THE JOVIAN PLANETS

Jupiter, Saturn, Uranus, Neptune

HISTORY

• known to ancients
• Galileo observes 4 moons of Jupiter, rings of Saturn
• Jupiter is the 3rd brightest night-sky object after Moon and Venus

Uranus

• discovered telescopically by William Herschel in 1781 (actually barely visible to naked eye)

Neptune

• predicted from observed perturbations of Uranus's orbit (not elliptical): 1845, Adams; 1846, Leverrier
• Observed by Galle, 1846

Remark: Pluto's existence discovered based on perturbations of orbits of Uranus and Neptune, but the calculations were wrong!
Calculation: Percival Lowell; Discovery: Clyde Tombaugh, 1930

SPACE PROBES

• Pioneer 10 and 11 (Jupiter, Saturn; early 70's) first man-made object to leave solar system
• Voyager 1 and 2 (Grand Tour of Jupiter, Saturn, Uranus, Neptune; launched late 70's, arrive late 80's)
• Galileo: Two part spacecraft arrived late 1995. First permanent orbiter of outer planets. Dropped probe into atmosphere.
• Cassini: en route to Saturn

PHYSICAL FEATURES

Size: all very large. Jupiter is 11 times Earth's diameter, has 1400 times Earth's volume

Mass: all have many moons, so mass can be deduced from Newton's law of gravitation. (Note difference from terrestrial planets, which have few moons)

Density: low compared to Terrestrial planets.

• Conclude: mostly gas (hydrogen and helium) compressed by gravity.
• Saturn would float on water
• Uranus and Neptune slightly denser, must have rocky cores

Actually, Jupiter and Saturn also have rocky cores: see below.

ROTATION

• very fast, esp. for such large bodies (Jupiter, Saturn, 10 hours; Uranus and Neptune, 17 hours)
• gaseous bodies, different parts rotate at different speeds
• equatorial bulge due to rotation is smaller than one would expect if there were no core, so deduce rocky cores of Jupiter and Saturn

AXIAL TILT

Jupiter: small (no seasons)

Saturn: similar to Earth's

Neptune: slightly larger than Earth's

Uranus: 98°! (retrograde)

• unlike any other body in solar system. Perhaps due to collision? (But then it's hard to explain why moon system and rings have same tilt)
• Seasons and daylight are very peculiar on Uranus: almost all of the planet is "the land of the midnight sun" like Earth's polar regions.

RINGS

All the Jovian planets have rings.

Very thin, 10-100 m thick, but as much as 200,000 km across

Those of Saturn are largest, first observed by Galileo

• Christian Huygens determines that they're rings in 1659.
• Theory says they can't be solid, liquid or gas. James Clerk Maxwell suggests that they're made of small chunks orbiting in concentric circles. Confirmed, James Keeler, 1895, by Doppler shift.

Rings of Uranus discovered by occultation of star, 1977; rings of Jupiter discovered by Voyager 1 in 1979, of Neptune by Voyager 2 in 1989.

Rings may be short lived (on time scale of solar system), which means that they must form fairly frequently. A moon may pass too close to a planet (Roche limit) and be destroyed by tidal forces. This may happen to Nereid (moon of Neptune) within 100 million years

ATMOSPHERES: JUPITER

composition determined by spectroscopic observation

• mostly hydrogen (86%), some helium (14%); traces of methane, water, ammonia
• large gravity allows Jupiter to retain light gases (hydrogen, helium) lost by terrestrial planets
• theory says interior is mostly composed of same (no direct evidence)

visual features

clouds

• three cloud layers predicted: ammonia (NH₃), ammonium hydrosulfide (NH₄HS), water (H₂O); not confirmed by Galileo
• colors most likely due to compounds of sulfur or phosphorus; chemistry controlled by solar radiation, like smog on earth

bands

• due to convection in atmosphere
• "zones" and "belts" are bright or dark tops of regions of rising or sinking gas
• Jovian weather (mostly) circles the planet because of high rotation rate
• bands also exhibit east-west flow

storms: white and brown ovals, Great Red Spot

• Great Red Spot, first observed in 1600's by Robert Hooke; size of Earth

ATMOSPHERES OF SATURN, URANUS, AND NEPTUNE

Saturn

• belt structure fainter. Cloud layers thicker due to weaker gravity
• Storms are rarer. White spot seen, 1990.
• Helium precipitation? perhaps.

Uranus, Neptune

• Ammonia frozen out; more methane. Methane absorbs red light, leads to bluish color.
• almost no band structure on Uranus.

Temperature of cloud tops on Jupiter, Saturn and Neptune indicate that planets are heated from within. Leftover heat from formation? differentiation?

Much about the Jovian planets' atmospheres is still not understood.

MAGNETOSPHERES

• very strong (Jupiter's extends past orbit of Saturn!)
• associated with strong radiation.
• Oddly inclined and offset from centers of planets (esp. Uranus and Neptune)
• Indicate presence of conducting cores: see internal structure

INTERNAL STRUCTURE

gas of increasing density, pressure

in Jupiter and Saturn, conditions are such that hydrogen becomes metallic

Uranus and Neptune probably have conducting slushy layers

All have rocky cores about the size of Earth

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