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Uranus
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
Uranus
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
Uranus’s Seasons
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.
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)
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.
ethane/etc fall
ethane/acetylene haze formed
Methane condenses
ethane/acetylene breakdown under pressure
Methane diffuses up
UV < 1600A
Clouds
• methane ice (CH4)
•
•
•
•
•
<1bar
(limit of probing)
Probably have:
Methane droplets?
hydrogen Sulfide (H2S)
more H2S, so no NH3 clouds?
NH4 hydrosulfide (NH4SH)
water
Solution Cloud
100bar
2-5bar
15-30bar
15-50bar
to a few
Processes
Micrometeorites bring CO and H2O to
the stratosphere.
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 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
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
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
Rings of Uranus
Uranus has a system of thin, dark rings
How Uranus’ Rings are Discovered
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
Uranus’s Rings and Small Satellites
The Moons of Uranus
Oberon
Titania
1610 km
1550 km
Umbriel
1190 km
Ariel
1160 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.
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.
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.