Uranus and Neptune• Uranus: general information

– Discovered in 1781 (Herschel)– Radius about 4x that of Earth– Mass about 14.5x that of Earth– Nearly featureless atmosphere– Thin, dark rings– 27 moons

• Neptune: general information– Discovered in 1846– Radius about 4x that of Earth– Mass about 17x that of Earth– Has internal heat & an active

atmosphere– 13 moons (Voyager 2, NASA/JPL)

(Voyager 2, NASA/JPL)

Uranus and Neptune• Uranus and Neptune have nearly identical sizes and

internal structures• Both appear bluish-green in color

– Sunlight passes through an outer haze layer and reflects off the cloud tops below

– The haze is rich in methane, which strongly absorbs red and orange light, letting blue and green light pass through

• This bluish-green light is what gets reflected by the clouds below

– An additional factor is blue-light scattering in the haze layer

(Neptune & Triton, Voyager 2, NASA/JPL)

Spacecraft Visits• Voyager 2 fly-bys

– Uranus: January 1986– Neptune: August 1989

• Hubble Space Telescope studies– Long-term monitoring of atmospheric weather patterns– Infrared imaging studies of their atmospheres, rings, and

moons

(HST image of Neptune and its largest moon Triton, NASA/JPL)

Orbit and Rotation of Uranus• Average distance from Sun: 19.2 AU• Orbital period: 84 years• Orbital eccentricity e = 0.05 • Rotation axis tilted by about 980

– Tilted on its side in its orbital plane– Because intensity of sunlight is so weak

at Uranus, and since interior of Uranus is a good insulator (holding heat), this has little effect on seasonal temperature variations

(copyright Calvin J. Hamilton)

SunUranus

Orbit and Rotation of Neptune• Average distance from Sun: 30.1 AU• Orbital period: 165 years• Orbital eccentricity e = 0.01 (nearly circular) • Rotation axis tilted by 300

– Similar to Earth, Mars, and Saturn– Has seasonal pattern of solar heating similar to Earth’s

• Rotational period: 16.1 hours– Has significant magnetic field– Similar to Uranus

(copyright Calvin J. Hamilton)

Interiors of Uranus and Neptune• Interior structures of both planets are very similar

• Atmosphere composition: molecular hydrogen (H2), Helium (He), and methane (CH4)

• Thick, slushy mantle of water, ammonia, and methane ices• Rocky cores about the size of Earth • Both planets lack the deep metallic hydrogen mantles found

on Jupiter and Saturn because of their smaller size (pressure doesn’t get large enough)

Atmosphere of Uranus• Uranus appears as a virtually featureless hazy

bluish-green ball– The reason is a lack of internal heat, unlike the other gas

giants– There is a lack of convection in the atmosphere maybe

due to layered interior (like an onion)…?– Clouds are cold and don’t billow up above the top layer of

haze– Results in a generally uniform appearance– Occasional clouds/storms

seen by HST in the infrared

(Voyager 2, NASA/JPL)

Atmosphere of Uranus

(NASA/JPL)

Atmosphere of Neptune• Neptune, like Jupiter and Saturn, radiates about

2.7x as much energy as it receives from the Sun– Creates atmospheric convection and weather on Neptune

• Belts and bright clouds of methane ice• Dark oval cyclonic storms

(NASA/JPL)

Atmosphere of Neptune

(Voyager 2, NASA/JPL)

Atmosphere of Neptune: Great Dark Spot• Large storm that

appeared in the mid- 1980s and then vanished by 1995

(Voyager 2, NASA/JPL)

Neptune’s Great Dark Spot

(Voyager 2, NASA/JPL)

Rings of Uranus

(Voyager 2, NASA/JPL, copyright Calvin J. Hamilton)

Rings of Uranus

(Voyager 2, NASA/JPL)

Rings of Uranus

(Voyager 2, NASA/JPL)

Rings of Neptune

(Voyager 2, NASA/JPL)

Rings of Neptune

(Voyager 2, NASA/JPL)

Pluto• General information

– Discovered in 1930 (Tombaugh)– Only planet not visited by a spacecraft– Receives only 1/1600th the sunlight received by Earth– Surface temperature of 35–45 K (–378 to –396 0F)– Density suggests a rocky core surrounded by an icy

mantle

– Very tenuous CH4 atmosphere• Surface pressure is 100,000x smaller than Earth’s

– Surface is covered with frozen N2 mixed with CH4 and traces of CO and H2O

Orbit and Rotation of Pluto• Average distance from Sun: 39.5 AU• Orbital period: 248.5 years• Orbital eccentricity e = 0.25 (largest of the

planets)– Closer to the Sun than Neptune for 20 years out

of its orbit around the Sun

• Orbital inclination: 170 • Rotation axis tilted by

about 1200

– Affects views from Earth strongly

(copyright Calvin J. Hamilton)

(www.nineplanets.org)

Charon• Pluto has one large moon: Charon

– Discovered in 1978 (Naval Observatory)– Largest moon/planet size ratio in the solar system (radius

of 625 km for Charon vs. 1150 km for Pluto)

• Pluto and Charon experience synchronous rotation with each other– Always keep the same face toward each other

(HST, NASA/JPL)

Pluto and Charon Size Comparison

Pluto

Charon

Surface of Pluto

(HST, NASA/JPL, ESA)

NASA’s New Horizons Mission• Launched in January 2006• Scheduled for Pluto fly-by (first ever) in 2015• Will be followed by encounters with objects in the

distant Kuiper Belt (about 50 AU from Sun) in 2016 – 2020

• See the following website for mission details:

pluto.jhuapl.edu/