Jupiter& its system Saturn& its system

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• Jupiter & its system• Saturn & its system

Chapter 14Comparative Planetology of

the Jovian Planets

Lecture 21 ASTA01

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• The worlds of the outer solar system can be studied from Earth. • However,

Lecture 21 ASTA01

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• Much of what scientists know has been radioed back to Earth from robot spacecraft.

• Voyager 2 flew past each of the outer planets in the 1970s and 1980s. • The Galileo spacecraft circled Jupiter dozens of

times in the late 1990s. • The Cassini/Huygens orbiter and probe arrived

at Saturn in 2004.• Throughout this discussion, you will find images and

data returned by these robotic explorers.

Space probes

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A Travel Guide to the Outer Planets

• You are about to visit five worlds that are truly unearthly.• This travel guide will warn you about what to

expect.

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The Outer Planets

• The outermost planets in our solar system are Jupiter, Saturn, Uranus, and Neptune. • These are often called the “Jovian planets,”

meaning that they are like Jupiter (named after a Roman god also known as Jove).

The Outer Planets• The figure below compares these four

worlds.

The Outer Planets

• Jupiter is the largest of the Jovian worlds.• It is over 11 times the diameter of Earth.

The Outer Planets

• Saturn is a bit smaller than Jupiter.• Uranus and Neptune are quite a bit

smaller than Jupiter.

The Outer Planets

• The other feature you will notice immediately is Saturn’s rings. • They are bright and beautiful and composed

of billions of ice particles.

The Outer Planets

• Jupiter, Uranus, and Neptune have rings too.• However, they are not easily detected from

Earth and are not visible here.

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Atmospheres and Interiors

• The four Jovian worlds have hydrogen-rich atmospheres filled with clouds. • On Jupiter and Saturn, you can see that the

clouds form stripes that circle each planet. • You will find traces of these same types of features

on Uranus and Neptune, but they are not very distinct.

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• Models based on observations indicate that, below their atmospheres, Jupiter and Saturn are mostly liquid. • So, the old fashioned term for these planets –

the gas giants – should probably be changed to the liquid giants.


• Uranus and Neptune are sometimes called the ice giants.• They are rich in water in both solid and liquid

forms.

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• Only near their centres do the Jovian planets have cores of dense material with the composition of rock and metal.

• Composition, but not the solid state…• None of the worlds has a definite solid surface

on which you could walk.

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• You have learned that the Jovian planets have low density because they formed in the outer solar nebula where water vapour could freeze to form ice particles.• The ice accumulated into proto-planets with

density lower than the rocky terrestrial planets and asteroids. • Once these planets grew massive enough, they

could draw in even lower-density hydrogen and helium gas directly from the nebula by gravitational collapse.

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Satellite Systems

• You can’t really land your spaceship on the Jovian worlds.• You might, however, be able to land on one of

their moons. • All the outer solar system planets have extensive

moon systems (as known in 2012):• Jupiter has 67+ moons, • Saturn has 62+ moons• Uranus has 37+ moons• Neptune has 13+ moons

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Satellite Systems

• In many cases, the moons interact gravitationally.• They mutually adjust their orbits.• They also affect the planetary ring systems.

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Satellite Systems

• Some of the moons are geologically active now.• Others show signs of past activity.• Of course, geological activity depends on heat flow

from the interior.• So, you might ponder what could be heating the insides

of these small objects.• In fact, it is the mutual interaction of moons with

themselves and with their host planets via gravitation that causes the heating

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Jupiter

• Jupiter, named for the Roman king of the gods, can be very bright in the night sky.• Its cloud belts and four largest moons can be

seen through even a small telescope.

Jupiter

• Jupiter is the largest and most massive of the Jovian planets.• It contains 71 percent

of all the planetary matter in the entire solar system.

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Jupiter

• You used Earth, the largest of the terrestrial planets, as the basis for comparison with the others.• Similarly, you can examine Jupiter in detail as

a standard in your comparative study of the other Jovian planets.

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The Interior

• Jupiter is only 1.3 times denser than water.• For comparison, Earth is more than 5.5 times

denser than water.• This gives astronomers a clue about the average

composition of the planet’s interior.

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The Interior

• Jupiter’s shape also gives information about its interior.• Jupiter and the other Jovian planets are all

flattened.• A world with a large solid, rocky core and mantle

would not be flattened much by rotation. • A largely liquid planet, though, would flatten

significantly (independent of chemical composition)

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Polar radius 66854+-10 km < Equatorial rad 71492+-4 km

93.5 : 100

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Jupiter’s orbit and the zodiac

Jupiter orbits the Sun at a distance of 5.2 AU, completing each orbit in about 11.9 years – which means that it goes through each zodiacal region in one year.

That may be why we have 12 signs of zodiac, and 2*12 hours in a day,

but we don’t really know…

The zodiac with 12 signs appeared in Chaldea (Babilonian) astrology in ~7th century BCE

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The Interior

• Despite the fact that Jupiter is the most massive of all the planets, its sidereal rotation period is merely 10 hours, making its spinning rate the fastest of all the planets. • There is nowhere to stand on Jupiter’s surface, a

fact that is easily forgotten when viewing Jupiter through a telescope or binoculars.• Models indicate that the interior is mostly

liquid hydrogen.

