April 8 - April 21
Liquids and gases are the focus of this module as atmospheres, "big mushies" and energy transfer are studied.
We mostly ignored the specifics of atmosphere formation (for solid worlds) in the last module; time to look in more detail. Atmospheres (fluids - gases and liquids) are bound to a world by gravity. The outer parts may escape the gravity, atom by atom, if there's enough energy for some to achieve escape velocity. The atmosphere is kept "up" (kept from collapsing onto the surface) by pressure - basically, the atoms & molecules hit each other and bounce, if they're warm enough. This is as true of Sun (see "gravitational equilibrium) as Earth's atmosphere. Sunlight (mostly) is the source of energy for atmospheres, so the absorption, reflection, or transmission of sunlight at different energies (frequencies) is important. The angles of sunlight on a spherical world result in uneven heating, which leads to circulation. If the world is turning (normally true), a "coriolis effect" leads to the formation of bands.
We can look at the structure in several ways. First, we can look at composition: close to a solid surface may be a well-mixed part of the atmosphere (homosphere). Further out, or the only atmosphere if the atmosphere isn't thick, the atoms and molecules reach heights that depend on their masses, so you get different proportions of chemicals at different heights (heterosphere). The book doesn't discuss this, but meteorology sources will, and it's parallel to the core-mantle-crust structure of the interior of a solid world. The parallel to the layers by structure is so strongly related to light and temperature that we group them, and that's the scheme of atmospheric structure we mostly use.
In general ,we expect a gas to get colder as it gets farther from the center of a world - the pressure and temperature both drop as you go up from a surface. On a world with a surface and a transparent atmosphere, the temperature pattern is quite dramatic - sunlight gets through and heats the surface, which heats the atmosphere in contact with the surface by conduction, and so the air closest to the surface should be warmest. Some things modify this pattern. The solid surface will generally be a thermal emitter (remember thermal spectra?) primarily in the infrared range. If there are gases in the atmosphere that absorb IR, they'll recycle some of that energy emitted by the surface and raise the overall temperature. If there are gases in the atmosphere that absorb UV (Earth only, as far as we know), there may be a layer of the atmosphere that's warmer than it would otherwise be. On Earth, that's the stratosphere. Please note that the text defines the stratosphere differently than a meteorology source would (only the part where the temperature is steady or increasing with height is the meteorological "stratosphere"). Sun is also giving off x-rays, which will basically ionize any atom they hit, so the outer part of any atmosphere (until you've gone far enough in for all the solar x-rays to have been absorbed) will have extra energy, and get warmer as you go out. The end result is the generic atmospheric structure depicted in the text. Compare the ones for the giant planets - the outer structures are the same. It's just that the giant planets keep going on down.
Energy transport - or at least the amount of convection - is tied to the temperature structure. It is beyond this course to go into details, just know that the troposphere on Earth tends to have a lot of convection, and the temperature change in the stratosphere pretty much prevents that convection from going higher.
The text describes the processes that add and remove gas from atmospheres. Note that some are parallel (adding or subtracting). Some are not named very well - you might want to find less misleading nicknames. Most important, some add or remove matter from the world as a whole, while others just move the matter from the atmosphere to the solid body or vice versa.
The Cosmic Perspective
Chapter 10 - note that the definition of stratosphere doesn't match the meteorology definition
Chapter 11 - special note of section 11.4
Chapter 13 - section 3
Chapter 14 - sections 1 & 2
Answers due at the end of the first week of the module:
1. Why is the atmosphere of Earth so different (chemically) than the atmospheres of Venus and Mars?
2. The big mushies (Jupiter, Saturn, Uranus, Neptune - and also Sun) are not quite spherical but slightly squashed. Why? (Hint: you answered a similar question when describing the formation of our Solar System.)
3. How can (or rather, how did) volcanoes affect the evolution of Earth's oceans and atmosphere?
4. What is the heat balance like on Jupiter? (Where does the energy come from and where does it end up?)
1. Read assigned Readings.
2. Contribute to the Main Discussion & your Team Conference
3. Submit answers to study questions by end of first week.
4. Submit homework Individually by the end of the second week. It should contain:
1- What's the source of the atmospheres of Mars, Earth, and Venus. How does this source differ from the source of the Jovian planets?
2 - How do storms on Earth compare to storms on the Jovian giants?
3 - What determines the wind patterns on a world? (For example, Earth has 6 major wind bands - 3 per hemisphere - while Jupiter has many - and other worlds have - well, you tell us what your world has.)
Montana State University
last updated 4/7/02