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The Giant Planets The Outer Planets • Hydrogen-rich atmospheres • Belt-zone circulation • Shallow atmospheres • Interiors mostly liquid H2 • Large satellite systems The Outer Planets This figure shows the solar system from a vantage point that emphasizes the relationship of the outer planets to the rest of the system Spacecraft Exploration of the Outer Solar System Pioneer 10 & 11 and Voyager 1 & 2 flew through the outer solar system. This is Voyager. Galileo orbited Jupiter. Cassini orbited Saturn Jupiter Interior Magnetosphere Atmosphere History Rings Roche Limit Galilean Satellites Io Europa Ganymede Callisto Earth Jupiter Semi-major Axis 1 A.U. 5.203 A.U. Inclination 0° 1° 18’ Orbital period 1.000 tropical year 11.86 tropical year Orbital eccentricity 0.017 0.048 Rotational period 23 h 56 min 4.1 s 9 h 50.5 min Tilt 23° 27’ 3° 7’ Radius 6378 km 71,492 km Mass 5.97 x1024 kg 1.90 x 1027 kg Bulk density 5.52 g/cm3 1.33 g/cm3 Atmosphere N2, O2 H2, He, CH4, NH3 Albedo 0.40 0.51 Surface temperature 250-300 K 110-150 K Escape speed 11.2 km/s 59.6 km/s Magnetic moment (equator) 8 x 1010 G.km3 1.3 x 1015 G.km3 Jupiter Largest and most massive planet in the solar system: Contains almost 3/4 of all planetary matter in the solar system. Visual image Infrared falsecolor image Most striking features visible from Earth: Multicolored cloud belts and the Great Red Spot Jupiter’s Interior Jupiter’s composition consists mostly of hydrogen and helium. Because of high pressure, H2 is compressed into a liquid, and even into a metallic (quantum degenerate) state. He, which is heavier than H2, continues to sink toward the center. It is in a sense a continuance of gravitational infall and it generates heat. T ~ 30,000 K Electron energy Degenerate Matter A point is reached below the liquid H2 boundary where the gravitational pressure forces a phase change from liquid to degenerate matter. Gravity forces electrons into the lowest energy levels of the atoms. Pressure in the degenerate core comes from electrons that cannot be packed arbitrarily close together (Pauli exclusion principle). Liquid metallic hydrogen is an excellent conductor of electricity and heat. Jupiter’s Magnetic Field Discovered through observations of decimeter (radio) radiation Magnetic field at least 10 times stronger than Earth’s magnetic field. Magnetosphere over 100 times larger than Earth’s magnetosphere. Extremely intense radiation belts. Very high energy particles can be trapped; radiation flux corresponds to ~100 times a lethal dose for humans! Jupiter’s Magnetosphere Jupiter is surrounded by belts of charged particles, much like the Van Allen belts but vastly larger A plasma torus is produced in the orbit of Io Jupiter’s Magnetosphere The magnetosphere extends beyond the orbit of Saturn Aurorae on Jupiter Just like on Earth, Jupiter’s magnetosphere produces aurorae concentrated in rings around the magnetic poles. ~1000 times more powerful than aurorae on Earth. Jupiter’s Atmosphere Jupiter’s liquid hydrogen ocean has no surface. There is a gradual transition from a gaseous to a liquid phase as temperature and pressure combine to exceed the critical point. Jupiter shows limb darkening ⇒ hydrogen atmosphere extends above the cloud layers. There is only a very thin atmosphere above the cloud layers. There is a transition to a liquid hydrogen zone ~1000 km below the clouds. Clouds in Jupiter’s Atmosphere There are 3 layers of clouds: 1. Ammonia (NH3) crystals 2. Ammonia hydrosulfide (NH4SH) 3. Water crystals The Cloud Belts of Jupiter Dark belts or bands alternate with bright zones. Zones are higher and cooler than the belts; they are regions of rising warm gas. Belts are lower and hotter than the zones; they are regions of sinking cool gas. The Cloud Belts on Jupiter Just like on Earth, high-and low-pressure zones are bounded by high-pressure winds (jet streams). • Hadley circulation forms three wind zones on Earth and five on Jupiter in each hemisphere. • Jupiter’s rapid rotation and size causes the winds to be parallel to the equator. • This gross structure has remained unchanged for over 300 years. The Atmosphere of Jupiter However, real wind speeds are complicated. This graph shows wind speed with respect to internal rotation rate at a single point in time. There