Scholars Online Astronomy - Chapter 12: The Moons of Jupiter and Saturn
Reading: Astronomy, Chapter 12: Jupiter and Saturn: Lords of the Planets
Study Notes: notes on your assigned reading from the text
- 12.1: Jupiter and Saturn are both superior, naked-eye planets. Earth-based observations without telescopes established their periods; once telescopes were available, astronomers were not only able to observe Jupiter's moons and Saturn's rings and moon Titan, but also establish the two planets' masses using Kepler's laws. When at opposition (closest to Earth and fully illuminated by the sun), Earth-based telescopes can observe bands, storms, and the shadow of its moons on Jupiter's upper atmosphere. Saturn has less dramatic banding, but the divisions of the rings were clearly visible in early telescope observations.
- 12.2: Because they are not solid material like the terrestrial planets, Jupiter and Saturn display differential rotation: segments of matter at different latitudes rotate with different angular speeds, more slowly at the poles than at the equator. Both planets have high speeds of rotation, around 10 hours for Jupiter, and around 10 hours, 15-30 minutes for Saturn. Since they are much larger than Earth, this means particles in their "surfaces" are moving with much higher linear velocities. Because hydrogen and helium spectral lines are prominent in UV light blocked by Earth's atmosophere, determining the composition of the two planets came only with flybys of Voyager spacecraft.
- 12.3: Our detailed information about both planets comes from Voyager 1 and 2 flyby missions (1970s-1980s) and ecent Cassini ang Galileo missions. These missions confirmed changes in the size and structure of the Great Red Spot, a giant high-pressure storm observed since the 1600s, which appears to rotate once every 6 days. Saturn has fewer storms, and these are generally less stable.
- 12.4: Currents in the planetary atmospheres are the result of covenction caused by internal core heat and by solar energy interacting with the high atmosphere. Using thermodynamic laws (heat always flows from hotter areas to colder areas), the conclusion from heat flow within Jupiter and Saturn requires that the cores be warmer than the upper atmosphere. It is important to realize that with the light zones, we see the tops of Jupiter's atmosphere, but with the dark belts we look into lower, warmer levels. Both Jupiter and Saturn have the same layered atomospheres: (NH3, NH4SH, H2, NH3 and H2, but the depths are different, due to Jupiter's higher temperature and greater mass.
- 12.5: The Galileo Probe descended into Jupiter's atmosphere in 1995, relaying information for several hours before going silent. Key findings were less water vapor than expected,possibly explained if the probe descended through a hot spot area. Jupiter has higher-than-solar amounts of carbon, nitrogen and sulfur, as expected if planitesimals and debris were pulled in by its mass. Unexplained are higher than expected amounts of argon, krypton, and xenon, challenging models of distributions of substances during the formation of the solar system.
- 12.6: Both planets are oblate: their equatorial diameters are greater than their polar diameters. The mass of the planet, rotational period, and distribution of atmosphere governs the degree of oblateness, so the core structure can be determined from these observables.
- 12.7: Radio telescope observations detect decimetric and synchrotorn radiation, a result of electrons moving in strong magnetic fields at near-light speeds. We assume the rocky cores of both planets are surrounded by liquid metallic hydrogen. The resulting magnetosphere for Jupiter is huge, capturing ionized particles from Io's eruptions and the solar wind to create plasma. Saturn's magnetosphere effect is smaller.
- 12.8: Earth-based telescopes detected rings around Saturn in 1655 (Christiaan Huygens), divided into three bands (Cassini 1675). Because of the tilt of the planet, we see the rings at different angles, sometimes from above or below, sometimes edge-on.
- 12.9: Ring particles are highly reflective ice or ice-coated rock, based on infrared spectrum measurements form Earth, Voyager, and Cassini spacecraft. Rings lie below the Roche limit, moons above the Roche limit. This limit defines the point at which tidal forces due to the planetary gravitational field would pull the near side more than the far side with sufficient force to break up the chemical bonds holding the material together.
- 12.10: Saturn's rings are actually many ringlets. Using light scattering analysis, astronomers can measure the size of particles in each ring. The outer, less dense rings appear to be outgassings from moons Io and Enceladus.
- 12.11: The ring structure is defined by interactions of the gravitational pull of Saturn and the gravitational pull of its many moons, which creates a resonance pattern. Moons like Mimas which move more quickly than the ring below them pull the ring apart (interaction between Mimas and Saturn makes it impossible for particles to remain within the area defined by the Cassini division). Moons like Pandora which move more slowly than the ring below them tend to drag on the ring particles and "herd" them into the ring.
Key Formulae to Know
- There are no key formulae for this chapter.
Read the following weblecture before chat: The Gas Giants and Planetary Atmospheres
Planetarium Program: Use the features of your planetarium program to view Jupiter and Saturn from space.
- View each planet from high above its north pole through two revolutions about the Sun, and note how it rotates.
- Explore the atomspheres of each planet. Identify any features you can find on one of the planets but not on the other.
Optional Website:Visit NASA's Voyager website for information about the current an continuing Voyager 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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