14a. Uranus & Neptune

1. 14a. Uranus & Neptune • The discovery of Uranus & Neptune • Uranus is oddly tilted & nearly featureless • Neptune is cold & blue • Uranus & Neptune are like yet dislike Jupiter • The magnetic fields of Uranus & Neptune • The dark rings of Uranus & Neptune • Ancient tidal heating in some Uranian moons • Surprisingly young surface on Neptune’s Triton

2. Earth, Uranus, Neptune & Pluto

3. The Discovery of Uranus • Uranus was discovered by accident • William Herschel sees a faint fuzzy blue object • Initial conclusion A distant comet 13 March 1781 • Final conclusion A distant planet End of 1781 • Earlier sightings of Uranus • Uranus is plotted on at least 20 prior star charts • It is barely visible under ideal seeing conditions • It moves an average of only ~ 0.011° per day

4. Uranus Data: Numbers • Diameter: 51,120.km 4.01 . Earth • Mass: 8.7 . 1025 kg 14.6 . Earth • Density: 1.3 . water 0.24 . Earth • Orbit: 2.9 . 109 km 19.19 AU • Day: 17h.12m 00s 0.72 . Earth • Year: 84.04 years 84.04 . Earth

5. Uranus Data: Special Features • Uranus is the third Jovian planet from the Sun • Uranus is the second smallest Jovian planet • Uranus has no solid surface • Uranus has a featureless blue atmosphere • Methane (CH4) gives Uranus its light blue color • Uranus’ interior consists of three layers… • Atmosphere Very thin • Mantle Liquid water, methane & ammonia • Core“Metal” & “rock” • Uranus has 5 size & 10 small known moons • Largest 5 moons are ~50% “ice” & ~50% “rock” • Smallest 10 moons may be captured asteroids

6. Uranus Data (Table 14-1)

7. Uranus Is Tilted & Featureless • Axial tilt of Uranus • Rotated ~ 98° to the plane of Uranus’s orbit • Rotational axis lies almost in the orbital plane • Slightly retrograde axial rotation • Most exaggerated seasons of all planets in the Solar System • Each pole of Uranus can point almost directly toward the Sun • Cloud features of Uranus seen by Voyager 2 • Very faint cloud markings Jan. 1986 • Required extensive computer processing to see • Composed mostly of low altitude methane (CH4) • Atmospheric composition • ~ 82.5 % H2 Colorless • ~ 15.2 % He Colorless • ~ 2.3 % CH4 Distinctive blue • UV from sunlight converts some methane into hydrocarbon haze

8. Uranus: The Ultimate in Seasonality

9. Storms & Rings (HST, 1998)

10. The Discovery of Neptune • Observations • Uranus’s orbit could not be accurately predicted • Discrepancies of ~ 2 arc minutes by 1830 • Uranus initially moved slightly faster than expected • Uranus later moved slightly slower than expected • Two possible explanations • Newtonian mechanics does not work • An undiscovered planet is causing the discrepancies • John Couch Adams completes calculations Oct 1845 • UrbainLeVerrier completes calculations 23 Sep 1846 • Johann Galle discovers Neptune 23 Sep 1846

11. Neptune Data: Numbers • Diameter: 49,528.km 3.88 . Earth • Mass: 1.0 . 1026 kg 17.23 . Earth • Density: 1.7 . water 0.30 . Earth • Orbit: 4.5 . 109 km 30.06 AU • Day: 16h.06m 36s 0.67 . Earth • Year: 164.8 years 164.8 . Earth

12. Neptune Data: Special Features • Neptune is farthest Jovian planet from the Sun • Neptune is the smallest Jovian planet • Neptune has no solid surface • Neptune has a colorful & dynamic atmosphere • Methane (CH4) gives Neptune its deep blue color • Great Dark Spot • Neptune’s interior consists of three layers • Atmosphere Very thin • Mantle Liquid water, methane & ammonia • Core“Metal” & “rock” • Neptune has 1 large, 2 medium, 5 small moons • Triton is ~25% “ice” & ~75% “rock” (like Pluto ! !)

