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/