto theradar- the duneson Venus also to be composedof "sand"size
darklinearstreaksseenon Venus(Figure5g).
material,i.e., grainsmovedin saltation.
13,332
GRFZLEY ET AL.: AEOLIAN FEATURES ON VENUS
Fig. 14b. Enlargement
of areaindicatedin Figure14a, showingdunesandradar-brightstreaks.
The Aglaonicedunefieldis in theso-called"crater-farm",
an
areaof relativelyabundant
impactcraters.Consequently,
the
surfacein this area is expectedto be a sourceof sand-size/J
materialfrom the ejectageneratedby the impactevent. The
dunefield is locatedin an "outflow"depositextending~250
km north from Aglaonice. Although the origin of crater
outflow featureson Venus is unclear,they couldresultfrom
turbulentlyemplacedejectaand/oroutflowof lava [Phillips
et al., this issue]. Regardlessof the process,at Aglaonice,
outflowmaterialevidentlyhasbeenreworkedby thewind to
field appearto originatefrom small (-200 m), radar-bright
cones. The streaksmost likely consistof the samehigh
radarreflectivitymaterialas thecones. The sharpbrighiness
boundaryin the middle of the image shownin Figure 14a
does not appear to be associatedwith any topographic
change,suggestingthat the brighinessis associatedwith a
change in dielectric constant. Measurementsacrossthe
boundaryshowa changein backscatter
of 6 dB (a factorof 4
in returned power) which, in turn, correspondsto a
significantchangein the dielectric constant. If the lowform both dunes and wind streaks. Similar aeolian features
reflectivityareasareassumedto havea dielectricconstantof
of 5 to
havenot beenidentifiedat otheroutflowdepositsthusfar in 2 to 3, thebrightareawouldhavea dielectricconstant
the analysesof Magellandata. The Aglaonicedepositmay 11. We suggestthatthebrightareais composedof materials
have been initially less consolidatedor may be more thatare of a differentcompositionthanthe surfaceon which
theyoccur.
weatheredthanothercrateroutflowdeposits.
The northern dune field (termed Fortuna-Meshkene0is
The most likely source for the material forming the
located in a valley between Ishtar Terra and Meshkenet Fortuna-Meshkenetdunesis debris from the surrounding
regionsof complexlydeformedtessera. The N-S trending
Tessera.
It covers
~17,120
km2 andhas-40 radar-bright
appearsto haveserved
linear wind streaksthat occurwithin the field (Figure14a). low-lyingregionbetweenthe tesserae
The orientation of the dunes and the wind streaks in the
as a trap for weathereddebris. Some materialalsomay be
southernpart of the field indicatea southeastto northwest derivedfroma nearbyparabolichalocrater. A similarvalley
wind flow thatshiftsto a westwardflow in thenorthernpart to the east contains no visible aeolian features.
of the field. Two brightstreaksneara 12-km-diameter
crater
Basedupon look angle effects of terrestrialdunesusing
northof thedunefield alsosuggestwestwardwind flow. The airix)me,Seasat,andshuttleimagingradarimages,dunesare
dunesrangefrom 0.5 to 10 km in length,are •0.2 km wide, bright in low look angle radar imagesbecauseof quasiand havean averagespacingof 0.5 km. The spacingof the specularreflectionsfrom smoothdune facesthat are neardunesincreases
towardthewesternandeasternmarginsof the normalto the radarbeam,wherethe incidenceangleat the
duneface is zero [Blomand Elachi, 1981, 1987] as shownin
valley.
