Rings, Moons, and Pluto

Rings, Moons, and Pluto • All the giant planets have moons that orbit them like planets in a miniature solar system • There are 60 satellites known in the outer solar system • Several of the moons are larger than Pluto and have atmospheres of their own • Pluto resembles these satellites more that it resembles the other 8 planets • All the giant planets have rings • Rings consist of billions of small particles or moonlets Lecture 13

The Jupiter System • Jupiter has 16 satellites • 4 large moons discovered by Galileo • Callisto, Ganymede, Europa, Io • Europa and Io are about the size of the Moon • Ganymede and Callisto are bigger than Mercury • Remaining 12 moons are much smaller • The inner 4 orbit inside the orbit of Io • Of the remaining 8 small moons • 4 have highly inclined orbits • 4 have retrograde orbits • May be captured objects Lecture 13

The Orbits of Jupiter’s Moons • Simulation Lecture 13

The Saturn System • Saturn has 19 known satellites • The largest is Titan, which is almost as big as Juptier’s Ganymede • Titan is the only moon with a substantial atmosphere (more later) • 6 other moons have regular orbits • Several moons orbit near the rings • 2 distant, irregular moons, one retrograde • The rings of Saturn are spectacular • Broad, flat, few gaps • Rings are a collection of icy fragments with sizes ranging from a ping pong ball to a basketball Lecture 13

The Orbits of Saturn’s Moons • Simulation Lecture 13

The Uranus System • The ring and satellite system of Uranus is tilted 98 degrees just like the planet itself • 11 rings • Only discovered in 1977 • Narrow ribbons with broad gaps • Composed of icy, dark material • 18 moons • The 5 largest satellites are similar to the 6 regular satellites of Saturn • The 13 smaller moons are very dark • Simulation Lecture 13

The Neptune System • Neptune has 8 satellites • 6 regular satellites close to the planet and two irregular satellites farther out • Triton is a relatively large moon in a retrograde orbit • Triton has an atmosphere and active volcanoes • Neptune has rings • Narrow and faint • Composed of dark, icy material • Simulation Lecture 13

The Largest Satellites Lecture 13

Callisto, an Ancient and Primitive World • Callisto orbits Jupiter at a distance of 2 million km and circles Jupiter in 17 days • Callisto has the same rotational period as orbital period like the Moon • The surface temperature of Callisto is -140 degrees centigrade • Callisto is about the same size as Mercury but with only 1/3 the mass of Mercury • Callisto has much less rocky materiel than Mercury • Callisto is undifferentiated • Callisto was frozen solid before differentiation was completed Lecture 13

The Ice of Callisto • The surface of Callisto is composed of water ice • This ice is much colder than ice on Earth and does not flow like Earth’s glaciers • The surface of Callisto is covered with impact craters • There is no geological activity on Callisto • Generally the craters on Callisto resemble the crater on the Moon • When one looks in detail at the crater, they show erosion • Results from sublimation of water leaving behind a dusty material Lecture 13

Portrait of Callisto • Callisto was a nymph, beloved of Zeus and hated by Hera. Hera changed her into a bear and Zeus then placed her in the sky as the constellation Ursa Major. Lecture 13

Ganymede, the Largest Satellite • About 1/4 of the surface of Ganymede is cratered • The remaining surface area was formed more recently • Sparse, fresh craters • About 1 billion years old • Younger than the lunar maria or martial plains • Ganymede is differentiated • Has a rocky core about the size of the Moon • Has a magnetic field • The young surface is the result of tectonic and volcanic forces • Some features formed when the crust cracked and water flooded craters • Mountain ranges were formed from compression of the crust • Impact craters were split and pulled apart • May have been caused by tidal forces from Jupiter Lecture 13

Portrait of Ganymede • Ganymede was a Trojan boy of great beauty whom Zeus carried away to be cup bearer to the gods. Lecture 13

Europa, the Satellite with an Ocean • Europa is similar to the Moon • Predominantly rock • Heat from the forming Jupiter combined with continuous heating from tidal forces evaporated most of the water on Europa • However, Europa retains an ice-covered surface • There are very few impact craters on Europa • Surface is younger than a few million years • Europa is better able to erase impact craters than Earth • Cracks and long ridges on the icy surface suggest that there is liquid water under the surface • Heat is derived from tidal forces • May be sufficient for primitive life Lecture 13

Portrait of Europa • Europa was a Phoenician princess abducted to Crete by Zeus, who had assumed the form of a white bull, and by him the mother of Minos. Lecture 13

Io, a Volcanic Satellite • Io is the inner most satellite of Jupiter • Similar to the Moon in size and density • Io has the highest level of volcanism in the solar system • Volcanic eruptions are visible on the surface of Io • The lava is similar to lava on Earth • Sometimes the lava comes in contact with frozen layers of sulfur and sulfur dioxide and produces huge plumes • The surface of Io is constantly changing • Tidal heating keeps Io active Lecture 13

Portrait of Io • Io was a maiden who was loved by Zeus (Jupiter) and transformed into a heifer in a vain attempt to hide her from the jealous Hera. Lecture 13

