The soft landers Venera 9 and 10 transmitted information to Earth about conditions on the surface of Venus. Temperature measurements in both locations ranged from 730° to 740°K, and pressure measurements ranged from 88 to 94 atm. Wind velocities were 0.5 to 1 m/sec. The radioactive content of material below the spacecraft was similar to that of Earth's basalt. The hardness of some of the formations was estimated to be comparable to Earth's hard rocks. The density of a hard formation was measured by Venera 10 to be 2.8 ± 0.1 g/cm3.

Panoramic television cameras provided photographs of the landing areas. The types of computer enhancements to which the photographs were subjected are briefly discussed.

Venera 9 landed on a slope composed of slabs and a fine-grained matrix; Venera 10 landed on a plain composed of scattered outcrops separated by a fine-grained matrix. Detailed descriptions of the landing areas are presented.

The available chemical and dynamic information about the conditions at the surface of the planet indicates that the atmosphere has a high efficiency in lifting and transporting loose small particles but that very little ablation of hard material is to be expected.

Assuming thermodynamic equilibrium between atmosphere and surface materials, solid-gas interaction may occur and produce some disintegration of hard rocks and (or) some lithification of loose material, respectively by the increase in volume of newly formed minerals and by the formation of films or crusts. A vertical thermal gradient and vertical movements of gases or rocks are necessary. It is concluded, however, that the scale of chemical changes may be limited.

The photographs show that at least two types of geomorphic degradation occur. The slabs of Venera 9 appear to be mass wasting downhill, and the outcrops of Venera 10 show evidence of rounding of corners and smoothing of surfaces. The mass wasting is likely to be caused by gravity and perhaps activated by quakes or other geologic processes. The rounding and smoothing processes are probably due to some atmospheric action.

Six possible origins of the slabs and outcrops are discussed: (1) surface lava extrusion, (2) igneous intrusion later exposed by erosion, (3) pyroclastic fall, (4) impact lithification, (5) sedimentary rock lithified at depth and later exposed by erosion, and (6) lithification (or metamorphism) of loose material by atmospheric action at the surface.

A model of the surface geology of Venus is presented in which fine material is gently moved by Venusian winds. Hard material is occasionally formed at the surface, either by lithification through atmospheric processes or by volcanic falls. Removal of loose material by winds will cause some of the lithified units to stand out as positive relief features and be subjected to rounding of corners and smoothing of surfaces. If a slope exists, the lithified units will break into slabs and mass waste downhill.

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