are continual changes to this graph with time. The Atmosphere of Jupiter The Great Red Spot is an anticyclonic storm that has existed for at least 300 years and probably much longer Rotating storms on Jupiter are anticyclones, i.e. they circulate around high pressure not low. The Atmosphere of Jupiter Lightning-like flashes have been seen; also shorter-lived rotating storms One example: a Brown Oval, is really a large gap in clouds The Atmosphere of Jupiter Recently, three white storms were observed to merge into a single storm, which then turned red. This may provide some clues to the dynamics behind Jupiter’s cloud movements. Atmospheric Heating Jupiter radiates more energy than it receives from the Sun because the core is still radiating heat caused by gravitational compression. Internal heating feeds energy to storms from below which causes them to be anticyclones. Could Jupiter have been a star? No; it is far too cool and too small for that. It would need to be about 80 times more massive to be even a very small star. The History of Jupiter • Formed from cold gas in the outer solar nebula, where ices were able to condense. • Rapid growth • In the interior, hydrogen became metallic (very good electrical conductor) • Rapid rotation ⇒ strong magnetic field • Soon able to trap gas directly by gravity • Rapid rotation and large size ⇒ beltzone cloud pattern • Heavy materials sank to the center • Dust from meteorite impacts onto inner satellites were trapped to form rings The History of Jupiter Jupiter’s energy output was larger in the past; it may have been 100 times brighter than the Moon as seen from Earth A dwarf star in Jupiter’s place probably would have made stable planetary orbits impossible Jupiter played an invaluable role in sweeping the solar system clear of debris before too much reached Earth—otherwise life on Earth might not have been possible Jupiter’s Ring Galileo spacecraft image of Jupiter’s ring, illuminated from behind Not only Saturn, but all four gas giants have rings. Rings must be constantly resupplied with new dust. Jupiter’s ring: dark and reddish; it was discovered by the Voyager 1 spacecraft. Composed of microscopic particles of rocky material Location: Inside the Roche limit. Ring material cannot be old because radiation pressure and Jupiter’s magnetic field force dust particles to spiral down into the planet. The Roche Limit The Roche limit is defined by the radius where the tidal force between two particles = the gravitational force between them. Particles cannot accrete inside this radius. All stable ring systems are inside this limit. Jupiter’s Family of Satellites Over 60 natural satellites are now known; new ones are still being discovered. The four largest satellites were discovered by Galileo and are called the Galilean satellites Io Europa Ganymede Callisto They have interesting and diverse individual geologies. Jupiter’s Family of Satellites Jupiter with Io and Europa. Note the relative sizes! Comparison of Medium-size Bodies in the Solar System Mass (kg) Mercury Ganymede Titan Callisto Io Moon Europa Triton Pluto 3.30 x 1023 1.47 x 1023 1.34 x 1023 1.06 x 1023 8.89 x 1022 7.35 x 1022 4.92 x 1022 2.16 x 1022 1.1 x 1022 7.8 x 10-5 2.36 x 10-4 5.7 x 10-4 4.7 x 10-5 1.2 x 10-2 2.5 x 10-5 2.1 x 10-4 Mass ratio (ms/mp) Radius (km) 2439 2631 2575 2400 1815 1738 1569 1350 1140 Bulk density (g/cm3) 5.43 1.93 1.88 1.83 3.55 3.34 3.04 2.1 2.1? 15.1 20.25 26.60 5.95 60 9.47 14.04 Orbital radius / planetary radius (rs/Rp) The Galilean Satellites of Jupiter Interiors Io Io Most active of all Galilean satellites No impact craters are visible at all. Over 100 active volcanoes! Activity powered by tidal interactions with Jupiter. Average density ~3.55 g/cm3 ⇒ Interior is mostly rock. Io Yellow, orange and browns are probably from sulfur and sulfur compounds in the ejecta. White is probably from SO2 ice. Io Cause of volcanism: Gravity! Io is very close to Jupiter, and it also feels gravitational forces from Europa. The tidal forces are huge, and provide the energy for the volcanoes. The tidal forces oscillate to act like a pump that heats up the interior. Io Volcanic eruptions eject charged particles; these interact with Jupiter’s magnetosphere and form a plasma torus. Particles in the torus can travel to Jupiter along magnetic field lines. Europa Average