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Synopsis






Venus, with similar mass and uncompressed density to the Earth, is our“twin planet”. It probably has a similar bulk composition. The best estimate of crustal thickness is 30 km. There is a strong correlation between gravity and topography: large volcanoes are supported by the crust and the rocks are strong due to the absence of water. There is no evidence for the operation of plate tectonics. The terrain is mostly rolling basaltic plains cut by some long meandering channels probably eroded by fluid lavas. There are a large variety of volcanoes, as wel as coronae, probably the surface expression of hot spots or plumes. Two small continent-size regions (Ishtar and Aphrodite Terrae) are apparently uplifted by compressional tectonics, while there are highly deformed regions (tesserae).Venus has a distinctive cratering record; for example, small impactors are effec-tively filtered out by the dense atmosphere. There are 940 mostly fresh impact craters, randomly distributed, that yield an age for the surface of 750 Myr. The craters are superimposed on a secondary basaltic crust that obliterated any pre-750 Myr terrain. The previous 85% of venusian geological history is completely obscured. This episodic event, possibly taking over 100 Myr, contrasts with the continuous recycling of the terrestrial plate-tectonic regime, although both planets likely have similar heat production with similar K, U and Th abundances. The absence of plate tectonics on Venus is due to a combination of smaller size, that places the basalt– eclogite transition too deep and to the absence of an asthenosphere, due to lack of water. The crustal composition is basaltic, occasion-ally alkali-rich, high in K, U and Th. Minor occurrences of “ pancake” domes may represent felsic differentiates or may merely be basaltic domes flattened under the 95 bar atmosphere. Venus appears much less differentiated than the Earth with less than 25% of the terrestrial atmospheric 40 Ar content (derived from 40K).

Is there a decipherable geological sequence or history on the planet? Probably not. Despite the presence of 940 craters, they are not abundant enough to establish a relative sequence among the tesserae, plains, volcanoes or coronae. Attempts at constructing a geological chronology fall back on lithological correlations, a hazar-dous procedure. This twin of the Earth with similar heat production has had a totally different tectonic history due mostly to lack of water. The 200 ppm water in the venusian atmosphere has a high D/H ratio possibly indicating early loss, but perhaps was inherited from comets. Whether the deficiency is due to early loss or failure to accrete significant water, a conclusion that we favor, remains controversial. Venus remains as a cautionary tale for seekers after“Earth-like ” planets, with habitability depending on apparently trivial differences in mass and water content


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