Titan, a Satellite with an Atmosphere • Titan, the largest satellite of Saturn, was discovered in 1655 by the Dutch Astronomer Christian Huygens, the first moon to be found after Galileo saw the four large moons of Jupiter • Titan is roughly the same size as Callisto and Ganymede but its composition is unknown • Titan has a substantial atmosphere • Has a pressure 1.6 times Earth • Mostly nitrogen • Contains other gases • Carbon dioxide, methane, ethane, propane • Hydrogen cyanide, cyanogen, cyanoacetylene • Active chemistry thought to be the progenitor of life on Earth Lecture 13

The Structure of Titan’s Atmosphere • Titan has multiple layers of clouds • These clouds completely obscure the surface of Titan • The surface temperature is about 90 K • Methane can exist as solid, liquid, gas at these temperatures • Organic compounds are stable in Titan’s atmosphere • May hold chemical history dating back billions of years • Cassini and Huygens will try to find some answers in 2004 Lecture 13

Portrait of Titan • In Greek mythology the Titans were a family of giants, the children of Uranus and Gaia, who sought to rule the heavens but were overthrown and supplanted by the family of Zeus. Lecture 13

Triton and its Volcanoes • Triton is the largest satellite of Neptune • Triton has a diameter of 2720 km and a density of 2.1 g/cm3 • Suggests 75% rock and 25% water ice • The surface of Triton is very cold • 35 to 40 K • The surface of Triton is made of frozen water, nitrogen, methane, and carbon monoxide • Impact craters on Triton are erased by “lava” flows • Melted ices • The southern pole of Triton is covered with a polar ice cap • In summer, the ice cap is heated and geysers are created 10 km high Lecture 13

Portrait of Triton • In Greek mythology, Triton is a god of the sea, the son of Poseidon (Neptune); usually portrayed as having the head and trunk of a man and the tail of a fish. Lecture 13

Discovery of Pluto • Pluto was discovered through a careful systematic search rather than having a good prediction of where to look as was the case for Neptune • Percival Lowell spent the last 10 years of his life (he died in 1916) searching unsuccessfully for the ninth planet • In 1930, Clyde Tombaugh found the ninth planet after an exhaustive search using photographic plates and the blinker method • Tombaugh worked at the Lowell Observatory • Used a camera donated by Lowell’s brother • Recent measurements show there are no more planets • IRAS, Voyager, Pioneer Lecture 13

Pluto’s Motion and Satellite • Pluto’s orbit has a large inclination to the ecliptic (17) • Pluto’s orbit takes it closer to the Sun than Neptune but there is no chance of collision because of the angle of inclination • Pluto requires 248.6 years to circle the Sun • Pluto’s rotational axis is tilted 90 degrees similar to Uranus • Pluto has a moon, Charon • Retrograde orbit in the ecliptic (not along Pluto’s equator) • About half the size of Pluto • Orbital period and rotational period are the same as Pluto’s rotational period • When Pluto is close to the Sun, it’s atmosphere gets thicker Lecture 13

The Origin of Pluto • Pluto is different from the other planets • Pluto’s orbit and composition is very different from the other outer planets • Pluto resembles the moons rather than the planets • The presence of Charon is puzzling • Some astronomers speculate that the strange orbits of Pluto, Charon, Nereid, and Triton are the result of violent collisions during the early history of the outer solar system Lecture 13

Portrait of Pluto and Charon • In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. • Charon is named for the mythological figure who ferried the dead across the River Styx into Hades (the underworld). Photo taken by the Hubble Space Telescope Lecture 13

Planetary Rings • All four of the giant planets have rings • The rings are made of billions of individual fragments each orbiting the planet • There are two formation possibilities • Breakup • The tidal force of the large planet destroyed existing moons • Prevention • The tidal forces of the large planet prevented material from coalescing into moons Lecture 13

The Rings of the Giant Planets Lecture 13

The Rings of Saturn • The rings of Saturn are very broad and extraordinarily thin • 70,000 km wide • 20 m thick • Ring particles are composed of water ice with sizes ranging from sand to house-size boulders • Saturn has three main rings, A, B, C • Saturn has narrow rings also that resemble those of Uranus and Neptune Lecture 13

Portrait of Saturn and its Rings • In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Photo of Saturn and its rings taken by Voyager 2 Dione Rhea Lecture 13

The Rings of Uranus and Neptune • The rings of Uranus are narrow and black making them almost invisible from Earth • The 9 main rings were discovered by watching a star pass behind Uranus • Two more rings were discovered by Voyager in 1986 • The outermost ring is the Epsilon Ring • Contains more mass than the other rings combined • Rings are made of black material • May be carbon or some hydrocarbon • Similar to its ten smaller inner moons Lecture 13

Satellite Ring Interactions • Gravitational resonances with small inner moons cause structure in rings and may be responsible for the existence of the rings • The narrow F ring of Saturn shows strands with bends and kinks • Must be under the influence of two shepherd moons, Pandora and Prometheus • Narrow gaps in Saturn’s rings are associated with small moons that clear lanes in the ring material Lecture 13

Rings, Moons, and Pluto