Key Ideas: A Warm Up Exercise. A Warm Up Exercise. The Moons of Saturn. Saturn s Moons

Transcription

1 Saturn s Moons Key Ideas: Saturn is the second largest Jovian Planet Gas planet much like Jupiter 62 moons (13 larger than 50 km in diameter) Titan only giant moon Largest of Saturn s moons Thick Nitrogen (80%) & Methane (3%) atmosphere Methane almost certainly plays a role similar to water on Earth Only solar system moon with an atmosphere Enceledus Geysers A Warm Up Exercise Which Galilean satellite of Jupiter does not contain a fairly thick layer of water ice at or near the surface? a) Titan b) Callisto c) Io d) Europa e) Ganymede A Warm Up Exercise Which Galilean satellite of Jupiter does not contain a fairly thick layer of water ice at or near the surface? a) Titan b) Callisto c) Io d) Europa e) Ganymede The Moons of Saturn Saturn has 62 moons (13 larger than 50km in diameter): 1 giant moon: Titan Thick Nitrogen and Methane atmosphere Only solar system moon with an atmosphere Medium & smaller moons: Range in size from 20 to 1500 km. Moons larger than 300 km are spherical. Moons smaller than 300 km are irregular. Mostly icy, or mixes of rock & ice, with densities of 3000 to 1500 kg/m 3 All have heavily cratered, old surfaces. 1

2 Tiny irregular moons of Saturn Large Moons of Saturn Titan Helene Epimetheus Calypso Mimas Enceladus Tethys Janus Telesto Pandora Prometheus Dione Hyperion Iapetus Rhea Phoebe Titan Titan Saturn s only giant moon: Radius: 2575 km (2 nd largest after Ganymede) Mean Density: ~1900 kg/m 3 Icy mantle over a rocky core. Titan has a substantial atmosphere: Only Solar System moon with an atmosphere. Hazy clouds of methane, nitrogen, & suspended aerosols hide the surface at visible wavelengths. Titan s Atmosphere Composition: ~80% N 2 (nitrogen) ~3% CH 4 (methane) Mix of argon and hydrocarbons like Ethane Cold, dense atmosphere: Temperature: 94 K (-290º F) Surface pressure: ~1.6 Earth atmospheres. Clouds of methane & N 2 ices High layer of hydrocarbon smog complex chemistry driven by ultraviolet radiation from the Sun acting on the methane Titan s Seasons Titan is tidally locked (like the Moon) to Saturn It also has seasons because Saturn (and its rings and moons) are inclined relative to their orbit around the Sun by about 27 o 2

3 Atmosphere of Titan Is methane Titan s "water"? Methane may play the same role on Titan that water does on the Earth: 94 K is between the boiling & freezing points of Methane Get gaseous methane in atmosphere. Methane condenses into clouds that could rain liquid methane. Could have standing methane lakes or ponds. Could get methane snow and polar caps. Computer enhanced view showing sunlight scattered by Titan s atmosphere. Cassini/Huygens data seem to show the likely answer is Yes Cassini Radar Images 60 km impact crater note asymmetry of ejecta pattern (winds? erosion?) Cassini Radar Images A bigger (440km) impact basin in general the surface of Titan seems pretty young erosion/weather? Cassini Radar Images North Pole Probably lakes near poles very smooth regions reflecting almost no radar Evidence for Activity Infrared spectrographic images from Cassini, possible evidence for Cryovolcanoes release of ammonia, methane, other volatiles from the interior; Changes in lakes, possibly evidence for rain, plus detection of liquid ethane likely that you get rain in winter, evaporate in summer; 3

4 Huygens Lander Images Changed region Seems to be shrinking Where liquid ethane was detected The Bigger Moons Aside from Titan, they are ice balls Diameter (km) Density (kg/m 3 ) Titan Rhea Iapetus Dione Tethys Enceladus Mimas Lot s of orbital resonances Mimas/Tethys, Enceledus/Dione are in 2:1 resonances, Titan:Hyperion (143km) are in a 3:4 resonance All the big moons rotate synchronously Trojan Moons When you have two massive bodies in a roughly circular orbit, there are locations where you can put a third, low mass body and it will stay there these are known as stable Lagrange points (some are unstable). Where the 1 st body is far more massive than the 2 nd, the stable points are roughly 60 o ahead and behind of the 2 nd body on the same orbit Telesto (34x28x26) Saturn Tethys Calypso (34x22x22) In essence it s a type of 1:1 resonance Rhea & Dione (1530 and 1120km diameters) Both have bright, heavily cratered leading hemispheres and dark, less cratered trailing hemispheres. Mostly water, 1/3 rock. Leading Trailing The theory is that after the early bombardment, the surface was remade leaving the streak pattern of the trailing side. The satellites were tidally locked by then, so there was a distinct front and back for subsequent impacts. Dione light-colored wisps are thought to be geologically young fractures in the icy crust 4

5 Iapetus (1460km diameter) One hemisphere is the darkest surface in the solar system, the other is one of the most reflective. The albedo of the leading hemisphere is , while that of the trailing hemisphere is 0.5 Likely material knocked off of Phoebe (220km diameter, albedo 0.06). Phoebe itself is peculiar because it s orbit is retrograde and close to the ecliptic rather than the ring plane (captured asteroid?) Dark side gets darker by absorbing more sunlight, heating up, evaporating any remaining ices at surface Enceladus (500km diameter) Most reflective body in the solar system (albedo > 0.9) surface mostly clean ice Surface shows regions of smooth plains and cracks without craters parts of the surface must be as young as 100 million years Tidal heating is probably the cause (like Io, Enceladus is in a 2:1 resonance with Dione), but estimates of the amount of heating are too small to melt water. Some other, lower melting point material? Enceladus Up Close Another view Enceladus Geysers mostly water ice with CO 2 and CH 4 probably helps to feed E ring people are very curious about this phenomenon have flown Cassini through geyser and down to 25 km! above surface where they originate 5

