Class 10 - Giant Planets I
Reading: The text is pretty skimpy on the giant planets - Chapter 8 of Hartmann pp 191- 195 and pp230-231 (1 whole page out of 428 on the giant planet interiors - 99.5% of the mass of the solar system!). So, I have a handout reading (pick up in class).
Before we delve into the details of our vast knowledge of the giant planets, let's begin by organizing knowledge (or potential knowledge) by generating a HEIRARCHICAL list of questions we might want to ask - starting with the simplest questions and moving down to more complex questions. In the class we generated a list (well, actually, a chaotic mess on the blackboard) by working in randomly-generated groups. Here, I will just give you my list (based on the class' response and the list I made before class).
Level 0 - names!
Level 1 - What are the basic, gross, physical properties of the planet? Size, location, orbit (6-orbital elements), shape (oblateness), mass (with size ->bulk density), spin rate and tilt, magnetic field (Y/N), Albedo, satellites, etc; Comparisons - are there systematic differences/similarities?
Familiarize yourself with JSUN....
| Planet | Orbital Distance (in AU) | Mass (Relative to Earth, in ME) | Radius (Relative to Earth, in RE) | Density (Relative to Water, in g/cm3) | Rotation or Spin Period (hours) | Tilt(tilt) | Moons | Composition |
| Jupiter | 5.20 | 317 | 11.2 | 1.33 | 9.8 | 3.1 | 63 | Mostly Hydrogen & Helium |
| Saturn | 9.53 | 90 | 9.4 | 0.70 | 10.6 | 26.7 | 33 | Mostly Hydrogen & Helium |
| Uranus | 19.2 | 14 | 4.11 | 1.32 | -17.3 | 97.9 | 27 | H, He, Hydrogen compounds & rocks |
| Neptune | 30.1 | 17 | 3.92 | 1.64 | 16.1 | 28.8 | 13 | H, He, Hydrogen compounds & rocks |
Level 1.5 - HOW do we measure these properties?
Level 2 - What are the properties of the observable "surface" properties? ("surface" really means depth of clouds, atmosphere at which light is reflected) i.e. the atmospheric properties - composition, temperature, weather - we are going to totally ignore these since they are covered in the companion course (ASTR 3720 Planetary Atmospheres).
Level 3 - What is the basic structure of the interior? Using the simplest assumptions and the basic observables, what can we learn about the interiors? e.g. pressure, density, temperature - we will discuss this in the next few classes.
Level 4 - What are missing from these basic models? Existance of a core? Hydrogen phase transition? Helium rain? Water/Ammonia/Methane electrical conductivity? Convection / dynamics? Radiative transfer - i.e. opacity effects? Magnetic dynamo?
Level 5 - What is the origin and evolution of the Giant Planets? Astronomy 101 picture - GPs vs TPs, the snow line, cosmic abundances, etc.
Level 6 - What's missing from the basic description of evolution? Did the hydrogen capture happen during or after the ice-core accretion? How much gravitational settling occured after accretion? How much material (from comets?) was added afterwards? When did the satellites form? What were the giant planets' roles in the subsequent / present re-arranging of material in the solar system?
Additional issues to consider:
- Magnetic fields - description, generation mechanism, magnetospheres
- Isotope ratios
- Heat fluxes - particularly Uranus (vs. Neptune)
- Degrees of internal mixing
- Uranus' tilt, Neptune's very low eccentricity
- How far down this list is our current understanding?