density: ~3 g/cm3 ⇒ composition: mostly rock and metal; icy surface. Close to Jupiter ⇒ should be hit by many meteoroid impacts; but few craters visible. ⇒ Active surface; impact craters are rapidly erased. The Surface of Europa Cracked surface and high albedo (reflectivity) provide additional evidence for geological activity. The Interior of Europa Europa is too small to retain its internal heat ⇒ Heating mostly from tidal interaction with Jupiter. Core is not molten ⇒ No magnetic field. Europa has a liquid water ocean ~15 km below the icy surface. Some speculate that life could exist in this submerged ocean. Ganymede Craters (Palimpsests) Bright terrain Dark surface Ganymede Average density = ~1.9 g/cm3 Largest of the 4 Galilean satellites. • Rocky core • Ice-rich mantle • Crust of ice• 1/3 of surface old, dark, cratered rest: bright, young, grooved terrain Bright terrain probably formed through flooding when the surface broke Ganymede Ganymede is the largest satellite in the solar system – it is larger than Pluto and larger in diameter than Mercury It has a history similar to Earth’s Moon, but made of dirty water ice instead of rock Ganymede Areas once thought to be flat turn out to have structure; they might result from a form of plate tectonics Callisto Tidally locked to Jupiter, like all of Jupiter’s satellites. Average density: ~1.79 g/cm3 ⇒ composition: mixture of ice and rocks Dark surface, heavily pocked with craters (palimpsests). No metallic core: Callisto never differentiated to form core and mantle. Layer of liquid water, ~10 km thick, ~100 km below surface, probably heated by radioactive decay. ⇒ No magnetic field. Saturn Principal Features Atmosphere Rings Satellites Titan Small Satellites Jupiter Saturn Semi-major Axis 5.203 A.U. 9.522 A.U. Inclination 1° 18’ 2° 9’ Orbital period 11.86 tropical year 29.46 tropical year Orbital eccentricity 0.048 0.054 Rotational period 9 h 50.5 min 10 h 14 min Tilt 3° 7’ 26° 44’ Radius 71,492 km 60,268 km Mass 1.90 x 1027 kg 5.68 x 1026 kg Bulk density 1.33 g/cm3 0.69 g/cm3 Atmosphere H2, He, CH4, NH3 H2, He, CH4, NH3 Albedo 0.51 0.50 Surface temperature 110-150 K 95 K Escape speed 59.6 km/s 35.6 km/s Magnetic moment (equator) 1.3 x 1015 G.km3 4 x 1013 G.km3 Spacecraft Exploration of Saturn Besides Voyager 1 & 2, the Cassini spacecraft is orbiting among Saturn’s satellites; it used many gravity assists to get there. Saturn Mass: ~1/3 of mass of Jupiter Radius: ~16% smaller than Jupiter Average density ≈ 0.69 g/cm3: Less than water! Rotates about as fast as Jupiter, but is twice as oblate ⇒ No large core of heavy elements. Mostly H2 and He; liquid hydrogen and liquid metallic hydrogen core. Saturn radiates ~1.8 times the energy received from the Sun. Probably heated by liquid helium droplets falling towards center, i.e. gravitational collapse Saturn Because the rotational axis of Saturn is tilted 26° 44’ to the ecliptic plane, the view of its rings from Earth changes as Saturn orbits the Sun. When they are aligned with the ecliptic plane, they practically disappear because they are so thin. Saturn’s Atmosphere Saturn’s atmosphere also shows zone and band structure like Jupiter, but coloration is much more subdued than on Jupiter. It is mostly H2, He, CH4, and NH3; the He fraction is much less than on Jupiter. This true-color image shows the delicate coloration of the cloud patterns on Saturn. Saturn’s Atmosphere Three-layered cloud structure, just like Jupiter Main differences from Jupiter More belts and zones (7) Much stronger winds: Winds up to ~500 m/s near the equator! Saturn’s Atmosphere Wind patterns on Saturn are like those on Jupiter, with zonal flow. Seven on Saturn compared to 5 on Jupiter and 3 on Earth. Saturn’s Atmosphere Anticyclones are rare on Saturn; they do not form often and they quickly dissipate when they do. Colors are more subtle compared to Jupiter probably because it is colder. Saturn’s Atmosphere This image shows what is thought to be a vast thunderstorm on Saturn, as well as the polar vortex at Saturn’s south pole. Saturn’s Magnetosphere Saturn also has a strong magnetic field, but only 5% as strong as Jupiter’s Creates aurorae. Saturn’s Rings Ring consists of 3 main segments A, B, and C Rings Separated by divisions: visually empty regions Rings could not have been formed together with Saturn