Answer to Question 1
Many features on Venus testify to its volcanic history. Radar maps reveal shield volcanoes and long lava channels. Other volcanic features include the coronae, circular bulges up to 2600 km (1600 mi) in diameter bordered by fractures, volcanoes, and lava flows. Coronae appear to be produced by rising convection currents of molten magma that push up under the crust. When the magma withdraws, the crust sinks back, but the circular fractures mark the edge of the corona.
Answer to Question 2
Calculations show that Venus and Earth should have outgassed about the same amount of carbon dioxide, but Earth's oceans have dissolved most of Earth's carbon dioxide and converted it to sediments such as limestone. This suggests that the main difference between Earth and Venus is the lack of water on Venus that would have removed carbon dioxide from the atmosphere. There is evidence that Venus had oceans when it was young, but being closer to the Sun it was warmer, and the carbon dioxide in the atmosphere created a greenhouse effect that made the planet even warmer. That process could have vaporized any oceans that did exist and reduced the ability of the planet to purge its atmosphere of carbon dioxide. As more carbon dioxide was outgassed, the greenhouse effect grew even more severe. Thus, Venus was trapped in what is called a runaway greenhouse effect.
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