VENUS: THE WANRE OF THE SURFACE FROM VEWERA PANORACUS. J. 0. &win, J. W. Mead, M. T. Zuber, and P. Helfenstein, Dept. of Geological Sciences, Brown Univ., Providence, RI 02912. Images of the surface of Venus obtained by the Soviet Venera 9, 10, 13, and 14 landers have been analyzed to provide a basis for understanding the nature of geologic processes operating there. The four spacecraft landed in the Beta-Phoebe region at median elevations in the upland rolling plains province (Table 1). The landing potnts are each separated by distances of more than a thousand km (1-4). The Venera panoramas were digitized and transformed into various per- spectives in order to facilitate analysis and comparison with other planetary surfaces (5.6). Table I1 swrmarizes the detailed observations made fran the panoramas. We discuss some of the mre significant features below. Bedrock is exposed at the Venera 10, 13, and 14 sites and is characterized by semi-continuous, flat polygonal to subrounded patches up to several m in width. The bedrock surface is often dominated by sub-horizontal to horizontal layered plates with thick- nesses of several an and abundant linear and polygonal vertical fractures. Angular to subangular platy blocks in the 5 to 70 an range dominate the Venera 9 site and occur much less frequently at the other sites (3,7). Blocks appear to share many characteristics with the exposed bedrock and are interpreted to be largely derived from it. Soils (particles< 1 an) are abundant at the Ven- era 9, 10, and 13 sites, but are uncomnon at Venera 14. Features indicative of a strong eolian influence (mats, dunes, wind tails) are not observed. A striking aspect of the Venera landing sites is their extreme similarity despite separation distances of thousands of Ian. Several hypo- theses are considered for the origin of the bedrock surfaces (1-4,6) and we investigate in de- tail the hypothesis that bedrock originated from surface lava flows. In thls interpretation, the broadly platy nature of the surface is analogous to the rolling and undulating nature of terres- trial pahoehoe flows caused by the formation and deformation of a semi-solid crust. The layering is interpreted to be foned by a canbination of upper thermal boundary la er fonatfon and hori- zontal sheets formed by cooling and shearing during flow emplacmnt (8.91. Vertical fractures are attributed largely to joint patterns formed during cooling. This interpretation made on the basis of surface morphology is consistent with Venera 13 and 14 geochemical results which repor- ted high potassium basalt and tholeiitic basalt compositions, respectively. Additional theoreti- cal grounds for investigating the lava-flow hypothesis are discussed by Wilson et al. (9). If this interpretation is correct, large areas of the Beta-Phoebe region are likelycharac- terized by lava flows. Recent high-resolution (1-2 km per cell) radar ddta of thls region ac- quired at Arecibo supports the presence of volcanism and the possibility of vast flows. Finally, the relative freshness of features observed by Ve~ra14 suggests that sat bedrock SU~~WSam pcologiully young, or that erosion rates are low. TABLE I. Sunrury of Vemra 9. 10, 13 and 14 Lander Mission Results r f Chrrrcterlstic Venerr 9 Venra 10 Venra 13 Venrrr 11

Location (lat.. long. )* 31.7'. 290.8' 16.0'. 291.0~ -7.6', 303.5' -13.2'. 310.1° Landlng Date Oct. 22, 1975 Oct. 25, 1975 Rrch 1. 1982 March 5. 1982 Surface Operation ~irne~ 50 44.5 100 59.5 (min. ) Mean ~lovatlon' {Lan > 6051) 2.1 t 0.4 1.5 + 0.6 1.4 t 0.3 1.0 t 0.3 fmean i std.dev.1 Clcan Roughnesst (Om) 4.0~r1.3~ 3.1~t0.8~3.0Ot0.3~ ~.9~t0.1'

Hean Reflectivltyf 0.10 i 0.03 0.10 t 0'03 0.15 t 0.08 0.14 t 0.04 Model Density (g/cm ) 1.9 1.9 2.6 2.4 Olelectrlc Constant 3.8 3.8 5.2 4.8 Surface Albedo 0'03-0.12 0.02-0.04 0.03-0.085 0.04-0.11 Surface Pressure (atm) -90 91 89.5 93.5 Average Wind Speed (me') 0.4 - 0.7 0.8 - 1,3 0.3 0.6 (1.3 - 0.6 Solar Zenith Angle 33O 2 7' -35' -35O #umber ef Full Panoramas' 1 1 11 6 Surface Temperature (XI 728 737 738 743

Error on speclfled coordlnrtls Is loto 2' I From Pioneer-Venus masulwmts for regions tlOQ km from landing coordinates 1 Coaplete (*holr) prnerart (-I@' in rziwth) t Length of tlr iuws m trmsalttod from surface

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Garvin, J.B. et al.

