Chapter 11 Jovian Planet Systems

11.1 A Different Kind of Planet Our goals for learning: Are jovian planets all alike? What are jovian planets like on the inside? What is the weather like on jovian planets? Do jovian planets have magnetospheres like Earth’s?

Are jovian planets all alike?

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

Density Differences Uranus and Neptune are denser than Saturn because they have less H/He, proportionately

Density Differences But that explanation doesn’t work for Jupiter….

Jupiter Jupiter’s appearance and physical properties Jupiter is the largest planet both in diameter and mass more than10x Earth’s diameter and 300x the mass Dense, richly colored parallel cloud bands cloak the planet Atmosphere is mainly H, He, CH4, NH3, and H2O Clouds appear to be particles of water, ice, and ammonia compounds Bright colors of clouds may come from complex organic molecules with composition still unknown Jupiter rotates once about every 10 hours with this fast rotation leading to a significant equatorial bulge Figure 9.1

Jupiter Jupiter’s interior Jupiter’s average density is 1.3 g/cm3 – indicates an interior composed mainly of very light elements Using average density, Jupiter’s shape, and its gravitational attraction on its moons and passing spacecraft, Jupiter’s interior can be calculated Interior becomes increasingly more dense as center is approached with its gaseous upper layers turning to liquid hydrogen about 10,000 km below the surface Deeper still, liquid hydrogen compresses into liquid metallic hydrogen, a material scientists only recently created in tiny high-pressure chambers An iron rocky core, a few times bigger than the Earth, probably resides at the center Figure 9.2

Jupiter Jupiter’s interior (continued) Jupiter’s atmosphere Jupiter, with a core temperature of about 30,000 K, emits more energy than it receives Possibly due to heat left over from its creation Planet may still be shrinking in size converting gravitational energy into heat Jupiter’s atmosphere General convection pattern: Heat within Jupiter carries gas to the top of the atmosphere High altitude gas radiates into space, cools and sinks Coriolis effect turns rising and sinking gases into powerful jet streams (about 300 km/hr) that are seen as cloud belts Adjacent belts, with different relative speeds, create vortices of various colors, the largest being the Great Red Spot, which has persisted for over 300 years Figure 9.3, Figure 9.4

Jupiter Jupiter’s atmosphere (continued) Convection in the deep metallic liquid hydrogen layer coupled with Jupiter’s rapid rotation creates a powerful magnetic field 20,000x stronger than the Earth’s field, it is the largest planetary magnetic field Jupiter’s auroral activity and intense radio emissions are indicative of its magnetic field Magnetic field also traps charged particles into far above the planet in regions resembling the Earth’s Van Allen radiation belts Lightning in clouds has been observed Figure 9.5

Jupiter Jupiter’s ring Jupiter has a thin ring made of tiny particles of rock dust and held in orbit by Jupiter’s gravity Solar radiation and collisions with charged particles trapped in Jupiter’s magnetic field exert a friction on the ring dust that will eventually cause the dust the drift into the atmosphere To maintain the ring, new dust must be provided – possibly from collision fragments ejected from the Jovian moons Figure 9.6

Saturn Saturn’s Appearance and Physical Properties Saturn is the second largest planet with a diameter and mass more than10x Earth’s diameter and 95x the mass Its average density of 0.7 g/cm3 is less than than of water Low density, like Jupiter, suggests a composition mostly of hydrogen an its compounds Internal structures similar to Jupiter Saturn radiates more energy than it receives, but unlike Jupiter, this energy probably comes from the conversion of gravitational energy from falling helium droplets as they condense in Saturn’s interior Saturn’s atmosphere looks different than Jupiter’s because Saturn is cold enough for ammonia gas to freeze into cloud particles that veil its atmosphere’s deeper layers Figure 9.10, Figure 9.11

Saturn Saturn’s Rings Rings are wide but thin Main band extends from about 30,000 km above its atmosphere to about twice Saturn’s radius (136,000 km) Faint rings can be seen closer to Saturn as well as farther away Thickness of rings: a few hundred meters Rings not solid, but made of a swarm of individual bodies Sizes range from centimeters to meters Composition mainly water, ice, and carbon compounds and is not uniform across rings Rings structures into ringlets and gaps Large gaps probably due to resonances with Saturn’s moons located beyond the rings Narrow gaps due to complex interaction between ring particles and tiny moons in the rings Figure 9.12, Figure 9.13

