March 25 - April 7
During this module, students will examine the interiors of the terrestrial and jovian worlds of the solar system, and the interior of the Sun. Energy transport will be an important topic.
Introduction & Perspective
Even though the "big mushies" are fluid worlds, we'll take a look at some aspects of their interiors here, along with the interiors of solid-surface worlds.
When we look at layers inside solid worlds, we can look two main ways. If we look at composition, we generally see three layers: core, mantle, and crust. These occur when a world is massive enough and warm enough to differentiate - the heaviest material sinks to the middle, the least dense floats to the outside. (Differentiation also turns gravitational potential energy into heat, warming the interior.) For active worlds, we can determine layers based on structure rather than composition - in which case we'll get different layers. Unfortunately, they're similar and have sometimes the same names. For Earth, we get a solid inner core (solid despite high temperature due to enormous pressure), a fluid outer core, a mantle (eek, there's that same word used just a bit differently) that is essentially solid but undergoes very slow convection, a more fluid (plastic) aesthenosphere, and a fairly solid lithosphere. (If you want to understand solid-that-moves, a thick water + cornstarch mixture makes a good tactile model.)
The energy for the solid (or fluid) body of a world comes from formation (collisions and differentiation), radioactive decay (if there's enough metal), and tidal heating (if it's orbiting close to a massive body). The text leaves out the tidal effects when discussing terrestrial worlds; put it into your general scheme because there are more worlds than five. (Where does sunlight come in? It really only affects the surface and atmosphere of a world. The energy of the bulk of worlds acts at a much slower pace.) There are a number of ways energy gets from the interior of a world to the outside, where it radiates into space. These processes are probably familiar - conduction and convection. (I group "eruption" with convection in my mind.) "Radiation" is the other main kind of energy transfer, but it doesn't work very quickly inside an opaque object - the light just doesn't get very far. However, there's a thick layer of our Sun where radiation is still the main way energy gets out - and the light doesn't get very far before being absorbed, so it takes a very, very long time for the energy to get out. Where things are transparent, radiation tends to be a very fast way of transporting energy. If matter can move, convection is the next fastest form of energy transfer. Conduction is fairly slow, but works for solid bodies. Note that these forms of energy transfer are general - they don't apply just to interiors of solid worlds, but to the giant planets and Sun, and to atmospheres (M6), and to everyday life.
Magnetic fields in worlds can seem hard to understand - that may be because they aren't completely understood yet! However, the basic pattern is pretty clear - you only get a strong global magnetic field if there's some kind of fluid circulating in the interior of a world. This works for Jupiter and Sun as well as Earth. Mars, for a counterexample, has only bits and pieces of magnetic field in the surface rock now. A global magnetic field then interacts with gases around the world, and can protect the world against, say, changes in the solar wind (this is Sun's exosphere - M6 - but tied to magnetic
The Cosmic Perspective
Study Questions and Problems:
Answers due at the end of the first week of the module:
1. (a) What is differentiation? (b) Why is a large world more likely to differentiate than a small one? (c) What other properties, in addition to size, are likely to be important in determining whether (or how much) a world differentiates?
2. Of the sources of internal heat, which are most likely to be important for (a) a large terrestrial world, and (b) a medium icy moon orbiting a giant planet?
3. How do worlds get rid of the heat from their interiors? (a) Give several examples, preferably for the terrestrial planets. (b) Are some processes faster or more efficient than others? (If so, which?) (c) How do these relate to surface features?
4. Some meteorites are almost completely metal. Why do we think these came from (a) fairly large asteroids that were (b) pretty thoroughly destroyed?
Assigned Individual & Team Activities:
1. Read the assigned readings.
2. Contribute to the main discussion.
3. Contribute to your group's discussion.
4. Answer the study questions by the end of the first week.
5. Submit homework by the end of the second week. It should contain:
Possible Discussion Questions:
1. How do the magnetic fields of different planets compare?
2. How is the potential for life on a world effected by the presence or absence of a magnetic field?
last updated 3/20/02