The Outermost Planets. The 7 Wanderers known since Antiquity. Uranus and Neptune distinctly Blue-ish!

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1 The Outermost Planets The 7 Wanderers known since Antiquity. Uranus and Neptune distinctly Blue-ish!

2 Uranus Uranus and 3 of its moons, barely visible from Earth. Discovered by William Herschel (Accidentally!) First new planet for > 3000 years! Featureless planet, just a few wispy clouds! View from Voyager

3 Neptune Elliptical Orbit of Uranus is perturbed: There must be another planet! Adams 1845 and Le Verrier 1846 predicted Neptune's position. First Seen by Galle in 1846.

4 Property Uranus Neptune Radius 25559km (4.0 R E ) 24766km (3.88 R E ) Mass M E Average Density 1271kg/m 3 (0.23) 1638kg/m 3 (0.297) Gravity 0.91 Earth s 1.14 Earth s Escape Speed 21.3km/s 23.5km/s Temperature (at Cloud tops) M E 56K / -217C 59K / -214C Eccentricity

5 Property Uranus Neptune Sidereal Orbital Period years years Sidereal Rotation Period days Retrograde! 0.67 days Axial Tilt # Moons Magnetic axis tilt Similar but with significant differences!

6 Uranus Peculiar Rotation Axis of rotation nearly in the orbital plane! Extreme seasons and weird days! Reason for such a strange tilt not known.

7 Consequences In Winter: Darkness for 42 years! In Summer: No night, sun moves in circles, with altering radii. Other Rotational Features Differential Rotation (~17.2 hours) Atmosphere moves faster at Poles (~14.2 hrs) than at Equator (16.5hrs). No zones or belts: Wind in same direction.

8 The Atmospheres Composition of Uranus and Neptune H 2 ~84% Helium ~14% Methane ~2% (Uranus); 3% Neptune x10 more than on Jupiter and Saturn. Less ammonia and more methane as distance from Sun increases: Ammonia freezes out at ~70K. Neptune is Bluer as it has more Methane.

9 Weather on Uranus South pole region is reflecting sunlight - highest wind speeds ~500km/hr. Spots: high clouds or storms ~1000km across. Temperature variation from pole to equator is small : Efficient heat transfer. No internal source of Heat. False colour Infra-red view from Hubble (1998) Ring System!

10 Weather on Neptune 1989 CH 4 ice clouds Voyager 2 sees Great Dark Spot storm system as big as Earth. (Similar to Jupiter)

11 Weather on Neptune in 1998 Dark spot disappeared! New storms appeared... Wind speeds greater than 1500km/hr, driven by internal heat from gravitational collapse. Still shrinking! Faint zones and belts: Convection cycles.

12 Interiors of Uranus and Neptune Very similar structures. No metallic Hydrogen not enough pressure. Relatively larger cores - (than Jupiter and Saturn.) Slushy interiors of conducting ammonia in H 2 0.

13 Strange Magnetic Fields.

14 Strange Magnetic Fields II. Magnetic fields aligned at strange angles to rotation axis. Why? (Don t really know!) Field in transition of flipping? Fields generated differently? (No metallic H) Presumably via the slushy conducting water.

15 The Moons of Uranus and its rings

16 The Moons of Uranus Oberon and Titania discovered in 1789 by William Herschel. Miranda by Gerard Kuiper in only Earth s Moon The five largest Moons to scale.

17 General Features of Major Moons Diameters from ~ km. All have average densities of ~1500kg/m 3 ice and rock. Geologically dead, but scarred and cratered. Dull less reflective. Radiation Darkening : Light induced chemistry of surface molecules to give dark organic material. Voyager 2 found 10 more moons. And now the moon count is up to 27.

18 Moons of Uranus Titania Oberon Umbriel

19 Ariel

20 Miranda The strange inner moon (~5 R Uranus ). Signs of a painful geological history: surface ridges, valleys, faults, craters. Tidal heating remelting? Collisions? Lots to discover.