Saturn Origin of Planetary Rings The Roche Limit Rings once thought to be left over remains from a planet’s formation However, ring lifetime is short since, as pointed out for Jupiter, they are subject to frictional forces that spiral particles into the planet’s atmosphere For rings to persist they must be replenished The Roche Limit Any object held together solely by gravity will break apart by tidal forces if it gets too close to the planet. Distance of breakup is called the Roche limit and is 2.44 planetary radii if object and planet have the same density All planetary rings lie near their planet’s Roche limit Existence of side-by-side ringlets of different compositions indicates rings supplied by varied comets and asteroids Objects bonded together chemically will survive Roche limit Figure 9.14 Animation: Roche breakup of a moon

Uranus Introduction Uranus’s Atmosphere Unknown to the ancients, even though visible to the naked eye, Uranus was not discovered until 1781 by Sir William Herschel While small relative to Jupiter/Saturn, Uranus is 4x larger in diameter than Earth and has 15x the mass At 19 AU, Uranus is difficult to study from Earth, but even close up images from Voyager reveal a rather featureless object Uranus’s Atmosphere Atmosphere is rich in hydrogen and methane Methane gas and ice are responsible for the blue color of Uranus’s atmosphere Figure 9.17, Figure 9.18

Uranus Uranus’s Interior With a density of 1.2 g/cm3 and smaller size, Uranus must contain proportionally fewer light elements than Jupiter/Saturn However, its density is too low for it to contain much rock or iron Consequently, Uranus’s interior probably contains water, methane, and ammonia, which would also help explain the spectrum of Uranus Size of equatorial bulge supports the idea that the interior is mostly water and other hydrogen-rich molecules and that it may have a rock/iron core It is currently not known if the core formed first and attracted lighter gases that condensed on it, or the core formed by differentiation after the planet formed. Figure 9.19

Uranus Uranus’s Rings and Moons Rings Moons Neptune is encircled by a set of narrow rings composed meter-sized objects These objects are very dark, implying they are rich in carbon particles or organic-like materials The extremely narrow rings may be held in place by shepherding satellites Moons Uranus has 5 large moons and several small ones that form a regular system Moons probably composed of ice and rock and many show heavy cratering Miranda is very unique in that it appears to have been torn apart and reassembled Figure 9.20, Figure 9.21

Uranus Uranus’s Odd Tilt Uranus’s spin axis is tipped so that it nearly lies in its orbital plane The orbits of Uranus’s moons are similarly tilted This odd tilt suggests that Uranus was struck during its formation and splashed out material to form the moons The large tilt together with Uranus’s fast spin rate gives an odd night/day pattern over the course of a Uranian year The uneven heating pattern may be the reason Uranus does not have cloud bands Figure 9.22

Neptune Introduction Neptune’s Structure Neptune is the outermost of the Jovian giants and similar in size to Uranus A deep blue world with cloud bands and vortex structures – the Great “Dark” Spot being, at one time, the most prominent feature Neptune was discovered from predictions made by John C. Adams and Urbain Leverrie, who calculated its orbit based on disturbances in Uranus’s orbit Neptune’s Structure Neptune’s interior is probably similar to Uranus’s – mostly ordinary water surrounded by a thin atmosphere rich in hydrogen and its compounds and probably has a rock/iron core Figure 9.23

Neptune Neptune’s Atmosphere Neptune’s blue, like Uranus, comes from methane in its atmosphere Unlike Uranus, Neptune has cloud belts Like Jupiter/Saturn, Neptune radiates more energy than it gains from the Sun The deep interior heat source drives convective currents which then lead, via the Coriolis effect, to the visible atmospheric belts Neptune’s winds are extremely fast reaching speeds of 2200 km/hr and creating visible vortex structures Figure 9.24

Neptune Neptune’s Rings and Moons Rings Neptune, like the other giant planets, has rings They are probably the debris from small satellites or comets that have collided and broken up They contain more dust than the Saturn/Uranus rings The rings are not distributed uniformly around the ring indicating they are relatively new Figure 9.25

