Scholars Online Astronomy - Chapter 14: Uranus, Neptune, Pluto, and the Kuiper Belt: Remote Worlds
Reading: Astronomy, Chapter 14: Uranus, Neptune, Pluto and the Kuiper Belt: Remote Worlds
Study Notes: notes on your assigned reading from the text
14.1 Uranus is actually visible by the naked eye, but confirming that it is a moving planet with a predictable orbit takes telescopic observations, which were made by William Herschel in 1957. There's a fun (very small) movie with the announcement to the British Royal Society. Uranus' orbit was disturbed by some massive object; Jeanne Laverrier and John Couch Adams were able to predict the likely location of the source of the perturbation. For some insight into the work they did, take a look at the Mathematical discovery of planets. Adams identified an object not already on the star charts on 23 September 1846.
14.2 Uranus' axis of rotation is tilted so far over that it is actually rotating "backwards". Some of the diagrams at SolStation Uranus can help if you are having trouble with this concept.
Where Jupiter and Saturn have very similar compositions, the atmosphere of Uranus lacks ammonia, probably due to difference in temperature. Uranus is cold enough that ammonia freezes and precipitate out of the atmosphere. Lack of heat from an internal thermal energy source results in Uranus having a relatively uniform atmosphere with little bending or circulation. In other words, there is no heat create the convection currents that give rise to the storms we see on Jupiter and the banding on Saturn.
14.3 Neptune has features similar to Uranus in composition, temperature, and energy; but since its rotational axis is more conventional, and since it has an internal heat source due to continuing gravitational contraction, its atmospheric banding is similar to Saturn. new chemical composition of its atmosphere is similar to that of Uranus, with high concentrations of liquid and frozen methane.
14.4 Since Uranus and Neptune contain a higher proportion of heavy elements than Saturn and Neptune (and are denser), we need a model of how they formed to account for the difference. Two possibilities are:
- Uranus and Neptune partially formed closer to Saturn and Jupiter, then were pushed further out where they stopped collecting mass.
- Uranus and Neptune formed directly from the gases of the solar nebula.
14.5 Unlike Saturn and Jupiter, the magnetic fields of Uranus and Neptune have axes that are very different from the rotational axis of the planets. This Animation of Uranus Magnetic field will help show how the rotation axis differs from the magnetic field axis.
14.6 All gas giant planets have ring systems, but the rings of Jupiter, Uranus, and Neptune are small collections of dust-like particles to small snowballs only a few centimeter in diameter. Uranus' rings were discovered during an occultation, when they passed in front of another star. Neptune's rings were discovered by Voyager 2.
14.7 Both Uranus and Neptune have some large spherical moons, as well as small, irregular moons that are probably captured asteroids. The five moderate sized moons of Uranus are:
- Miranda: Hemispherical deformity; heavily cratered on one side, with ridges and resurfaced terrain, possibly due to cataclysmic event powered by tidal heating.
- Ariel: Cracked, with superficial ice lava flows, possibly due to tidal heating.
- Umbriel: Geologically dead, with many impact craters.
- Titania: Cracked, with superficial ice lava flows, possibly due to tidal heating..
- Oberon: Geologically dead, with many impact craters.
The rest of Uranus' moons appear to be captured asteroids. Those within the orbit of Miranda have prograde orbits; those beyond the orbit of Oberon have retrograde orbits, all except one. This pattern would appear to indicate that it is easier for planets to pass an asteroid and capture it as retrograde when it is further from the planet's center.
14.8 Neptune has three main moons, only one of which (Triton) is large, and many smaller moons which appear to be captured asteroids. Triton has a severe orbital inclination but a relatively circular retrograde orbit, so that Neptune is tidal forces cause Triton to slow down. It will gradually spiral in toward the planet and reach Neptune's Roche limit after which it will disintegrate. Triton is otherwise remarkable, as it is the third moon in the solar system (after Io and Titan) to have an atmosphere.
14.9 We include Pluto here in our survey, even though recent deliberations by the IAU (International Astronomical Union) have reclassified Pluto as a dwarf planet, rather than a planet. By definition, pilots must have enough mass to collapse to a sphere under their own gravity (which Pluto does), must be in an orbit around the sun and not some other body (which Pluto is), and must have swept through their neighborhood free of debris (which Pluto has not). Pluto has a companion "Moon" which is nearly half its size; some astronomers consider this a double planet, rather than a planet (Pluto) with its moon (Charon).
14.10 The discovery of Pluto and the search for other objects in the same area of the sky has led to the realization that there are many thousands of objects beyond Neptune. These objects belong to the Kuiper belt and include not only Pluto and Charon, but also Eris (larger than Pluto), Objects 2005 FY9 [now called Makemake] and 2003 EL61 [now called Haumea], Sedna, and Quaoar. These objects all appear to be massive enough to have formed spheres while collapsing under their own weight. Several (Eris and 2003 EL61) have moons of their own.
Key Formulae to Know
- There are no key formulae for this chapter.
Read the following weblecture before chat: Moon Systems, Rings
- Moons of Uranus: Set your planetarium program to observe Uranus from Earth at the current time, and adjust it so that Uranus will stay centered in your field of view as time passes, then increase magnification until you can see the five primary moons of Uranus. Set the dates to 1986 the advance rate at 5 minutes per step and watch the moons for at least 48 hours. [You may want to remove the stars from your view so that you can focus on the moons.] Then set the time for 2007 and watch the moons. How do the orbits differ? Do all the moons' orbits lie in the same plane?
- Neptune: Make similar observations of Neptunes moons Triton and Nereid over time, sing the current time and a 1 hour flow interval. The the two moons orbit Neptune in the smae direction? How does the shape of Triton's orbit differ from the shape of Nereid's orbit?
- Pluto: Observe Pluto and Charon from a near-Pluto location. How does Charon rotate relative to its revolution around Pluto? How does Pluto rotate relative to this orbit?
Optional Websites:Explore the SETI Planetary Rings Node, part of JPL's planetary data system. This website allows scientists to gain access to data on planetary rings and moons to carry out their own research.
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Go over the list of Key Words and Key Ideas at the end of the chapter. If you don't remember the definition of the key word, review its use (the page number on which it is explained is given).
- Read through the Review Questions and be prepared to discuss them in class. If any of them confuses you, ask about it!
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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