The surface of Venus is mostly covered by volcanic materials. Volcanic surface features, such as vast lava plains, fields of small lava domes, and large shield volcanoes are common. The new land of Beta Regio contains massive shield volcanoes such as Rhea Mons and Theia Mons and is still evolving. The two largest surface features on Venus are Aphrodite Terra (about the size of Africa, bright area near the equator in the southern hemisphere), and Ishtar Terra (about the size of Australia, too near the north pole to see on the map). Though the surface is more rugged than Earth's, there is much less variation in altitude - 60% of the Venusian surface is within 500 m of the planet's datum. The highest point on Venus, Maxwell Montes in Ishtar Terra, is an exception to this rule, as it rises about 11 km above the datum. The lowest point is Diana Chasma in Aphrodite Terra, almost 3 km below the datum. Compare Venusian stratigraphy to the extremes found on Mars. Venus is also home to the longest channel in the solar system, which meanders around for 6800 km, longer than the Nile River on Earth.
The presence of lava channels over 6,000 kilometers long suggests river-like flows of extremely low-viscosity lava that probably erupted at a high rate. Large pancake-shaped volcanic domes suggest the presence of a type of lava produced by extensive evolution of crustal rocks.
The typical signs of terrestrial plate tectonics - continental drift and basin floor spreading - are not in evidence on Venus. The planet's tectonics is dominated by a system of global rift zones and numerous broad, low domical structures called coronae, produced by the upwelling and subsidence of magma from the mantle.
Although Venus has a dense atmosphere, the surface reveals no evidence of substantial wind erosion, and only evidence of limited wind transport of dust and sand. This contrasts with Mars, where there is a thin atmosphere, but substantial evidence of wind erosion and transport of dust and sand. Preliminary results from Venera 9 indicated clouds 30-40 km thick with bases at 30-35 km altitude; atmospheric constituents including HCl, HF, Br, and I (wouldn't want to get a whiff of that in your face mask!); surface pressure about 90 (earth) atmospheres; surface temperature 485 ° C; light levels comparable to those at earth midlatitudes on a cloudy summer day, and successful TV photography showing shadows, no apparent dust in the air, and a variety of 30-40 cm rocks which were not eroded. Preliminary results from Venera 10 provided an atmospheric profile as follows: (altitude [km]/pressure [earth atmospheres]/temperature [° C]) 42/3.3/158, 15/37/363, and 0/92/465. The surface wind speed was 3.5 m/s (about 8 mph), but don't be lulled--the density of the air is so high that a light breeze would feel like a gale. Contrast this with Mars' very thin atmosphere, in which near-supersonic winds would just feel like a light breeze.
For a discussion of the runaway greenhouse effect and resurfacing of Venus which seems to have taken place in the recent geologic past (500 MYA), see also the Jul?99 article in Scientific American.
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