The bright wind streaksin the Fortuna-Meshkenetdune Figure 14c. At Magellanwavelength(12.6 cm), a radarecho
GREELEY
ETAL.: AF_•LIANFEATURF3
ONVENUS
13,333
Fig. 14c. Radarimageof the dunefield in the Gran Desierto,Sonora,Mexico; areashownis ~25 by 15 km; radar
"look direction"is from the southwest(left) towardthe northeast(right) (Seasatimagerevolution1312).
from a duneis possibleonly whena sandsurfaceseveral betweenthe dunesthemselves.One possibilityis that the
wavelengths
ona sideis nearlyperpendicular
to theimaging duneslie on a roughbasement.If thisis thecase,therough
radar. Becausewindblownsandon Earth has an angle of basementwould showas brighton the radarimage,whereas
reposeof about33ø,radarbackscatter
from duneslipfacesis the smoothdunefaceswouldappeardark. In thiscase,the
possibleonlyat lookangleslessthan33ø wherea duneslope dunes need not be near-normal to the radar illumination.
is normal to the incident beam [Blom and Elachi, 1987].
Anotherpossibilityis that the duneshavea lower dielectric
terrainso theyappeardarker.
Assuminga similarslopegeometryfor Venusiandunes,only constantthanthe surrounding
thosedunesviewedat look angles-33 ø and with slip faces In either case, the periodic image brightnesspattern is
with imagesof sandduneson Earth.
orientedapproximately
perpendicular
to theradarillumination consistent
will yield a radar backscatteron Magellan images. The
Aglaonicedunefield hasan incidenceangleof ~34ø, andthe
4. YARDANGS
dune slopes are oriented perpendicular to the radar
illumination,thus satisfyingthe necessaryslopegeometry Yardangsare streamlinedhills thatresultfrom wind erosion
for viewing. The Fortuna-Meshkenet
dunefield wasviewed of rockandinduratedsediments.A field of possibleyardangs
with an incidenceangleof ~22ø, and most,althoughnot all, has been identified on Venus at 9øN, 60.5øE, about 300 km
of the dunesare orientednearly perpendicularto the radar southeastof the crater Mead (Figure 17a). The region
illumination.
But this dune field also has faces oriented
surrounding
Mead containsthehighestconcentration
of windparallel to the radar illumination. Studiesby Blom and related features on Venus identified to date and is discussed in
Elachi [1981, 1987] have shown that dune faces that are not more detail in section5.1. The yardanglikefeatureson
near-normalto the radarilluminationwill not returna quasi- Venusconsistof setsof slightlysinuous,parallelridgesand
specularreflectionto the radar. Instead,the dunesbecome grooves.The featuresaverage25 km longby 0.5 km wide,
invisiblewhennot imagednear-normal.This wouldimply with spacingbetweenthe ridgesrangingfrom 0.5 km to 2
thatin orderto be interpreted
asdunes,otherscattering
effects km. Unlike wind streaks,they havewell-definedboundaries
must be involved, such as change in the roughnessor anddo not originatefrom topographic
features,suchas hills.
composition(i.e., dielectric constant)acrossthe dunesor The proposedyardangs
occurin two sets,eachcomposed
of
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ETAL.: AEOIJANFEA•
ON VENUS
Fig. 14d. Enlargement
of somedunesin Figure14a. Thedunes
onEarthin Figure14candtheVenusian
dunes
shownherearebothdisplayed
at thesamescale.The imageis centered
at 67.7øN,90ø5E.
about a hundred individual
features.
Both sets indicate a
northeast-southwest
wind regime.
Yardangsoccur in many desertregionson Earth and in
some areas of Mars (Figure 17b). Yardangs commonly
develop on relatively soft depositsthat are sufficiently
cohesiveto retainsteepslopes.Lakebedsediments
andsome
volcanicashdepositsare typicalmaterialsin whichyardangs
form [McCauleyet al., 1977]. On Earth, yardangsoccurin
clustersoriented parallel to the prevailing winds which
formed them but are.also controlledby structuralfeatures,
suchasjoints, and erosionalpatternssuchas streamvalleys.
The shape of yardangsis the result of several factors,
includinglithology,structure,wind flow field, surrounding
topography,and the supplyof agentsof abrasion[Ward and
Greeley,1984].
The possibleyardangson Venus suggestthe presenceof
relativelyfriabledepositsthathavebeensubjectedto erosion
andthatagentsof erosionby the wind havebeenactivein the
pastor are currentlyactive. The sourceof the materialthat
formedthe possibleyardangsis interpretedto originatefrom
the formationof Mead crater. The yardangsindicatethat
continuingwinds in this region have reworkedthe Mead
depositsover time.