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The Interior

• However, if you jumped into Jupiter carrying a rubber raft expecting an ocean, you would be disappointed.• The base of the atmosphere is so hot and the

pressure is so high that there is no sudden boundary between liquid and gas.• As you fell deeper through the atmosphere, you

would find the gas density increased around you until you were sinking through a liquid.• You would, however, never splash into a distinct

liquid surface because the transition from gas to liquid takes place very gradually.

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The Interior

• Under very high pressure, liquid hydrogen becomes liquid metallic hydrogen.• This is a very good conductor of electricity.• Most of Jupiter’s interior is composed of this

unusual material.

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The Interior

• That large mass of conducting liquid is stirred by convection currents and spun by the planet’s rapid rotation.• As a result, it drives the dynamo effect and

generates a powerful magnetic field.• Jupiter’s field is over 10 times stronger than

Earth’s.

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The Interior

• A planet’s magnetic field deflects the solar wind and dominates a volume of space around the planet called the magnetosphere.

The Interior

• The strong magnetic field around Jupiter traps particle streams from the solar wind in radiation belts a billion times more intense than the Van Allen belts that surround Earth.

The Interior

• The spacecraft that have flown through these regions received over 1000 times the radiation that would have been lethal for a human.

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The Interior

• At Jupiter’s centre, a so-called rocky core contains heavier elements, such as iron, nickel, and silicon.• With a temperature four times hotter than the

surface of the Sun (25000 K) and a pressure of 50 million atm (Earth’s air pressure at sea level), this material is unlike any rock on Earth.• The term rocky core refers to the chemical

composition, not to the properties of the material.

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The Interior

Jupiter’s Complex Atmosphere

The red spot, observed by Galileo is a permanent(?) hurricane

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• #1. Jupiter’s extensive magnetosphere is responsible for auroras around the magnetic poles.

• #2. Jupiter’s rings, discovered in 1979 by the Voyager I space probe, are relatively close to the planet.

• #3. the pattern of coloured cloud bands circling the planet is called belt–zone circulation.

This pattern is related to the high- and low-pressure areas found in Earth’s atmosphere.


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#4. The belt–zone circulation is related to the high- and low-pressure areas found in Earth’s atmosphere. The positions of the cloud layers are at certain temperatures within the atmosphere where ammonia (NH3), ammonium hydrosulphide (NH4SH), and water (H2O) can condense.

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#5. It is not yet known whether the Great Red Spot is long-lived or will disappear in a 100 years, whether it is deeply rooted or a shallow circulation like earth hurricanes.

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Jupiter’s Ring

• Astronomers have known for centuries that Saturn has rings.

• Jupiter’s ring, though, was not discovered until 1979, when the Voyager 1 spacecraft sent back photos.

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Jupiter’s Rings

• Jupiter’s rings were not discovered until 1979, when the Voyager 1 spacecraft sent back photos. Less than 1 percent as bright as Saturn’s icy rings, Jupiter’s rings. This indicates that they are rocky rather than icy. Ring particles are mostly microscopic, very dark (1/100 brightness of Saturn’s rings) and reddish.

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Jupiter’s Rings

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Jupiter’s Rings

• Photos show that they are very bright when illuminated from behind.• That is, it is scattering light forward. • Large particles do not scatter light forward.

So, a ring filled with basketball-size particles would look dark when illuminated from behind.

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Jupiter’s Ring

• Forward scattering of visible light shows you that the ring is mostly made of tiny grains, with diameters approximately equal to the wavelengths of visible light (0.5 micrometers or microns)• This would be about the size of particles in

cigarette smoke.

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Jupiter’s Ring

• The rings orbit inside the Roche limit.• This is the distance from a planet within

which a moon cannot hold itself together by its own gravity.

• If a moon comes inside the Roche limit, the tidal forces overcome the moon’s gravity and pull the moon apart.• Also, raw material for a moon cannot coalesce

inside the Roche limit.

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Jupiter’s Ring

• The Roche limit is about 2.4 times the planet’s radius, depending somewhat on the relative densities of the planet and the moon material.• Jupiter’s rings lie inside the limit for the planet.• Those of Saturn, Uranus, and Neptune too lie

within the respective planetary limits.

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Jupiter’s Ring

• Now you can understand Jupiter’s dusty rings.• If a dust speck gets knocked loose from a

larger rock inside the Roche limit, the rock’s gravity cannot hold the dust speck.

• Also, the billions of dust specks in the ring can’t pull themselves together to make a new moon because of tidal forces inside the Roche limit.

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Jupiter’s Ring

• We can be sure that Jupiter’s ring particles are not old.• The pressure of sunlight and the planet’s

powerful magnetic field alter the orbits of the particles.• Images show faint ring material extending down

toward the cloud tops – evidently dust specks spiralling into the planet.• Dust is also destroyed by the intense radiation

around Jupiter that grinds the dust specks down to nothing in a century or so.

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Jupiter’s Ring

• The rings you see today, therefore, can’t be material left over from the formation of Jupiter.• The rings of Jupiter must be continuously

resupplied with new dust.• Observations made by the Galileo spacecraft

provide evidence that the source of ring material is micrometeorites eroding small moons orbiting near, or within, the rings.

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Jupiter’s Ring

• The rings around Saturn, Uranus, and Neptune are also known to be short-lived.• They too must be resupplied by new material,

probably eroded from nearby moons.

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Jupiter’s Ring• In addition to supplying the rings with particles, moons:• Confine the rings gravitationally,• Alter their shapes. • For comparison: Mimas, Cassini gap 1:2 resonance with

Mimas

Jupiter’s Family of Moons

• Jupiter has four large moons and at least 60 smaller moons.• Larger telescopes and modern techniques are

rapidly finding more small moons orbiting the Jovian planets.

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• Most of the small moons are probably captured asteroids.

• In contrast, the four largest (Galilean)moons are clearly related to each other and probably formed with Jupiter.

Jupiter’s Family of Moons: Ganymede + Callisto

• The outermost Galilean moons, Ganymede and Callisto, are about the size of Mercury – one and a half times the size of Earth’s moon.• In fact, Ganymede is the largest moon in the

solar system.


• They have low densities – only 1.9 and 1.8 g/cm3 respectively. • This must mean that they consist roughly of

half rock and half ice.

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• Observations of their gravitational fields by the Galileo spacecraft reveal that both have rocky or metallic cores and lower-density icy exteriors. • So, they have both differentiated.

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• Both moons interact with Jupiter’s magnetic field in a way that shows they probably have mineral-rich layers of liquid water 100 km or more below their icy crusts.


• Callisto’s surface and most of Ganymede’s surface appear old.• This is because they are heavily cratered and

very dark.


• The continuous blast of micrometeorites evaporates surface ice, leaving behind embedded minerals to form a dark skin, like the grimy crust on an old snowbank.• So, surfaces get darker with age.


• More recent impacts dig up cleaner ice and leave bright craters.


• Ganymede has some younger, brighter grooved terrain believed to be systems of faults in the brittle crust. • Some sets of grooves overlap other sets of

grooves.• This suggests extended episodes of geological activity.

Jupiter’s Family of Moons: Europa

• The density of the next moon inward, Europa, is 3 g/cm3.This is high enough to mean that it is mostly rock with a thin icy crust.


• Europa’s visible surface is made of very clean ice, contains very few craters, has long cracks in the icy crust, and has complicated terrain that resembles blocks of ice in Earth’s Arctic Ocean.


• The pattern of mountainlike folds on the surface suggests that the icy crust breaks as the moon is flexed by tides.


• Europa’s gravitational influence on the Galileo spacecraft reveals that a liquid-water ocean perhaps 200-km deep lies below the 10- to 100-km-thick crust.


• The lack of craters shows you that it is an active world where craters are quickly erased.

Jupiter’s Family of Moons: Io

• Images from spacecraft reveal that Io, the innermost of the Galilean moons, has over 100 volcanic vents on its surface.


• The active volcanoes throw sulphur-rich gas and ash high above the surface.• The ash falls back to bury the surface at a rate of a

few millimetres a year.

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• That explains why you see no impact craters on Io. They are covered up as fast as they form.

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• Io’s density is 3.6 g/cm3.• Thus, it is not ice but rather rock and metal. • Its gravitational influence on the passing Galileo

spacecraft revealed that it is differentiated into a large metallic core, a rocky mantle, and a low-density crust.

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• The activity seen in the Galilean moons must be driven by energy flowing outward.• Yet, these objects are too small to have remained hot from

its formation. • The solution is hidden in the orbital dynamics

The system of JupiterIs a miniature of a Planetary system, But more resonant

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• Io’s volcanism is driven by tidal heating.• Io follows a slightly elliptical orbit, caused by its

interactions with the other moons (1:2:4 resonance with Europa and Ganymede)• Jupiter’s gravitational field flexes Io with tides.• The resulting friction heats its interior. • That heat flowing outward causes the “widespread

& recurrent volcanism”• That prediction was published by Peale,

Cassen and Reynolds (1979) ~2 weeks

before Voyager 1 arrived at Io (where a

Moon-like surface was expected!)


• Europa is not as active as Io. However, it too must have a heat source – presumably tidal heating like Io• Maybe the warm underground ocean can sustain

life?• Or Ganymede’s ocean?

• Or Callisto’s?

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• Ganymede is no longer active.• When it was younger,

though, it must have had internal heat to break the crust and produce the grooved terrain.

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The History of Jupiter

• The four Galilean moons seem to have formed like a mini-solar system in a disk of gas and dust around the forming planet.• The innermost, Io, is densest.• The densities of the others decrease as you move

away from Jupiter, similar to the way the densities of the planets decrease with distance from the Sun.

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The History of Jupiter’s moons

• Perhaps the inner moons incorporated less ice because they formed closer to the heat of the growing planet.• You can recognize that tidal heating also has

been important – and the intense warming of the inner moons could have driven off much of their ices.• Thus, two processes together may be responsible

for the differences in compositions of the Galilean moons.

Jupiter& its system Saturn& its system

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