Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 6 The Outer Solar System © 2010 Pearson Education, Inc. Are jovian planets all alike? © 2010 Pearson Education, Inc. Jovian Planet Composition • Jupiter and Saturn – Mostly H and He gas • Uranus and Neptune – Mostly hydrogen compounds: water (H2O), methane (CH4), ammonia (NH3) – Some H, He, and rock © 2010 Pearson Education, Inc. Density Differences • Uranus and Neptune are denser than Saturn because they have less H/He, proportionately. © 2010 Pearson Education, Inc. Density Differences • Uranus and Neptune are denser than Saturn because they have less H/He, proportionately. © 2010 Pearson Education, Inc. Rotation and Shape • Jovian planets are not quite spherical because of their rapid rotation. © 2010 Pearson Education, Inc. Interiors of Jovian Planets • No solid surface • Layers under high pressure and temperatures • Cores (~10 Earth masses) made of hydrogen compounds, metals, and rock • The layers are different for the different planets. WHY? © 2010 Pearson Education, Inc. What are jovian planets like on the inside? © 2010 Pearson Education, Inc. Inside Jupiter • High pressures inside Jupiter cause phase of hydrogen to change with depth. • Hydrogen acts like a metal at great depths because its electrons move freely. © 2010 Pearson Education, Inc. Inside Jupiter • Core is thought to be made of rock, metals, and hydrogen compounds. • Core is about same size as Earth but 10 times as massive. © 2010 Pearson Education, Inc. Comparing Jovian Interiors • Models suggest cores of jovian planets have similar composition. • Lower pressures inside Uranus and Neptune mean no metallic hydrogen. © 2010 Pearson Education, Inc. Jupiter’s Internal Heat • Jupiter radiates twice as much energy as it receives from the Sun. • Energy probably comes from slow contraction of interior (releasing potential energy). © 2010 Pearson Education, Inc. Internal Heat of Other Planets • Saturn also radiates twice as much energy as it receives from the Sun. • Energy probably comes from differentiation (helium rain). • Neptune emits nearly twice as much energy as it receives, but the source of that energy remains mysterious. © 2010 Pearson Education, Inc. What is the weather like on jovian planets? © 2010 Pearson Education, Inc. Jupiter’s Atmosphere • Hydrogen compounds in Jupiter form clouds. • Different cloud layers correspond to freezing points of different hydrogen compounds. © 2010 Pearson Education, Inc. Jovian Planet Atmospheres • Other jovian planets have cloud layers similar to Jupiter’s. • Different compounds make clouds of different colors. © 2010 Pearson Education, Inc. Jupiter’s Colors • Ammonium sulfide clouds (NH4SH) reflect red/brown. • Ammonia, the highest, coldest layer, reflects white. © 2010 Pearson Education, Inc. Saturn’s Colors • Saturn’s layers are similar, but deeper in and farther from the Sun (more subdued). © 2010 Pearson Education, Inc. Methane on Uranus and Neptune • Methane gas of Neptune and Uranus absorbs red light but transmits blue light. • Blue light reflects off methane clouds, making those planes look blue. © 2010 Pearson Education, Inc. Weather on Jovian Planets • All the jovian planets have strong winds and storms. © 2010 Pearson Education, Inc. Jupiter’s Great Red Spot • Is a storm twice as wide as Earth • Has existed for at least three centuries © 2010 Pearson Education, Inc. Jovian Ring Systems • All four jovian planets have ring systems. • Others have smaller, darker ring particles than Saturn. © 2010 Pearson Education, Inc. Why do the jovian planets have rings? © 2010 Pearson Education, Inc. Why do the jovian planets have rings? • They formed from dust created in impacts on moons orbiting those planets. How do we know? © 2010 Pearson Education, Inc. How do we know? • Rings aren’t leftover from planet formation because the particles are too small to have survived for so long. • There must be a continuous replacement of tiny particles. • The most likely source is impacts with jovian moons. © 2010 Pearson Education, Inc. Ring Formation • Jovian planets all have rings because they possess many small moons close in. • Impacts on these moons are random. • Saturn’s incredible rings may be an “accident” of our time. © 2010 Pearson Education, Inc. What are Saturn’s rings like? © 2010 Pearson Education, Inc. What are Saturn’s rings like? • They are made up of numerous, tiny individual particles. • They orbit around Saturn’s equator. • They are very thin. © 2010 Pearson Education, Inc. Artist’s Conception of Rings Close-Up © 2010 Pearson Education, Inc. Spacecraft View of Ring Gaps © 2010 Pearson Education, Inc. What kinds of moons orbit the jovian planets? © 2010 Pearson Education, Inc. Sizes of Moons • Small moons (< 300 