Pioneer Venus Radar results: Geology from images and altimetry

Altimetry and radar image data obtained by the Pioneer Venus spacecraft and earth‐based radars have provided a first look at the global distribution of topographic relief, regional morphology, and surface roughness characteristics of Venus. From these data a preliminary interpretation of planetary crustal history can be made. A distinctly unimodal distribution of relief for Venus has been determined from the Pioneer Venus altimetry data. We have divided the surface of the planet into three provinces: ‘upland’ rolling plains, making up 65% of the surface, lying between planetary radii of 6051.0 km and 6053.0 km; highlands, about 8% of the surface, between planetary radii of 6053.0 km and 6062.1 km; and lowlands, about 27%, between planetary radii of 6049.0 km and 6051.0 km. The numerous dark circular features in the rolling plains province may be lava‐filled impact basins; their size‐frequency distribution is consistent with that obtained for ancient terrains on the moon and Mars. A ‘granitic’ composition for the rolling plains province is indicated by gamma ray spectrometry from the USSR Venera 8 lander; this province thus may represent most of the planet's ancient crustal material. Ishtar Terra (centered between latitudes 60° and 75°N at longitude 0°) and Aphrodite Terra (centered at latitude 5°S between longitudes 80° and 190°) compose most of the highland province; their highest points stand 11.1 and 5.7 km above planetary datum, respectively. Ishtar is made up of an uplifted plateau and a great volcanic construct; Aphrodite appears to be highly disrupted tectonically and degraded. Gravity and altimetry data indicate that the highlands are compensated isostatically, probably as a result of crustal thickening or lateral variations in the crust and mantle, that is, by either passive or dynamic mechanisms. The lowlands province of the planet includes several crudely circular low areas with low relief within the highlands. These areas are radar dark, have low backscattering efficiency and/or high radar absorptivity, and 1° to 2° meter‐scale rms slopes. All lowland regions may be covered by younger basaltic lavas that have filled depressions where the crust is thinner, like the basins and lowlands on the moon, Mars, and earth. An integrated global pattern of subduction troughs or mid‐basin ridges, indicative of active global plate tectonism, has not been identified from the altimetry data. However, complex ridge‐and‐trough regions east of Ishtar Terra and in southern Aphrodite Terra, and a tectonically disrupted region between Beta Regio and Aphrodite Terra, may be the result of large‐scale crustal motion. Beta Regio appears to consist of two giant irregular shield volcanoes; their relief profiles and the presence of a summit depression alined on an axial trough suggest a basaltic composition. This interpretation is supported by the ‘basaltic’ composition reported by the USSR Venera 9 and 10 for rocks directly east of Beta. The very low (440 m) center of mass‐center of figure offset determined for Venus from the Pioneer Venus data may have resulted from lateral crustal spreading or uniform distribution of the original low‐density crust.