Chapter 6 the Outer Solar System

Home , Tidal heating

6.1 Jovian Planets, Rings, and Moons Our goals for learning: • What are jovian planets like? • Why are jovian moons so active? What are jovian planets like?

Mostly H and He gas © 2015 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 planet because of different pressures due to their masses – different compression

• 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 • Core is about same size as Earth but 10 times as massive

• Jupiter and Saturn are mostly H, He • High pressures result in regions of liquid metallic hydrogen

© 2015 Pearson Education, Inc. Uranus and Neptune contain a higher proportion of heavy elements than Jupiter and Saturn Comparing Jovian Interiors • Models suggest that cores of jovian planets have similar composition. • Lower pressures inside Uranus and Neptune mean no metallic hydrogen.

• The jovian cores are very similar: ~ mass of 10 Earths • Uranus and Neptune are higher density than Jupiter and Saturn because they have less H, He. – The jovian planets differ in the amount of H/He gas accumulated.

Why did that amount differ?

Hypothesis 1: • LOCATION: The planet that forms in a denser part of the nebula forms its core first; also has more H, He nearby to accumulate • TIMING: Solar wind blew out remaining gases before Uranus and Neptune could gather large H, He atmospheres

Hypothesis 2: • Solar Nebula was too thin to form gas giants AT ALL beyond Saturn. • Uranus and Neptune formed closer in, were gravitationally nudged outward before they accreted large atmospheres of H, He

• Hydrogen compounds in Jupiter form clouds. • Different cloud layers correspond to freezing points of different hydrogen compounds. • Other jovian planets have similar cloud layers. © 2015 Pearson Education, Inc. Cloud layers

• Brown layers are deeper (warmer) • White layers are higher (cooler)

• They are less visible – lower down in atmosphere (Saturn colder than Jupiter) – more spread out due to less gravitational compression

• Methane gas absorbs red light but transmits blue light. • Blue light reflects off methane clouds, making those planets look blue.

All the jovian planets have strong winds and storms

• Jupiter and Saturn radiate more energy than they receive from Sun • From – Formation and continued contraction – radioactive decay – differentiation: He condensation (He rain)

20 Jupiter's Great Red Spot

• A storm twice as wide as Earth • Has existed for at least 3 centuries

• Neptune emits nearly twice as much energy as it receives, but the source of that energy remains mysterious • Uranus does not emit more energy than it receives, so does not have an internal heat source

• In 2005ish, Hubble Space Telescope took this UV photo • Uranus now has its equator to the Sun • Storms are breaking out in the previously shadowed Northern

• What are jovian planets like? – Jupiter and Saturn are mostly made of H and He gas. – Uranus and Neptune are mostly made of H compounds. – They have 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. Multiple cloud layers determine the colors of jovian planets. – All have strong storms and winds.

• What is the weather like on jovian planets? – Multiple cloud layers determine the colors of jovian planets. – All have strong storms and winds.

A. The composition changes from mostly ammonia in Jupiter and Saturn to mostly methane in Uranus and Neptune. B. All have about the same amount of hydrogen and helium but the proportion of rocks is greater in those planets closer to the Sun. C. The core mass decreases with the mass of the planet. D. The composition changes from mostly hydrogen in Jupiter and Saturn to mostly helium in Uranus and Neptune. E. All have cores of about the same mass, but differ in the amount of surrounding hydrogen and helium.

© 2015 Pearson Education, Inc. Jovian Planet Moons Our Goals For Learning: • What kinds of moons orbit the jovian planets? • Why are Jupiter's Galilean moons geologically active? • What geological activity do we see on Titan and other moons? • Why are jovian planet moons more geologically active than small rocky planets? What kinds of moons orbit the jovian planets?

• Small moons (< 300 km) – No geological activity • Medium-sized moons (300-1,500 km) – Geological activity in past • Large moons (> 1,500 km) – Ongoing geological activity

• Enough self-gravity to be spherical • Have substantial amounts of ice. • Formed in orbit around jovian planets. (except Triton) • Circular orbits in same direction as planet rotation. (except Triton)

•Rock melts at higher •Ice melts at lower temperatures temperatures •Only large rocky •Tidal heating can melt planets have enough internal ice, driving

• Far more numerous than the medium and large moons • Not enough gravity to be spherical: “potato-shaped" • Captured asteroids/comets, so orbits don’t follow usual patterns.