5. CORRELATIONSOF AEOLIAN FEA•
and dust can be producedfrom impact crateting, volcanic
eruptions, and weathering by chemical and physical
processes. Mechanical weathering of rock and particle
generationon Venusalso may occurwith the formationof
tesseraterrain,ridge belts,coronae,and rift zones. Figure 1
shows that the distribution
of aeolian features on Venus is
not random;rather,they appearto be associated
with certain
impactcratersand sometectonicallydeformedterrains. To
illustrate theseassociations,we discussthe Mead and Carson
craterareasand deformedterrainsin TellusRegioandOvda.
-!
WITH GEOLtX;Y
The association of wind streaks and other aeolian features
with local andregionalgeologycan provideinformationon Fig. 15. Wind tunnel results for flow over raised rim crater,
thepossiblesources
of windblownmaterial.In general,sand generatinglong depositionalfeaturein wake of crater.
GREELEY ET AL.: AF.DEIAN FEA•
ON VENUS
13,335
Fig. 16. Aglaonice
dunefield,centered
at 24.8øS;areashownis ~78 by 180km. Thisdunefield,indicated
by the
specular
patternat "A", is locatedwithinanoutflowassociated
withtheAglaonice
impactcrater.Radar-dark
linear
streaks
sweepacrossthearea,suggesting
windsfromtheeast(right)towardthewest(left). If thiswindorientation
is correct,theproposed
duneswouldbe transverse
forms(MagellanMRPS 34032).
Theseregionswerechosenon thebasisof theabundance
and contrast,radar-brightstreaksin the area have emissivity
varietyof streaks
andareunique,ratherthantypical,regions valuesashighas0.827. Fresnelreflectivityvalues(corrected
on Venus. For each area, we have assessedthe local for the effectsof diffusescattering[seePettengillet al., this
geology,possiblesourcesof particulatematter,and the issue]) generally have values that are close to the unit
complementof emissivity,with a range of valuesfrom
development
of thewind streaks.
0.107on brightstreaksto 0.138 on darkstreaks.The overall
5.1. Association
of AeolianFeaturesWithImpactCraters
loweremissivityfor theregionsuggests
thatif thedepositis
thendifferences
in mineralogymayaccountfor
Mead Crater. Mead (Figure 18a andPlate 1) is 275 km in fine-grained,
diameterand is the largestpreservedimpactcraterfoundon thehigherdielectricconstant.
CarsonCrater. Carson(Figure 19a andPlate2) is oneof
Venus. It is characterized
by two tings,a radar-brightfloor,
by a lowand little apparentejecta [see Phillips et al., this issue; severalimpactcraterson Venusthatis surrounded
backscatter,low-emissivityparabolichalo [Phillips et al.,
Schaber et al., this issue]. The area surroundingMead
on plainsunits
includesmotfledradar-bright
and-darkplainsandhasa large this issue]. The radar-darkhalo is superposed
concentration of wind streaks. Most of the streaks in the
composedof lobatevolcanicflows. Within the halo and in
regionare darklinearanddarktransverse-ragged
forms. A the immediateregion(within a 500-kin radiusof the crater),
few radar-darkwispy streaksare foundwest of Mead and numerouswind streaksare found that appearto be eithera
abouta dozenradar-bright
fan-shaped
streaksoccurnortheast directresultof the impactprocessor a resultof subsequent
and southeast of the crater. Most streaks form in the lee of
redistributionof fine-grainedmaterials. The streaksare
ridgesin thelow-lyingplains,althoughsomestreaks
formin locatedin low-lyingplainswestof AlphaRegioand include
fan, -darkwispy,and-darklinearforms. Streaks
the lee of small hills (Figure 18b). Streaksrangein length radar-bright
from a few kilometersto > 100 km. Streakssurrounding rangein lengthfrom tensof kilometersto > 100 kin. Many
Mead indicatewind flow towardthe equator,at leastfor the of the streaksform in the lee of small cones,althoughsome
time when the streaks were formed. In addition to the wind
streaks form in associationwith ridges. Most streaks
indicatewindsflowing towardthe equator,althoughothers
streaks,thefield of possibleyardangsdescribedin section4
is locatedsoutheast
of the crater. Immediatelyeastof Mead indicate flow in random directions.
is a gap in Magellandatacausedby superiorconjunction. Emissivityvalueson theparbolichalorangefrom 0.778 to
Consequently,
thefull extentof aeolianfeatures
in theareais 0.830, comparedwith an averagevalue of 0.844 for the
not known at this time.
surrounding
terrain. Valuesof 00, emissivity,reflectivity,
The surfacearoundMeadcraterappears
to beblanketed
with and rms slope for some streaksin the Carson area are
fine particlesthat were probablyproducedat the time of includedin Table 3. As in the Mead region,the dark streaks
impact.In addition,thecrateris surrounded
by a faintradar- show higher reflectivitiesthan the bright streaksand the
plains. Emissivityvaluesappearto be strongly
dark halo, visible in the emissivity data (Plate 1). The surrounding
generalareashowslowerradarbackscatter
andemissivity influencedby the positionof the streakrelative to the lowvalues (-20.8 riB, 0.804, respectively)than the average emissivityparabolichalo.
(-15 dB, 0.860,respectively)
for theVenusiansurfaceimaged Although Mead and Carsonare unusualin their high
at the sameincidenceangle(45ø in the first mappingcycle). concentrationof aeolian features, wind streakson Venus are
Moreover, some of the lowest emissivity values in the most commonlyfound near impact craters. Most streaks
regioncorrespond
to concentrations
of wind streaks.For around craters form within 5 to 6 crater diameters of a crater
particulate
example,theradar-dark
regionassociated
withthestreaks
near or cratercluster,indicatingthatimpact-produced
the center of Figure 18 has an emissivityof 0.808. In materialmaybe locallysignificant.However,theamountof
13,336
GREELEY ET AL.: AEOLIAN FEATURESON VENUS
•
20 km
.
Fig. 17a. Venusyardangs,
centered
at 9øN,60.7øE;areashownis -200 km by 200km (Magellan
MRPS37879).
finematerialproduced
by impacts
on Venusis notthought
to
be significant
whenaveraged
overtheentireglobe[Garvin,
1990]. Parabolichalo craters,suchas Carsonand Mead,
most commonlyhave adjacentstreaks,with over 70% of
these craters having associatedaeolian features. The
occurrence
of streaks
aroundimpactcratersthataresurrounded
by parabolas
or darkhaloesandaroundthediffusedeposits
thoughtto be "failed impacts"[Phillipset al., 1991, this
issue]mayindicaterelativelyyoungregionsthathavenotyet
been"homogenized"
by theVenusenvironmen[The impact
ß
.
.
..
craterswhere wind streaksare found may representthe
youngestcraterson the planet, indicatingthat the streaks
may be usefulas stratigraphic
markers. We interpretthe
high correlationof streakswith impact craters,and the
generallack of streaksin otherregions,to indicatethat the
impactprocessis the mostefficientproducerof particulate
matter on Venus.
From observationsand analyses of aeolian features
associated
with Mead,Carson,andotherlargeimpactcraters
and from considerations
of the propertiesof the surfaces
where aeolian featuresoccur, we suggestthe following
scenario:Priorto impact,a bow shockwasproducedin the
Venusianatmosphere
by the incomingbolide. Becauseof
the highdensityof theatmosphere,
sucha bow shockwould
Fig. 17b. Yardangsand mesasin the westemThatsisregionof be capableof producingsubstantialturbulencewhere it
Mars; area shown is -190 by 190 km (Viking Orbiter frame interceptedthe surfaceand probablywas responsiblefor
44B37).
generating
anddislodging
weathered
debrisandinjectingsand
GREELEY ET AL.: AFDLIAN FEATURF3 ON VENUS
13,337
Fig. 18a. Mageilanradarimage,centeredat 15øN,59.1øE,of the Fig. 18b. Detail of linear streaks northeast of Mead.
area northeastof the impact crater Mead (lower left cornerof Interfingering bright and dark streaks complicate the
image). Modificationof the surface,indicatedby wind streaks, interpretationof depositionalversus erosional origins; area
extends as much as 150 km from the crater.
