1. JOVIAN (GAS GIANT) PLANETS

2. SATURN Nearly as big as Jupiter, and it rings very nicely.
Cassini image of Saturn, edge-on rings, their shadows,
& moons Dione & Enceladus

3. SATURN: Ringed and Very Gassy
Mass is 95.2 times that of Earth.
Radius is 60,268 km or 9.45 times Earth's Density is only g/cm3 or of Earth's.
The most distant naked eye planet.
Period of years Semi-major axis, a = 9.54 AU, e = Inclination to ecliptic = 2.49 deg
9 moons discovered from the ground.
No solid surface: differential rotation Magnetic (interior) spin period: 10h 40m Spins so fast & is so gassy that its ellipticity > 10% axial tilt = 26.7 deg

4. Rings and Orbit Rings are prominent; tilted at 27 degrees so seen from
different angles from Earth; nearly invisible when edge-on

5. Saturn's Atmosphere and Interior are Very Similar to Jupiter's
BUT the main interior differences include:
Lower density: less mass so less compression and relatively more gaseous
Molecular H2 layer  30,000 km thick Metallic H layer  15,000 km thick Rocky/metallic core, R  15,000 km
Central: T = 15,000 K; P = 5 x 106 atm
Strong magnetic field, but still only ~5% of Jupiter's.

6. Saturn’s Interior

7. Saturn’s Atmospheric Differences from Jupiter
Fewer belts and zones (due to lower T) but faster winds (to 1500 km/h) Thicker cloud layers (lower gravity) Less coloration (probably due to lower T)
In atm: H2 over 92%, He almost 8%; Why?
He has settled or “rained” downwards At the lower T and P of Saturn's atm, He doesn't dissolve in H2 Separation of He and compression of whole planet yields much more heat, so Saturn's surface T is 97 K, not 74 K.

8. Saturn’s Atmosphere
Real color from HST showing storm near equator and pole at earlier time.
False color image from IR: blue, haze free; green, more haze; orange, high clouds

9. Atmospheric Structure and Fast Winds

10. URANUS and NEPTUNE Most Distant Official Planets are NEARLY TWINS
MU = M ; MN = M .
RU = 25,559 km or 4.01 times Earth's.
RN = 24,766 km or 3.88 times Earth's.
U = 1.27 g cm-3 ; N = 1.64 g cm-3 .

11. As seen from Earth, URANUS
Discovered by Wm. Herschel in 1781 (barely visible to naked eye, greenish, nearly featureless)
Period of years
Semi-major axis: AU, e = 0.047
5 moons discovered from the ground pre-space; 10 by Voyager, 2 more by 200-inch.
Tilted by 98 deg  LONG seasons
Magnetic (interior) spin period: 17.2 h (retrograde)

12. Seasons on Uranus
Axis almost in orbital plane so equatorial zones have two summers near equinoxes and two winters near solstices: poles have 42 yr of light and 42 years of dark

13. Rotation on Uranus HST shows cloud motions over 8 hours
IR HST picture shows clouds and rings

14. As seen from Earth, NEPTUNE
Predicted by John Adams (1845) and Urbain Leverrier (1846): producing growing error in Uranus's position noticed over the years
Found in the right place by Johann Galle in Sept bluish, tiny disk; Voyager showed some bands
P=163.7 yr; a = AU; e=0.009
2 moons discovered from the ground; 11 from space
Magnetic (interior) spin period: 16.1 h; surface 17.3 h

15. ATMOSPHERES Temperatures: Uranus, 58 K; Neptune 59 K
Both Uranus and Neptune are about 84% H2 and 14% He.
Methane (CH4 ) is most of the remainder Methane absorbs red light, so planet looks blueish/green More CH4 in Neptune, so it's bluer Ammonia is frozen out in these very cold atmospheres
No internal heat source for Uranus, so washed out bands and zones covered by a haze Fairly strong winds distribute Sun's heat so sunny side isn't much hotter than shady side.
Neptune has an internal heat source: probably extra methane has helped to trap internal heat of formation more noticeable bands and some storms

16. Neptune’s Dark Spot (from Voyager) and Clouds from IR HST views showing brightening as more sunlight hits South side

17. Interiors of Uranus and Neptune
Below atmospheres, each about 1/3 of R:
MOLECULAR HYDROGEN (unlikely to convert to metal)
below that, SLUSH, including water ice and dissolved ammonia: an electrically conducting fluid
ROCKY CORE (maybe 10 M )

18. Magnetic Fields
Despite not having (much) metallic hydrogen both have substantial internal fields (100 times earth's, 1/10th Saturn)
Off-center and misaligned with spin axis (60O for Uranus, 46O for Neptune, vs. 10O for Jupiter and well-aligned for Saturn).
Why?: currently reversing; reflecting a blow; ionic NH3 ?