21 Miranda Surface detail: cliffs, stripes, few craters

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23 Neptune s Main Moons Triton discovered in Diameter 2710km, 0.29 Moon mass, Orbits at ~14.3 R Neptune Has retrograde orbit of 5.88 days. Nereid discovered by Kuiper in Diameter 340km, orbit period ~360 days Orbits at ~223 R neptune Eccentric orbit. Very different moons! Neptune has 13 known moons...

24 Triton

25 Triton Retrograde motion means: 1. Origin may be capture. 2. But spiralling into Neptune: Doomed to tidal break-up in ~100 million years. New ring system!

26 Triton Frozen Ice water Lake Captured into elliptical orbit; Neptune s tidal forces made orbits circular over time. Resultant stresses melted interior ice volcanoes, geysers. Temperature: -235C. Thin Nitrogen atmosphere.

27 The Rings of Uranus Discovered in 1977 by Stellar Occultation.

28 Thin, Well-separated Rings 9 thin rings discovered by ground-based observations. In 1986, Voyager 2 discovered 2 more Narrow ~10km wide rings, only 10 s m thick. Not very reflective dark.

29 Image enhanced view of the rings of Uranus revealing fine structure

30 Outer Epsilon Ring Also has 2 Shepherd satellites: Cordelia and Ophelia. Others proposed for other thin rings but not yet observed. Keeps ring narrow. Close-up shows internal structure. ~30 km wide

31 Neptune s View from Voyager 2 Rings Like Uranus rings, first seen by Stellar Occultation 5 rings: Galle*, Leverrier, Lassel*, Arago and Adams: * Broader Saturn-like rings. Cold, faint rings contain methane ice, radiation darkened into other carbon compounds.

32 Pluto By late 1800 s Neptune s and Uranus orbits were apparently sufficiently perturbed to postulate the existence of another planet. Eventually discovered in 1930 by Tombaugh. Despite Lowell s earlier predictions, Pluto is too small to account for the irregularities! Hence it was a Lucky discovery! Pluto Roman god of the dead

33 Pluto s Strange 248-year Orbit High ~0.25 Eccentricity Orbital plane inclined at ~17.2 to Ecliptic Perihelion: 29.7AU; Aphelion 49.4 AU

34 Other Properties Orbit period is exactly 1.5x that of Neptune. Planets orbits locked in a 3:2 resonance: 2 Pluto orbits = 3 Neptune orbits No chance of a collision! Mass: M E ; Radius: 1137km (0.18 R E ) Gravity: G E ; Escape Speed: 1.2km/s Axial Tilt: 118 ; Temperature: ~50K, 223C Rotation period: days Retrograde!

35 Pluto s Moon: View from Hubble (1990) Charon Discovered 1978 Charon : Mythical boatman who ferried the dead to Hades Pluto s realm. Radius ~650km, Mass ~1/6 of Pluto. Relatively Big! Orbits at 19700km, ~17.3R P

36 More images of Pluto and Charon!

37 Pluto and Charon.and now Nix and Hydra Strongly Tidally locked to each other! Pluto s ~ 6.4 day rotation = Charon s orbital period and its rotational period. Always keep same face to each other. Moon does not rise or set! (Fixed in Sky) Two more moons, Nix and Hydra, have recently been confirmed!

38 Charon s Synchronous Orbit

39 Pluto is Similar to Triton Density of Pluto 2100 kg/m 3 Rock and Ice (like Triton) Solid ices cover the planet: Nitrogen (N 2 ), Methane (CH 4 ), Carbon Monoxide (CO). Like Triton, has a very thin atmosphere. Charon has water ice surface polar caps.

40 Pluto s Surface Structure Hubble s enhanced UV/ Visible images. Icy polar caps Darker regions: Craters? Old dirty snow? Charon also has contrasts

41 The Origin of Pluto-Charon Co-formation or Capture theories both possible. Could have been formed along with the other planets. But icy space debris beyond Neptune is known to exists hence collisions possible. (Such debris origin of short-period comets.) At these large distances. more speculation than hard evidence.