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Moons, Pluto, and Moons, Pluto, and RingsRings

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Ring and Satellite Systems (1)The The ringsrings and and moonsmoons in the in the outer solar systemouter solar system are different in are different in compositioncomposition from objects in from objects in the inner solar systemthe inner solar system

Most contain dark, organic compounds mixed Most contain dark, organic compounds mixed with ice and rockwith ice and rock

The presence of dark The presence of dark material implies that material implies that they reflect very they reflect very little lightlittle light

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Ring and Satellite Systems (2)MostMost satellites in the satellites in the outer solar systemouter solar system are in are in directdirect or or regularregular orbits orbits

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

A number of them have A number of them have irregularirregular orbits orbitsThey orbit in a They orbit in a retrograderetrograde (east-to-west) (east-to-west) direction, or else have orbits with high direction, or else have orbits with high eccentricityeccentricity or high or high inclinationinclination

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

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Jupiter's MoonsTo date, Jupiter has To date, Jupiter has 6363 known satellites known satellites

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

Europa and Io are the size of Europa and Io are the size of our Moonour Moon

Ganymede and Callisto are Ganymede and Callisto are bigger than Mercurybigger than Mercury

The rest are much The rest are much smallersmaller

More moons will likely be More moons will likely be found in the futurefound in the future

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The Galilean Moons

Callisto, Ganymede, Europa, IoCallisto, Ganymede, Europa, Io

They were first seen by Galileo (the They were first seen by Galileo (the astronomer)astronomer)

Studied by the Galileo space probe and Studied by the Galileo space probe and by Hubble Space Telescope by Hubble Space Telescope

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

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

Io

Europa

Ganymede

Callisto

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

2 million km from Jupiter2 million km from Jupiter

Noon-time surface Noon-time surface temperature: 130 K temperature: 130 K (140°C below freezing!)(140°C below freezing!)Its diameter: 4820 km, Its diameter: 4820 km, about the same as Mercury’sabout the same as Mercury’sIts mass: about 1/3 of Mercury’s massIts mass: about 1/3 of Mercury’s mass

So it’s 1/3 as dense as MercurySo it’s 1/3 as dense as MercuryIt has far less rocky and metallic materials than It has far less rocky and metallic materials than do the terrestrial planetsdo the terrestrial planetsIt is composed largely of iceIt is composed largely of ice

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

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

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

Callisto is covered with impact cratersCallisto is covered with impact cratersAlthough they look similar to craters on Although they look similar to craters on the Moon, they formed very differentlythe Moon, they formed very differently

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

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

The lighter areas are younger than the darker onesThe lighter areas are younger than the darker ones

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

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

It’s geologically active, powered by internal heatIt’s geologically active, powered by internal heat

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Ganymede: Largest Moon (2)It has a diameter of 5262 km It has a diameter of 5262 km

Slightly larger than Mercury Slightly larger than Mercury

Why is Ganymede very different from Callisto? Why is Ganymede very different from Callisto? Possible explanationPossible explanation

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

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

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Ganymede: Old Dark Terrain

OldOld:: it is it is covered with covered with craterscraters

Dark: ice Dark: ice covered with covered with dust from dust from meteoroid meteoroid impactsimpacts

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Ganymede: New Bright Terrain

New: suggested by New: suggested by fewer craters fewer craters

Bright: due to Bright: due to fracturing of fracturing of the icy surface the icy surface

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

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

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

Actual colorActual color Enhanced colorEnhanced color

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Evidence for Warm Oceans on Europa

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

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

It also has smooth areas where water may have flowed up It also has smooth areas where water may have flowed up and refrozen and refrozen

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

Enhanced colorEnhanced color

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Io

Io: Volcanic Moon 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

Jupiter

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Volcanism on Io

Io has no impact craters They must have been eradicated by its volcanic activity

Volcanoes produce plumes of material that extend up to 280 km above the surface The colors on Io come from sulfur (yellow, black, red) and from sulfur dioxide (SO2, 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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Pele Volcano

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Saturn’s MoonsTo date, Saturn has To date, Saturn has 3333 known known satellitessatellites

The largest is TitanThe largest is TitanAlmost as big as GanymedeAlmost as big as Ganymede

The only satellite with a substantial The only satellite with a substantial atmosphereatmosphere

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Some of Saturn’s Moons

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

The thick atmosphere makes its The thick atmosphere makes its surface impossible to see surface impossible to see

Why does Titan have an atmosphere? Why does Titan have an atmosphere? Possible reasons:Possible reasons:

