The Outer Planets

• Most of what we know about the outer planets came from Voyagers 1&2
• 1979 Voyager 2 reached Jupiter
• 1980 Voyager 1 reached Saturn
• 1986 Voyager 2 reached Uranus
• 1989 Voyager 2 reached Neptune
• 1995 Galileo dropped a probe into the atmosphere of Jupiter

• Made mostly of 75% Hydrogen and 24% Helium gas 1% other elements
• VERY massive compared to Earth
• Since they are so massive, the gravity is very strong.
• Since the gravity is so strong, they are able to hold on to light weight gases
• For example: Jupiter is 300 time s the mass of the earth!

• Evidence gathered by the Galileo Space probe mission to Jupiter indicates that gas giants may have a solid core about the size of Earth. As you go down through the atmosphere pressure increases to incredibly levels due to the weight of the atmosphere

• Pluto is small, rocky and covered with ices

• It is thought that the cores of the gas giants may be solid made mainly of rock, different ices and frozen carbon dioxide.
• Pressure from the atmosphere above would cause things that are normally gases like carbon dioxide to become solid, even though the temperatures are MUCH higher than we would think ice could exist
• The cores of the gas giants may be several times the mass of the earth

• *18,000 km thick!
• Made of H & He
• Has colorful bands made by cloud layers and strong winds
• Great Red Spot is 3X the size of the Earth
• Is a storm similar to a hurricane
• Has been going on for almost 400 years! (Galileo saw it!!)

• Most of what we know of the moons comes from the Galileo spacecraft that arrived in the Jupiter system in 1995

• Has 17 moons in all
• 4 Galilean moons (the ones that Galileo saw in 1600's)
• Io - covered by volcanoes that are constantly erupting resulting in lava flows that change the shape of the surface constantly.
• Europa - covered by ice that shows ridges and cracks, MAY HAVE liquid water under the ice!!
• Ganymeade - largest moon in the solar system, covered by ice and craters
• Callisto - icy, covered by craters. Is the most heavily cratered body in the solar system.
• All of the Galilean moons are larger than earth's moon

• 2^nd largest planet
• smaller than Jupiter
• Voyager probes showed that the atmosphere is made of mainly H & He
• Has clouds and storms like Jupiter, but not as strong
• The only planet less dense than water! If you had a tub big enough it would float!

• Galileo saw things "sticking out" of Saturn turned out to be rings
• Cassini studied Saturn a few decades later than Galileo with a better telescope and saw that they were rings.
• Until the Voyager mission, Saturn was thought to be the only planet with rings. The Voyager spacecraft showed that ALL OF THE GAS GIANTS have rings! The rings around Jupiter, Uranus, and Neptune are too thin and faint to be seen from Earth even with the most powerful telescopes.
• Saturn's rings are made of chunks of ice and rock orbiting the planet
• Once thought to be a few "solid rings", Voyager 1 showed them to be many smaller ring grouped together

• Titan - Saturn's largest moon is bigger than Earth's moon
• Discovered in 1665- we didn't know it was a moon until Voyager 1
• Titan has a very thick atmosphere
• Saturn has a total of 19 moons! Several of which were discovered by Voyager 1

• About 4 times the diameter of Earth, mush smaller than Jupiter
• 2 X as far from the sun as Saturn
• Looks Bluish because of Methane in the atmosphere

• 1781 discovered by Wm Herschel the first planet discovered since ancient times
• He thought it was a comet, then people calculated its orbit, using Kepler's laws found its orbit was too regular, it had to be a planet.

• Voyager2 sent back most of what we know about Uranus in 1986
• Rotates in 17 hours!
• Axis is tilted nearly 90 degrees! It rotates on its side!
• This axis of rotation is what lead scientists to theorize that Uranus may have been hit by a comet in early development which knocked it on its side.

• 19 in all
• 5 largest are icy and cratered. These have been known since the late 1700's
• Some of the large moons have recent lava flows, indicating a hot interior in the "recent past".
• Voyager 2 discovered 10 moons that were previously unknown
• 1997 the HST discovered 2 more moons, making a total of 19

• 30 X as far from the sun as Earth!!
• Discovered by Math!
• Uranus' orbit was not behaving the way it should
• 1846 French and English mathematicians calculated that the problem with Uranus' orbit could be explained if there were another large planet out about where Neptune is… there was!

• 1989 Voyager 2 gave us almost all we know about Neptune
• has a great dark spot similar to Jupiter's
• HST images 5 years later showed the spot was gone

• 8 moons that we know of
• Triton is the largest. Has ice caps and a thick atmosphere.
• Appears to have "ice volcanoes" material that erupts from under the ice sheets.

• Pluto is Not a Gas Giant!!
• It has a solid surface
• Pluto is only about 2/3 the size of earth's moon!! Since its moon, Charon, is 1/2 the size of Pluto, some call it a double planet
• It takes Pluto 249 years to revolve around the sun.

• 1930 Clyde Tombaugh discovered Pluto, he was searching for something that was affecting Neptune's orbit.
• 1978 Charon was discovered

• Pluto is so small that some do not consider it a planet
• There are many small rocky objects out past Neptune, maybe Pluto is just one of them
• There is some evidence that Pluto may have been a moon of Neptune that was hit by something , fractured and knocked out of orbit.
• It is small and misshapen, Charon is nearly the size of Pluto
• Its orbit is tilted at such a strange angle to the rest of the planets and the orbit is much more elliptical than the other planets, might suggest that it did not form at the same time and place as the other planets
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