White box outlines
shownis centeredat 15øN, 60.4øE (Magellan MRPS 39821).
the locationof Figure 18b. Numberson image correspondto
Table 3 (MagellanMRPS 39820).
and dust into the atmosphere. Some of the dark patches
describedby Phillips et al. [1991, this issue]as "failed
impacts"couldrepresentparticulatematerialbothfrom the
disruptedbolide and from weatheredmaterial dislodged
locally. Thosebolidesreachingthe surface-generated
ejecta
of a wide rangeof particlesizes;togetherwith the material
raisedby thebow shock,a symmetrical
hemisphere
of debris
expandedfrom thepointof impact. As the massloftedinto
the atmosphere,
fine particleswerecaughtby theprevailing
easterlywindsand distributedtowardthe westto form the
radar-darkparaboliccollar. Althoughmanywind streaksat
Mead crater (and to somedegreeat Carson)are oriented
westward,somearerandomlyorientedandmayreflectlocal
turbulence,or formation at a time not associatedwith the
impact.
obstacles. The radar propertiesof the streaks(Table 3)
indicatethatthebrightstreakshavehigherradarbackscatter
crosssectionsthanthe surrounding
plains,whereasthe dark
streaks have lower backscatter cross sections than the
surrounding
plains.In addition,thedarkeststreakareasshow
emissivityvalues-0.02 lower than the surroundings,
suggesting
slightlyhigherdielectricconstants
for the less
darkstreaks.Theappearance
of boththebrightanddarkareas
in the SAR is therefore probably a result of roughness
differences,rather than differencesin dielectricconstant. In
thisregion,the darkandbrightstreaksbehindtheridgesare
interpretedto representadjacentareasof deposition(dark
streaks)andscour(brightstreaks).
Hestia-Rupes/Ovda
Regiois alsotectonically
deformedand
has a concentrationof streaks(Figure 21). Wind streaks
extenduphillfrom tessera
blocks,indicatingupslopewinds..
5.2. Associationof Aeolian Features With Tectonically As detailed by Arvidson et al. [this issue], the streaks
delineate an elevation contourboundaryin which streaks
DeformedAreas
occurwest of the boundaryat lower elevations.To the east
Wind streaks and other aeolian features are found in some (at higherelevations)the plainshave enhanced
backscatter
areasthathavebeentectonically
deformed.For example,the crosssections,and as discussedby Arvidsonet al. [1991],
area southwestof Tellus Regio, shown in Figure 20, this variation may be related to elevation-dependent
containsabundantwind streaksthat do not appear to be weatheringreactions.Abovea criticalelevation(6054 km
related to an impact crater (the nearest impact crater, [Pettengillet al., 1983]) the plainsare inferredto be bright
Voynich,is> 1000kmaway).Consequently,
theparticulate becauseof the presenceof high dielectricmaterialswhich
material associatedwith the wind streaks was probably maybe stableat thelowertemperatures
andpressures
found
generated
fromthecomplexlydeformed,
ruggedterrainof at higherelevations.The streaksdevelopedbecausewinds
thesematerialswithin the elevationzonewhere
TellusRegio. The streaksin thisregionincluderadar-dark segregated
linearstreaksandradar-brightand-darkfan-shaped
streaks. theweatheringreactionoccurs.
