Structure and Atmosphere
Venus' diameter is 95% of Earth's at 12104 km (7521 mi). We cannot see its surface because it is shrouded in thick clouds of sulfuric acid. The Magellan Venus orbiter has been instrumental in providing us with radar maps of Venus' surface from probes that have parachuted through its clouds and into its thick atmosphere. Because of the extreme conditions these probes did not last long, but we can see from this data that Venus is dominated by two main upland regions that are comparable to Earth's continents. Aphrodite Terra near the equator is over half the size of Africa and Ishtar Terra in the far North is roughly the size of Australia. Ishtar is home of the highest point, the Maxwell Montes mountain range, which is 11 km (7 mi) high, taller than Earth's Mount Everest. Recent lava flows indicate a high likelihood of currently active volcanoes making it the only planet apart from our own that has active vulcanism (Neptune's Triton and Jupiter's Io also have volcanoes).
The craters that dot its surface are small in number but large in size,
indicating a surface of a relatively young geological age of less than 800
million years. Since small meteorites burn up in the thick atmosphere, only
large ones manage to impact resulting in Venusian craters measuring up to
300 km (185 mi) in diameter. Some of these craters have been formed by
volcanic activity. The presence of lava channels over 6000 km (3750 mi) 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 interior of Venus is believed to be similar to that of the Earth with a metallic core and silicate mantle. Unlike the Earth, Venus has a very small magnetic field apart from that induced by the effect of the solar wind. Extensive fault-line networks cover the planet, probably the result of the same crustal flexing that produces plate tectonics on Earth. But on Venus the surface temperature is sufficient to weaken the rock, which cracks just about everywhere, preventing the formation of major plates and large earthquake faults like the San Andreas fault in California. 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 up-welling and subsidence of magma from the mantle.
Venus has an atmosphere which, at the surface, has a pressure 90 times that of the Earth's. Unlike the Earth's atmosphere, which is mainly composed of nitrogen and oxygen, Venus's atmosphere is made up of 97% carbon dioxide with most of the remainder being nitrogen and argon. One consequence of the preponderance of carbon dioxide in the atmosphere is that Venus suffers from the severe effects of 'greenhouse effect'. This means that the carbon dioxide in the atmosphere is transparent to the light and heat coming from the Sun but is opaque to the long wavelength infrared radiation coming from the hot planet. Even though a large percentage of heat radiation is reflected from the top of the clouds and never reaches the surface of the planet, the heat that does penetrate is captured by the thick cloud covering. Less than half of the infrared radiation is released back to space. the effect is to raise the temperature of the planet by a massive 500°C (900°F). Compare this to Earth, where its own 'greenhouse effect' raises the temperature by only 35°C (63°F). The temperature difference between equator and poles is only a few degrees.
Venus is covered with clouds made of sulphuric acid, rather than the
water vapour clouds found on Earth. These clouds permanently shroud Venus'
volcanic surface, which has been radar mapped by spacecraft and from Earth-based
telescope. At ultraviolet wavelengths cloud patterns become distinctive.
In particular, a horizontal "V" shaped cloud feature is visible near the
equator. This global feature might indicate atmospheric waves, analogous
to high and low pressure cells on Earth. Bright clouds toward Venus' poles
appear to follow latitude lines. The polar regions are bright, possibly
showing a haze of small particles overlying the main clouds. The dark
regions show the location of enhanced sulphur dioxide near the cloud
tops.
At high altitude winds reach speeds as high as 360 km/hr (225 mph) and the clouds circle the planet in only four Earth days. The circulation is in the same direction - west to east - as Venus' slow rotation of 243 days, where as Earth's winds blow in both directions - west to east and east to west - in six alternating bands. Close to the surface, however, the wind is much gentler and the surface reveals no evidence of substantial wind erosion. Only limited wind transport of dust and sand is evident. This contrasts with Mars, where there is a thin atmosphere, but substantial evidence of wind erosion and transport of dust and sand.
According to planet evolution theory and NASA spacecraft data, Venus may have had hot planet-spanning oceans for hundreds of millions of years before it lost them. The theoretical history for Earth's twin planet solves several Venus mysteries and explains many of the extreme conditions found today. Called the "wet greenhouse" theory, it suggests that Venus had relatively Earth-like conditions during its early history. It also suggests (as many scientists now believe) that Venus, Earth, and Mars formed from similar interstellar materials, and that all three originally had substantial amounts of water. Spacecraft may yet find geological and erosion evidence of early bodies of water on Venus, as they already have on Mars.
The "wet greenhouse" theory with its oceans accounts for the almost completely waterless state of the present-day planet. It explains where the planet's missing oxygen is stored. Ironically, the presence of early oceans on Venus may well account for the incredibly dry condition of today's planet.
Venus' original atmosphere of about 4 billion years ago is believed to have been much like it's atmosphere today: many times denser than Earth's and mostly carbon dioxide. The "wet greenhouse" theory suggests that this enormous primordial atmosphere was reduced to a small part of its original mass by ocean planet interactions. This would have left Venus' atmosphere about the same size of Earth's. The thin Earth-like atmosphere then lasted several hundred million years and eventual loss of most of the water from such a thin Earth-like atmosphere would have stripped the planet of its water, leaving it bone dry as Venus is today.
