1. The Outer Worlds

4. Uranus was discovered by chance
Uranus recognized as a planet in 1781 by William Herschel
while scanning the sky for nearby objects with measurable parallax: discovered Uranus as slightly extended object, ~ 3.7 arc seconds in diameter.

5. Neptune’ Discovery
Discovered in 1846 at position predicted from gravitational disturbances on Uranus’s orbit by John Couch Adams and Urbain Jean Leverrier. (But don’t forget Galle!)
Blue-green color from methane in the atmosphere
4 times Earth’s diameter; 4 % smaller than Uranus

6. The Atmospheres of Uranus and Neptune
Outer atmospheres of Uranus and Neptune are similar to those of Jupiter and Saturn
Uranus and Neptune are cold enough that ammonia freezes; methane dominates and gives the characteristic blue color

7. The Atmospheres of Uranus and Neptune
Uranus is very cold; clouds only in lower, warmer layers.
Few features are visible:

8. The Atmosphere of Uranus
Like other gas giants: No surface.
Gradual transition from gas phase to fluid interior.
Mostly H; 15 % He, a few % Methane, ammonia and water vapor.
Optical view from Earth: Blue color due to methane, absorbing longer wavelengths
Cloud structures only visible after artificial computer enhancement of optical images taken from Voyager spacecraft.

9. The Structure of Uranus’ Atmosphere
Only one layer of Methane clouds (in contrast to 3 cloud layers on Jupiter and Saturn).
3 cloud layers in Jupiter and Saturn form at relatively high temperatures that occur only very deep in Uranus’ atmosphere.
Uranus’ cloud layer difficult to see because of thick atmosphere above it.
Also shows belt-zone structure
 Belt-zone cloud structure must be dominated by planet’s rotation, not by incidence angle of sun light!

10. The Atmospheres of Uranus and Neptune
Neptune has storm systems similar to those on Jupiter, but fewer:

11. The Atmospheres of Uranus and Neptune
Band structure of Neptune is more visible, and Neptune has internal heat source of unknown origin:

12. Cloud Structure of Uranus
Hubble Space Telescope image of Uranus shows cloud structures not present during Voyager’s passage in 1986.
 Possibly due to seasonal changes of the cloud structures.

13. Neptune is a cold, bluish world with Jupiter-like atmospheric features
No white ammonia clouds are seen on Uranus or Neptune
Presumably the low temperatures have caused almost all the ammonia to precipitate into the interiors of the planets
All of these planets’ clouds are composed of methane
Much more cloud activity is seen on Neptune than on Uranus.
This is because Uranus lacks a substantial internal heat source.

14. Neptune’s Clouds
Much more cloud activity is seen on Neptune than on Uranus
This is because Uranus lacks a substantial internal heat source

16. Exaggerated Seasons On Uranus
Uranus’s axis of rotation lies nearly in the plane of its orbit, producing greatly exaggerated seasonal changes on the planet
This unusual orientation may be the result of a collision with a planetlike object early in the history of our solar system. Such a collision could have knocked Uranus on its side

18. The Motion of Uranus
19.18 AU
97.9o
Very unusual orientation of rotation axis: Almost in the orbital plane.
Possibly result of impact of a large planetesimal during the phase of planet formation.
Large portions of the planet exposed to “eternal” sunlight for many years, then complete darkness for many years!

19. Uranus and Neptune contain a higher proportion of heavy elements than Jupiter and Saturn
Both Uranus and Neptune may have a rocky core surrounded by a mantle of water and ammonia
Electric currents in the mantles may generate the magnetic fields of the planets

20. Magnetospheres and Internal Structure
Comparison of the interiors of the Jovian planets.

21. The Magnetic Field of Uranus
No metallic core  no magnetic field was expected.
But actually, magnetic field of ~ 75 % of Earth’s magnetic field strength was discovered:
Offset from center: ~ 30 % of planet’s radius!
Inclined by ~ 60o against axis of rotation.
Possibly due to dynamo in liquid-water/ammonia/methane solution in Uranus’ interior.
Magnetosphere with weak radiation belts; allows determination of rotation period: hr.

22. Magnetospheres and Internal Structure
Uranus and Neptune both have substantial magnetic fields, but at a large angle to their rotation axes.
The rectangle within each planet shows a bar magnet that would produce a similar field. Note that both Uranus’s and Neptune’s are significantly off center.

