uranus. hubble-space-telescope image (rings artificially enhanced) 4 times larger than earth (in...
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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
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?
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
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.
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
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
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
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.
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