prepared by: jennifer west department of physics and astronomy university of manitoba backman seeds...

Prepared by: Jennifer West

Department of Physics and AstronomyUniversity of Manitoba

Backman Seeds Ghose Milosevic-Zdjelar Read

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Chapter 14 Comparative Planetology

of the Jovian Planets

Saturn, Uranus, Neptune & their satellite systems

Dwarf planets: Trans-Neptunians

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• 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.

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Saturn

• The Roman god Saturn was celebrated in a weeklong Saturnalia at the time of the winter solstice in late December

• Romans took over that tradition from Mesopotamia (god Marduk’s festival)

• Early Christians took over the Roman holiday to celebrate Xmas.

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Saturn

• Saturn is most famous for its beautiful rings.• These are easily visible through the telescopes of

modern amateur astronomers. • Amateurs recently discovered a storm on Saturn

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Saturn

• Saturn’s orbital radius is almost twice that of Jupiter (9.5 AU) and Saturn takes almost 30 years to orbit the Sun.

• Earth is shown

for comaprison

of sizes

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Saturn

• Interestingly, Saturn’s density is

ρ = 0.69 g/cm3,

One often hears that Saturn would float on water

Actually, that’s not true, the lake would fall toward and wrap around Saturn..

Low density means it’s made mostly of hydrogen + helium gas

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Saturn

• The two Voyager spacecraft flew past Saturn in 1979.• The Cassini spacecraft went into orbit around

Saturn in 2004 on an extended exploration of the planet, its rings, and its moons.

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Saturn the Planet

• Saturn shows only faint belt-zone circulation.• However, Voyager, Hubble Space Telescope,

and Cassini images show that belts and zones are present and that the associated winds blow up to three times faster than on Jupiter.

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Saturn the Planet

• Belts and zones on Saturn are less visible because they occur deeper in the cold atmosphere – below a layer of methane haze (CH4)

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Saturn the Planet

• The low density of Saturn suggests that it is, like Jupiter, rich in hydrogen and helium.• Photos show that Saturn is the most oblate of

the planets.• That evidence shows that its interior is mostly

liquid with a small core of heavy elements.

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Saturn the Planet

• As its internal pressure is lower, Saturn has less liquid metallic hydrogen than Jupiter.• Perhaps this is why its magnetic field is 20 times

weaker than Jupiter’s.• Like Jupiter, Saturn radiates more energy than it

receives from the Sun.• Models predict that it has a very hot interior.• Scientists suspect that He rains down toward

the center of Saturn in the form of droplets.

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Saturn’s Rings

• There are several important points to note about the icy rings of Saturn.

#1 Cassini division at 2:1 resonance with moon Mimas

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Saturn’s Rings

• #2. The rings are made up of billions of ice particles, each in its own orbit around the planet.• Particles you observe now can’t be as old as

Saturn. They must be replenished now and then by impacts on Saturn’s moons or other processes.

• The same is true of the rings around the other Jovian planets.

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Saturn’s Rings

• #3. The gravitational effects of small moons can confine some rings in narrow strands or keep the edges of rings sharp.• Moons can also produce waves in the rings

that are visible as tightly wound ringlets.

Daphnis,8 km moonOpens theEncke gap

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Saturn’s Rings

• #4. The ring particles are confined in a very thin plane (10 m only!!!) spread among small moons and confined by gravitational interactions with larger moons.

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Saturn’s Rings & moons

• The rings of Saturn & other Jovian worlds are created by and controlled by the planet’s moons.• Without the moons, there would be no rings.

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Saturn’s Family of Moons

• Saturn currently (2012) has 62 known moons.• They contain mixtures of ice and rock. Many are

small. Many are probably captured objects.

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Saturn’s Family of Moons: Titan

• The largest of Saturn’s moons is Titan.• It is a bit larger than Mercury.• Its density suggests that it must contain a

rocky core under a thick mantle of ices.• Titan is so cold that its gas molecules do not

travel fast enough to escape.

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• It has an atmosphere that is about 1.5 times denser than Earth’s but is composed mostly of N2, with traces of argon (Ar) and methane (CH4).

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• Sunlight converts some of the methane into complex carbon-rich molecules.• These collect into small particles, filling the

atmosphere with orange smog.

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• These particles settle

slowly downward to coat

the surface with what has

been described as dark

organic goo, composed

of carbon-rich molecules.

Dunes on Earth (upper)

& Titan (lower)

Titan

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• Titan’s surface is mainly composed of ices of water and methane at T= -180°C (93 K).

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• The Cassini spacecraft dropped the Huygens probe into Titan’s atmosphere.• It photographed dark drainage channels – rivers of

liquid methane (CH4)

• It falls as rain

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• Such methane downpours may be rare, though. No direct evidence of liquid methane was detected as the probe descended. But the orbiter found lakes!

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• Later radar images made by the Cassini orbiter have detected what appear to be lakes presumably containing liquid methane: the first extraterrestrial river discovered flows into lake Ontario on Titan.• Infrared images suggest the presence of

methane volcanoes that replenish the methane in the atmosphere.• So, Titan must have some internal heat source to

power the activity.

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Saturn’s Family of Moons: Iapetus

Iapetus close-up by Cassini space probe

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• Most of the remaining moons of Saturn:• Are small and icy,• Have no atmospheres,• Are heavily cratered, and• Have dark, ancient surfaces (are not active)

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Saturn’s Family of Moons: Pandora (diameter 84km)

• Most of the remaining moons of Saturn:• Are small and icy,• Have no atmospheres,• Are heavily cratered, and• Have dark, ancient surfaces.

