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1 Lecture 15 Moons and Rings of the Outer Planets January 7d, 2014 2 Rings of Outer Planets • Rings are not solid but are fragments of material – Saturn: Ice and ice-coated rock (bright) – Others: Dusty ice, rocky material (dark) • Very thin – Saturn rings ~2 km thick. • Rings can have many gaps due to small satellites . – Saturn and Uranus 3 Rings of Jupiter •Very thin and made of small, dark particles. 4 Rings of Saturn Flash movie 5 Saturn’s Rings Ring structure in natural color, photographed by Cassini probe July 23, 2004. Click on image for Astronomy Picture of the Day site, or here for JPL information 6 Saturn’s Rings (false color) Photo taken by Voyager 2 on August 17, 1981. Click on image for more information 7 Newly Discovered Saturnian Ring • Nearly invisible ring in the plane of the moon Pheobe’s orbit, tilted 27° from Saturn’s equatorial plane • Discovered by the infrared Spitzer Space Telescope and announced 6 October 2009 • Extends from 128 to 207 Saturnian radii and is about 40 radii thick • Contributes to the two-tone coloring of the moon Iapetus • Click here for more info about the artist’s rendering 8 Rings of Uranus • Uranus -- rings discovered through stellar occultation – Rings block light from star as Uranus moves by. 9 Rings of Uranus Shepherd satellites keep rings thin by means of small gravitational forces. • Inner moon: tugs forward on ring particles, boosting them to larger orbit • Outer moon: tugs backward on ring particles, slowing them to smaller orbit. 10 Rings of Neptune 11 How are Rings Formed? • Tidal forces increase the closer one gets to the planet. • If the moon is too close, tidal force can break apart moon • Roche limit = distance from planet where tidal forces are equal to internal forces – Rings form inside Roche limit – Moons form outside Roche limit – Only small objects can form inside Roche limit 12 Roche Limit 13 How Old are Saturn’s Rings? • Rings are likely recent. • Modeling shows rings of Saturn cannot remain stable for billions of years. • Likely that a small moon or comet strayed into Roche limit of Saturn and was destroyed ~50 million years ago. 14 What is the Roche limit? A. The mass a planet must exceed to have satellites. B. The minimum mass for a planet to be composed of hydrogen. C. The maximum orbit distance for a moon before being captured by the Sun’s gravity. D. The distance at which a moon held together by gravity will be broken apart by tidal forces. E. The maximum distance astronomers can see into a planet’s clouds. 15 What is the Roche limit? A. The mass a planet must exceed to have satellites. B. The minimum mass for a planet to be composed of hydrogen. C. The maximum orbit distance for a moon before being captured by the Sun’s gravity. D. The distance at which a moon held together by gravity will be broken apart by tidal forces. E. The maximum distance astronomers can see into a planet’s clouds. 16 Moons of Jupiter • 63 known moons • 4 large moons -- Galilean Moons – Provide look at mini-solar system – Density decreases outward. (3570-1865 kg/m3) – All have synchronous rotation 17 Basic Information on Galilean Moons Moon Radius 1815 km Io 1.04 Rmoon 1569 km Europa 0.9 R moon 2631 km Ganymede 1.5 R moon 2400 km Callisto 1.4 R moon Mass 1.22 Mmoon Density (kg/m3) 3570 Dist (M km) 0.4 0.65 Mmoon 2967 0.7 2.01 Mmoon 1940 1 1.47 Mmoon 1865 1.8 18 Io • Densest • Sulfur compounds give surface orange color. • Extremely volcanically active • Few craters • Why so active? – Orbit not circular but elliptical. – tidal forces continually changing, flexing interior of Io. – Flexing produces large amounts of heat, melting the interior. 