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


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


The Orbits of Jupiter’s MoonsThe Orbits of Jupiter’s Moons• Simulation


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


The Orbits of Saturn’s MoonsThe Orbits of Saturn’s Moons• Simulation


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


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


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


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


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


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.


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


Portrait of GanymedePortrait of Ganymede• Ganymede

was a Trojan boy of great beauty whom Zeus carried away to be cup bearer to the gods.


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


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.


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


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.


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


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


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.


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


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.


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


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


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


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


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


The Rings of the Giant PlanetsThe Rings of the Giant Planets


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


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


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


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

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...