Planetary Rings (cont.) + Galilean Satellites

1. Planetary Rings (cont.) + Galilean Satellites Rings / Galilean Satellites 4/10/07

2. Announcements • Reading Assignment • Finish Chapter 15 • 5th homework due Thursday. • Reminder about term paper – due April 17. • A sample “planet fact sheet” has been posted on the website • Details of turnitin.com • Go to www.turnit.com • Click on “new users” • Usertype  student • Class ID: 1868418 • Password: Section2 • Mission Updates? • Next study-group session is tomorrow, from 10:30AM-12:00Noon – in room 330. Rings / Galilean Satellites 4/10/07

3. All Rings Have Gaps:Why? Rings / Galilean Satellites 4/10/07

4. Rings, Gaps, and Resonances • Gaps can occur in rings because the positions of particles in that ring are in a resonance with the planet and a moon • Example: The Cassini division is located at a point that is in a 2:1 resonance with Saturn’s moon Mimas • Note that it is not completely devoid of particles!! The Cassini division between Saturn’s A and B rings Rings / Galilean Satellites 4/10/07

5. The gaps in rings are similar to Kirkwood gaps • Gaps in the number of asteroids in the asteroid belt • We will discuss this more in two weeks Distribution of asteroid periods showing gaps, like in planetary rings Rings / Galilean Satellites 4/10/07

6. Cassini Found Density Waves in Saturn’s Rings Cassini images of spiral density waves in Saturn’s Rings Rings / Galilean Satellites 4/10/07

7. Shepherd moons • Small moons can also act to “shepherd” the ring particles (by gravity) into certain locations. • Such moons are known as shepherd moons • Saturn may have many undiscovered shepherd satellites Saturn’s faint F ring, which is just outside the A ring, is kept narrow by the gravitational pull of shepherd satellites Rings / Galilean Satellites 4/10/07

8. Moons can even orbit a planet within one of the gaps in the rings, as in the case of Saturn’s moon Pan which orbits within the Encke gap Rings / Galilean Satellites 4/10/07

9. Rings / Galilean Satellites 4/10/07

10. Different parts of the Rings have different colors • Different ring colors are caused by light scattering off of different sized particles • Light scattering depends on the size and composition of the particles in the ring The outer ring of Uranus has a moon embedded in it. The moon sweeps up larger pieces of material, leaving dust. Dust more effectively scatters blue light Rings / Galilean Satellites 4/10/07

11. The Color of Saturn’s Rings reveal that the ring particles do not migrate outside of their orbits Cassini image Natural color Voyager 1 image Colors are strongly enhanced Rings / Galilean Satellites 4/10/07

12. The Roche Limit • Particles in a ring never coalesce to form a moon. • Planetary rings are within the Roche limit of the planet. • Tidal forces are greater than the gravitational forces among the ring particles • Question: why don’t the ring particles (chunks of ice and rock) break apart? Rings / Galilean Satellites 4/10/07

13. Rings Are Probably Short-Lived • Gravity of the Sun and planet’s satellites constantly disturbs orbits of particles • Collisions probably happen frequently • Particles should drift out of the ring • Rings around outer planets have probably come and gone repeatedly during the history of Solar System Rings / Galilean Satellites 4/10/07

14. Saturn’s Rings • Most spectacular • Most massive (by 105) • Brightest • albedo of 0.8 • The planet itself has an albedo of 0.5 • Probably composed of ices • Youngest? • Sizes of the particles range from micrometers to a about ten meters • Besides gaps, get spokes, waves Rings / Galilean Satellites 4/10/07

15. Earth-based observations reveal three broad ringsencircling Saturn Rings / Galilean Satellites 4/10/07

16. Spokes in Saturn’s Rings • Voyager I found changing dark structures (directed outward) in the rings. • given the term "spokes" • They rotate with the magnetic field • It has been proposed that electrostatic repulsion between ring particles may play a role. Rings / Galilean Satellites 4/10/07

17. Rings / Galilean Satellites 4/10/07

18. Jupiter and Saturn’s Moons Rings / Galilean Satellites 4/10/07

19. Galilean Satellites Rings / Galilean Satellites 4/10/07

20. Earth-based views • Easily seen through a small telescope • Even binoculars • Their movement can be seen throughout the night • They make shadows on Jupiter when they transit • They can also disappear behind Jupiter during an occulation Rings / Galilean Satellites 4/10/07

21. Made by amateur -- Antonio Cidado (the movie was made using individual frames taken during a single night’s observing) Rings / Galilean Satellites 4/10/07

22. Galilean Satellites and Kepler’s Laws • Jupiter’s moons obey Kepler’s laws, just as the planets do in orbit about the Sun. • The orbital period is related to the semi-major axis (Newton’s form of Kepler’s 3rd law) • Note that this is slightly DIFFERENT in form from what we used for the planets (why?) Rings / Galilean Satellites 4/10/07

23. Orbital Resonances among Galilean Satelites • Io, Europa, and Ganymede are in an orbital resonance such their orbital periods are in the proportion 1:2:4 • Io revolves around Jupiter twice for every complete orbit of Europa • Europa revolves around Jupiter twice for every complete orbit of Ganymede • Io completes 4 orbits in this time Rings / Galilean Satellites 4/10/07

24. Jupiter’s Satellites Rings / Galilean Satellites 4/10/07

25. You may notice … • Io’s orbital period: 1.769 days • Europa’s orbital period: 3.551 days but … 3.551/1.769 = 2.00735 ??? (not 2) We know that the orbital resonance is exactly 1:2. So what is the explanation for this apparent discrepancy? (The answer is subtle and you are not required to know it) Rings / Galilean Satellites 4/10/07

26. Jupiter’s Satellites Rings / Galilean Satellites 4/10/07

27. Solar-system in Miniature ? • The density of the Galilean satellites decreases with distance from Jupiter • This is analogous to planets in the solar system • Close to the Sun are the Terrestrial planets (high density) • Farther away are the Gas Giants (low density) Planet densities Rings / Galilean Satellites 4/10/07

28. The Galilean satellites probably formed in a similar fashion to our solar system but on a smaller scale Rings / Galilean Satellites 4/10/07