THE TECTONICS OF VENUS

Solid Venus has several differences from solid Earth: a mild variation in topography, with marked departures of some kilometres confined to less than 10% of the surface; no interconnected system of ridges, such as would be associated with a spreading lithospheric boundary layer; a high correlation of gravity with topography; and a ratio of gravity to topography implying compensation depths in excess of 100 km for maj or features. This high admittance ratio implies a stiff upper mantle, as also indicated by the high depth-diameter ratio of craters. The det ails of Magellan radar imagery enable some inference of event sequence s. The morphology generally indicates a more regional scale of change than on Earth. The principal chronometer is impact crater distribution . The number of impact craters identified is 914, which is equivalent to about 500 Ma's infall of bodies big enough to penetrate the atmosph ere. But the number of craters per unit area has but slightly more var iations from the mean than random and only a mild negative correlation with topographic elevation. Only one-third of the craters evidence te ctonic or volcanic modification. Hence the rate of resurfacing over th e last few 100 Ma must be quite slight compared to Earth's. The main d ebate is whether Venerean tectonic activity is in monotonic decline, o r whether it is episodic on a 500 Ma time-scale. Evidencing some curre nt tectonic and volcanic activity are a few limited regions of marked topographic and geoidal highs, exceeding 8 km and 80 m respectively, w ith steep slopes. Clearly, plate tectonics does not exist on Venus. Th e underlying cause of this different evolution appears to be the lack of water. This dryness makes the upper mantle stiff enough to regional ize the tectonics and inhibit recycling of crust. The lack of water al so prevents erosion and thus the recycling of secondary differentiates . These deficiencies in recycling imply a strong net upward differenti ation of heat sources and a thick crust, now allowed by new results on dry diabase rheology.