Pedagoguery

Venus is commonly referred to as Earth's twin, and in many respects, it is. It's diameter is 95% of Earth's, and its mass is 81% of Earth's. It has a similar density, indicating a similar composition. However, it has very different conditions on its surface. How did Venus get to be the way it is?

Given the physical similarities of the two planets, it is quite likely that they started with similar inventories of carbon dioxide and water. Most of Earth's carbon is locked up in carbonate rock found deep under the oceans. By contrast, most of Venus' carbon dioxide remains free, resulting in an atmospheric abundance some 250,000 times greater than Earth's resulting in a greenhouse effect that makes the surface temperature of Venus some 900°F greater than it otherwise would be (as compared to Earth's greenhouse effect difference of about 60°F).

One reason for the difference are the oceans on Earth, which provide a huge sink for carbon dioxide. Carbon dioxide dissolved in seawater provides some of the raw material for diatoms to create their carbonate skeletons. It is these skeletons that, when the diatom dies, fall to the bottom of the sea to eventually be compressed into carbonate rock like limestone.

One of the main differences between Venus and Earth are the length of their respective days. Earth's is 24 hours long, while Venus' day is 243 Earth days long. What's more, Venus rotates in the opposite direction of the other planets. It is likely the result of a terrible collision early in the formation of Venus, that essentially halted the rotation of the young planet. This lack of fast rotation has a significant consequence: Venus lacks a significant magnetic field since it does not have significant rotation to stir up the molten iron in its core to form a dynamo.

Water vapor tends to distribute itself fairly evenly in an atmosphere. There are barriers to its movement, such as the temperature inversion at the top of Earth's troposphere that keep most of Earth's water vapor confined to the lower part of the atmosphere, but it can cross that barrier. When the water vapor gets high enough in the atmosphere, solar ultraviolet can disassociate the molecule into hydrogen and oxygen atoms. The hydrogen atoms will preferentially escape, due to the fact that they are least massive of the atoms and thus the average kinetic energy of the atoms means they are moving faster. This process, however, is slow, even at Venus' distance from the Sun, and so cannot account for all of Venus' water loss. Here, the lack of a magnetic field is key. In Earth's case, the magnetic field serves to deflect the oncoming solar wind. The solar wind is comprised of charged particles, mainly hydrogen and helium nuclei, that are expelled from the Sun's atmosphere. So, in the presence of a magnetic field, the solar wind is deflected away, and does not interact strongly with the Earth's atmosphere, except at the poles. In the case of Venus, however, it is much different. The solar wind directly impacts the upper atmosphere of Venus. Especially along the terminator line, the high energy particles of the solar wind can strip atoms out of the upper atmosphere. The Venus Express probe has observed a tail of ionized gas, including hydrogen and oxygen, emanating from the night side of Venus, lending evidence to this theory.

So, the early history of Venus probably was something similar to Earth's. After the planet cooled, the atmosphere, which was probably produced primarily by volcanic out gassing, consisted of carbon dioxide, water vapor, sulfur dioxide, carbon monoxide, nitrogen, hydrogen, methane, ammonia, and chlorine. As the planet further cooled, the water vapor condensed out to form oceans. Being closer to the Sun, Venus was warmer than the Earth, but not as warm as today for a couple of reasons. First, the atmosphere was much thinner, having just been formed. Secondly, the Sun was not as hot then as it is today. As a star ages, the abundance of helium in its core increases, forcing it to contract slightly and thus increase its temperature to maintain equilibrium. The increased warmth of Venus as compared with Earth meant that the atmosphere was more humid. Water vapor is a power greenhouse gas, so as more water vapor was added to the atmosphere, both through evaporation and volcanic out gassing, temperatures rose. A positive feedback loop developed, causing greater evaporation and eventually kicking more and more water vapor into the upper atmosphere. There, the molecules were disassociated, hydrogen was stripped away by the solar wind, and eventually what was left behind was the parched hellhole we see today.

Next time, the colors of alien vegetation.