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

Transcription

1 Planetary Rings (cont.) + Galilean Satellites

2 Announcements Reading Assignment Finish Chapter 15 5 th 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 Click on new users Usertype student Class ID: Password: Section2 Mission Updates? Next study-group session is tomorrow, from 10:30AM-12:00Noon in room 330.

3 All Rings Have Gaps: Why?

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

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

6 Cassini Found Density Waves in Saturn s Rings Cassini images of spiral density waves in Saturn s Rings

7 Small moons can also act to shepherd the ring particles (by gravity) into certain locations. Shepherd moons 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

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

9

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

11 The Color of Saturn s Rings reveal that the ring particles do not migrate outside of their orbits Voyager 1 image Colors are strongly enhanced Cassini image Natural color

12 Particles in a ring never coalesce to form a moon. Planetary rings are within the Roche limit of the planet. The Roche Limit 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?

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

14 Most spectacular Most massive (by 10 5 ) 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 Saturn s Rings Besides gaps, get spokes, waves

15 Earth-based observations reveal three broad rings encircling Saturn

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.

17

18 Jupiter and Saturn s Moons

19 Galilean Satellites

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

21 Made by amateur -- Antonio Cidado (the movie was made using individual frames taken during a single night s observing)

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 3 rd law) Note that this is slightly DIFFERENT in form from what we used for the planets (why?)

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

24 Jupiter s Satellites Io Europa Ganymede Callisto Distance (J. radii) Period (days) Mass (Lunar) Density (g/cc)

25 You may notice Io s orbital period: days Europa s orbital period: days but 3.551/1.769 = ??? (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)

26 Jupiter s Satellites Io Europa Ganymede Callisto Distance (J. radii) Period (days) Mass (Lunar) Density (g/cc)

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

28 The Galilean satellites probably formed in a similar fashion to our solar system but on a smaller scale