Uranus

Uranus

Hubble-Space-Telescope image (rings artificially enhanced)

4 times larger than Earth (in radius)

14.5 times the mass of Earth

Faint rings, indirectly detected through

occultations

Discovered in 1781

Rotation axis inclined by 970 against the orbital axis:

Uranus is “rolling” on its orbit

Exploration of the Outer Worlds

New Horizons

Uranus

Discovery of UranusUranus 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 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

Uranus’s Storms

Uranus’s Seasons

The Atmosphere of UranusLike 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.

Composition

83% H2 15% He 2% CH4

Traces of N2, HCN, H2S, NH3, CO

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

Surface/Atmosphere of Uranus

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)

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.

Processes

Ethane (C2H6), ethylene (C2H4), acetylene (C2H2) from methane photodissociation.

Met

hane

dif

fuse

s up

ethane/acetylene breakdown under pressure

etha

ne/e

tc f

all

Methane condenses

ethane/acetylene haze formed

UV < 1600A

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.

The Interior of UranusAverage 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

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

Internal Structure of Uranus

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 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

The magnetic field of Uranus

The Rings of UranusRings 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

Uranus has a system of thin, dark rings

Rings of Uranus

How Uranus’ Rings are Discovered

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

Uranus’ Satellites

Uranus’s Rings and Small Satellites

The Moons of UranusOberon

Umbriel Ariel

Miranda

Titania

1550 km

480 km

1610 km

1190 km1160 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.

Miranda

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.

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.

Oberon

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.

Umbriel