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Crrater production and obliteration are modeled for the plains of Venus, using (1) the observed distribution of Venus-crossing asteroids and comets; (2) viscous relaxation of crater topography; and (3) erosion and burial by atmospheric, volcanic, and tectonic processes. Crater lifetimes are assumed to be proportional to crater depths for both classes of obliterative processes although the individual criteria vary. An average crater retention age between 0.4 to 2.0 Gyr is estimated for plains, under the assumption that craters are produced and not removed. The range is driven by uncertainty in identifying degraded impact as opposed to volcanic craters. On the other hand, crater retention ages greater than about 1.6 Gyr are unlikely if viscous relaxation operates without loading of crater floor by burial. The preferred model has plains subject to crater production and obliteration processes that vary over both space and time.

Venera 15 and 16 spacecraft images show clear evidence of major crustal disruptions on Venus which have been interpreted to indicate crustal divergence. Complementary to the divergent zones are mountain belts that border the continent-like high terrains. The requisite transcurrent motions appear to be manifested as diffuse shear zones. The rift zones form an interconnected transpolar system which ties in with previously recognized equatorial disruption zones, suggesting a global tectonic network. Several independent lines of evidence suggest that the tectonism may be geologically young.

Recent stereographic methods have been applied to Voyager 1 images in order to determine the shape and large-scale topography of Io. The best triaxial figure of semiaxes of 1830.0, 1818.7, and 1815.3 kilometers is consistent with a differentiated satellite in hydrostatic equilibrium. It is suggested that many of the broad topographic swells and basins noted may be due to isostatic responses to thermal changes in the lithosphere-asthenosphere system. Results support the picture of increasing heat flow converting the basal lithosphere into a lower-density asthenosphere, resulting in isostatic uplift. It is shown that lithospheric thicknesses ranging from 5-100 km may results in elevation variations of up to 1 km.

The nonconcentric nature, anomalous depth, and terraced morphology of the nested craters that compose Cleopatra Patera are more closely analogous to volcanic craters (calderas) than multiring impact structures. Associated deposits northeast and downslope of the Patera first recognized on Venera 15/16 radar images are interpreted as volcanic plains related to, and perhaps cogenetic with, Cleopatra. The plains lavas probably originated as effusions from a radial rift zone or ring fissures. Rim deposits surrounding Cleopatra are asymmetric along the structural fabric of the region, also indicating fissure-type eruptions. Finally, a volcanic origin not only is easily reconciled with the tectonic setting of the Patera, it is almost required by the correlation between the Patera and regional structural trend.