19 Volcanic Plumes on Io - Galileo photojournal.jpl.nasa.gov/catalog/PIA01652 20 Eruption at Tvashtar Catena, Io Galileo photojournal.jpl.nasa.gov/catalog/PIA02545 21 Europa • Smooth, icy surface with cracks similar to polar regions on Earth. • Few craters. • Thin, oxygen atmosphere • Tidal forces acting on Europa may have melted water under ice creating liquid ocean under the surface ice. • Rocky core (from density). photojournal.jpl.nasa.gov/catalog/PIA00502 22 Europa - Galileo photojournal.jpl.nasa.gov/catalog/PIA00291 23 Ganymede • Largest satellite (larger than Mercury or Pluto) • Low density = mainly ice. • Many craters. • Surface is dust-covered ice. • Grooves and ridges may be due to crustal motion • Magnetic field • Possible sub-surface ocean photojournal.jpl.nasa.gov/catalog/PIA00716 24 Grooved Terrain on Ganymede Galileo photojournal.jpl.nasa.gov/catalog/PIA01616 photojournal.jpl.nasa.gov/catalog/PIA01611 25 Callisto • Many craters • No recent geologic activity. • Frozen ice and some rock. photojournal.jpl.nasa.gov/catalog/PIA03456 • Magnetic field • Possible sub-surface ocean. photojournal.jpl.nasa.gov/catalog/PIA00745 26 Possible Interiors of the Galilean Moons Europa Io Ganymede Callisto photojournal.jpl.nasa.gov/catalog/PIA01082 27 Comparison of Gallilean Satellites 28 What energy source powers Io’s active volcanoes? Chemical reactions Tidal forces Roche energy Thermonuclear reactions 0 of 78 ct io ar re a ee m on uc le Ro ch 0% ns ne rg y 0% Th er Ti da l fo rc io ns lr ea ct ica 0% es 0% Ch em A. B. C. D. 90 29 What is Callisto’s orbit speed if its orbit semi-major axis is 1.88×106 km and its orbit period is 16.7 days? A. 1.01 km/s B. 8.19 km/s C. 8.88 km/s D. 2,403 km/s 30 What is Callisto’s orbit speed if its orbit semi-major axis is 1.88×106 km and its orbit period is 16.7 days? A. 1.01 km/s B. 8.19 km/s C. 8.88 km/s D. 2,403 km/s 2 a v P 6 2 1.88 10 km 16.7 d 86400 s/d v 8.19 km/s 31 Moons of Saturn • • • • 61 known moons Most are small and irregular. Some spherical moons. Largest moon = Titan 32 Missions to Saturn • Voyager 1 and 2 • Cassini / Huygens 33 Titan • Largest of 61 moons of Saturn • Second largest moon in Solar System. • Dense Nitrogen atmosphere. • Featureless clouds and purple-blue haze. • Organic compounds found in atmosphere Image courtesy of NASA. Click on image for more information. 34 Basic Information on Titan • Mass = 1.8 Mmoon • Radius = 2575 km (1.48 Rmoon) • Density = 1880 kg/m3) • Distance = 1.2 million km 35 Cassini - Huygens 37 Methane Oceans and Streams on Titan Best guess of landing spot Mosaic of Huygens Images Interactive Figure http://www.esa.int/SPECIALS/Cassini-Huygens/SEMC8Q71Y3E_0.html 38 Mimas Image taken by Cassini spacecraft August 2, 2005. Click here for more information 39 Iapetus These two global images of Iapetus show the extreme brightness dichotomy on the surface of Iapetus. Images taken by Cassini spacecraft December 27, 2004 (left panel) and September 10, 2007. Click here for more information. 40 Moons of Uranus • • • • 27 known moons Most smaller, mainly made of ice Heavily cratered, dirty surface Some have cracks due to possible past activity (Miranda) 41 Titania and Oberon (Uranus) Images courtesy of NASA. Click on images for more information 42 Miranda One of 27 moons of Uranus Although smallest among the five main satellites of Uranus, Miranda’s bizarre surface makes it the most interesting and memorable. Astronomers can only guess at the violent events that led to the unusual wrinkles and bands. 43 Triton • One of 13 known moons of Neptune • Orbits retrograde (possibly captured?) • Thin Nitrogen atmosphere • Water ice surface • Jets of Nitrogen erupting from surface were observed by Voyager 2 44 Basic Information on Triton • • • • • Mass = 0.29 Mmoon Radius = 1350 km (0.78 Rmoon) Density = 2070 kg/m3 Distance = 0.35 million km Only slightly larger than Pluto and nearly identical in composition… the two likely share the same origin. 45 Triton – Nitrogen Geysers Image taken by Voyager 2 in August 1989. Click on image for more information. Additional view 46 Moons that have few surface craters probably A. have warm interiors. B. have been shielded from impacts by their host planet. C. are captured asteroids. D. have had their smaller craters obliterated by larger impacts. 47 Moons that have few surface craters probably A. have warm interiors. B. have been shielded from impacts by their host planet. C. are captured asteroids. D. have had their smaller craters obliterated by larger impacts.