Jovian Planets: Jupiter and Saturn

Shortened Version Feb 15, 2011

Jovian Planets (Gas Giants)

12,757 km(1 DE)

142,984 km(11 DE)

120,536 km(9.4 DE)

51,118 km(4.0 DE)

49,528 km(3.8 DE)

The outer planets are much bigger than the terrestrial ones, and are gigantic low-density gas balls unlike the high density rocky terrestrial planets.

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Distance Comparison

1 AU 5.2 AU 9.6 AU 19.2 AU 30.1 AU

3.7% as much sunlight as Earth

1.1% as much sunlight as Earth

0.3% as much sunlight as Earth

0.1% as much sunlight as Earth

Sun

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Jupiter: Atmosphere 3 - Composition

• 1930s (Rupert Wildt) based on low density (1.3 g/cm3), suggests primarily made of Hydrogen (H) and Helium (He)

• Confirmed by spacecraft in 1970s– Why couldn’t we see spectral lines of H & He from the earth?

• atmosphere (by mass): 75% H, 24% He, 1% other• atmosphere + interior: 71% H, 24% He, 5% other• Note Sun: 73% H, 25% He, 2% other• Hence its similar to original solar nebula!

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Jupiter: Storms

• Great Red Spot– about size of Earth– At largest, has been

seen at 3x Earth size– Rotates ccw– more than 300 yrs old

• White ovals– smaller, less long-lived– also rotate ccw

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Storms in Saturn’s atmosphere seem to be shorter-lived

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Jupiter’s atmosphere is

divided into light-colored zones and

dark-colored belts.

High-speed winds blow

eats or west at the boundaries between them.

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Saturn: Atmosphere 3

• Composition of clouds same as Jupiter

• Atmosphere depleted in He (96.3% H, 3.3% He, 0.4% other)=> where is missing He?

• Similar easterly and westerly winds, but at equator, Saturn’s winds are much faster~1100 miles/hr!!!

Wind speed in meters/second

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Jupiter: Belts and Zones

• Alternating light/dark stripes in atmosphere

• Convection – warm air rises from below, cools off, then sinks (same as Earth’s troposphere)

• Rapid rotation stretches these convection cells across the planet

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Saturn: Atmosphere 2 10

Internal heat drives convection

• Jupiter & Saturn radiate more heat than they receive from the sun.

• Energy from formation (accretion).

• Saturn being smaller than Jupiter should have cooled off more by now.

• Helium Rain

– Saturn is cold enough that He can condense into droplets

– These droplets “rain” down through the H

• release energy (convert gravitational energy to heat)

=> Also explains why atmosphere is depleted in He

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The oblateness of Jupiter and Saturn reveals their rocky cores

• Jupiter probably has a rocky core several times more massive than the Earth

• The core is surrounded by a layer of liquid “ices” (water, ammonia, methane, and associated compounds)

• On top of this is a layer of helium and liquid metallic hydrogen and an outermost layer composed primarily of ordinary hydrogen and helium

• Saturn’s internal structure is similar to that of Jupiter, but its core makes up a larger fraction of its volume and its liquid metallic hydrogen mantle is shallower than that of Jupiter

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Jupiter: Magnetic Field

• Enormous – 30 million km across– if we could see it from Earth, it

would appear 16x larger than the full moon in the sky

• Also very strong, but more variable in strength than Earth’s

• First detected by “synchrotron radiation”– radar signals emitted by charged particles trapped in

magnetic field lines

• These particles also cascade into Jupiter’s atmosphere creating aurora, like on Earth

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Saturn: Magnetic Field• Weaker than Jupiter’s

– probably because liquid metallic H region is smaller

• Aligned perfectly with rotation axis

• Contains fewer charged particles– no Io-like active moon to

supply– icy ring material takes

particles out of magnetic field lines

=> weaker aurora

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• 1610: Galileo looks at Saturn, notices it is not circular.

• Bulges on sides disappear in 1612, reappear in 1613

• 1655: With a better telescope, Huygens discovers ring, tilted by 27°

Saturn’s Rings 15

•Last crossing 1995-6

•Next crossing 2008-9

•Rings estimated to beonly 30 meters thick!

Saturn’s Rings are Thin 16

• Huygens explains why the rings disappear. They are very thin, and when seen edge twice a saturn year, they vanish!

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1675 Cassini observes the “division” in the rings, separating the brighter “B” ring from the fainter outer “A” ring

Cassini Division 18

Earth-based observations reveal three broad rings encircling Saturn

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A more subtle division is found in the “A” ring

Encke Division (1838) 20

• 1857 Maxwell argues cannot be solid (centrifugal forces would tear it apart)

• 1895 Keeler measures Doppler velocities, each “ringlet” has different speed

• Rings consist of many “moonlets”, each orbiting the central planet.

• 80% reflectivity implies they are icy particles

• Probably from an icy moon pulled apart, only 100 million years ago.

Rings are not solid! 21

• The moon’s own gravity tends to hold it together.

• Tidal forces from planet tend to pull it apart

• The “Roche Limit” is the critical distance where the two balance

• Saturn’s moons are (mostly) outside of Roche Limit

• Saturn’s rings are (mostly) inside the Roche limit.

Roche Limit (2.4 Saturn Radii) 22

• From scattering of radio signals through the rings, estimate average particles are 10 cm in size

• Some are as small as 1 cm, some as big as 5 meters.

• In comparison the ring particles of the other planets are:

– Jupiter’s are small dust particles smaller than 1 mm

– Uranus particles are perhaps 1 meter in size

– Neptune size unknown (similar to Uranus?)

1970 Voyager Probes 23

Cassini Saturn Orbit Insertion (SOI)

June 30, 2004, 7:30 - 9:15 PDT

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Spectra of Saturn’s main ringsshowing only H2O ice

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Rings of Jupiter

View of Jupiter from behind, looking toward the Sun

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Jupiter’s rings

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• Jupiter has three very thin rings• As with Neptune and Uranus, Jupiter’s rings are made of tiny black particles• The material in Jupiter’s rings is probably coming from the small moons Metis,

Adrastea, and others.• Material is being blasted off those small moons by the impact of

micrometeoroids attracted to Jupiter.

Saturn’s Rings: Natural Color

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Gaps in Saturn’s Rings

Gaps in the rings are caused by resonances with the satellitesExample: Mimas causes the Cassini Division

Mimas makes one revolution in 23 hours. A ring particle in The Cassini Division makes one revolution in 11 ½ hours, or two revolutions in one Mimas period. This is resonance.

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What is “resonantmotion”?

When you push someone ina swing, each time theycome back, you give a newpush to keep them going.This is resonant motion.

Each time a ring particle comes between Saturn and Mimas, it gets a pull fromMimas, causing its orbit tobecome eccentric. This increases the likelihood thatit will collide with anotherparticle and be destroyed.

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Encke Division

Probably created not by resonance, but by the small moon “Pan” which shares the orbit.

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F Ring from Cassini probe close approach, July 3, 2004 33

Saturn’s F-ring from Cassini34

F Ring Shepherd Moons

These two moons keep the narrow F ring in place.

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Things to do

•Add more Pioneer and Voyager history (follow “the planets” video),

along with Galileo and Cassini probe results.

•Should we separate Jupiter from Saturn?