Titan is large enough to have a strong Titan is large enough to have a strong gravitational field gravitational field Titan is cold enough so that the gas in the Titan is cold enough so that the gas in the atmosphere is slow movingatmosphere is slow moving

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

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

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

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

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After separating

from Cassini, the Huygens probe landed

on Titanin Jan. 2005

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Some of Huygens’ Images

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Uranus SystemTo date, it has To date, it has 2727 known known satellitessatellites

none are really largenone are really large Its rings and satellites are Its rings and satellites are tilted at 98tilted at 98oo just like the just like the planet itselfplanet itselfIt has 11 ringsIt has 11 rings

composed of very dark composed of very dark particlesparticlesdiscovered 1977discovered 1977consist of narrow ribbons of consist of narrow ribbons of material with broad gapsmaterial with broad gaps

very different from the very different from the rings of Saturnrings of Saturn

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Neptune’s MoonsTo date, it has 13 known satellitesTo date, it has 13 known satellites

6 are regular, close to the planet6 are regular, close to the planet2 are irregular, farther out2 are irregular, farther out

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

the only large moon in the solar system that circles its the only large moon in the solar system that circles its planet in a direction opposite to the planet's rotation (a planet in a direction opposite to the planet's rotation (a retrograde orbit)retrograde orbit)

may once have been an independent may once have been an independent object that Neptune capturedobject that Neptune captured

has an atmosphere and active has an atmosphere and active volcanismvolcanismbears some resemblance to Plutobears some resemblance to Pluto

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Triton: Icy WorldIts surface has the Its surface has the coldest temperature coldest temperature (between 35 and 40K) of (between 35 and 40K) of any of the worlds our any of the worlds our robot spacecrafts have visitedrobot spacecrafts have visited

Its surface material is made of frozen water, Its surface material is made of frozen water, nitrogen, methane, and carbon nitrogen, methane, and carbon monoxidemonoxide

Triton has a very thin nitrogen Triton has a very thin nitrogen atmosphere, with a atmosphere, with a pressure 16 millionths of our pressure 16 millionths of our atmospheric pressureatmospheric pressure

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Ice volcanoes on Ice volcanoes on TritonTriton: plume rising 8 km above the surface and extending 140 km "downwind"

On Triton: eruptions of volatile gasses like nitrogen or methane driven by seasonal heating from the Sun

On Earth, Venus, Mars: rocky magma driven by internal heat

On Io: sulfur compounds driven by tidal interactions with Jupiter

plume

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Pluto (1)Discovered through systematic searchDiscovered through systematic search

at P. Lowell observatory in 1930at P. Lowell observatory in 1930

Highest inclination to the ecliptic (17°)Highest inclination to the ecliptic (17°)

Largest eccentricity ~ 0.248Largest eccentricity ~ 0.248

Average distance from the Sun ~40 AU Average distance from the Sun ~40 AU Perihelion closer than NeptunePerihelion closer than Neptune

Orbital period ~248.6 earth yearsOrbital period ~248.6 earth years

Rotation: ~6.4 days on its sideRotation: ~6.4 days on its side

Pluto's diameter 2240 kmPluto's diameter 2240 km

Only 1 known satellite: CharonOnly 1 known satellite: CharonCharon’s orbit is locked to Pluto, Charon’s orbit is locked to Pluto, revolving and rotating at the revolving and rotating at the same time as Pluto rotatessame time as Pluto rotates

HST Picture

Pluto

Charon

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Pluto (2)The only planet not yet The only planet not yet visited by spacecraftvisited by spacecraftVery faint from Earth Very faint from Earth

observation requires the best telescopesobservation requires the best telescopes

Diameter is ~2190 km (60% of the Moon)Diameter is ~2190 km (60% of the Moon)Density is ~2.1 g/cmDensity is ~2.1 g/cm33

Mixture of rocky material and water ice Mixture of rocky material and water ice Similar to Neptune’s moon TritonSimilar to Neptune’s moon TritonHas a highly reflective surfaceHas a highly reflective surface

frozen methane, carbon monoxide, & nitrogenfrozen methane, carbon monoxide, & nitrogen

Surface temperature between 50 and 60KSurface temperature between 50 and 60KHas a tenuous atmosphereHas a tenuous atmosphere

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Quaoar – New planet?Its orbit is more circular than Pluto'sIts orbit is more circular than Pluto's

It is closer to the eclipticIt is closer to the ecliptic7.9° inclinatiion compared to Pluto's 17°7.9° inclinatiion compared to Pluto's 17°

