JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. E8, PAGES 13,319-13,345, AUGUST 25, 1992 Aeolian Featureson Venus' Preliminary Magellan Results RONALDGREELEY, • RAYMOND E. ARVIDSON,2 CHARLES ELACHI, 3 MAUREEN A. GERINGER,• JEFFREY J. PLAUT,3 R. STEPHENSAUNDERS, • GERALD SCHUBERT, 4ELLEN R. STOFAN,• Emc J.P. THOUVENOT,3's STEPHEND. WALL)• ANDCATHERINE M. WEITZ• Magellan synthetic aperture radar data reveal numerous surface features that are attributed to aeolian, or wind processes. Wind streaksare the most common aeolian feature. They consistof radar backscatterpatterns that are high, low, or mixed in relation to the surfaceon which they occur. A data base of more than 3400 wind streaksshows that low backscatterlinear forms (long, narrow streaks)are the most common and that most streaksoccur between 17øS to 30øS and 5øN to 53øN on smoothplains. Moreover, most streaksare associatedwith depositsfrom certain impact cratersand some tectonicallydeformed terrains. We infer that both of these geological settingsprovide fine particulatematerial that can be entrainedby the low-velocity winds on Venus. Turbulenceand wind patternsgenerated by the topographicfeatures with which many streaksare associatedcan account for differencesin particle distributionsand in the patternsof the wind streaks. Thus, some high backscatterstreaks are consideredto be zonesthat are swept free of sedimentaryparticles to expose rough bedrock;other high backscatterstreaks may be lag depositsof densematerials from 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 aeolianfeatures on Venusincludg[ yardangs(?) and dune fields. The Aglaonicedune field, centeredat 25øS, 340øE, covers~1290 km': and is locatedin 90øE,anejecta coversflow ~17,120channel •n•the in aAglaonice valley betweenimpactIshtar crater. Terra The andMeshkenet MeshkenetduneTessera. field, located Wind at streaks67øN, associatedwith both dune fields suggestthat the dunesare of transverseforms in which the dune crestsare perpendicularto the prevailingwinds. Dunes on Venus signal the presenceof sand-size (~60 to 2,000 gm) grains. The possibleyardangs are found at 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 using (primarily) cycle 1 Magellan radar data (Figure 1). Aeolian,or wind-related,processes on the surfaceof Venus Aeolianfeatures include possible (1) dunefields, (2) yardangs have been debatedfor more than two decades,and many (wind-erodedhills), and (3)various types of wind streaks investigatorspredicted that aeolian features would eventually (surface patterns of contrasting radar backscattercross be found (reviewed by Greeley and Arvidson [1990]). sections). We describe these aeolian features and their Althoughimages of the surfacereturned from SovietVenera characteristicsas seenon Magellan radar imagesand assess landersand measurementsof near-surfacewinds suggested the geologicalsettings and properties of the surfacein which localmodification of the surfaceby wind, definitiveevidence they occur. We alsodiscuss the possiblemodes of formation for more widespreadaeolian activity was not observeduntil of the mostcommon aeolian features (wind streaks),drawing the Magellan mission[Saunders et al., 1991]. Preliminary on terrestrialexamples, Martian analogs,and resultsfrom analysesof Magellan radar imagesrevealed several regions wind tunnelsimulations. We thenconsider the relationships where wind-related features are abundant, as well as other between aeolian features and patterns of atmospheric isolatedoccurrences [Arvidson et al., 1991]. circulation on Venus. For thisreport, about 44% of the surfaceof Venushas been searched in a reconnaissance mode for wind-related features 1.1. Background 1Departmentof Geology, Arizona State University, Tempe. Wind-relatedfeatures observed on planetary images provide 2Departmentof Earthand Planetary Sciences, Washington direct evidence for the interactionof the atmospherewith the University, St. Louis, Missouri. surface. The presenceof depositionalaeolian features, such 3Jet Pro pulsion Laboratory, Pasade na, California. 