23. The Magnetosphere of Uranus
Rapid rotation and large inclination deform magnetosphere into a corkscrew shape.
UV images
During Voyager 2 flyby: South pole pointed towards sun; direct interaction of solar wind with magnetosphere  Bright aurorae!

24. Uranus and Neptune each have a system of thin, dark rings

25. Apparent motion of star behind Uranus and rings
The Rings of Uranus
Rings of Uranus and Neptune are similar to Jupiter’s rings.
Confined by shepherd moons; consist of dark material.
Apparent motion of star behind Uranus and rings
Rings of Uranus were discovered through occultations of a background star

27. Uranus’s rings are narrow:

28. Two shepherd moons keep the epsilon ring from diffusing:

29. The Rings of Neptune
Neptune has five rings, three narrow and two wide:

30. The Rings of Neptune Interrupted between denser segments (arcs)
Ring material must be regularly re-supplied by dust from meteorite impacts on the moons.
Interrupted between denser segments (arcs)
Made of dark material, visible in forward-scattered light.
Focused by small shepherd moons embedded in the ring structure.

31. The moons of Uranus – 27 at present
The first two were discovered by William Herschel in 1787, and named, by his son, after characters from Shakespeare’s A Midsummer Nights Dream, Titania and Oberon.
Two more moons were found by William Lassell in 1851 and named Ariel and Umbriel
G. Kuiper discovered Miranda in 1948.
All the moons of Uranus are named after characters from Shakespeare or Alexander Pope.
Voyager 2’s flyby in January 1986 led to the discovery of another 10.
Six additional moons have since been discovered by telescope.

32. The Moons of Uranus 5 largest moons visible from Earth.
10 more discovered by Voyager 2; more are still being found.
Dark surfaces, probably ice darkened by dust from meteorite impacts.
5 largest moons all tidally locked to Uranus.

33. Moons of Uranus

35. Interiors of Uranus’s Moons
Large rock cores surrounded by icy mantles.

36. The Surfaces of Uranus’s Moons
Oberon
Titania
Old, inactive, cratered surface,
Largest moon
but probably active past.
Heavily cratered surface, but no very large craters.
Long fault across the surface.
Dirty water may have flooded floors of some craters.
Active phase with internal melting might have flooded craters.

37. The Surfaces of Uranus’s Moons
Umbriel
Ariel
Dark, cratered surface
Brightest surface of 5 largest moons
Clear signs of geological activity
No faults or other signs of surface activity
Crossed by faults over 10 km deep
Possibly heated by tidal interactions with Miranda and Umbriel.

38. Uranus’s Moon Miranda Most unusual of the 5 moons detected from Earth
Ovoids: Oval groove patterns, probably associated with convection currents in the mantle, but not with impacts.
20 km high cliff near the equator
Surface features are old; Miranda is no longer geologically active.

39. MIRANDA

40. The Moons of Neptune Unusual orbits:
Two moons (Triton and Nereid) visible from Earth; 6 more discovered by Voyager 2
Triton: Only satellite in the solar system orbiting clockwise, i.e. “backward”.
Nereid: Highly eccentric orbit; very long orbital period (359.4 d).

41. Triton is a frigid, icy world with a young surface and a tenuous atmosphere
Neptune has 13 satellites, one of which (Triton) is comparable in size to our Moon or the Galilean satellites of Jupiter
Triton has a young, icy surface indicative of tectonic activity
The energy for this activity may have been provided by tidal heating that occurred when Triton was captured by Neptune’s gravity into a retrograde orbit
Triton has a tenuous nitrogen atmosphere

42. The Surface of Triton
Very low temperature (34.5 K)
 Triton can hold a tenuous atmosphere of nitrogen and some methane; 105 times less dense than Earth’s atmosphere.
Surface composed of ices: nitrogen, methane, carbon monoxide, carbon dioxide.
Possibly cyclic nitrogen ice deposition and re-vaporizing on Triton’s south pole, similar to CO2 ice polar cap cycles on Mars.
Dark smudges on the nitrogen ice surface, probably due to methane rising from below surface, forming carbon-rich deposits when exposed to sun light.

43. The Surface of Triton (2)
Ongoing surface activity: Surface features probably not more than 100 million years old.
Large basins might have been flooded multiple times by liquids from the interior.
Ice equivalent of greenhouse effect may be one of the heat sources for Triton’s geological activity.