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Moons, Moons, Pluto, Pluto, & & RingsRings

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Ring and Satellite Systems (1)Ring and Satellite Systems (1)The rings and moons in the outer solar system are different in composition from objects in the inner solar system

Most of these rings and moons contain dark, organic compounds mixed with ice and rockThe presence of dark material implies that the objects reflect very little light

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Ring and Satellite Systems (2)Ring and Satellite Systems (2)Most satellites in the outer solar system are in direct or regular orbits

They revolve about their planet in a west-to-east direction and in the plane of the planet’s equator

A number of them have irregular orbitsThey orbit in a retrograde (east-to-west) direction, or else have orbits with high eccentricity or high inclination

These satellites are usually smaller, located relatively far from their planetThey were probably formed far away and subsequently captured by the planet they now orbit

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Jupiter's MoonsJupiter's MoonsTo date, Jupiter has 63 known satellites

The largest four are Callisto, Ganymede, Europa, and Io

Europa and Io are the size of our MoonGanymede and Callisto are bigger than Mercury

The rest are much smallerMore moons will likely be found in the future

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Galilean MoonsGalilean MoonsThey were first seen by Galileo, using his telescopes

most recently studied by the Galileo spacecraft and by the Hubble Space Telescope

The combined data about the moons have shown important similarities to the terrestrial planets

The differences between the moons seem to be mostly due to distance from Jupiter

IoIo

EuropaEuropa

GanymedeGanymede

CallistoCallisto

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Major Satellites in Solar Major Satellites in Solar SystemSystem

Callisto: Cratered World (1)Callisto: Cratered World (1)This is the outermost of the Galilean satellites

2 million km from JupiterIts noon-time surface temperature is 130 K (140°C below freezing!)Its diameter is 4820 km, about the same as Mercury’sIts mass is about 1/3 of Mercury’s

So it’s 1/3 as dense as MercuryThis means that Callisto has far less rocky and metallic materials than do the terrestrial planetsIt must instead be composed largely of ice

Movie from a mosaic of images taken by the Voyager spacecraft

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Callisto: Cratered World (2)Callisto: Cratered World (2)Callisto seems not to have fully differentiated (separated into layers of different density materials)

The details of gravitational pull on the Galileo spacecraft suggest that Callisto lacks a dense coreThis is surprising to astronomers!

All big icy moons are expected to be differentiated (they should have differentiated more easily than rocky ones) Yet Callisto appears to have been frozen solid before the differentiation was complete

It is covered with impact cratersAlthough they look similar to craters on the Moon, they formed very differently

Callisto lacks the interior forces to drive geological change — it’s geologically dead

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Ganymede: Largest Moon (1)Ganymede: Largest Moon (1)It is the largest satellite in the solar systemIt is cratered, but less so than CallistoCrater counting suggests that ¾ of the surface may have formed more recently than the lunar maria

The lighter areas are thought to be younger than the darker ones

Ganymede is geologically very different from CallistoIt is a differentiated world, like terrestrial planets, with

a core about the size of our Moona mantle and crust of ice “floating” above the corea magnetic field, a signature of a partly molten interior

It is geologically active, powered by internal heat

Ganymede: Largest Ganymede: Largest Moon (2)Moon (2)

It has a diameter of 5262 km Slightly larger than Mercury

Why is Ganymede very different from Callisto? Possible reasons

Their small difference in size and internal heating may have led to the significant differences between the two moons nowThe gravity of Jupiter may be responsible for Ganymede’s continuing geological activity

It is close enough to Jupiter that tidal forces from Jupiter may have occasionally heated its interior This could have triggered major convulsions on its crust

Movie from a mosaic of images taken by the Voyager spacecraft

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Europa: Moon with Ocean (1)Europa: Moon with Ocean (1)Its surface is covered with a thick layer of water ice

For the most part, it is smooth, but is crisscrossed with cracks and low ridges often stretching for thousands of kilometers across icy plains

Under the ice, there may be liquid water or slush If so, tidal heating may be responsible for keeping the water in liquid form

Actual colorActual color Enhanced colorEnhanced color

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Evidence for Warm Oceans on Evidence for Warm Oceans on EuropaEuropa

Galileo images appear to confirm the existence of a global ocean on Europa

It has ridges and multiple-line features that may have formed when thick layers of ice were broken up into giant icebergs and ice flows and then refrozen in place It also has smooth areas where water may have flowed up and refrozen

If it has liquid water, could life exist on Europa?

