Rings, Moons, and Pluto All the giant planets have moons that orbit them like planets in a miniature...
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Transcript of Rings, Moons, and Pluto All the giant planets have moons that orbit them like planets in a miniature...
1ISP 205 - Astronomy Gary D. Westfall Lecture 13
Rings, Moons, and PlutoRings, 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
2ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Jupiter SystemThe 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
3ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Orbits of Jupiter’s MoonsThe Orbits of Jupiter’s Moons• Simulation
4ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Saturn SystemThe 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
5ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Orbits of Saturn’s MoonsThe Orbits of Saturn’s Moons• Simulation
6ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Uranus SystemThe 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
7ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Neptune SystemThe 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
8ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Largest SatellitesThe Largest SatellitesName Diameter
(km)Mass
(Moon=1)Density (g/cm3)
Reflectivity (%)
Moon 3476 1.0 3.3 12
Callisto 4820 1.5 1.8 20
Ganymede 5270 2.0 1.9 40
Europa 3130 0.7 3.0 70
Io 3640 1.2 3.5 60
Titan 5150 1.9 1.9 20
9ISP 205 - Astronomy Gary D. Westfall Lecture 13
Callisto, an Ancient and Primitive WorldCallisto, 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
10ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Ice of CallistoThe 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
11ISP 205 - Astronomy Gary D. Westfall Lecture 13
Portrait of CallistoPortrait 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.
12ISP 205 - Astronomy Gary D. Westfall Lecture 13
Ganymede, the Largest SatelliteGanymede, 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
13ISP 205 - Astronomy Gary D. Westfall Lecture 13
Portrait of GanymedePortrait of Ganymede• Ganymede
was a Trojan boy of great beauty whom Zeus carried away to be cup bearer to the gods.
14ISP 205 - Astronomy Gary D. Westfall Lecture 13
Europa, the Satellite with an OceanEuropa, 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
15ISP 205 - Astronomy Gary D. Westfall Lecture 13
Portrait of EuropaPortrait 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.
16ISP 205 - Astronomy Gary D. Westfall Lecture 13
Io, a Volcanic SatelliteIo, 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
17ISP 205 - Astronomy Gary D. Westfall Lecture 13
Portrait of IoPortrait 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.
18ISP 205 - Astronomy Gary D. Westfall Lecture 13
Titan, a Satellite with an Titan, a Satellite with an AtmosphereAtmosphere• 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
19ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Structure of Titan’s AtmosphereThe 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
20ISP 205 - Astronomy Gary D. Westfall Lecture 13
Portrait of TitanPortrait 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.
21ISP 205 - Astronomy Gary D. Westfall Lecture 13
Triton and its VolcanoesTriton 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
22ISP 205 - Astronomy Gary D. Westfall Lecture 13
Portrait of TritonPortrait 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.
23ISP 205 - Astronomy Gary D. Westfall Lecture 13
Discovery of PlutoDiscovery 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
24ISP 205 - Astronomy Gary D. Westfall Lecture 13
Pluto’s Motion and SatellitePluto’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
25ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Origin of PlutoThe 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
26ISP 205 - Astronomy Gary D. Westfall Lecture 13
Portrait of Pluto and CharonPortrait 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
27ISP 205 - Astronomy Gary D. Westfall Lecture 13
Planetary RingsPlanetary 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
28ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Rings of the Giant PlanetsThe Rings of the Giant Planets
29ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Rings of SaturnThe 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
30ISP 205 - Astronomy Gary D. Westfall Lecture 13
Portrait of Saturn and its RingsPortrait 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
Rhea
Dione
31ISP 205 - Astronomy Gary D. Westfall Lecture 13
The Rings of Uranus and NeptuneThe 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
32ISP 205 - Astronomy Gary D. Westfall Lecture 13
Satellite Ring InteractionsSatellite 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