Its diameter is 1280 Its diameter is 1280 kmkm

compared to Pluto's compared to Pluto's 2240 km2240 km

Possibly Pluto and Possibly Pluto and Quaoar are both Quaoar are both Kuiper-belt objectsKuiper-belt objects

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Pluto (3)Pluto is not like the terrestrial or jovian Pluto is not like the terrestrial or jovian planetsplanets

Pluto, Quaoar, Charon, and possibly Triton, Pluto, Quaoar, Charon, and possibly Triton, are examples of are examples of Kuiper-beltKuiper-belt objects objects

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

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Planetary Rings (1)All four giant planets have ringsAll four giant planets have rings

Each ring is a system of billions of small Each ring is a system of billions of small particles (particles (moonletsmoonlets))

Each ring displays complicated structure Each ring displays complicated structure related to the interaction between the rings related to the interaction between the rings and satellitesand satellites

The four ring systems The four ring systems are very different from are very different from each other in mass, each other in mass, structure, and structure, and compositioncomposition

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Planetary Rings (2)

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

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

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

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Planetary Rings (3)

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What causes Rings?Each ring is a collection of vast Each ring is a collection of vast numbers of objects and particlesnumbers of objects and particles

Each particle obeys Kepler’s lawsEach particle obeys Kepler’s lawsInner particles revolve fasterInner particles revolve fasterRing does not rotate as a solid bodyRing does not rotate as a solid bodyBetter to consider the revolution of individual moonletsBetter to consider the revolution of individual moonletsParticles within the ring are close to one another Particles within the ring are close to one another Exert mutual gravitational influence, even collide in low Exert mutual gravitational influence, even collide in low speed collisionsspeed collisionsGives rise to waves that move across the ringsGives rise to waves that move across the rings

Two basic theories of how the rings came to beTwo basic theories of how the rings came to beBreakup hypothesis:Breakup hypothesis: remains of a shattered satellite remains of a shattered satelliteAnother hypothesisAnother hypothesis: particles that were unable to fuse : particles that were unable to fuse into a single bodyinto a single body

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Ring Causes ContinuedIn either theory, the gravitation of the planet In either theory, the gravitation of the planet plays an important roleplays an important role

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

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

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

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Rings of SaturnConsists of many rings and subrings: Consists of many rings and subrings: A, B, CA, B, C

B Ring: brightest, most closely B Ring: brightest, most closely packed particlespacked particlesA and C rings: translucentA and C rings: translucentTotal mass of B estimated to be similar to that of an icy Total mass of B estimated to be similar to that of an icy satellite 250 km in a diametersatellite 250 km in a diameterA & B separated by a wide gap called Cassini divisionA & B separated by a wide gap called Cassini division

Rings are broad and very thinRings are broad and very thinWidth of main ring ~70000 km, thickness ~20 mWidth of main ring ~70000 km, thickness ~20 mRing particles composed mainly of water iceRing particles composed mainly of water iceParticles range from grains the size of sand up to Particles range from grains the size of sand up to house-sized bouldershouse-sized bouldersA handful of narrow rings ~100-km wide, in addition A handful of narrow rings ~100-km wide, in addition to the main ringsto the main rings

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Rings of Saturn

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Narrow and blackNarrow and blackAlmost invisible from EarthAlmost invisible from EarthNine rings discovered (1977) during Nine rings discovered (1977) during observation of a star – occultationobservation of a star – occultationFirst seen by Voyager (1986)First seen by Voyager (1986)Outermost and most massive called Outermost and most massive called EpsilonEpsilon

100 km wide, ~ 100 m thick100 km wide, ~ 100 m thick51000 km from the planet51000 km from the planet

Other rings much smaller: 10 km wideOther rings much smaller: 10 km wideParticles are very darkParticles are very dark

black carbon and hydrocarbon black carbon and hydrocarbon compoundscompounds

Rings of Neptune are similar but even Rings of Neptune are similar but even more tenuousmore tenuous

Rings of Uranus and Neptune

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Satellite-Ring InteractionsEach ring has an intricate structure, as Each ring has an intricate structure, as discovered by Voyagerdiscovered by Voyager

Structures due to mainly gravitational effects Structures due to mainly gravitational effects of satellitesof satellites

Without satellites, the rings would be flat and Without satellites, the rings would be flat and featurelessfeatureless

There could even be no rings at all…There could even be no rings at all…

Gaps in Saturn’s A-ring result from gravitational Gaps in Saturn’s A-ring result from gravitational resonances with smaller inner satellites, resonances with smaller inner satellites, especially Mimasespecially Mimas