6 Mimas (392km diameter) Huge impact crater, Herschel 130 km across, walls 5km high, crater 10km deep, central peak 6km high fracture cracks on far side (like Calores Basin on Mercury). Relatively few craters probably was completely remelted at some point by an even bigger impact than Herschel, erasing the craters from the initial bombardment These tiger stripes are the source of the geysers 180K in stripes, 70K elsewhere Cassini s View Saturn s Rings 6

7 A Warm Up Exercise The gravitational field of Saturn s moon Titan is weaker than Mercury s gravity. Why does Titan keep and atmosphere when Mercury does not? a) Titan has a strong internal heat source b) Titan s density is less than Mercury s c) Titan has a faster orbital speed d) Titan is warmer than the night side of Mercury e) Titan is cooler than the day side of Mercury A Warm Up Exercise The gravitational field of Saturn s moon Titan is weaker than Mercury s gravity. Why does Titan keep and atmosphere when Mercury does not? a) Titan has a strong internal heat source b) Titan s density is less than Mercury s c) Titan has a faster orbital speed d) Titan is warmer than the night side of Mercury e) Titan is cooler than the day side of Mercury The Rings of Saturn Rings were first seen in 1610 by Galileo as bulges appearing on either side of Saturn. 1659: Christiaan Huygens used a better telescope, showing them to be rings. 1675: Dominiq Cassini found it was two rings, separated by the Cassini Division. Better telescopes in later centuries showed the rings to be divided into many sub-rings. Voyager finally showed thousands of ringlets. The rings are orbiting chunks of ice. Ringlets viewed by Voyager 2 (color enhanced) Rings are not solid, but instead composed of billions of tiny chunks of ice and rock: Chunks orbit independently around Saturn like tiny ice moons. Vary in size from a few centimeters up to many meters across. All packed close together, continually colliding, giving them worn, irregular shapes. Shiny ices make the rings appear very bright. 7

8 In fact.. All Jovian planets have rings: Jupiter: faint, dusty rings Saturn: bright, spectacular rings Uranus: dark, thin rings Neptune: dark, thin rings & ring arcs Ring properties: Bands of orbiting dusty particles and ice balls Shepherd moons Roche (tidal) radius All but Saturn s were discovered recently Uranus & Neptune have thin, dark rings: Discovered in 1977 & 1985 by watching stars disappear behind them (stellar occultation ) Jupiter has faint, dusty rings: Discovered in 1979 by Voyager 1 & 2. Can be seen from Earth using infrared images. Jupiter s Ring Saturn s Bright, Broad Rings Most elaborate of the Jovian ring systems: Broad bands of bright icy material. Broad gaps (Cassini Division & Encke Gap) Extend from 73,000 km to 140,000 km from the center of Saturn ( R Saturn ) The rings are very thin: Thickness is <100 meters it s like a sheet of paper 1 mm thick & 10 km wide! Galileo image mosaic C Ring B Ring A Ring Cassini Division Encke Gap F Ring 8

9 These two moons switch inner/outer every orbit (50km apart)! Close-up of the B-Ring Enceledus and the E ring Orbiting Iceballs Rings are not solid: Billions of ice balls in independent orbits. Range in size from centimeters to 5 meters not very many large pieces Total mass ~10 6 M Earth About the mass of a small icy/rocky moon a few 100 km in size. As the iceballs collide: Stick into larger ice balls Chip off fragments or break into smaller chunks Thin Rings of Uranus & Neptune Narrow, dark rings separated by large gaps. Uranus: Dark, narrow rings only a few km wide. Wide epsilon ring is only ~100 km wide. Neptune: Faint, very dark rings only a few km wide. Ring arcs : clumps in the outermost 9

10 Uranus Rings Backlit by the Sun as seen by Voyager 2 Rings of Neptune Shepherd Moons Thin rings should not last for very long: Collisions between ring particles will cause them to spread out over time Backlit by the Sun as seen by Voyager 2 Thin rings can be gravitationally confined by a pair of shepherd moons : Outer moon s gravity drags on ring particles, causing them to fall inwards into lower orbits Inner moon s gravity accelerates ring particles, moving them into higher orbits. Shepherd moons of Saturn s F-ring Pandora Prometheus 10

11 Shepherd Moons of Uranus Epsilon Ring Ophelia Rings and Resonances Many structures in rings are governed by orbital resonances with the planet s moons. Cordelia Example: Saturn s Cassini Division. The Cassini division is in a 2:1 resonance with the moon Mimas. Mimas orbits every 22 h, iceballs in the Cassini Division orbit every 11 h. Every 2 orbits, iceballs in the Cassini Division get tugged towards Mimas, clearing a gap. B Ring 2:1 resonance with Mimas Cassini Division A Ring The Origin of the Rings The rings are not very massive: Jupiter, Uranus, & Neptune s rings have the mass of an icy moon ~10 km across. Saturn s rings have the mass of a mid-sized icy moon ~200 km across. Possible origins of the ring material: An icy moon that came too close and was disrupted by tidal forces. A moon that couldn t form because of tides. Roche Radius Distance where tidal forces on a moon equal the gravitational force holding it together: If a moon gets closer than the Roche radius of its parent planet, it will be pulled apart by tides. Icy particles orbiting inside the Roche radius of a planet cannot aggregate into a moonlet. All rings are located inside the Roche Radii of their parent planets. 11