The Outer Worlds

Uranus was discovered by chance

• Uranus recognized as a planet in 1781 by William Herschel

while scanning the sky for nearby objects with measurable parallax: discovered Uranus as slightly extended object, ~ 3.7 arc seconds in diameter.

Neptune’ DiscoveryDiscovered in 1846 at position predicted from gravitational disturbances on Uranus’s orbit by John Couch Adams and Urbain Jean Leverrier. (But don’t forget Galle!)

Blue-green color from methane in the atmosphere

4 times Earth’s diameter; 4 % smaller than Uranus

The Atmospheres of Uranus and Neptune

Outer atmospheres of Uranus and Neptune are similar to those of Jupiter and Saturn

Uranus and Neptune are cold enough that ammonia freezes; methane dominates and gives the characteristic blue color

Uranus is very cold; clouds only in lower, warmer layers.

The Atmospheres of Uranus and Neptune

Few features are visible:

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 due

to methane, absorbing longer

wavelengths

Cloud structures only visible after artificial computer enhancement of optical images

taken from Voyager spacecraft.

The Structure of Uranus’ Atmosphere

Only one layer of Methane clouds (in

contrast to 3 cloud layers on Jupiter and Saturn).

3 cloud layers in Jupiter and Saturn form at relatively high temperatures that occur only very deep in

Uranus’ atmosphere.

Uranus’ cloud layer difficult to see because of thick atmosphere above it.

Also shows belt-zone structure

Belt-zone cloud structure must be dominated by planet’s rotation, not by incidence angle of sun light!

The Atmospheres of Uranus and Neptune

Neptune has storm systems similar to those on Jupiter, but fewer:

The Atmospheres of Uranus and Neptune

Band structure of Neptune is more visible, and Neptune has internal heat source of unknown origin:

Cloud Structure of Uranus

Hubble Space Telescope image of Uranus shows cloud structures not present during Voyager’s passage in 1986.

Possibly due to seasonal changes of the cloud structures.

Neptune is a cold, bluish world with Jupiter-likeatmospheric features

• No white ammonia clouds are seen on Uranus or Neptune

• Presumably the low temperatures have caused almost all the ammonia to precipitate into the interiors of the planets

• All of these planets’ clouds are composed of methane

• Much more cloud activity is seen on Neptune than on Uranus.

• This is because Uranus lacks a substantial internal heat source.

Neptune’s Clouds

• Much more cloud activity is seen on Neptune than on Uranus

• This is because Uranus lacks a substantial internal heat source

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 planetlike object early in the history of our solar system. Such a collision could have knocked Uranus on its side

The Motion of Uranus

Very unusual orientation of rotation

axis: Almost in the orbital plane.

Large portions of the planet exposed to

“eternal” sunlight for many years, then

complete darkness for many years!

Possibly result of impact of a large

planetesimal during the phase of planet

formation.

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Uranus and Neptune contain a higher proportionof heavy elements than Jupiter and Saturn

• Both Uranus and Neptune 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

Magnetospheres and Internal Structure

Comparison of the interiors of the Jovian 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.

Magnetosphere with weak radiation belts; allows determination of rotation period: 17.24 hr.

Magnetospheres and Internal Structure

Uranus and Neptune both have substantial magnetic fields, but at a large angle to their rotation axes.

The rectangle within each planet shows a bar magnet that would produce a similar field. Note that both Uranus’s and Neptune’s are significantly off center.

The Magnetosphere of Uranus

Rapid rotation and large inclination deform magnetosphere into a corkscrew shape.

During Voyager 2 flyby: South pole pointed towards sun; direct interaction of solar wind with magnetosphere Bright aurorae!

UV images

Uranus and Neptune each have a system of thin,dark rings

The Rings of UranusRings of Uranus and Neptune 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’s rings are narrow:

Two shepherd moons keep the epsilon ring from diffusing:

The Rings of Neptune

Neptune has five rings, three narrow and two wide:

The Rings of Neptune

Made of dark material, visible in forward-scattered light.

Focused by small shepherd moons embedded in the ring structure.

Ring material must be regularly re-supplied by dust from meteorite impacts on the moons.

Interrupted between denser segments (arcs)

The moons of Uranus – 27 at present

• The first two were discovered by William Herschel in 1787, and named, by his son, after characters from Shakespeare’s A Midsummer Nights Dream, Titania and Oberon.

• Two more moons were found by William Lassell in 1851 and named Ariel and Umbriel

• G. Kuiper discovered Miranda in 1948. • All the moons of Uranus are named after

characters from Shakespeare or Alexander Pope.

• Voyager 2’s flyby in January 1986 led to the discovery of another 10.

• Six additional moons have since been discovered by telescope.

The Moons of Uranus

5 largest moons visible from Earth.

10 more discovered by Voyager 2; more are still being found.

Dark surfaces, probably ice darkened by dust from meteorite impacts.

5 largest moons all tidally locked to Uranus.

Moons of Uranus

Interiors of Uranus’s Moons

Large rock cores surrounded by icy mantles.

The Surfaces of Uranus’s Moons

Oberon

Old, inactive, cratered surface,

Titania

but probably active past.

Long fault across the surface.

Dirty water may have flooded floors of some craters.

Largest moon

Heavily cratered surface, but no very large craters.

Active phase with internal melting might have flooded craters.

The Surfaces of Uranus’s Moons

Umbriel

Dark, cratered surface

Ariel

No faults or other signs of surface activity

Brightest surface of 5 largest moons

Clear signs of geological activity

Crossed by faults over 10 km deepPossibly heated by tidal interactions

with Miranda and Umbriel.

Uranus’s Moon Miranda

Most unusual of the 5 moons detected from Earth

Ovoids: Oval groove patterns, probably associated with convection currents in the

mantle, but not with impacts.

20 km high cliff near the equator

Surface features are old; Miranda is no longer geologically active.

MIRANDA

The Moons of Neptune

Two moons (Triton and Nereid) visible from Earth; 6 more discovered by Voyager 2

Unusual orbits:

Triton: Only satellite in the solar system orbiting clockwise, i.e. “backward”.

Nereid: Highly eccentric orbit; very long orbital period (359.4 d).

Triton is a frigid, icy world with a young surfaceand a tenuous atmosphere

• Neptune has 13 satellites, one of which (Triton) is comparable in size to our Moon or the Galilean satellites of Jupiter

• Triton has a young, icy surface indicative of tectonic activity

• The energy for this activity may have been provided by tidal heating that occurred when Triton was captured by Neptune’s gravity into a retrograde orbit

• Triton has a tenuous nitrogen atmosphere

The Surface of Triton

Triton can hold a tenuous atmosphere of nitrogen and some methane; 105 times less dense than Earth’s atmosphere.

Very low temperature (34.5 K)

Surface composed of ices: nitrogen, methane, carbon monoxide, carbon dioxide.

Possibly cyclic nitrogen ice deposition and re-vaporizing on Triton’s south pole, similar to CO2 ice polar cap cycles on Mars.

Dark smudges on the nitrogen ice surface, probably due to methane rising from below surface, forming carbon-rich deposits when

exposed to sun light.

The Surface of Triton (2)

Ongoing surface activity: Surface features probably not more than 100 million years old.

Large basins might have been flooded multiple times by liquids from the interior.

Ice equivalent of greenhouse effect may be one of the heat sources for Triton’s geological activity.