4Departmentof Earthand SpaceSciences, Institute of as dunes,shows areas where particles capable of movement Geophysics and Planetary Physics, University of California, by the wind occur and gives indicationsof weathering Los Angeles. processes.The identificationof the type and orientationof 5CentreNational de la RechercheScientifique, Toulouse, aeolianfeatures provides clues to the physicalproperties of France. surfacematerials where they occurand the wind directionat the time of their formation. Assessmentof their age Copyright 1992 by the American GeophysicalUnion. providesinsight into pastwind regimesand climates. Wind streaksare amongthe mostcommon aeolian feature Papernumber 92JE00980. observedon 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 180 195 210 225 240 255 270 285 300 315 330 345 0 15 30 45 60 75 90 105 120 135 150 165 180 l' '"l""l"",'"',"" l''" l''"l''"l''"l''"l''" l''"l''"l"" l""l''"l''"l'"'l'"' f""l''",''",""l''"l''" l"",'"'l''",""l'"'l''"l""l'"'l''"l''"l'"'l''"l""l""l''" l""l'"'l''"l""l'"'l''"l""l''"l''"l''"l''"l''"l''"l''"l''" I""I'"'I''" I""I'"'I''"I''"I'"'I''" l""l''"l''"l''"l""[' '" I""I'"'I ?::•::' ) Fan Bright •-•-•:.....I•'•'H-:':T'A -15 -15 -3O "•:•'"'•"'•.•,...•,•.•...• '• •------'•-•::•,..•,,•,-"•..............'.•:• -3O -45 -45 -6O -6O •.•.:•:•.•,• Fig. 1. Map of Venus showingorientation of wind streaksequal-area by 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 through50 ø (superiorconjunction) and longitudes -160 ø through330 ø (digital basemap from U.S. GeologicalSurvey). patternsin which loosematerials commonly <1 m thick are 2h). Wind streaks of several types also occur on Mars distributedby sediment-movingwinds. Typically,they are (Figures 2e and 2f), as reviewedby Greeley et al. [ 1992]. associatedwith wind patternsand turbulencegenerated by Thomaset al. [ 1981] derived a classificationof Martian wind topographicfeatures such as smallhills. On Earth and Mars, streaksbased on (1) their upwindsources (sediment deposit or wind streaksare visibleon opticalimages because of albedo topographicobstacle), (2) their albedo contrast(bright or contrastsrelated to particle size or compositionand to dark) in relation to the backgroundsurface, and (3) special exposuresof bedrock. Wind streaksare alsovisible on Earth morphologic or compositional features. Some, termed on radarimages [Greeley et al., 1989;Saunders et al., 1990], variable features,appear, disappear,or changetheir size, wherefactors such as bedrock exposures and sedimentcover shape, and orientation on time scalesof weeks to years resultin contrastingradar backscattercross sections. Wind [Thomas and Veverka, 1979, etc.]. Martian wind streaks streakson Earth rangein lengthfrom a few centimetersfor 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 patternsdeveloped in the lee of hills and small mountains Mesogaearegion [Veverka et al., 1976]. (Figures2a-2d) and in associationwith impact craters (Figure Regardlessof type,mode of formation,or planetwhere they Fig. 2. (Opposite)Typical wind streaks on Earth,Mars, and Venus; arrows indicate prevailing wind direction. (a) Amboy,California, optical image showing cinder cone (460 m in diameter)and dark streak. Prevailing wind is fromthe west(left); generalbackground consists of pahoehoebasaltic lava flowsand is mantledwith windblown sand(whim areas). Area of streakis darkbasalt swept free of sanddue to windturbulence shed from flow aroundthe cone(from Greeley and Iversen [1987]; air photo AXL-26K-36 taken January 10, 1953). (b) Seasatradar image (revolution882) of Amboy,California, showing radar-bright streak corresponding to dark areas on Figure2a thatis relativelyfree of windblownsand. Dark areas correspond to concentrations of sand and (extending toward the top of thepicture from the cone) basalt pebbles and cinders. Radar illumination from the bottom. (c) SIR-Aradar image of the Altiplano,Bolivia, showing radar-dark streaks as longas 15 km formedin associationwith hills (bright features).Contrasts in radarbackscatter cross sections result from differencesin sandmantles, dune forms, and vegetation,all relatedto erosionand deposition patterns generated by windflow around the hills. Prevailingwinds arefrom the Pacific Ocean to thewest (left) (SIR-A: DT-31). (d) SIR-Aradar image of linearstreaks southeast of LaskarGan, Afghanistan,formed