36 Io's Volcanoes

• Volcanic eruptions continue to change Io's surface.

• Volcanic plumes on Io. Photographed by Voyager in 1970s • Io is the most volcanically active body in the solar system, but why?

Io is squished and stretched as it orbits Jupiter.

But why is its orbit so elliptical?

Every seven days, these three moons line up.

The tugs add up over time, making all three orbits elliptical. Orbital Resonances • 4 Io orbits • 2 Europa orbits • 1 Ganymede orbit • 4:2:1 Resonance

Animation Europa's Ocean: Waterworld?

100 km thick water ocean; best possibility for life?

• Largest moon in the solar system • Clear evidence of geological activity; possible subsurface liquid water? • Tidal heating plus heat from radioactive decay?

• "Classic" cratered iceball • No tidal heating, no orbital resonances • But it has evidence of subsurface liquid water?! – Magnetic field…

A. Auroras B. Infrared light C.Jupiter pulls harder on one side than the other D.Volcanoes

Not to scale!

160 mi across

1,600 miles across To scale! © 2015 Pearson Education, Inc. What mechanism is most responsible for generating the internal heat of Io that drives its volcanic activity? A. radioactive decay B. tidal heating C. accretion D. differentiation E. bombardment

• Titan is the only moon in the solar system that has a thick atmosphere. • It consists mostly of nitrogen (95%) with some argon, methane, and ethane. • Very cold: 93K (-180C)

• The Huygens probe provided a first look at Titan's surface in early 2005. • It had liquid methane, "rocks" made of ice.

• Radar imaging of Titan's surface has revealed dark, smooth regions that may be lakes of liquid methane.

• Differentiated, but cold today Medium Moons of Saturn

Fountains of ice particles and water vapor from the surface of Enceladus indicate that geological activity is ongoing.

• Similar to Pluto, but larger • Captured into orbit: – orbits in retrograde, inclined orbit – Spiraling in: will be torn apart • Evidence for past geological activity – from tidal heating as orbit changed

• Varying amounts of geological activity occur. • Moon Miranda has large tectonic features and few craters (episode of tidal heating in past?).

• Ice melts at lower temperatures. • Tidal heating can melt internal ice, driving activity.

• Not all have same origins Jupiter and Saturn’s Small Moons

• Jupiter: – 4 inner in regular orbits – 59 outer in inclined retrograde orbits: captured asteroids • Saturn – 55 small – Some in close regular orbits: remnants of collisions? – Most in outer, inclined retrograde orbits: captured asteroids Uranus’s Small Moons

• 13 small inner moons – prograde orbits – Orbits are changing! – Due to close encounters – all will collide within 100 million years so less than 100 million years old – Origin unknown: possibly from fragmentation of other moons? • 9 small outer moons – retrograde orbits – captures Neptune’s Small Moons

• 7 small inner moons: prograde, equatorial – Re-accreted after collisions of original moons when Triton was captured? • 6 small outer moons: large, inclined orbits – 3 prograde – 3 retrograde What have we learned?

• What kinds of moons orbit the jovian planets? – Moons of many sizes – Level of geological activity depends on size. • Why are Jupiter's Galilean moons geologically active? – Tidal heating drives activity, leading to Io's volcanoes and ice geology on other moons.

• What geological activity do we see on Titan and other moons? – Titan is the only moon with a thick atmosphere. – Many other icy moons show signs of geological activity. • Why are jovian planet moons more geologically active than small rocky planets? – Ice melts and deforms at lower temperatures, enabling tidal heating to drive activity.

• They are made up of numerous, tiny individual particles. • They orbit over Saturn's equator. • They are very thin.

We see different view of rings depending on orientation of Earth, Saturn

• Some small moons create gaps within rings.

Pair of small moons can force particles into a narrow ring

• Orbital resonance with a larger moon can also produce a gap

• Cassini division is due to tugs from resonance with Mimas (1:2 resonance)

© 2015 Pearson Education, Inc. The full set of rings, imaged as Saturn eclipsed the Sun from the vantage of the Cassini spacecraft, 1,200,000 km (746,000 mi) distant, on 19 July 2013 (brightness is exaggerated). Earth appears as a dot at 4 o'clock, between the G and E rings. (Wikipedia/NASA/JPL) 83 Jovian Ring Systems • All four jovian planets have ring systems • Others have smaller, darker ring particles than Saturn

• Radiation darkened methane ice, small particles

• They formed from dust created in impacts on moons orbiting those planets.

How do we know this?

• Rings aren't leftover from planet formation because the particles are too small to have survived this long. • There must be a continuous replacement of tiny particles. • The most likely source is impacts with the jovian moons.

© 2015 Pearson Education, Inc. Ring Formation • Jovian planets all have rings because they possess many small moons close-in. • Impacts on these moons are random; produce ring particles • Origins of Saturn's incredible rings is unknown

• What are Saturn's rings like? – They are made up of countless individual ice particles. – They are extremely thin with many gaps. • Why do the jovian planets have rings? – Ring systems of other jovian planets are much fainter with smaller, darker, less numerous particles. – Ring particles are probably debris from moons.

A. orbiting in the equatorial plane of their planet. B. known to exist for all of the jovian planets. C. composed of a large number of individual particles that orbit their planet in accord with Kepler's third law. D. all of the above

© 2015 Pearson Education, Inc. 6.2 Asteroids, Comets, and the Impact Threat Our Goals for Learning: • Why are asteroids and comets grouped into three distinct regions? • Do small bodies pose an impact threat to Earth? Asteroid Facts

• Largest is Ceres, diameter ~1,000 km • 150,000 in catalogs, and probably over a million with diameter >1 km.

• Asteroids are rocky leftovers of planet formation. • 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.

• 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.

© 2015 Pearson Education, Inc. Which explanation for the asteroid belt seems the most plausible? A. The belt is where all the asteroids happened to form. B. The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter. C. The belt is where all the asteroids happened to survive.

A. about twice that of Earth B. about the same as that of Jupiter C. less than any terrestrial planet D. about the same as that of Earth E. more than all the planets combined

• Icy-rocky objects (comets) on orderly orbits at 30-100 AU in disk of solar system • The largest of these, Eris dwarf planet Eris, is the same size as Pluto

• 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. • Not large: solar nebula was thin

• Oort Cloud: Comets 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.

• Most comets remain perpetually frozen in the outer solar system.

• Dirty snowballs: – H compounds – CO, CO2 ices – complex molecules including organics

Comet Wild 2

• Only comets that enter the inner solar system grow tails.

© 2015 Pearson Education, Inc. Anatomy of a Comet Comet Hale-Bopp • Coma is atmosphere that comes from heated nucleus • Plasma tail is gas escaping from coma, pushed by solar wind • Dust tail is pushed by photons Nucleus of Halley’s Comet © 2015 Pearson Education, Inc. Meteor Showers

• 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.

© 2015 Pearson Education, Inc. What have we learned? • What are comets like? – Comets are like dirty snowballs – Most are far from Sun and do not have tails – Tails grow when comet nears Sun and nucleus heats up • Where do comets come from? – Comets in plane of solar system come from Kuiper Belt – Comets on random orbits come from Oort cloud

• 15 February 2013 • 40,000 mph; 13,000 tonnes; 20 m across • Exploded 20 mi up; 500 kilotons TNT • Recovered 1200 lb piece from lake bottom "2 Cheljabinsk meteorite fragment" by Didier Descouens - Own work. Licensed under CC BY-SA 4.0 via Commons - https://commons.wikimedia.org/wiki/File:2_Cheljabinsk_meteorite_fra • Video gment.jpg#/media/File:2_Cheljabinsk_meteorite_fragment.jpg Frequency of Impacts

• Small impacts happen almost daily. • Impacts large enough to cause mass extinctions are many millions of years apart.

• 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 ~ millions of years • Major damage ~ tens to hundreds of years

Credit: Don Davis Mass Extinctions

• Fossil record shows occasional large dips in the diversity of species: mass extinctions. • The most recent was 65 million years ago, ending the reign of the dinosaurs. – 99% of organisms died – 75% of species went extinct

• Iridium is very rare in Earth surface rocks but is 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.

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

• Geologists found a large subsurface crater about 65 million years old in Mexico.

• Hot rock debris rains down: fires • Tsunami • Dust, smoke in atmosphere – global cooling (months) – plants die • vaporized rock: increased CO2 in atmosphere; increased greenhouse effect (decades) • Nitrous oxides created in atmosphere – killed marine organisms – acid rain

• We haven’t seen it yet. • We have found 1682 potentially hazardous asteroids (PHAs) as of March 6, 2016 – PHA: larger than 100m; come within 0.05 AU of Earth • 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…