0 Uranus 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)
Exploration of the Outer Worlds New Horizons
Discovery of Uranus Uranus was discovered by chance Uranus recognized as a planet in 1781 by William Herschel
Appearance • To the unaided eye, it appears uniform, but there are slight color variations • Photochemical + meteor/ring dust haze in stratosphere?
Appearance • In IR, features are more pronounced
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
0 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. Cloud structures only visible after artificial computer enhancement of optical images taken from Voyager spacecraft. Optical view from Earth: Blue color of methane
Composition 83% H2 15% He 2% CH4 Traces of N2, HCN, H2S, NH3, CO
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
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
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)
0 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.
UV < 1600A ethane/acetylene haze formed Methane diffuses up Methane condenses ethane/etc fall ethane/acetylene breakdown under pressure Processes Ethane (C2H6), ethylene (C2H4), acetylene (C2H2) from methane photodissociation.
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 15-50bar • Solution Cloud to a few 100bar
Processes Micrometeorites bring CO and H2O to the stratosphere.
0 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
Internal Structure of Uranus Uranus contains a higher proportionof 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
0 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.
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
0 The Rings of Uranus Rings of Uranus are similar to Jupiter’s rings. Confined by shepherd moons; consist of dark material. Rings of Uranus were discovered through occultations of a background star Apparent motion of star behind Uranus and rings
Rings of Uranus Uranus has a system of thin, dark rings
Uranus’ Satellites 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
0 The Moons of Uranus Oberon Titania 1610 km 1550 km Ariel Umbriel 1160 km 1190 km Miranda 480 km
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.
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.
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.
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.
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.