Scholars Online Astronomy - Chapter 13: Jupiter and Saturn's Moons
Reading: Astronomy, Chapter 13: Jupiter and Saturn's Moons of Ice and Fire
Study Notes: notes on your assigned reading from the text
13.1 The "Galilean" satellites are the four largest moons of Jupiter: Io, Europa, Ganymede, Callisto (see comparison charts below), which orbit Jupiter with rotation:revolution (spin-orbit 1:1) periods of 1.8 days to 16.7 days. The inner satellites has a 1:2:4 ratio for their periods. Occultations of the moons with Jupiter and each other were used to establish the lunar diameters.
13.2 Detailed observation of the moons comes from Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and Galileo. Using data on how near encounters altered the trajectory of the satellites, astrononers determined the densities of the moons and proposed models for their composition. Ganymede and Callisto have densities less than 2000 kg/m3, so are thought to be a mix of rock and ice. Io and Europa are both around 3000-3600 kg/m3, similar to Earth's density, and are thought to be primarily rock.
13.3 The variation in density and composition can be explained using a model of gravitational collapse around Jupiter similar to that around the Sun. In both cases, the center of the collapse was warmer than the surrounding space, so lighter elements and ice would evaporate near the center and be found only on the more distant planets from the sun, or more distant moons from Jupiter.
13.4 Io has no impact craters. Its elongation along an axis through its equator causes it to "nod" or wobble as it moves on an elliptical orbit around Jupiter. Counter forces from tidal drag by Ganymede and Europa keep it in 1:1 spin-orbit coupling. This gives rise to volcanic activity in two forms: geysers erupting and ejecting sulfur and sulfur dioxide, and slower ultramfic (enhanced with magnesium and iron) lava flows.
13.5 Emissions from Io's volcanoes escape the moon and collect in a torus, orbiting Jupiter independently but along the same orbital path as Io. Jupiter's magnetic field interacts with the gases in the torus, creating an electric current flow from Jupiter through flux tubes to the Io torus. This field is not strong enough to account for Io's magnetosphere, suggesting that it has its own dynamo.
13.6 Europa's surface is a layer of cracked ice, possibly floating above a liquid ocean, with brown debris, possible from recent impacts or from upwelling of salt or minerals dissolved in interior ocean. Its mass indicates a large rocky core. Markings on surface ice possibly result of crustal expansion and shrinking due to tidal forces. May have induced magnetic field from interaction with Jupiter's field.
13.7 Ganymede has two kinds of terrain, dark terrain marked by many craters and furrows, and light terrain marked by fewer craters. Its surface structures are puzzling, given the lack of heat sources (too small for radioactivity to generate enough heat for geological activity, too far from Jupiter for tidal stresses to generate heat.) Its internal heat (indicated by the presence of a magnetic field and the conclusion that there must be a molten metallic layer or core to serve as a dynamo) may be the result of changes to its orbit in the past.
In contrast, Callisto is heavily cratered with larger craters. It appears undifferentiated, but response to Jupiter's magnetic field by producing an induced magnetic field of its own indicates the presence of liquid, which could only happen if the interior is hotter than expected from structural models, density, or solar energy available.
13.8 Saturn's moon Titan is the only Solar System moon with a significant atmosphere, one of heavy gas molecules which it retains through its own gravitational attraction. Nitrogen in Titan's atmosphere is retained when ammonia (NH3) outgassed from below the surface breaks down under UV radiation; the hydrogen escapes. Titan appears to still have some slight methane rainfall, and ethane lakes near the north pole (where it is winter), indicating a possible migration of ethane similar to the seasonal migration of CO2 on Mars. Surface features include "sand" dunes of ice/polymer and volcanoes. Heat for evident volcanic activity is likely the result of radioactivity in the core, where radioactive isotopes are in significant concentration. Multiple series of methane outgassing events accounts for current levels of methane.
13.9 Jupiter's satellites include much smaller, non-spherical rocky objects. The four major ones of this group are Metis, Adrastea, Amalthea, and Thebe, which all orbit Jupiter inside the orbit of Io, and appear to be captured asteroids. A cloud of over 55 other asteroid-type moons orbit Jupiter beyond the orbit of Callisto; 48 of these have retrograde orbits.
13.10 Besides the "terrestrial world" Titan, Saturn has six moderate-sized satellites large enough to be spherical under their own mass, and over fifty small, asteroid-type satellites. Those with prograde orbits in or near the rings appear to be aggregation of ring materials. The remainder by and large have retrograde orbits and appear to be captured asteroids. The six middle-size moons all have unique surface features that challenge current models for the formation of such objects. Enceladus has volcanoes.
Key Formulae to Know
- There are no new key formulae for this chapter.
Read the following weblecture before chat: The Moons of the Gas Giants (including Neptune and Uranus)
- Moons of Jupiter: Set your planetarium program to observe Jupiter from Earth at the current time, and adjust it so that Jupiter will stay centered in your field of view as time passes, then increase magnification until you can see all four Galilean moons. [You may want to remove the stars from your view so that you can focus on the moons.] Make a drawing of their positions. With a time increase of two hours, step through four days (96 hours), drawing the locations of the moons relative to Jupiter for each interval. Use this information to determine the period of each moon. Using Kepler's laws, determine which moons are furthers and whichclosets to the planet. Do the moons pass in front of Jupiter? Disappear behind Jupiter?
- Moons of Saturn: Set your planetarium program to observe Saturn from Earth at the current time, and adjust it so that Saturn will stay centered in your field of view as time passes, then increase magnification until you can see Titan and at least two other Saturn moons. Step through time at two-hour intervals. How does the motion of Saturn's moons compare to the motion of Jupiter's moons?
- From the Moons: If your planetarium supports this, view Jupiter from Io and Ganymede, then view Saturn from Titan and Enceladus over several hours. Compare the motions of the planet as seen from each satellite. Why are there differences in the speed and direction of motion?
- Orbits: If your planetarium permits, use it to draw the orbits of Titan and at least three of Saturn's smaller moons. How do these orbits differ? Why is there such a radical difference in orbits?
Optional Websites:Visit NASA's Saturn website for information about the Cassini and Huygens missions.
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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