Neptune Neptune’s Rings and Moons (continued) Moons Neptune has six small moons orbiting close to the planet and two moons farther out One of the two is Triton Triton’s orbit is “backwards” and is highly tilted with respect to Neptune’s equator – Triton is perhaps a captured planetesimal from the Kuiper belt Triton is large enough and far enough from the some to retain an atmosphere Triton has some craters with dark steaks extending from them – at least one of which originates from a geyser caught in eruption by the passing Voyager II The material in the geyser is thought to be a mixture of nitrogen, ice and carbon compounds heated beneath the surface by sunlight until it expands and bursts to the surface Figure 9.26

Pluto Survey Discovered by Clyde Tombaugh in 1930 by scanning millions of star images over the course of a year Pluto’s large distance and very small size makes it difficult to study, even in the largest telescopes In 1978, James Christy discovered Charon, Pluto’s moon The orbiting combination of Pluto and Charon allows an accurate measurement of their masses – Pluto is the least massive planet Charon’s steeply tilted orbit implies that Pluto is highly tilted as well Charon takes 6.4 days to orbit Pluto once Pluto rotates with the same period of 6.4 days Figure 9.27

Pluto Survey (continued) The recent eclipses of Pluto with Charon has allowed the radii of both objects to be determined Pluto is 1/5 the diameter of Earth Charon is relatively large being about ½ Pluto’s diameter From these masses and diameters, Pluto’s density is 2.1 g/cm3 suggesting an object of water, ice, and rock Very little is known of Pluto’s surface, but computer analysis of eclipse images suggest a bright south pole, perhaps a frozen methane cap Pluto also has a tenuous atmosphere of N2, CO and traces of CH4 Pluto was once thought to be a moon of Neptune that escaped; now it is thought Neptune captured Pluto, a remnant planetesimal Figure 9.28

Sizes of Jovian Planets Greater compression is why Jupiter is not much larger than Saturn even though it is three times more massive Jovian planets with even more mass can be smaller than Jupiter

Rotation and Shape Jovian planets are not quite spherical because of their rapid rotation

What are jovian planets like on the inside?

Interiors of Jovian Planets No solid surface. Layers under high pressure and temperatures. Cores (~10 Earth masses) made of hydrogen compounds, metals & rock The layers are different for the different planets. WHY?

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

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

Comparing Jovian Interiors Models suggest cores of jovian planets have similar composition Lower pressures inside Uranus and Neptune mean no metallic hydrogen

Jupiter’s Internal Heat Jupiter radiates twice as much energy it receives from Sun (it’s radiation would kill an unprotected astronaut in minutes) Energy probably comes from slow contraction of interior (releasing potential energy)

Internal Heat of Other Planets Saturn also radiates twice as much energy it receives from 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 As Jupiter and Saturn cool, the interior of the planets will approach temperatures where hydrogen and helium no longer mix. This process, which is likely to have already occurred in Saturn, could lead to the formation of helium droplets that would "rain down" towards the center of the planet and provide an additional source of heat.

What is the weather like on jovian planets?

Jupiter’s Atmosphere Hydrogen compounds in Jupiter form clouds Different cloud layers correspond to freezing points of different hydrogen compounds NH3 NH4SH H2O

Jovian Planet Atmospheres Other jovian planets have cloud layers similar to Jupiter’s Different compounds make clouds of different colors

Jupiter’s colors Ammonium sulfide clouds (NH4SH) reflect red/brown. Ammonia, the highest, coldest layer, reflects white.

Saturn’s colors Saturn’s layers are similar, but deeper in and farther from the Sun --- more subdued.

Methane on Uranus and Neptune Methane gas of Neptune and Uranus absorb red light but transmit blue light Blue light reflects off methane clouds, making those planes look blue

Jupiter’s Bands White ammonia clouds form where air rises Coriolis effect changes N-S flow to E-W winds Between white clouds we see deeper reddish clouds of NH4SH Warmer red bands are brighter in IR

Jupiter’s Great Red Spot A storm twice as wide as Earth Has existed for at least 3 centuries

Weather on Jovian Planets All the jovian planets have strong winds and storms

Do jovian planets have magnetospheres like Earth’s?