Pandora

Diameter 84 km

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• The moon Enceladus, however, shows signs of recent geological activity• Some parts of its

surface contain 1000

times fewer craters

than other regions.

It also has

ice volcanoes!

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Saturn’s Family of Moons: Enceladus

• Infrared observations show that its south polar region is unusually warm and venting water and ice geysers, which have been directly imaged.

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• Evidently, a reservoir of liquid waters lies only tens of metres below the surface. • At some point in its history, the moon must have

been caught in a resonance with another moon and was warmed by tidal heating.

Planet Uranus

• Now that you are familiar with the gas giants in our solar system, you will be able to appreciate how weird the ice giants – Uranus and Neptune – are.

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Uranus

• Uranus was discovered in 1781 serendipitously (accidentally) by astronomer William Herschel, a German expatriate living in England.

• He named it George’s Star – after his patron the English King George III.

• Continental astronomers – especially the French – refused to accept a planet named after an English king. They called it Herschel.

Planet Uranus

• Years later, the name Uranus was suggested – the oldest of the Greek gods (god of the heavens)

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Uranus

• Uranus orbits the Sun at an average of 19.2 AU, almost twice as far away as Saturn. • Once again, we see how greatly spaced the

Jovian planets are compared to the inner Terrestrial planets, and that the next one is about 1.8-2 times further than its inner neighbour

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Planet Uranus

• Uranus is only a third of the diameter of Jupiter and only 1/20 as massive.• Being four times farther from the Sun, its

atmosphere is over 100°C colder than Jupiter’s. [1 K = 1°C (temp. difference), but Kelvin scale starts from absolute zero or T = 0 K = -273°C]

• T (Jupiter) ~ 150 K (110 K w/o internal heat) • T (Saturn) ~ 70 K • T (Uranus) ~ 50 K • T (Neptune) ~ 40 K

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Planet Uranus

• Uranus never grew massive enough to capture large amounts of gas from the nebula as Jupiter and Saturn did. • So, it has much less hydrogen and helium. • Its internal pressure is enough lower than Jupiter’s

that it should not contain any liquid metallic hydrogen.

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Planet Uranus

• Oblateness and mean density => a small core of heavy elements and a deep mantle of partly solid water.

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Planet Uranus

• Although referred to as ice, this material would not be anything like ice on Earth – given the temperatures and pressures inside Uranus.

• The internal structure and composition of

Uranus and Neptune are similar

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Planet Uranus

• The mantle also contains rocky material and dissolved ammonia and methane.• Circulation in this electrically conducting

mantle may generate the planet’s peculiar magnetic field, which is highly inclined to its axis of rotation.• Above the mantle lies the deep hydrogen and

helium atmosphere.

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Planet Uranus

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Planet Uranus

• Uranus’s odd rotation may have been produced when it was struck by a very large planetesimal late in its formation. • Alternatively, it could due to tidal interactions

with the other giant planets, as it migrated outward early in the history of the solar system.

Planet Uranus

• Voyager 2 photos show a nearly featureless ball.• The atmosphere is mostly hydrogen and helium.• However, traces of methane absorb red light,

making the atmosphere look green-blue.

Planet Uranus

• Computer enhancement revealed a few clouds and bands around the south pole.

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Planet Uranus

• In the decades since Voyager 2 flew past Uranus, spring has come to the northern hemisphere of Uranus and autumn to the southern hemisphere.

Planet Uranus

• Images made by the Hubble Space Telescope and modern Earth-based telescopes reveal changing clouds and cloud bands in both hemispheres.

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Planet Uranus

• Infrared measurements show that Uranus is radiating about the same amount of energy that it receives from the Sun.• Thus, it has much less heat flowing out of its

interior than Jupiter or Saturn (or Neptune).

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Planet Uranus

• This may account for its limited atmospheric activity.• Astronomers are not sure why it differs in this

respect from the other Jovian worlds.

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The Uranian Moons

• The five major moons of Uranus are smaller than Earth’s Moon and have old, dark, cratered surfaces. • A few have deep cracks produced, perhaps,

when the interior froze and expanded.• In some cases, liquid water “lava” appears to

have erupted and smoothed over some regions.

The Uranian Moons: Miranda

• Miranda, the innermost moon, has 1/7 of the diameter of Earth’s moon.• Its surface is marked

by grooves called ovoids.

It looks like it was broken

apart and reassembled

by a giant collision. But

it could just be ice geology.

The Uranian Moons: Miranda

• The ovoids may have been caused by internal heat driving convection in the icy mantle. • By counting craters on

the ovoids, astronomers conclude that the entire surface is old, and the moon is no longer active.

The Uranian rings

• The rings of Uranus:• Are dark and faint,• Contain little dust,• Are confined by

shepherd satellites, and• Must be continuously

resupplied with material from the moons.

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The Uranian rings

• The rings are not easily visible from Earth.• The first hint that Uranus had rings came from

occultations.• This is the passage of the planet in front of a star.

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The Uranian rings

• Most of what astronomers know about the rings comes from the observations of the Voyager 2 spacecraft.• Their composition appears to be water ice

mixed with methane that has been darkened by exposure to radiation.

• In 2006, astronomers found two new, very faint rings orbiting far outside the previously known rings

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The Uranian rings

• The newly discovered satellite Mab appears to be the source of particles for the larger ring.• The smaller of the new rings is confined

between the orbits of the moons Portia and Rosalind.• Note that the International Astronomical Union (IAU)

has declared that the moons of Uranus are to be named after characters in Shakespeare’s plays.

prepared by: jennifer west department of physics and astronomy university of manitoba backman seeds...

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