Reading:
What happened this morning? Yes, it snowed - then melted, evaporated, sublimed, re-condensed..... All within hours.
We are most familiar with water - the substance that goes through different phases - vapor, liquid, solid - here on Earth. But farther out in the solar system we need to think about other volatiles that change phases at different temperatures and pressures of other atmospheres. e.g. SO2 in Io's atmosphere, N2, CH4 in the atmospheres of Pluto and Triton.
Plot of vapor pressure vs. temperature for water. Triple Point = square. Dashed line is for super-cooled water and vapor.
What are units of hPa? Hectopascal?
Another version - from Goody and Walker - Fig 5-1 (a US book - so it's vapor instead of vapour).
These pressures are partial pressures of water vapor. e.g. At a temperature of -10C there is a partial pressure of water vapor of 2 mbar. So, how many water molecules are there in the atmosphere at -10C compared with the number of N2 molecules in the Earth's atmosphere?
These plots are for equilibrium. What do we mean by EQUILIBRIUM? What's happening to the molecules? What happens if you move away from equilibrium - by changing T or P rapidly?
The Ice-liquid line is not vertical - it has a -ve slope. So, what happens when you put a huge pressure on ice? e.g. with heavy skater on very narrow skate-blade? It turns to water - and the skater skates along.
What happens when you heat ice? heat water? cool vapor? cool water?
To go from ice to liquid you need to add energy - to break up the bonds of the ice crystals. This is the LATENT HEAT OF FUSION. Similarly, when water freezes, the same energy of fusion is released.
Same for liquid and vapor - LATENT HEAT OF VAPORIZATION.
What is the amount of heat? What are the values of these latent heats?
Here is a great explanation from a South African site at PHYSCHEM.co.za with a table for various materials.
We will be returning to these concepts several times when we look at evaporation/condensation cycles at various planets.
Sulfur dioxide SO2 atmosphere. How does the vapor pressure of SO2 vary with temperature on Io?
STRONG dependence on temperature - 20 degrees in temperature produce 4 orders of magnitude variation in vapor pressure. Huge change. Local atmospheres near hot spots.
How do we detect SO2 in Io's atmosphere? SO2 has a complex spectrum in the UV
Here are some HST observations of the spectrum of Io
These are images in an SO2 absorption band... so bright = less SO2, dark = more!
Varies dramatically with time
These are absorption bands of S2- observed in a volcanic plume by HST.
.... now let's look at some pretty pictures of Io and its peculiar role in the magnetosphere of Jupiter.
Here are some movies of Io's aurora taken by Cassini.