Jupiter’s Magnetosphere Jupiter’s strong magnetic field gives it an enormous magnetosphere (14 times stronger than Earth’s) Gases escaping Io feed the donut-shaped Io torus

Other Magnetospheres All the jovian planets have substantial magnetospheres, but Jupiter’s is largest by far

Thought Question Jupiter does not have a large metal core like the Earth. How can it have a magnetic field? a) The magnetic field is left over from when Jupiter accreted b) Its magnetic field comes from the Sun c) It has metallic hydrogen inside, which circulates and makes a magnetic field d) That’s why its magnetic field is weak

What have we learned? Are jovian planets all alike? Jupiter and Saturn are mostly H and He gas Uranus and Nepture 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

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 largest by far

11.2 A Wealth of Worlds: Satellites of Ice and Rock Our goals for learning: What kinds of moons orbit jovian planets? Why are Jupiter’s Galilean moons so geologically active? What is remarkable about Titan and other major moons of the outer solar system? Why are small icy moons more geologically active than small rocky planets?

Planetary Fact Sheet - Metric

What kinds of moons orbit the jovian planets?

Sizes of Moons 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

Medium & 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.

Small Moons Far more numerous than the medium and large moons. Not enough gravity to be spherical: “potato-shaped”

Small Moons Captured asteroids or comets, so orbits do not follow usual patterns.

How are Jupiter’s Galilean moons so geologically active?

Io’s Volcanic Activity Io is the most volcanically active body in the solar system, but how?

Io’s Volcanoes Volcanic eruptions continue to change Io’s surface

Tidal Heating Io is squished and stretched as it orbits Jupiter But why is its orbit so elliptical?

The tugs add up over time, making all 3 orbits elliptical. Orbital Resonances Every 7 days, these 3 moons line up.

Europa’s Ocean: Waterworld?

Tidal stresses crack Europa’s surface ice.

Europa’s interior also warmed by tidal heating Based on Galileo spacecraft measurements of the strength of gravity over different positions of Europa and theoretical modeling of the interior.

Ganymede Largest moon in the solar system Clear evidence of geological activity Tidal heating plus heat from radio-active decay? Largest moon Figure might work better if there were one with out the callouts.

Callisto “Classic” cratered iceball. No tidal heating, no orbital resonances. But it has magnetic field !?

Thought Question How does Io get heated by Jupiter? a) Auroras b) Infrared Light c) Jupiter pulls harder on one side than the other d) Volcanoes

What is remarkable about Titan a major moon of Saturn?

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

Titan’s Surface Huygens probe provided first look at Titan’s surface in early 2005 Liquid methane, “rocks” made of ice

Medium Moons of Saturn Almost all show evidence of past volcanism and/or tectonics

Medium Moons of Uranus Varying amounts of geological activity Moon Miranda has large tectonic features and few craters (episode of tidal heating in past?)

Neptune’s Moon Triton Similar to Pluto, but larger Evidence for past geological activity

Why are small icy moons more geologically active than small rocky planets?

Rocky Planets vs. Icy Moons Rock melts at higher temperatures Only large rocky planets have enough heat for activity Ice melts at lower temperatures Tidal heating can melt internal ice, driving activity

What have we learned? What kinds of moons orbit jovian planets? Moons of many sizes Level of geological activity depends on size Why are Jupiter’s Galilean moons so geologically active? Tidal heating drives activity, leading to Io’s volcanoes and ice geology on other moons

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

11.3 Jovian Planet Rings Our goals for learning: What are Saturn’s rings like? How do other jovian ring systems compare to Saturn’s? Why do the jovian planets have rings?

What are Saturn’s rings like?

What are Saturn’s rings like? They are made up of numerous, tiny individual particles They orbit over Saturn’s equator They are very thin

Earth-based view

Spacecraft view of ring gaps Voyager

Artist’s conception of close-up

Gap Moons Some small moons create gaps within rings

Shepherd Moons Pair of small moons can force particles into a narrow ring

Resonance Gaps Orbital resonance with a larger moon can also produce a gap Ie 4:3

How do other jovian ring systems compare to Saturn’s?

Jovian Ring Systems All four jovian planets have ring systems Others have smaller, darker ring particles than Saturn

Why do the jovian planets have rings?

Why do the jovian planets have rings? They formed from dust created in impacts on moons orbiting those planets How do we know that?

How do we know? 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.

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

What have we learned? What are Saturn’s rings like? Made up of countless individual ice particles Extremely thin with many gaps How do other jovian ring systems compare to Saturn’s? Much fainter ring systems with smaller, darker, less numerous particles Why do the jovian planets have rings? Ring particles are probably debris from moons

How the Universe Works Videos How the Universe Works: Jupiter How the Universe Works: Saturn