Wind streaks are also associated with a few other
Wind streaksrangefrom~ 10 to > 50 km in length.All of
thestreaksarein low-lyingplainsimmediatelyadjacentto a tectonicallydeformedregions,suchas Alpha and Laima
andin someridgebelt regions.Tesseraeandridge
zoneof complexlydeformedterrainthatextendsfrom the tesserae
anduplifted,resultingin mechanical
mainbodyof the highlands.Streaksin thisregiontendto beltshavebeenfractured
form in the lee of ridges in the plains, with some dark erosion that may have produced sufficient fine-grained
material apparentlyaccumulatingbehind topographic material to form aeolian features. On the other hand, no
13,338
G•
ET AL: AF•LIAN FEATUR• ON VENUS
100 km
Plate 1. Image data with emissivityvaluesshownin color overlay in the vicinity of Mead crater (circularpurple
areain upperright). Emissivityvaluesin the centralpart of Mead are as low as 0.702, while a large region around
the crater has a mean emissivityof 0.804, which is roughly 0.05 less than a typical plains surface. Values < 0.788
are shown as violet; >_0.840 are shown as red.
aeolian features have been identified around coronae and rift
largeamountsof volatiles,andmaybe relativelyrare[Garvin
zones,bothof which are characterized
by extensivetectonic et al., 1982; Itead and Wilson, 1986]. Alternatively,
deformation. However, both rift zones and coronae are far volcanicparticulatematter may be easily weldedand thus
less complexly deformedthan tesseraeand, thus, may not more difficult to rework with the wind.
producesufficientsedimentto form aeolianfeatures. In
On mostplanets,moreparticulatematteris generatedwith
addition,theseterrainslack the steepouterslopesof tesserae age as surfacefeaturesweatherand erode. In general,a
that may enhancemasswastingand the generationof local relativelylargeamountof particulatematteron Venusseems
winds. Aeolian featuresmay be morecommonlylocatedin to indicatea youngerage,asin thecaseof theparabolichalo
the lee of ridge belts due to their unique topographic craters. Arvidson et al. [this issue] have observedthat lava
characteristics
servingas an obstacleto the wind, ratherthan flows tendto becomemore "homogenized"
over time. This
theirassociation
with significantmechanical
erosion.
processmay be relatedto somematerialbindingprocesson
Wind streaksform least frequently in associationwith the surface.For example,laboratoryexperiments
showthat
volcanic features. Streaks,however, do form in the wake of
in the high-temperature,high-pressureenvironmentof
some volcanic cones, but in many casesthe particulate Venus,particulatematerialtendsto "coldweld"andbecomes
materialmay also have been derivedfrom nearbyimpact more difficult to move by the wind [Marshall et al., 1991].
craters.Explosiveeruptionson Venusare thoughtto require Therefore,rexentimpacts(i.e., Carson),regionsof ongoing
GREELEYET AL.: AEOLIAN FEATURF_S
ON VENUS
13,339
tectonic activity (i.e., Tellus), or elevated regions with
uniqueweathering
characteristics
(i.e.,Ovda)maybe someof
the relativelyfew siteson the planetwhereloosematerialis
presentandis movedby thewind.
6. CONSTR•
ON MODELS OF ATMOSPHERIC CIRCULATION
As discussed
in section1, atmospheric
circulationnearthe
surfaceof Venuscouldbe predominantly:(1) a Hadley cell
with equatorward
surfacewinds,(2) an anti-Halleycell with
surfacewinds away from the subsolarregion, (3) a zonal
circulationwith westwardsurfacewinds, or (4) something
more complex. The globalpatternof wind streakazimuths
may provide clues as to which of thesemodels (if any)
characterize the near-surface circulation.
Figure 9 shows that many streaks are oriented with
equatorward
components.This is particularlyevidentin the
southwardorientationsof streaksin the northernhemisphere
regionsat (25øN,335øE),(10øN,65øE),and(20øN 100øE)and
in the northward
orientations
of streaks in the southern
hemisphereat 20øS, 345øE. Northernhemispherestreak
azimuthsare clearlyconcentrated
around180øE;73% of the
total population of northern hemisphere streaks have
azimuthsbetween90ø and 270ø. Similarly, 77% of the
southernhemispherestreakshave azimuthsbetween0ø and
90øEand 270ø and360øE. Northernhemisphere
streaksare
predominantly oriented southward whereas southern
hemisphere
streaksaremainlyorientednorthward.
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
The streak orientations are consistent with a near-surface
Hadley cell circulation, and accordingly, they provide
observational support for the existence of such an
atmospheric circulation pattern. This conclusion is
preliminaryand shouldbe treatedcircumspectly
until more
completewind streakdata for the whole planetare obtained
and analyzed. Possibleobservationalbiases,suchas those
associated
with imaginggeometry,needto be assessed
more
carefully. For example, the orientation of Magellan
spacecraftgroundtracksmight result in preferentialradar
detectionof streakswhichlie parallelto them.
The latitudinal distributionof wind streaksmay contain
informationon the strengthof the equatorward
nearsurface
winds in the Hadley cell or on the polewardextentof the
Hadleycirculation.There is a strongconcentration
of wind
streaksin the southernhemispherebetween17ø and 36øS
latitude (Figure 7). Wind streaks are more uniformly
distributed
withlatitudein thenorthernhemisphere,
although
there is a concentration of streaks between 23 ø and 30øN
latitude(Figure7). The areabetween17øSand 36øSis 30%
of the area of the southernhemisphere,but 57% of the
southernhemispherewind streaksare concentratedthere.
Similarly, the area between 23øN and 30øN, 10% of the
northern hemisphere area, contains 20% of northern
hemispherewind streaks. The peaks in the latitudinal
distribution of wind streaks between 23øS and 30øS and 23øN
and30øNtendto suggestthatequatorward
Hadleycirculation
windsare strongest
at theselatitudes.The broaddistribution
of northernhemispherestreaksover all latitudes(Figure7)
suggestthatthe Hadleycell in the northernhemisphere
may
extendall the way to thepole;however,therelativelysmall
numberof streaksin the southernhemisphere
polewardof
i
i
i
i
13,340
GREELEYETAL.: AF_DLIAN
FEATURES
ONVENUS
Fig. 19a. Magellanradarimageof the areaaroundtheimpactcraterCarson,centeredat 25øS,345øE,radar-darkhalo
associated
with the clusterof craters,and the parabolic-shaped
"collar"associated
with Carson. Numberson image
correspond
to Table 3 (MagellanF-MIDR 25S345).
36øSimpliesweakerwindsor lesspolewardpenetrationof
7. SUMMARY AND CONCLUSIONS
the southern hemisphereHadley cell. There is some
Wind streaks are the most common aeolian feature on
sampling bias, however, becauseMagellan coverageis
Venus.
More than 3400 have been identified in this study.
incompleteover the southernlatitudesand southpolar area.
The implicationsof Figure 7 for atmosphericcirculation They occurin a varietyof shapesand includeradar-bright,
must be viewed with cautionat this stageof our analysis radar-dark,andmixedradarreflectivityformsin relationto the
because the latitudinal distribution of wind streaks can also
background
on whichtheyoccur. Most streaksare foundin
be influencedby otherfactorsthat wouldvary with latitude relativelysmoothplainsin latitudinalbandsof 23øSto 30øS
and 23øN to 30øN.
Wind streaks tend to be oriented
suchas the supplyof windblownmaterial.
with surfacewinds
Streak orientationsshow no preferentialalignmentsthat downwindtowardthe equator,consistent
would confirm or deny the existence of other lower relatedto a Hadleycirculationin theloweratmosphere.Data
patterns
relatedto subsolaratmosphere
motionssuchastheanti-Halleycirculationor the donotsupportHalleycirculation
althoughobservations
are limited
westwardzonal flow discussedearlier. For example,streak antisolarheatingcontrasts,
azimuthspointeastwardasfrequentlyastheypointwestward. to test this possibility. Wind streaksare found at all
Thus,the dominantcirculationof the cloudlevel atmosphere elevationson Venus. In contrastto pre-Magellanpredictions
doesnot penetrateto the surfacewith sufficientstrengthto of downslopewinds,mostwind streaksare foundon gentle
preferentially
organizethewindstreaks
towardthewest.The slopes(< 2ø) andarerandomlyorientedwith regardto slope.
in OvdaRegio,streaks
canbeoriented
upslope.
wind vectorat a fixed point on the surfacerotatesthrough Locallye'as
Botherosionaland alepositional
aeolianfeatureshavebeen
360ø duringthe diurnalcycleof the anti-Halleycirculation.
