Lesson Objectives:- Venus's surface
- Geological activity
- Venus's atmosphere
Venus' thick cloud cover makes it hard to see to the surface. However, Venus shows features of volcanoes and tectonics similar to Earth. Venus lacks erosion because it is too hot for any type of rain or snow. Also Venus has virtually no wind or weather because of its slow rotation.
Venus lacks plate tectonics, which are important for shaping most of Earth's major geological features such as mountain ranges and ocean trenches. This is a mystery to scientists as Venus has a thick, strong lithosphere. However, scientists have guessed that Venus' lithosphere is stronger than the Earth's and thus resistant to breaking into plates.
Internal heat on Venus should be similar to the Earth due to their similar sizes, which should lead to similar levels of geological activity.
In fact, there are relatively few impact craters meaning the surface is geologically young.
Furthermore, Venus's clouds contain sulfuric acid, which comes from volcanic outgassing. Over time, sulfur dioxide disappears from the air, so its presence means outgassing had to have happened less than 100 million years ago.
However, there is little evidence of erosion on Venus because it is too hot for rain and the planet rotates extremely slowly (once every 243 days), so there is no wind or weather.
One mystery of Venus is an apparent lack of plate tectonics. On Earth, stresses caused by convection currents in the mantle fractured our lithosphere into tectonic plates. The movements of these tectonic plates are responsible for shaping most of Earth's major geological features such as mountain ranges and ocean trenches, and also means that different regions of the Earth have different geological features and different ages.
On Venus, on the other hand, the entire planet looks to have aged uniformly. Scientists hypothesize that Venus has a thicker and stronger lithosphere than the Earth, so it has resisted fracturing caused by mantle convection.
Venus is a lot closer to the Sun, but the density of its clouds means its surface actually absorbs less sunlight than Earth. If it weren't for the greenhouse effect caused by a carbon-dioxide heavy atmosphere, it would actually be colder than Earth.
Why are the Earth's atmosphere and Venus's atmosphere so different?
Earth and Venus both experienced the same volcanic outgassing which released massive amounts of water vapor and carbon dioxide into the air. Water vapor and carbon dioxide, as you may recall, both function as greenhouse gases. The difference is, our water vapor condensed into rain and formed oceans. Liquid water breaks down carbon dioxide, so our carbon dioxide dissolved in the oceans and formed carbonate rocks.
Why didn't Venus form oceans? Venus's closer proximity to the Sun would have resulted in higher surface temperatures. Higher temperatures mean the air can hold more water, and also mean faster evaporation of water. This all results in a positive feedback loop - more water in the atmosphere traps more heat, which in turn leads to even more evaporation and more water vapor in the air. Without oceans, the carbon dioxide also accumulates, and the end result is a runaway greenhouse effect.
Where did the water vapor in the atmosphere go? The sun's ultraviolet rays break down water molecules in the air and cause their hydrogen atoms to escape into space. In the end, all of Venus's water disappeared into space and it was left with only carbon dioxide.