because material would have been blown away by particle streams from a hot Saturn at the time of formation. Composition of Saturn’s Rings Rings are composed of ice particles ranging from ~1 cm to ~1 m in size They move at large Keplerian velocities around Saturn They have small relative velocities (all moving in the same direction). Shepherd Satellites Some satellites in orbits close to the rings focus the ring material, keeping the rings confined. Rings Inner Satellites Shepherd Satellites Divisions and Resonances Besides some satellites acting as “shepherds,” some satellites have orbital periods that are in a small-number fractional multiple (e.g., 3:1) of the orbital period of material in the disk. This is an orbit-orbit resonance and disk material is cleared out by tidal forces ⇒ Divisions Electromagnetic Phenomena in Saturn’s Rings Radial spokes in the rings rotate with the rotation period of Saturn: Magnetized ring particles lifted out of the ring plane and rotating along with the magnetic-field structure Titan: Saturn’s Largest Satellite About the size of Jupiter’s satellite Ganymede. Rocky core, but also a large amount of ice. A thick atmosphere of N2 and Ar, hides the surface from direct view. Titan’s Atmosphere Because of the thick, hazy atmosphere, surface features are only visible in infrared images. Surface pressure: 50% greater than air pressure on Earth Surface temperature: 94 K (-290 oF) ⇒ Methane (CH4) and ethane (C2H6) are liquid! CH4 is gradually converted to C2H6 in the atmosphere ⇒ CH4 must be constantly replenished, probably through breakdown of ammonia (NH3). North Polar Lakes Many of the organic compounds in Titan’s atmosphere may have been precursors of life on Earth. Titan’s Atmosphere Trace chemicals in Titan’s atmosphere make it chemically complex Titan Some surface features on Titan are visible in this Cassini infrared image Titan The Huygens probe has landed on Titan and returned images directly from the surface Titan Based on measurements made by Cassini and Huygens, this is the current best estimate of the interior structure of Titan Saturn’s Mid-size Satellites Mimas, Enceladus, Tethys, Dione, and Rhea all orbit between 3 and 9 planetary radii from Saturn, and all are tidally locked—this means they have “leading” and “trailing” surfaces Iapetus orbits 59 radii away and is also tidally locked Saturn’s Mid-size Satellites Saturn’s smaller satellites formed of rock and ice; heavily cratered and appear geologically dead. Tethys: Iapetus: Enceladus: Heavily cratered; marked by 3 km deep, 1500 km long crack. Leading (upper right) side darker than rest of surface because of dark deposits. Possibly active; regions with fewer craters, containing parallel grooves, possibly filled with frozen water. Saturn’s Mid-size Satellites Dione and Tethys are similar, having icy, heavily cratered surfaces. Saturn’s Mid-size Satellites Rhea has a highly reflective, heavily cratered surface. Wispy features are on trailing side but not leading; origin not yet understood. Mimas is the closest satellite to Saturn, and it has a crater covering one-third of its surface, the result of an impact that must have almost destroyed the satellite. Saturn’s Small Satellites Masses of many small satellites are not well known Janus and Epimetheus share a single orbit Saturn’s Small Satellites Telesto and Calypso are located at the L4 and L5 Lagrangian points of Tethys, respectively Uranus Atmosphere Interior Rings Satellites Jupiter Uranus Semi-major Axis 5.203 A.U. 19.2 A.U. Inclination 1° 18’ 0° 46’ Orbital period 11.86 tropical year 84.01 tropical year Orbital eccentricity 0.048 0.048 Rotational period 9 h 50.5 min 17 h 14 min – retro Tilt 3° 7’ 97° 52’ Radius 71,492 km 25,559 km Mass 1.90 x 1027 kg 8.66 x 1025 kg Bulk density 1.33 g/cm3 1.27 g/cm3 Atmosphere H2, He, CH4, NH3 H2, He, CH4 Albedo 0.51 0.66 Surface temperature 110-150 K 58 K Escape speed 59.6 km/s 21.3 km/s Magnetic moment (equator) 1.3 x 1015 G.km3 3.8 x 1012 G.km3 The Discovery of Uranus Discovered in 1781 by William Herschel; first planet to be discovered in more than 2000 years Little detail can be seen from Earth; arrows point to three of Uranus’ satellites. Rotation of Uranus Uranus has the peculiarity of an axis of rotation which lies almost in the plane of its orbit. Seasonal variations are extreme. The