Enhanced colorEnhanced color

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It is the most volcanically active world in the solar system Io has an elliptical orbit, causing it to twist back and forth relative to Jupiter and experience tidal forces

This twisting and flexing is the likely source of Io’s internal heating that drives its massive volcanism

Io’s interior heat may also have produced a differentiated interior

Io probably has an iron core surrounded by a molten rocky mantle

Io, the movie

Io: Volcanic Moon Io: Volcanic Moon

Jupiter

Io

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Volcanism on IoVolcanism on IoIo has no impact craters

They must have been eradicated by its volcanic activity

Many of its volcanic eruptions produce plumes of material that extend hundreds of km out into space The colors on Io come from sulfur (yellow, black, red) and sulfur dioxide (white) Io’s volcanoes can be very long lived

Some have been observed for 20 years Loki volcano eruptsLoki volcano erupts

Lava fountainsLava fountains

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Infrared Images of IoInfrared Images of IoImages taken by the Keck telescope plus adaptive optics

Bright spots indicate hot lava erupted by volcanoes

Io in infrared, the movie

Galileo imageGalileo image

Without Without adaptive adaptive

opticsoptics

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Saturn’s MoonsSaturn’s MoonsTo date, Saturn has 46 known satellites

The largest is Titanalmost as big as Ganymede

Titan is the only satellite among the jovian moons that has a substantial atmosphere

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Some of Saturn’s MoonsSome of Saturn’s Moons

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Titan: Moon with Atmosphere (1)Titan: Moon with Atmosphere (1)It’s the second largest moon in the solar system It’s the only moon in the solar system with a substantial atmosphere

The thick atmosphere makes its surface impossible to see

Why does Titan have an atmosphere? Possible reasons:

Titan is massive enough to have strong gravity that can retain its atmospheric gases Titan is cold enough so that the gases in the atmosphere are slow moving and hence do not easily escape into space

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Titan: Moon with Titan: Moon with Atmosphere (2)Atmosphere (2)

Titan’s atmosphere has a pressure 1.6 times Earth's is comprised of mostly nitrogen, plus 6% argon and a few percent methanehas trace amounts of organic compounds (such as carbon monoxide, ethane, and hydrogen cyanide) and water has multiple layers of clouds

the bottom layer probably composed of methanethe top layer including a dark reddish haze or smog, which hides Titan’s surface from our view

Its surface has a temperature of about 90 KThis means that on Titan’s surface methane may exist in liquid or solid form, and there may even be seas or lakes of methane, as well as methane ice

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Cassini SpacecraftCassini Spacecraft

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After After separating separating

from Cassini, from Cassini, the Huygens the Huygens probe landed probe landed

on Titanon Titanin Jan. 2005in Jan. 2005

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Some of Cassini’s Images Some of Cassini’s Images

Cassini-Huygens mission’s website

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Uranus SystemUranus SystemTo date, it has 27 known satellites

none are really large

Its rings and satellites are tilted at 98° just like the planet itselfIt has 11 rings

composed of very dark particlesdiscovered in 1977consisting of narrow ribbons of material with broad gaps

very different from the rings of Saturn

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To date, it has 13 known satellites6 are regular, close to the planet2 are irregular, farther out

Its largest moon is Triton (in mythology, Triton is Neptune’s son)

It is the only large moon in the solar system that circles its planet in a direction opposite to the planet's rotation (a retrograde orbit)It may once have been an independent object that Neptune captured

Tritonhas an atmosphere and active volcanismbears some resemblance to Pluto

Neptune’s MoonsNeptune’s Moons

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Triton: Icy Triton: Icy World (1)World (1)It has an atmosphere and

active volcanismIts surface has the coldest temperature (between 35 and 40 K) of any of the worlds our robot spacecraft have visitedIts surface material is made of frozen water, nitrogen, methane, and carbon monoxideIt has a very thin nitrogen atmosphere, with a pressure of only 16 millionths of our atmospheric pressureTriton resembles Pluto in size, composition, and temperature

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On Triton, volcanic eruptions produce plumes of nitrogen gas and are driven by seasonal heating from the Sun

