1. Uranus

2. Uranus Discovered in 1781 4 times larger than Earth (in radius)
Discovered in 1781
4 times larger than Earth (in radius)
14.5 times the mass of Earth
Faint rings, indirectly detected through occultations
Rotation axis inclined by 970 against the orbital axis: Uranus is “rolling” on its orbit
Hubble-Space-Telescope image (rings artificially enhanced)

3. Exploration of the Outer Worlds
New Horizons

4. Uranus

5. Discovery of Uranus Uranus was discovered by chance
Uranus recognized as a planet in 1781 by William Herschel

6. Appearance
To the unaided eye, it appears uniform, but there are slight color variations
Photochemical + meteor/ring dust haze in stratosphere?

7. Appearance
In IR, features are more pronounced

8. Exaggerated Seasons On Uranus
Uranus’s Storms
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 planet-like object early in the history of our solar system. Such a collision could have knocked Uranus on its side

9. Uranus’s Seasons

10. 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 of methane
Cloud structures only visible after artificial computer enhancement of optical images taken from Voyager spacecraft.

11. Composition
83% H2 15% He 2% CH4
Traces of N2, HCN, H2S, NH3, CO

12. Surface/Atmosphere of Uranus
Uranus is nearly featureless and has an unusually tilted axis of rotation
Uranus has an atmosphere composed primarily of hydrogen, helium, and a few percent methane
Methane absorbs red light, giving Uranus its greenish-blue color

13. Zonal Winds Far fewer bands than on Jupiter and Saturn
Uranus -100m/s(10o), 0(-15o,30o), 175(-50o)
Less sunlight = less turbulence
= flow less disturbed

14. Temperature Tropopause ~ 55K
Stratosphere on Uranus is less than 75K until ~ 1mbar, but more recent IR observations have shown a warming in the stratosphere.
Methane, ethane, acetylene act as coolants in stratosphere, similar mesospheric temperature (150K)

15. The Structure of Uranus’ Atmosphere
Only one layer of Methane clouds (in contrast to 3 cloud layers on Jupiter and Saturn).
Cloud layer is very deep in Uranus’ atmosphere.
Uranus’ cloud layer difficult to see because of thick atmosphere above it.

16. Processes
Ethane (C2H6), ethylene (C2H4), acetylene (C2H2) from methane photodissociation.
Methane diffuses up
ethane/acetylene breakdown under pressure
ethane/etc fall
Methane condenses
ethane/acetylene haze formed
UV < 1600A

17. Clouds methane ice (CH4) <1bar (limit of probing) Probably have:
Methane droplets?
hydrogen Sulfide (H2S) 2-5bar
more H2S, so no NH3 clouds?
NH4 hydrosulfide (NH4SH) 15-30bar
water bar
Solution Cloud to a few 100bar

18. Processes
Micrometeorites bring CO and H2O to the stratosphere.

19. Ices of water, methane, and ammonia, mixed with hydrogen and silicates
The Interior of Uranus
Average density ≈ 1.29 g/cm3 → larger portion of rock and ice than Jupiter and Saturn.
Ices of water, methane, and ammonia, mixed with hydrogen and silicates

20. Internal Structure of Uranus
Uranus contains a higher proportion of heavy elements than Jupiter and Saturn
Uranus 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

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.

22. The magnetic fields of both Uranus is oriented at an unusual angle
The magnetic field of Uranus
The magnetic fields of both Uranus is oriented at an unusual angle
The magnetic axis of Uranus is steeply inclined from its axis of rotation
The magnetic and rotational axes of all the other planets are more nearly parallel
The magnetic field of Uranus is also offset from the center of the planet

23. Apparent motion of star behind Uranus and rings
The Rings of Uranus
Rings of Uranus 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

24. Uranus has a system of thin, dark rings
Rings of Uranus
Uranus has a system of thin, dark rings

25. How Uranus’ Rings are Discovered

26. Some of Uranus’ satellites show evidence of past tidal heating
Uranus has five satellites similar to the moderate-sized moons of Saturn, plus at least 22 more small satellites

27. Uranus’s Rings and Small Satellites

28. The Moons of Uranus Oberon Titania 1610 km 1550 km Ariel Umbriel
Oberon
Titania
1610 km
1550 km
Ariel
Umbriel
1160 km
1190 km
Miranda
480 km

29. Miranda
Miranda is a small satellite with a diameter of 470 kilometers (290 miles).
Its surface is unlike anything in the solar system with features that are jumbled together in a haphazard fashion. Miranda consists of huge fault canyons as deep as 20 kilometers (12 miles), terraced layers and a mixture of old and young surfaces.

30. Miranda

31. Ariel The brightest moon of Uranus.
The surface is pock-marked with craters, but the most outstanding features are long rift valleys stretching across the entire surface. Canyons much like the ones on Mars appear in the pictures. The canyon floors appear as though they have been smoothed by a fluid.

32. Titania
The largest moon of Uranus. It is marked by a few large impact basins, but is generally covered with small craters and very rough rocks.

33. Oberon
Characterized by an old, heavily cratered, and icy surface. The surface shows little evidence of internal activity other than some unknown dark material that apparently covers the floors of many craters.

34. Umbriel
The darkest satellite of Uranus. It is about the same size as Ariel and has about the same density. The surface appears to be old with large craters and does not change much from one location to another. Near the top is a puzzling bright ring called the fluorescent cheerio.