Atmosphere of Uranus Like other gas giants: No surface. Gradual transition from gas phase to liquid interior. Mostly H2; 15% He, a few % methane, ammonia and water vapor. Optical view from Earth: Blue color from methane, absorbing longer optical wavelengths Cloud structures not visible even after artificial computer enhancement of optical images taken from Voyager 2 spacecraft. Cloud Structures of Uranus A Hubble Space Telescope image of Uranus shows cloud structures in lower, warmer layers not visible to Voyager 2 in 1986. ⇒ Possible seasonal changes of the cloud structures. Interior of Uranus Deep hydrogen & helium atmosphere No liquid metallic hydrogen Ice/rock mantle Mantle may be slushy so that it can convect The Rings of Uranus Rings of Uranus and Neptune are similar to Jupiter’s rings. Confined by shepherd satellites; consist of dark material. Rings of Uranus were discovered using occultations of a background star Apparent motion of star behind Uranus and rings The Satellites of Uranus Umbriel 5 largest satellites: Miranda, Ariel, Umbriel, Titania, and Oberon Visible from Earth. Tidally locked to Uranus. 10 more discovered by Voyager 2; more are still being found. Ariel Miranda Dark surfaces, probably ice darkened by dust from meteorite impacts. Miranda Neptune Atmosphere Rings Satellites Triton Magnetospheres of the Giant Planets Jupiter Neptune Semi-major Axis 5.203 A.U. 30.07 A.U. Inclination 1° 18’ 1° 46’ Orbital period 11.86 tropical year 164.8 tropical year Orbital eccentricity 0.048 0.007 Rotational period 9 h 50.5 min 16 h 3 min Tilt 3° 7’ 29° 34’ Radius 71,492 km 25,269 km Mass 1.90 x 1027 kg 1.03 x 1026 kg Bulk density 1.33 g/cm3 1.64 g/cm3 Atmosphere H2, He, CH4, NH3 H2, He, CH4, C2H6 Albedo 0.51 0.62 Surface temperature 110-150 K 56 K Escape speed 59.6 km/s 23.8 km/s Magnetic moment (equator) 1.3 x 1015 G.km3 2 x 1012 G.km3 Neptune Discovered in 1846 by J. G. Galle at a position predicted from gravitational disturbances on Uranus’ orbit by J. C. Adams and U. J. Leverrier. Blue-green color from methane in the atmosphere 4 times Earth’s diameter 4% smaller than Uranus The Atmosphere of Neptune Cloud-belt structure with high-velocity winds; origin not well understood. Darker cyclonic disturbances, similar to Great Red Spot on Jupiter, but not long-lived. White cloud features of methane (CH4) ice crystals The Rings of Neptune Interrupted between denser segments (arcs) Made of dark material, visible in forward-scattered light. Ring material must be regularly resupplied by dust from meteorite impacts on the satellites. Focused by small shepherd satellites embedded in the ring structure. The Satellites of Neptune Two satellites (Triton and Nereid) are visible from Earth; 6 more discovered by Voyager 2 Unusual orbits: Triton: Only satellite in the solar system orbiting clockwise, i.e. retrograde. Nereid: Highly eccentric orbit; very long orbital period (359.4 d). The Surface of Triton Very low temperature (34.5 K) ⇒ Triton can hold a tenuous atmosphere of nitrogen (N2) and some methane (CH4); 105 times less dense than Earth’s atmosphere. Surface composed of ices: N2, CH4, carbon monoxide (CO), carbon dioxide (CO2). Possibly cyclic N2 ice deposition and re-vaporizing on Triton’s south pole, similar to CO2 ice polar cap cycles on Mars. Dark smudges on the N2 ice surface, probably from CH4 rising from below the surface, forming carbon-rich deposits when exposed to sunlight. The Surface of Triton Ongoing surface activity: Surface features probably not more than 100 million years old. Large basins might have been flooded multiple times by liquids from the interior. Ice equivalent of greenhouse effect may be one of the heat sources for Triton’s geological activity. Comparing the Rings of the Giant Planets Magnetospheres and Internal Structure of the Giant Planets Uranus and Neptune both have substantial magnetic fields, but at a large angle to their rotation axes. The rectangle within each planet shows a bar magnet that would produce a similar field. Note that both Uranus’ and Neptune’s are significantly off center. Magnetospheres and Internal Structure of the Giant Planets If magnetic fields of Uranus and Neptune are produced by dynamos, then the “slush” must be conducting and convecting. Interior structure of Uranus and Neptune, compared to that of Jupiter and Saturn.