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’ Discovery Discovered 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

The Atmospheres of Uranus and Neptune Uranus is very cold; clouds only in lower, warmer layers. Few features are visible:

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 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-like atmospheric 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 19.18 AU 97.9o Very unusual orientation of rotation axis: Almost in the orbital plane. Possibly result of impact of a large planetesimal during the phase of planet formation. Large portions of the planet exposed to “eternal” sunlight for many years, then complete darkness for many years!

Uranus and Neptune contain a higher proportion of 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. UV images During Voyager 2 flyby: South pole pointed towards sun; direct interaction of solar wind with magnetosphere  Bright aurorae!

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

Apparent motion of star behind Uranus and rings The Rings of Uranus Rings of Uranus and Neptune 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

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 Interrupted between denser segments (arcs) Ring material must be regularly re-supplied by dust from meteorite impacts on the moons. Interrupted between denser segments (arcs) Made of dark material, visible in forward-scattered light. Focused by small shepherd moons embedded in the ring structure.

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 Titania Old, inactive, cratered surface, Largest moon but probably active past. Heavily cratered surface, but no very large craters. Long fault across the surface. Dirty water may have flooded floors of some craters. Active phase with internal melting might have flooded craters.

The Surfaces of Uranus’s Moons Umbriel Ariel Dark, cratered surface Brightest surface of 5 largest moons Clear signs of geological activity No faults or other signs of surface activity Crossed by faults over 10 km deep Possibly 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 Unusual orbits: Two moons (Triton and Nereid) visible from Earth; 6 more discovered by Voyager 2 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 surface and 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 Very low temperature (34.5 K)  Triton can hold a tenuous atmosphere of nitrogen and some methane; 105 times less dense than Earth’s atmosphere. 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.