© 2010 Pearson Education, Inc. 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 (H 2 O), methane (CH 4 ), ammonia (NH 3 ) –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 (NH 4 SH) 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. But why is its orbit so elliptical?

© 2010 Pearson Education, Inc. The tugs add up over time, making all three orbits elliptical. Orbital Resonances Every 7 days, these three moons line up.

© 2010 Pearson Education, Inc. 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 radio- active 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 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. Kuiper belt: on orderly orbits from 30–100 AU in disk of solar system Oort cloud: on random orbits extending to about 50,000 AU Only a tiny number of comets enter the inner solar system. Most stay far from the Sun.

© 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 Dinosaur fossils in lower rock layers No dinosaur fossils in upper rock layers Thin layer containing the rare element iridium

© 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.