Physics 320 Lecture 19: Planetary Ring Systems

1. Physics 320 Lecture 19: Planetary Ring Systems Bin Chen NJITPhysics Department

2. Saturn’s Rings Io A short video about this image: https://www.youtube.com/watch?v=KuTukEleqvo True-color photo from Cassini in 2013

3. Galileo’s Discovery • In 1610, Galileo wrote a letter to the Duke of Tuscany: “…planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another.” • In 1612, Galileo observed Saturn again, and found the rings appeared to vanish. Mystified, Galileo wondered, "has Saturn swallowed his children?"

4. A brief history • 1610: first discovery by Galileo. • 1655: Christiaan Huygens became the first person to suggest that Saturn was surrounded by a ring. Huygens observed Saturn and wrote that “It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic”. • In 1675, Giovanni Cassinidetermined that it was composed of multiple smaller rings with gaps between them, the largest of which was later named the ”Cassini Division”. • In 1859, James Maxwell proposed that the rings must be composed of numerous small particles, which were later proven to be correct using spectroscopic observations. • 1970s–present: the space age. Pioneer 11 discovered the F ring in 1979. Voyager discovered the G ring. Cassini entered the orbit of Saturn in 2004 and is still observing (until mid 2017), which provided the most detailed images to date.

5. Structure of Saturn’s Rings

6. Saturn’s rings: physical properties • Consist of numerous small particles from ~1 cm to ~10 m • Composed of mostly water ice (>99%) • Clumpy and dynamic Artist concept of close-up view of Saturn’s ring particles (NASA/JPL)

7. Saturn’s rings are ridiculously thin! • When viewed edge-on, they practically disappear! • Saturn’s rings are 250,000 km wide, but only a few x 10 m thick in some places. A sheet of paper the size of San Francisco would have about the same ratio of width to depth. • A letter-sized paper is 100–10,000 x fatter than Saturn’s rings!

8. Why are Saturn’s rings so thin? • Particles with tilted orbits collide with each other inelastically, losing kinetic energy and angular momentum. • Orbits flatten out due to these collisions. • This continues to occur until the particles spread very thin to a few times the size of the largest particles. Artist concept of close-up view of Saturn’s ring particles (NASA/JPL)

9. Other phenomena of Saturn’s rings Pan, Encke gap, and edge waves Dark Spokes

10. Saturn’s Shepherd Moon Prometheus

11. Interactions between Saturn’s moons and rings Prometheus enters Saturn’s F ring

12. To recap: Phil Plait’s Crash Course on Saturn • https://www.youtube.com/watch?v=E8GNde5nCSg

13. Jupiter’s tenuous ring system Jupiter and its ring in infrared Galileo images (NASA/JPL)

14. Neptune’s rings

15. The rings of Uranus