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)

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 98 0 –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) Sun Uranus

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 30 0 –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 (H 2 ), Helium (He), and methane (CH 4 ) 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 1/1600 th the sunlight received by Earth –Surface temperature of 35–45 K (–378 to –396 0 F) –Density suggests a rocky core surrounded by an icy mantle –Very tenuous CH 4 atmosphere Surface pressure is 100,000x smaller than Earth’s –Surface is covered with frozen N 2 mixed with CH 4 and traces of CO and H 2 O

Orbit and Rotation of Pluto Average distance from Sun: 39.5 AU Orbital period: 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: 17 0 Rotation axis tilted by about –Affects views from Earth strongly (copyright Calvin J. Hamilton) (

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 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/