Lecture #31: Satellites and Rings II Ring Systems Overview of rings: Jupiter, Saturn, Uranus, Neptune. What are rings made of? How do they form? Reading: Chapter 11.3. The Main Point All of the giant planets have rings, which are just billions of small dusty, rocky, or icy moons traveling in similar orbits around a planet according to Kepler s laws. Astro 102/104 1 Astro 102/104 2 Overview All four of the giant planets have rings. Each ring system consists of billions of small "moonlets" orbiting close to their planet. Ring systems exhibit complex structures and patterns caused by interactions with larger moons. Properties of Ring Systems Outer Radius Mass Reflectivity Planet (km) Rplanet (kg) (%) Jupiter 128,000 1.8 ~10 10 ~5 Saturn 140,000 2.3 10 19 60 Uranus 51,000 2.2 10 14 ~5 Neptune 63,000 2.5 10 12 ~5 Astro 102/104 3 Planetary Ring Systems Astro 102/104 4 1
What are Rings? Billions of moons! marble-sized to house-sized (Saturn) dust-sized (Jupiter, Uranus, Neptune) Rings are collections of huge numbers of small bodies, each moving on its own, independent, Keplerian orbit. Because of Kepler's Third Law, inner ring particles must move faster than outer ring particles (differential rotation): - Rings don't rotate as a solid body. Particles collide & interact gravitationally with each other: - Waves and other phenomena are generated in the process. Astro 102/104 5 Rings and Tides Rings exist inside the "tidal stability limit" of a planet. Within this limit, known as the Roche limit, a planet's gravity should break up satellites. Only very small moons and rings exist within this zone. Cloud Tops Each planet's ring system drawn to a different scale. Astro 102/104 6 Where Do Rings Come From? Small impacts on small moons create dust-sized ring particles that disperse into an orbiting disk. Occasional larger impacts on small moons can shatter the moon and release many boulder-sized ed ring particles. Astro 102/104 7 Saturn's Rings One of the most beautiful sights in the solar system! Saturn's Ring System Outer Edge Width Ring (km) Rplanet (km) F 140,180 2.324 90 A 136,780 2.227 14,600 Cassini Division 122,170 2.025 4,590 B 117,580 1.949 25,580 C 92,000 1.525 17,490 CB AF Cassini Division Astro 102/104 8 2
There are actually thousands of rings and gaps. Gaps can be cleared by so-called gap moons orbiting within the rings. Other gaps are due to orbital resonances with small inner moons. Two nearby gap moons can shepherd a narrow ring. Some rings are dense & bright, others thin & "Spokes" seen in the rings transparent. are thought to result from interactions between ring Astro 102/104 particles and Saturn s 9 magnetosphere. Cassini Images of Saturn s Rings Rings in their natural color Cassini s s first image of the F ring False color image of two density waves in Saturn's Astro 102/104 10 A ring Rings are Thin! Saturn's rings are about 6 times wider than Earth. But they are less than 100 meters thick! Sometimes the rings appear edge-on from Earth. Saturn "ring plane crossing" observed with HST in 1996. Astro 102/104 11 Jupiter's Rings Discovered from infrared telescopes and Voyager 1. Very different from Saturn's! Very dark, very small particles. Three main components: Halo Broad, faint inner torus. Main Ring 7000 km wide. Edge radius is 129,000 km. Gossamer Rings Broad, faint outer disks. Dust particles from the small inner satellites of Jupiter appear to be the source of ring material. Astro 102/104 13 3
How were Jupiter s rings observed? Jupiter's rings are very dark and very faint. Hard to see against the glare of Jupiter itself. The best images were obtained when the Galileo spacecraft flew into Jupiter's shadow and looked back towards the Sun Amazing navigation feat! Astro 102/104 14 Light Scattering Observing rings at different viewing geometries provides a way to determine the sizes of ring particles. Astronomers have also used radar to estimate the sizes of ring particles. Confirmed by Voyager radio occultation data. Astro 102/104 15 Uranian Rings 10 main narrow rings, 1 diffuse ring. Ring Distance (km) Width (km) ------- ------------- ---------- 1986U2R 38000 2,500 6 41840 1-3 5 42230 2-3 4 42580 2-3 Alpha 44720 7-12 Beta 45670 7-12 Eta 47190 0-2 Gamma 47630 1-4 Delta 48290 3-9 1986U1R 50020 1-2 Epsilon 51140 20-100 Astro 102/104 16 (distance is from Uranus' center to the ring's inner edge) Neptune's Rings Distance Width Ring (km) (km) Name ------- -------- ----- ------- Diffuse 41900 15 Galle Inner 53200 15 LeVerrier Plateau 53200 5800 Lassell, Arago Main 62930 < 50 Adams (distance is from Neptune's center to the ring's inner edge) 3 main narrow rings, 1 diffuse. Main rings appear clumpy : Once thought to be arcs, not rings. Voyager showed that they are rings. Astro 102/104 17 4
Keeping it all together: Shepherds Small inner moons of the giant planets can gravitationally "shepherd" the ring particles, maintaining their stability. Pandora and Prometheus, Shepherding Saturn's tenuous F ring Two Uranian shepherds Astro 102/104 18 Ring Composition Saturn's rings are bright and slightly red: Spectra show evidence for water ice. Red component unknown... "dirt"? "organics"? Jupiter's, Uranus', and Neptune's rings are all very dark and gray: Not water ice? "Carbonaceous" material? Dark "organics"? Differences due to different ring origins? Astro 102/104 19 Summary All of the outer gas giant planets have ring systems. Saturn's rings are made up of billions of chunks of dirty ice, each obeying Kepler s laws. The rings of Jupiter, Uranus, and Neptune are composed of smaller (dust-sized) and much darker particles of unknown composition. Rings occupy regions within the tidal stability zones of their planets: Ongoing impacts on small moons create ring particles and explain why rings are common. Astro 102/104 20 Next Lecture... The Trans-Neptunian Region Pluto: Discovery. Physical characteristics. i Charon. Other Kuiper Belt Objects. Reading: Chapter 12.3 Astro 102/104 21 5