Accordingly,it is difficultto devisea testof thewind streak identified on Venus. The presence of possible dunes
datathatwouldprovideevidencefor or againsttheanti-Halley (alepositional
features)
providescluesto thenatureof someof
cell.
the sediments and their behavior in the aeolian environment.
GREELEY ET AL.: AEOLIAN FEATURES ON VENUS
13 341
50 km
Plate2. Image data for the sameareaas above,with emissivityvaluesshownin color overlay. Emissivityvalues
in the vicinity of the cratersare typically 0.05 lower than thosein the surroundingregions. Values < 0.784 are
shown as violet; > 0.844 are shown as red.
Sanddunesform only from sand-sizeparticles(60 to 2000
!.tmin diameter)which are transported
primarily in saltation
by thewind,regardless
of planetaryenvironment
[Greeleyand
lversen, 1985]. Consequently,if the featuresidentifiedin
Figures 14 and 16 are dunes,they signal the presenceof
sandgrainsand processes
that produceparticulatematerial.
Althoughthe identificationof yardangsis tentative,their
possible presencealso provides insight into the aeolian
regime on Venus. As features formed by wind erosion,
yardangsshow that windblown particles are capable of
erodingmaterial despitethe relatively low kinetic energy
produce•by slow-movingwinds.
In conclusion,the surfaceof Venusis characterized
by low
ratesof erosion,primarily due to the lack of water on the
surface.Mechanicalerosionthroughtectonicdeformationin
ridgebeltsandregionsof tesseramayproducesmallamounts
of particulatematterthatcanform aeolianfeatures.Volcanic
deposits may also play a small role in producing fine
material on Venus: many streaksform in associationwith
conesof probablevolcanicorigin. However, the primary
contributionto the productionof particulatematteron Venus
is from impact cratering.
Aeolian features form
predominantlynear impact craters,especially those with
associated
ejectahaloesor parabolas,or neardark deposits
thoughtto be "failed"impacts[Schaberet al., thisissue].
Continuinganalysisof the backscattercrosssectionsof
aeolianfeatureswill providefurtherinsightintoerosionaland
Fig. 19b. Detail of streaksnortheastof Carson. Note variation
in backscatter
from the coreto the edgeof the streaks.
depositionalprocesseson Venus. Data from Magellan's
extended
mission will
be used to assess the backscatter
characteristicsof aeolian features of different viewing
geometries
andto obtaina completeinventoryof wind-related
featureson Venus. Most significantly,over the next several
years, Magellan will provide the opportunity to detect
changesin aeolianfeaturesor the formationof new features,
providing further information on atmosphere/surface
interactions and the nature and evolution of surface materials
on Ven us.
'i'
i•:.
'•j•'•
,'i'.....:..
•
• "'
..................
'".....
'"
":
::•
"..:.
..,.:.....:.;:.::•
,:.•:..::.:.i::*?:
....
:.;.....
'*'*::: ' •:-','•'
13,344
GRI•LEY ET AL.: AEO•
Acknowledgements. We wish to thank the following for their
contributions to this report: R. Blom and T. Fart for helpful
discussions, D. Ball for photographic support, E. Lo for
computational support, S. Selkirk for figure preparation, and
S. Blixt for word processing.The Magellan Projectand partsof
the researchdescribedhere are carried out by the Jet Propulsion
Laboratory,California Institute of Technology,under contract
from the National Aeronautics and Space Administration, and
through contractsto individuals: JPL-958880 (Greeley) JPL958496 and NAGW 1874 (Schubert), and JPL-957415
(Arvidson). Support for Plaut was provided by the National
ResearchCouncil under the ResearchAssociateProgram.
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(ReceivedSeptember27, 1991;
revised April 27, 1992;
acceptedApril 28, 1992.)