km) – No geological activity • Medium-sized moons (300–1500 km) – Geological activity in past • Large moons (> 1500 km) – Ongoing geological activity © 2010 Pearson Education, Inc. Medium and Large Moons • Enough self-gravity to be spherical • Have substantial amounts of ice • Formed in orbit around jovian planets • Circular orbits in same direction as planet rotation © 2010 Pearson Education, Inc. Small Moons • They are captured asteroids or comets, so their orbits do not follow usual patterns. © 2010 Pearson Education, Inc. Why are Jupiter’s Galilean moons so geologically active? © 2010 Pearson Education, Inc. Io’s Volcanic Activity • Io is the most volcanically active body in the solar system, but why? © 2010 Pearson Education, Inc. Tidal Heating Io is squished and stretched as it orbits Jupiter. © 2010 Pearson Education, Inc. But why is its orbit so elliptical? Orbital Resonances Every 7 days, these three moons line up. © 2010 Pearson Education, Inc. The tugs add up over time, making all three orbits elliptical. Europa’s Ocean: Waterworld? © 2010 Pearson Education, Inc. Tidal stresses crack Europa’s surface ice. © 2010 Pearson Education, Inc. Europa’s interior also warmed by tidal heating. © 2010 Pearson Education, Inc. Ganymede • Largest moon in the solar system • Clear evidence of geological activity • Tidal heating plus heat from radioactive decay? © 2010 Pearson Education, Inc. Callisto • “Classic” cratered iceball • No tidal heating, no orbital resonances • But it has a magnetic field!? © 2010 Pearson Education, Inc. What is remarkable about Titan and other major moons of the outer solar system? © 2010 Pearson Education, Inc. Titan’s Atmosphere • Titan is the only moon in the solar system to have a thick atmosphere. • It consists mostly of nitrogen with some argon, methane, and ethane. © 2010 Pearson Education, Inc. Titan’s Surface • Huygens probe provided first look at Titan’s surface in early 2005. • It found liquid methane and “rocks” made of ice. © 2010 Pearson Education, Inc. Medium Moons of Saturn • Almost all of them show evidence of past volcanism and/or tectonics. © 2010 Pearson Education, Inc. Medium Moons of Saturn • Ice fountains of Enceladus suggest it may have a subsurface ocean. © 2010 Pearson Education, Inc. Medium Moons of Uranus • They have varying amounts of geological activity. • Miranda has large tectonic features and few craters (possibly indicating an episode of tidal heating in past). © 2010 Pearson Education, Inc. Neptune’s Moon Triton • Similar to Pluto, but larger • Evidence of past geological activity © 2010 Pearson Education, Inc. Asteroids, Comets, and the Impact Threat © 2010 Pearson Education, Inc. Asteroid Orbits • Most asteroids orbit in the asteroid belt between Mars and Jupiter. • Trojan asteroids follow Jupiter’s orbit. • Orbits of near-Earth asteroids cross Earth’s orbit. © 2010 Pearson Education, Inc. Origin of Asteroid Belt • Rocky planetesimals between Mars and Jupiter did not accrete into a planet. • Jupiter’s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet. © 2010 Pearson Education, Inc. Asteroid Facts • Asteroids are rocky leftovers of planet formation. • The largest is Ceres, diameter ~1000 kilometers. • 150,000 in catalogs, and probably over a million with diameter >1 kilometer. • Small asteroids are more common than large asteroids. • All the asteroids in the solar system wouldn’t add up to even a small terrestrial planet. © 2010 Pearson Education, Inc. Asteroids are cratered and not round. © 2010 Pearson Education, Inc. What are comets like? © 2010 Pearson Education, Inc. Comet Facts • Formed beyond the frost line, comets are icy counterparts to asteroids. • Nucleus of comet is a “dirty snowball.” • Most comets do not have tails. • Most comets remain perpetually frozen in the outer solar system. • Only comets that enter the inner solar system grow tails. © 2010 Pearson Education, Inc. How did they get there? • Kuiper belt comets formed in the Kuiper belt: flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets • Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction © 2010 Pearson Education, Inc. Anatomy of a Comet • A coma is the atmosphere that comes from a comet’s heated nucleus. • A plasma tail is gas escaping from coma, pushed by the solar wind. • A dust tail is pushed by photons. © 2010 Pearson Education, Inc. Only a tiny number of comets enter the inner solar system. Most stay far from the Sun. Oort cloud: on random orbits extending to about 50,000 AU Kuiper belt: on orderly orbits from 30–100 AU in disk of solar system © 2010 Pearson Education, Inc. Have we ever witnessed a major impact? © 2010 Pearson Education, Inc. Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. Tidal forces tore it apart during a previous encounter with Jupiter. © 2010 Pearson Education, Inc. Did an impact kill the dinosaurs? © 2010 Pearson Education, Inc. Mass Extinctions • Fossil record shows occasional large dips in the diversity of species: mass extinctions. • Most recent was 65 million years ago, ending the reign of the dinosaurs. © 2010 Pearson Education, Inc. Iridium: Evidence of an Impact • Iridium is very rare in Earth surface rocks but often found in meteorites. • Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact. • Dinosaur fossils all lie below this layer. © 2010 Pearson Education, Inc. Iridium Layer No dinosaur fossils in upper rock layers Thin layer containing the rare element iridium Dinosaur fossils in lower rock layers © 2010 Pearson Education, Inc. Consequences of an Impact • Meteorite 10 kilometers in size would send large amounts of debris into atmosphere. • Debris would reduce sunlight reaching Earth’s surface. • Resulting climate change may have caused mass extinction. © 2010 Pearson Education, Inc. Likely Impact Site • Geologists have found a large subsurface crater about 65 million years old in Mexico. © 2010 Pearson Education, Inc. Meteor Crater, Arizona: 50,000 years ago (50-meter object) © 2010 Pearson Education, Inc. Frequency of Impacts • Small impacts happen almost daily. • Impacts large enough to cause mass extinctions happen many millions of years apart. © 2010 Pearson Education, Inc. Facts about Impacts • Asteroids and comets have hit Earth. • A major impact is only a matter of time: not IF but WHEN. • Major impacts are very rare. • Extinction level events happen millions of years apart. • Major damage happen tens to hundreds of years apart. © 2010 Pearson Education, Inc. Is the impact threat a real danger or media hype? © 2010 Pearson Education, Inc. The asteroid with our name on it • We haven’t seen it yet. • Deflection is more probable with years of advance warning. • Control is critical: Breaking a big asteroid into a bunch of little asteroids is unlikely to help. • We get less advance warning of a killer comet…. © 2010 Pearson Education, Inc. What have we learned? • Are jovian planets all alike? – Jupiter and Saturn are mostly H and He gas. – Uranus and Neptune are mostly H compounds. • What are jovian planets like on the inside? – Layered interiors with very high pressure and cores made of rock, metals, and hydrogen compounds – Very high pressure in Jupiter and Saturn can produce metallic hydrogen. © 2010 Pearson Education, Inc. What have we learned? • What is the weather like on jovian planets? – Multiple cloud layers determine colors of jovian planets. – All have strong storms and winds. • Do jovian planets have magnetospheres like Earth’s? – All have substantial magnetospheres. – Jupiter’s is the largest by far. © 2010 Pearson Education, Inc. What have we learned? • What kinds of moons orbit the jovian planets? – Moons come in many sizes. – The level of geological activity depends on a moon’s size. • Why are Jupiter’s Galilean moons so geologically active? – Tidal heating drives geological activity, leading to Io’s volcanoes and ice geology on other moons. © 2010 Pearson Education, Inc. What have we learned? • What is special about Titan and other major moons of the solar system? – Titan is only moon with thick atmosphere. – Many other major moons show signs of geological activity. • Why are small icy moons more geologically active than small rocky planets? – Ice melts and deforms at lower temperatures, enabling tidal heating to drive activity. © 2010 Pearson Education, Inc. What have we learned? • What are Saturn’s rings like? – They are made up of countless individual ice particles. – They are extremely thin with many gaps. • How do other jovian ring systems compare to Saturn’s? – The other jovian planets have much fainter ring systems with smaller, darker, less numerous particles. • Why do the jovian planets have rings? – Ring particles are probably debris from moons. © 2010 Pearson Education, Inc. What have we learned? • Have we ever witnessed a major impact? – The most recent major impact happened in 1994, when fragments of comet SL9 hit Jupiter. • Did an impact kill the dinosaurs? – Iridium layer just above dinosaur fossils suggests that an impact caused mass extinction 65 million years ago. – A large crater of that age has been found in Mexico. © 2010 Pearson Education, Inc. What have we learned? • Is the impact threat a real danger or media hype? – Large impacts do happen, but they are rare. – They cause major extinctions about every 100 million years. • How do the jovian planets affect impact rates and life on Earth? – Jovian planets sometimes deflect comets toward Earth but send many more out to Oort cloud. © 2010 Pearson Education, Inc.