13. Neptune Data (Table 14-2)

14. Neptune Is Cold, Blue & Dynamic • Temperature • Neptune has an average temperature of ~ 55 K • Identical to Uranus yet ~ 50% farther from the Sun • Uranus must have a strong internal heat source • Color • Atmospheric composition • ~ 80 % H2 Colorless • ~ 18 % He Colorless • ~ 2 % CH4 Distinctive blue • Storms • The Great Dark Spot Aug. 1989 • Remarkably similar to Jupiter’s Great Red Spot • Gone when viewed by HST in 1994 • Cirrus clouds • Methane ice

15. Great Dark Spot & Clouds

16. Neptune’s Banded Atmosphere • Belts & zones • Belts Dark blue • Descending atmospheric regions • Zones Light blue • Ascending atmospheric regions

17. Neptune’s Clouds (HST, 1998)

18. Uranus & Neptune ⇔ Jupiter • Average density • Jupiter & Saturn Relatively low • Chemical composition very similar to the Sun • Uranus & Neptune Relatively high • Chemical composition very different from the Sun • Considerably deficient in H2 and He • Hypothesized Jovian planet formation process • Planetesimals accreted to form each planet’s core • Each planet’s core accreted H2 and He • Formation of Uranus & Neptune • Too massive to have formed so far from the Sun • Too few planetesimals at those great distances • Did Uranus & Neptune form closer to the Sun? • Did Jupiter/Saturn interactions fling them outward?

19. Uranus & Neptune Inner Structure

20. Uranus & Neptune Magnetic Fields • Orientation of magnetic fields • Uranus & Neptune magnetic fields are steeply tilted • Uranus ~ 59° • Neptune ~ 47° • Both fields are offset from center • Possible explanations • Magnetic fields might be undergoing reversal • This happens about every 11 years on the Sun • Simultaneous reversals are highly unlikely • Catastrophic collisions may be responsible • Much more likely for Uranus than for Neptune • Cause of magnetic fields • Axial rotation + circulation of interior conducting liquid • Probably ammonia dissolved in water • May be produced by multiple convection cells

21. Five Planetary Magnetic Fields

22. Uranus & Neptune Dark Rings • The rings of Uranus • Discovered by accident 10 March 1977 • Expected occultation of a faint star • Blocked out light 9 times as rings passed in front • Voyager 2 discovers 2 more rings January 1986 • Rings are ~10 km wide & well inside the Roche limit • Ring particles are 0.1 to 10.0 m wide & dark as coal • The rings of Neptune • Also discovered by stellar occultations • Cause of dark rings • Methane (CH4) ice can persist that far from the Sun • Radiation darkening may be responsible • Methane is dissociated into carbon & hydrogen atoms • Solid carbon remains & gaseous hydrogen escapes

23. Uranian Rings & Small Moons

24. Tidal Heating in Some Uranian Moons • Five moderate-sized satellites • Average densities ~ 1.5 g . cm-3 • Consistent with a mixture of rock & ice • All are very dark • Radiation darkening a distinct possibility • Miranda Unique in the Solar System • Multiple landscapes • Abundant heavily cratered terrain • Some dramatic terrain • Possible catastrophic impact with rearranged fragments • Possible tidal heating that permitted rocky crust to sink

25. Uranus’ Remarkable Miranda

26. Young Surface on Neptune’s Triton • Triton is Neptune’s only large satellite • 2,706 km diameter • Slightly smaller than Earth’s Moon • Retrograde orbit • Almost certainly captured into that orbit • Conspicuous absence of large craters • Mottled terrain near Triton’s south pole • Cantaloupe terrain away from Triton’s south pole • Triton’s unusual properties • Surface temperature of ~ 38 K • Cold enough for most N2 to freeze • Warm enough for little N2 atmosphere • Wind-blown deposits in some places • Tidal forces have Triton spiraling in toward Neptune • In ~ 100 million years Triton will be inside the Roche limit

27. Neptune’s Remarkable Moon Triton

28. Discovery Uranus discovered by accident Seen earlier but unrecognized Neptune discovered deliberately Perturbations of Uranus’s orbit Pluto discovered by accident No real perturbations of Neptune Planetary data Uranus & Neptune ~ 4x Earth’s diameter & ~15x mass Pluto ~ 0.2x Earth’s diameter & 0.2% mass Planetary characteristics Uranus & Neptune Blue due to methane absorption Relatively rich in rock & metal Strongly tilted magnetic fields Unusual features Uranus’s axis nearly in orbital plane Neptune’s internal energy source Pluto’s status as a true planet Satellites Uranus’s Miranda Distinctly different terrain types Neptune’s Triton Similarity to Pluto & Charon Important Concepts