Solar System Outer Planets
The Outer Planets • Our solar system’s outer planets are Jupiter, Saturn, Uranus, and Neptune. • Pluto has been reclassified as a dwarf planet, one of many beyond Neptune. • Space probes sent to the outer planets include the famous Voyager missions, Galileo, Cassini/Huygens, and New Horizons.
Jupiter • Jupiter is the largest planet in the solar system and the fifth planet from the Sun. • 1979, the twin Voyager 1 and Voyager 2 spacecraft flew past Jupiter, and in 1995 the Galileo orbiter reached Jupiter. • These spacecraft gathered information about Jupiter’s atmosphere and moons, and have added greatly to our knowledge of the Jupiter system. • Voyager revealed that Jupiter has a faint system of dusty rings, and one of the moons is highly volcanic.
Atmosphere • Jupiter is mostly made up of H and He, and is similar in composition to the Sun. There is also some NH3 (ammonia), CH4 (methane), and H2O. • Scientists hypothesize that the atmosphere of H and He changes to a planetwide ocean of liquid H and He toward the middle of the planet. • Below these layers there may be a solid rocky core, but due to the higher pressure the rock would be different from rocks here on Earth.
In pictures you can see bands of white, red, tan, and brown clouds in the atmosphere. • There are continuous storms on Jupiter’s surface, and the Great Red Spot is the most spectacular of these storms. Earth, for comparison Great Red Spot
The “Galilean” Moons • Jupiter is known to have at least 60 moons. • The four largest (discovered by Galileo) are Io, Europa, Ganymede, and Callisto. • They are generally as large as Earth’s moon or larger.
Io • Closest large moon to Jupiter, slightly larger than Earth’s moon • The gravity of Jupiter and nearby Europa exert a strong pull on Io. This tidal pull creates heating due to continuous friction in Io’s crust. • This makes Io the most volcanically active object in the solar system. • Sulfur compounds give its surface distinct reddish and orange colors. • Has a thin oxygen, sulfur, and sulfur dioxide atmosphere.
Europa • Mostly composed of rock with a thick, smooth crust of ice. The lack of craters indicates a very young, active surface. • Europa experiences tidal heating, but to a lesser degree than neighboring Io. • Studies indicate that a water ocean as deep as 50 km exists beneath Europa’s ice crust. Along with possibly Ganymede and Callisto, this may be the only place in the solar system where liquid water exists in large quantities.
Ganymede • Largest moon in the solar system (larger than the planet Mercury). • Has a crust of ice about 100 km thick, covered with grooves. • The icy crust might surround an ocean of water or slushy ice. • Has a rocky core and a very thin oxygen atmosphere. Earth and Moon for comparison Ganymede
Callisto • Most distant of ofJupiter’s four large moons. • Heavily cratered crust composed mainly of ice and rock that is several hundred kilometers thick. • Crust might surround a salty ocean around a rock core. • Thin atmosphere of CO2.
New Worlds to Explore? Our knowledge of the Jupiter system before the Space Age
Saturn • Sixth planet from the Sun. • Second largest planet in the solar system. • Has the lowest overall density (0.7 g/cm3), so low that the planet would float in water. • The twinVoyager probes flew by Saturn in 1980 and 1981. • Since 2004, the Cassini orbiter has been investigating the Saturn system, including the Huygens probe which landed on Titan.
Atmosphere • Saturn is a large, gas giant planet. • It has a thick outer atmosphere composed mostly of H and He. It also contains NH3, CH4, and water vapor. • As you go deeper into the atmosphere, the gases eventually turn to liquid H and He. Its composition is overall similar to Jupiter and the Sun. • Below the atmosphere and liquid layer, it may have a small, rocky core.
Rings and Moons • The Voyager probes showed us more detailed views of Saturn’s rings. Each ring is composed of thousand of ringlets, made of tiny ice and rock particles. • At least 60 moons orbit. Saturn’s largest moon, Titan, is larger than theplanet Mercury. • Titan has a thick atmosphere of N, Ar, and CH4.
Titan • Voyager1 could not see through Titan’s thick hazy atmosphere. • The Cassini mission included a smaller probe, Huygens, which parachuted to the surface of Titan in 2005. • Titan’s surface temperature allows methane to exist as a solid, liquid, or gas. Titan experiences a “methane” cycle which is similar to the water cycle on Earth. • Radar mapping by Cassini indicates a strangely Earthlike surface with river valleys and lakes of methane.
Uranus • Seventh planet from the Sun. • First planet discovered with the telescope; discovered in 1781 by Sir William Herschel. • Voyager 2 flew by Uranus in 1986. • It is a large, gaseous planet with 27 satellites and a system of thin, dark rings not visible from Earth • Two of Uranus’s moons, Cordeliaand Ophelia keep the last ring from dispersing into space.
Characteristics • Atmosphere is composed of H, He, and some methane (CH4) which gives it a pale blue-green color. • Few cloud bands and storm systems are visible; very featureless atmosphere. • Under the atmosphere it is thought to have a mantle of liquid and solid water, CH4, and NH3 surrounding a rocky core.
Its axis of rotation is tilted on its side compared with the other planets (about 90º from vertical). It is nearly parallel to the plane of its orbit. • As a result, Uranus rolls along like a ball as it orbits the the Sun.
Neptune • Now considered the eighth (and last) planet from the Sun. • Dwarf planet Pluto’s orbit crossesinside Neptune’s orbit on their journey around the sun. From 1979 to 1999 Pluto was closer to the Sun than Neptune. • Discovered in 1846 by Urbain Le Verrier and John Couch Adams. • Visited only by Voyager 2, which flew by in 1989.
Neptune’s Characteristics • Atmosphere is similar to that of Uranus. • Neptune has hurricane-like storms in its atmosphere that are similar to storms in Jupiter’s atmosphere, including one called the Great Dark Spotdiscovered in 1989 by Voyager 2. • In 1994, photos from Hubble Space Telescope indicated that the Great Dark Spot had disappeared.
Neptune’s Great Dark Spot, about the size of the Earth Photos take of Neptune by Hubble Space Telescope, there is no Great Dark Spot
The disappearance of the Great Dark Spot shows that Neptune’s atmosphere is active and changes quickly. • Under the atmosphere is thought to be a layer of liquid water, CH4, and NH3that might change to solid ice. • Voyager 2 detected six new moons making a total of eight moons.
Triton, the largest of Neptune’s moons. Temperature: - 235° C (- 391 ° F) • Triton has a thin nitrogen atmosphere and geyser activity.
Pluto (dwarf planet) • Long considered the ninth planet since its discovery in 1930 by Clyde Tombaugh. In 2006 it was demoted to a dwarf planet. • Pluto is a member of a group of objects that orbits in a disc-like zone beyond the orbit of Neptune known as the Kuiper Belt. • This distant realm is populated with thousands of miniature ice worlds which formed early in the history of our solar system. • Pluto isn’t even the largest of them. • Stay tuned: New Horizons will arrive in 2015.