The plumes can rise 10 km above the surface

On Earth, Venus, Mars, rocky magma is driven by internal heatOn Io, sulfur compounds are driven by tidal interactions with JupiterTriton movie

Triton: Icy Triton: Icy World (2)World (2)

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Pluto (1)Pluto (1)Discovered through systematic search

at P. Lowell observatory in 1930

Highest inclination to the ecliptic (17°)Largest eccentricity ~ 0.248Average distance from the Sun ~40 AU

Perihelion closer than Neptune

Orbital period ~248.6 earth yearsRotation: ~6.4 days on its sidePluto's diameter 2240 kmIt has only 1 known satellite, Charon

Charon’s orbit is locked to Pluto, revolving and rotating at the same time as Pluto rotates

Pluto

Charon

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Pluto (2)Pluto (2)It is the only planet not yet visited by spacecraftIt appears very faint from Earth

Its observation requires the best telescopesIts diameter is ~2190 km (60% of the Moon)Its density is ~2.1 g/cm3

It is composed of a mixture of rocky material and water ice

similar to Neptune’s moon TritonIt has a highly reflective surface

frozen methane, carbon monoxide, & nitrogen

Its surface temperature is between 50 and 60 KIt has a tenuous atmosphere

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Pluto (3)Pluto (3)Pluto is not like the terrestrial or jovian planetsPluto, Quaoar, Charon, and Triton are probably examples of Kuiper-belt objects

The Kuiper belt is a disk-shaped region of space beyond Neptune’s orbit

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Quaoar: New planet?Quaoar: New planet?Its orbit is more circular than Pluto'sIt is closer to the ecliptic

7.9° inclination compared to Pluto's 17°

Its diameter is 1280 km

compared to Pluto's 2240 km

Possibly Pluto and Quaoar are both Kuiper-belt objects

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QuaoarQuaoar

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Planetary Rings (1)Planetary Rings (1)All four giant planets have ringsEach ring is a system of billions of small particles (moonlets)Each ring displays complicated structure related to the interaction between the rings and satellitesThe four ring systems are very different from each other in mass, structure, and composition

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Planetary Rings Planetary Rings (2)(2)

Saturn’s ringsmade up of icy particles spread out into several vast, flat rings, with a great deal of fine structure

Neptune’s & Uranus’ ringsmade up of dark particles, confined to a few narrow rings, with broad empty gaps

Jupiter’s ringstransient dust bands, constantly renewed by erosion of dust grains from small satellites

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Planetary Rings (3)Planetary Rings (3)The dotted vertical line is the limit inside which the gravitational (tidal) force of a planet can break up a satelliteIn other words, only really small bodies survive inside the tidal stability limit

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What Causes Rings? What Causes Rings? (1)(1)

Each ring is a collection of vast numbers of objects and particles

Each particle obeys Kepler’s lawsThe inner particles revolve faster than those farther outThe ring does not rotate as a solid bodyParticles within the ring are close to one another The particles exert mutual gravitational influence and occasionally even touch in low speed collisions

Two basic theories on how the rings formed have been proposed

Breakup hypothesis: they are the remains of a shattered satelliteAnother hypothesis: the rings consist of particles that were unable to fuse into a single body

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What Causes Rings? (2)What Causes Rings? (2)In either theory, the gravitation of the planet plays an important role

Tidal forces for orbits close to the planet can tear bodies apart, or inhibit loose particles to come together

The rings of Saturn and Uranus are close to the planetsIn the breakup theory, a satellite, or a passing comet, may have come too close and been torn apart under tidal forces, or through some collision

Some scientists believe that some of the rings are young and must therefore be the result of a breakup

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Rings of Jovian PlanetsRings of Jovian Planets

Jupiter’s ringsJupiter’s rings

Saturn’s ringsSaturn’s rings

Uranus’ ringsUranus’ rings Neptune’s ringsNeptune’s rings

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Satellite-Ring InteractionsSatellite-Ring InteractionsEach ring has an intricate structure, as discovered by VoyagerThe rings’ structures are due mainly to the gravitational effects of the satellites

Without the satellites, the rings would be flat and featureless

There could even be no rings at all…Gaps in Saturn’s A-ring result from gravitational resonances with smaller inner satellites, especially Mimas