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Much of what I will cover for the Venutian atmosphere will be a review of previous Greenhouse material. This isn't a bad thing. This particular topic is very important for this class, so revisiting it isn't a bad idea. Also, it's our first section back after Spring Break. I don't want to cover really heavy material straight away.
Something to keep in mind is that we only came to understand the Greenhouse Effect (how it works and its importance here on Earth) by studying Venus. This particular topic is an excellent example of the importance of space exploration for understanding our own planet.
Venus is an absolute hellhole. The atmosphere on Venus is thick. Really thick. The atmospheric pressure at the surface of Venus is 90 bar. That's 90 times more atmospheric pressure than on the surface of the Earth, and approximately equivalent to the pressure that one feels 1 km underwater (the world record SCUBA depth is 332 meters). The temperature at the surface is 740K, sufficient to melt lead.
Venus is an excellent example of the fragility of the complex sequence of events that leads to habitable conditions on a planet. In many regards, Earth and Venus are twin sisters. They're approximately the same age, approximately the same mass, and they're located at approximately the same distance from the Sun (Venus is of course a bit closer). Earth became a life-sustaining paradise, and Venus became an absolute hellhole that rains sulfuric acid and has temperatures hot enough to melt lead. What went wrong?
When they were born, volcanic processes spewed H2O, CO2, SO2, N2, etc. into both the Venutian and Earth atmospheres.
Venus, being a little bit closer to the Sun, is slightly warmer than the Earth. As a result, more H2O remains in the atmosphere as water vapor instead of condensing into liquid water on the surface. H2O, as we've learned, is a very efficient greenhouse gas. Lots of water vapor in the atmosphere leads to increased heat, which evaporates any liquid water that may have condensed, which leads to more heat, etc.
Without liquid water on the surface, CO2 is not sequestered out of the atmosphere and contributes to greenhouse gassing as well. As a result, the planet cooks.
Earth, a bit cooler, has liquid water condense onto its surface. This liquid water forms oceans, which dissolves CO2 out of the atmosphere to form carbonate rocks. This reduces the greenhouse warming of the Earth, resulting in an atmosphere of mostly N2.
This is a review question. Oxygen is a byproduct of biological processes. We see an excess of oxygen in the Earth's atmosphere because life put it there.
The story of Venus' evolution that we just went over requires that Earth and Venus started with approximately the same amount of water. Where did all of that water go?
In the upper reaches of the atmosphere, water molecules can be dissociated. This means that they are broken into their constitudent pieces (O and H and H) by UV radiation. Do you guys remember what H2 is called? Answer: Deuterium (the same that is the critical step in the proton-proton chain).
On Venus, we see a much higher concentration of Deuterium than on Earth. What could this mean?
While, on Earth, much of the water remains in liquid form on the surface; Venus has all of its H2O in the atmosphere. This H2O is broken into O2 and Deuterium. Deuterium is heavier than normal hydrogen, and therefore has a higher escape velocity (it's harder for Deuterium to escape the atmosphere than it is for ordinary Hydrogen to escape). It makes sense then that the remaining Hydrogen in the Venutian atmosphere is enriched in Deuterium.
Furthermore, spacecraft have observed Hydrogen and Oxygen ions escaping from the planet into the solar wind. The flux of Hydrogen is twice that of oxygen, which is precisely what you'd expect if the source of these ions is dissociated water.
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The clouds on Venus form at altitudes between 20 and 80 km. It is at these altitudes that UV radiation can break H20, CO2, and SO2 molecules into the consitutent pieces, and those pieces ca re-form into H2SO4, which combines with water vapor to form sulfuric acid.
The lower altitudes are clear because the temperatures are so intense. Sulfuric acid droplets vaporize and break back into H20 and SO3, making the air clear.
How do we see through the Venutian clouds to study its surface? Answer: RADAR.
Given its size, Venus almost certainly has a differentiated, metal-rich core. It does not, however, have a magnetic field. Why might this be?
A magnetic field requires three things: