Venus: The nature of the surface from Venera panoramas

Abstract

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. The landing points are each separated by distances of more than a thousand kilometers. The Venera panoramas were digitized and transformed into various perspectives in order to facilitate analysis and comparison with other planetary surfaces. Bedrock is exposed at the Venera 10, 13, and 14 sites and is characterized by semicontinuous, flat polygonal to subrounded patches up to several meters in width. The bedrock surface is often dominated by subhorizontal to horizontal layered plates with thicknesses of several centimeters and abundant linear and polygonal vertical fractures. Angular to subangular layered to platy blocks in the 5‐ to 70‐cm range dominate the Venera 9 site and occur much less frequently at the other sites. Blocks appear to share many characteristics with the exposed bedrock and are interpreted to be largely derived from it. Soils (particles <1 cm) are abundant at the Venera 9, 10, and 13 sites but are uncommon at Venera 14. Features indicative of a strong eolian influence (moats, dunes, wind tails) are not observed. A striking aspect of the Venera landing sites is their extreme similarity despite separation distances of thousands of kilometers. Several hypotheses are considered for the origin of the bedrock surfaces, and we investigate in detail the hypothesis that bedrock originated from surface lava flows. In this interpretation, the broadly platy nature of the surface is analogous to the rolling and undulating nature of terrestrial pahoehoe flows caused by the formation and deformation of a semisolid crust. The layering is interpreted to be formed by a combination of upper thermal boundary layer formation and horizontal sheets formed by cooling and shearing during flow emplacement. 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 reported high potassium basalt and tholeiitic basalt compositions, respectively. If this interpretation is correct, large regions of the Beta‐Phoebe area are likely to be characterized by lava flows. The relative freshness of features observed by Venera 14 suggests that some bedrock surfaces are geologically young or that erosion rates are low.