Uranus was discovered in 1781 by Herschel; first planet to

be discovered in more than 2000 years

Little detail can be seen from Earth; arrows point to three

of Uranus’s moons

Discoveries of Uranus and Neptune

Slightly more detail can be seen in this image taken by

Voyager 2 at a distance of 1 million km

Neptune was discovered in 1846, after analysis of Uranus’s

orbit indicated its presence

Details of Neptune cannot be made out from Earth either;

arrows again point to moons

More detail is visible in these

Voyager 2 images, taken from a

distance of 1 million km

-cloud streaks ranging in width from 50 km to 200 km

Uranus and Neptune are very similar

Sizes of Jovian Planets relative to Earth. Uranus and Neptune are quite similar in their bulk properties, each one probably having a core about 10 times more massive than Earth. Jupiter and Saturn are both much larger, but their rocky cores are probably comparable in mass to those of Uranus and Neptune.

Uranus Neptune

Mass 14.5 x Earth 17.1 x Earth

Radius 4.0 x Earth 3.9 x Earth

Density 1300 kg/m3 1600 kg/m3

Physical Properties

Peculiarity of Uranus: Axis of rotation lies almost in the plane

of its orbit. Seasonal variations are extreme.

Orbital Properties

Seasons on Uranus. Because of Uranus’s axial tilt of 98°, the planet experiences the most extreme seasons known in the solar system.

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

Atmospheres

Uranus Color is natural and the upper atmosphere is nearly featureless, except for a few wispy clouds in the northern hemisphere.

Atmospheres

Uranus’s Rotation. Computer-enhanced

Hubble images, ~4 hours apart.

Nore: motion of a pair of bright clouds

(A & B) in Uranus’s southern

hemisphere. (The numbers at the top

give the time of each photo)

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

Atmosphere of Uranus

HST captured this image of a rare

dark spot on Uranus in 2006.

Uranus is very cold; clouds

only in lower, warmer layers

Infrared image of Uranus shows that

planet’s ring system, as well as a

number of clouds (pink and red

regions) in the upper atmosphere.

Band structure of

Neptune is more

visible; it had a

“Dark Spot” similar

to Jupiter’s storms

(now vanished)

Atmosphere of Neptune

Voyager 2

Hubble views (2011) taken at roughly 4-hour intervals, offering a full

view of the planet. The methane ice crystal cloud features tinted pink the

infrared image. Note the Great Dark Spot had disappeared.

Uranus and Neptune both have substantial

magnetic fields, but at a large angle to their

rotation axes.

Note that both

Uranus’s and

Neptune’s are

also significantly

off center.

Magnetospheres

Jovian Magnetic Fields.

Interior structure of Uranus and Neptune, compared to that of

Jupiter and Saturn:

Internal Structure

molecular

hydrogenMolecular

hydrogen

rocky

coremetallic

hydrogen

“slush”

Regular vs Irregular Planetary Moons

regular moon

-relatively close to planet

-orbit prograde,

-little orbital inclination (in equatorial plane of planet)

-small eccentricity (~circular) orbit

-believed to have formed in orbit about their planet

irregular moon

-distant orbit

-orbit random retrograde/prograde

-orbit inclined at high angle

-high eccentricity orbit

-believed to have been captured

by planet

•Uranus has 27 moons, five of which are major: Miranda,

Ariel, Umbriel, Titania, and Oberon

•Similar to Saturn’s medium-sized moons, except that all are

much less reflective

•Umbriel is the darkest

https://en.wikipedia

.org/wiki/Moons_o

f_Neptune

Miranda Ariel Umbriel Titania Oberon

Uranus moons

Uranus

moons/rings

Miranda

Umbriel

Titania

Oberon

Ariel

Miranda is the most unusual moon of Uranus;

origin of the cracks and grooves is unknown

Miranda. The asteroid-sized innermost moon of Uranus, photographed by Voyager 2. Miranda has a fractured surface, suggesting a violent past, but the cause of the grooves and cracks is currently unknown. Resolution in the inset is a remarkable 2 km. The long “canyon” near the bottom of the inset is nearly 20 km deep.

Neptune has 14 moons, but only two can be seen from

Earth: Triton and Nereid (7 regular: 7 irregular)

Triton is in a retrograde orbit; Nereid’s orbit is highly

eccentric (irregular/captured moons)

Triton’s surface has few craters, indicating an active surface

Moon’s of Neptune

Triton

https://en.wikipedia.org/wiki/Moon

s_of_Neptune

Nitrogen geysers have

been observed on Triton,

contributing to the

surface features

South polar region of Triton, showing a variety of terrains ranging from deep ridges and gashes to what appear to be lakes of frozen water, all indicative of past surface activity. The orange region at lower right is nitrogen frost, forming the moon’s polar cap. The long black streaks at bottom left were probably formed by geysers of liquid nitrogen on the surface.

Moon’s of Neptune

Also, there appear to be ice volcanoes on Triton

Roughly circular lake like feature on Triton may have been caused by the eruption of an ice volcano. The water “lava” has since solidified, leaving a smooth surface. The absence of craters implies that this eruption was a relatively recent event. Inset: computer-generated view along Triton’s surface, illustrating the topography of the area.

Moon’s of Neptune

Uranus and Neptune have faint ring systems, detected via

stellar occultation

Rings of the Outermost Jovian Planets

Occultation of Starlight. By carefully watching the dimming of distant starlight as a planet crosses the line of sight, astronomers can infer fine details about that planet. The rings of Uranus were discovered with this technique.

Uranus’s rings are narrow

Uranus’s Rings

Mrings of Uranus, as imaged by Voyager 2. All 9 rings known before the spacecraft’s arrival can be seen in this photo. The two rings discovered by Voyager 2 are too faint to be seen here. The inset at top shows a close-up of the Epsilon ring, revealing some internal structure. The width of this ring averages 30 km; special image processing has magnified the resolution in the inset to about 100 m—the size of a football field.

Two shepherd

moons keep the

Epsilon ring from

diffusing

Uranian Shepherd Moons. These two small moons, named Cordelia (U7) and Ophelia (U8), were discovered by Voyager 2 in 1986. They shepherd Uranus’s Epsilon ring, keeping it from diffusing away.

Neptune has five rings: three narrow and two wide

Neptune’s Faint Rings. In this long-exposure image, Neptune (center) is heavily overexposed and has been artificially obscured (by an instrument) to make the rings easier to see. One of the two fainter rings lies between the inner bright ring (Leverrier) and the planet. The others lie between the Leverrier ring and the outer bright (Adams) ring.

• Uranus and Neptune were discovered in the last 350

years

• Uranus and Neptune are similar: gaseous and cold

• Uranus’s spin axis is almost in the plane of its orbit

• Surface features are hard to discern on Uranus but are

more obvious on Neptune

• Uranus has no excess heat emission, but Neptune does

Summary

• Uranus’s mid-sized moons are similar to those of Saturn

• Neptune’s moon Triton has a retrograde orbit

• Uranus and Neptune both have faint ring systems