--TMLE 11 Observed Characteristics of ~eniraLander Sites '~ite Bedrock Fragnents (>I cm) . Fines/Soil (~1cn) ' evidence for bed- y g .Abundance of angular to subangular .Apparent bimodal distribution rock exposure layered and platy blocks in the 5-70 (fines below limits of resolution, cm range. coarse, -1 an). ' Several blocks have elongate, rounded ' Few intermediate fragments ridaes and other undulatory surfaces. between fines and 10 cm blocks. 'Some blocks polygonal in outline; 'Distributed between blocks, some are steeply inclined relative to little evidence of fillets horizon. around blocks. Finer fraction (1-5 cm) distributed in interblock areas

'Covers 40-601 of surface. 'Chly a few discrete fragments >5 cm. 'Fines distributed in extensive Exposed as semi -continuous, ' Located in areas dominated by fines, patches in low areas and in small generally flat. subrounded to not on bedrock. patches on bedrock surfaces. polygonal patches up to several ' Vislble frawnts in soil grade into ' Fines lower albedo than bedrock. meters in width. roughness at the scale of resolution ' Fines grade into small fragraents Surface rough; in nearf ield. in intermediate and far field. in size. 5-10 ca of topography, up to a aeter in background. 'Surface texture Is pitted, and also contains a cuspate scarp. - Linear and orthogonal fractures.

13 'Covers 20-50% of surface. 'Larger fragments are angular to sub- 'Finesdistributed in extensive 'Exposed as semi-continuous angular. layered and platy; locally patches between bedrock exposures generally flat polygonal to sub- distributed in patches associated and in local small patches in rounded patches. primarily with bedrock. lbny frag- bedrock pits and depressions. 'Surface rough in nearfield, bed- ment boundaries can be related to 'Fines larer albedo than bedrock. rock plate edges and surface shars adjacent bedrock fractures. 'Biadal distribution? 5-10 a Of topography; pits and 'Swller fragnents are located on and 'Soil occurs on surface of lander shallar linear depressions. within soil/fines patches and only ring on both sides of spacecraft 'Several rounded. elongate, and occasionally on bedrock. Occur in two and on lens cover. cuspate scarps. mdes: 1) in an annulus surrounding Linear fractures. the lander ring. The most rounded -Surface has a somewhat layered particles occur here; fra-nts lie appearance. on top of soil layer1 2) in soil patches between bedrock exposures. Particles mostly partly burid. .Msttwt prucity.of fines y 14 .COVWS almt of surface. -@ly a few discrete blocks >10 a. .Exwed as continuous areas of .(kc 50 cm block with layend/strtatd corprrrd to other sites. interlocktm). grnerally flat, texture. .Sam local accuulatiw 11 polygonal plates. .Fragments are angular to subangular; fractures In bedrock and In .Surface is rwgh at scale of several can be geometrically fitted isto front of spacecraft on am sf&. centimeters. adjacent bedrock. .Soil occurs on surface of .Surface structure dominated by .In farfield. fravnts appear in local lander ring only on am side of subhorizontal to horizontal lay- patches between extensive flat platy bed- spacecraft. ered plates with thicknesses of rock exposures. several centimeters. Some layers .In nearfield, smaller fragments have two show different albedo, with upper- modes of occurrence: 1) in depressions most layers darkest. Uppetnost and fractures in bedrock; 2) distributed plate in nearfield has hole or around lander ring on am side of space- window revealing underlying layer. craft. Some surface sublayers show tongue- likc overlaps. .Surface textures Include pitting, waviness and elongate, cuspate scarps. .hun&nt linear and polygonal fractures.

References: (1) Florensky, C.P. et al. (1977) GSA Bull. 99, 1537. (2) Keldysh, M.V. (ed.) (1979) NASA m-75706, 189 PP. (3)ilevsky, A.T. etT(1984) GSA Bull., in press. (4) Florensky, C.P. et al. (1983) Science 221, 57. (5) GXJ.B. et-m3)Planetar Re ort 3, 10-12.7~arvir1,J.B. et al. (1981) NASA TM-84211, 1m. (8) G.A. holcanoes. Prentice- all. ~xpp.(9) Milson, L. et 11. (1981) Pmc.I&-f Lunar Planet. Sci. Conf. 15th, in press, this volume. (10) Campbell, D. et -983) Science, in press.

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