Chapter 11: Chapter 11: Moons,Moons,Rings and Rings and PlutoPluto

Ring and Satellite SystemsRing and Satellite SystemsGeneral properties.General properties.

– Composition different from objects in the Composition different from objects in the inner solar systeminner solar system

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

Dark materials imply small reflectivityDark materials imply small reflectivity

– Most satellites in regular orbitsMost satellites in regular orbits West-to-east directionWest-to-east direction In the plane of the planet’s equatorIn the plane of the planet’s equator Likely to have formed at about the same time as Likely to have formed at about the same time as the planetthe planet

– Irregular satellitesIrregular satellites Retrograde (east-to-west), or else have orbits of Retrograde (east-to-west), or else have orbits of high eccentricity, or high inclinationhigh eccentricity, or high inclination

Usually smaller satellites, located relatively far Usually smaller satellites, located relatively far from their planetfrom their planet

Formed subsequently or capturedFormed subsequently or captured

Jupiter's MoonsJupiter's Moons

52 known satellites (as of about 2 years 52 known satellites (as of about 2 years ago)ago)– Four large moons (Callisto, Ganymede, Four large moons (Callisto, Ganymede, Europa, Io)Europa, Io)

Europa, Io are the size of our moonEuropa, Io are the size of our moon Ganymede, Callisto – the largest – are bigger than Ganymede, Callisto – the largest – are bigger than mercurymercury

– Many smaller moons.Many smaller moons.

Note: moon and satellite mean the same Note: moon and satellite mean the same thing in astronomy.thing in astronomy.

Large "Galilean" MoonsLarge "Galilean" MoonsIo

Europa

Ganymede

Callisto

– Callisto, Ganymede, Europa, IoCallisto, Ganymede, Europa, Io– first seen by Galileo (the first seen by Galileo (the astronomer)astronomer)

– studied by the Galileo space studied by the Galileo space probeprobe

– Hubble Space Telescope Hubble Space Telescope observationsobservations

– combined data has found combined data has found important similarities to the important similarities to the terrestrial planetsterrestrial planets

– differences between moons differences between moons mostly due to distance from mostly due to distance from JupiterJupiter

Callisto: Crater Callisto: Crater PackedPacked

Outermost of the Galilean Outermost of the Galilean satellites.satellites.

2 million km from Jupiter.2 million km from Jupiter. Noontime surface temperature: 130 K Noontime surface temperature: 130 K

(140(140ooC below freezing!)C below freezing!) Diameter 4820 km ~ same as MercuryDiameter 4820 km ~ same as Mercury Mass ~ 1/3 Mercury’s massMass ~ 1/3 Mercury’s mass Not as dense as MercuryNot as dense as Mercury Composed largely of ice.Composed largely of ice. Not fully differentiated.Not fully differentiated.

– From details of gravitational pull on From details of gravitational pull on Galileo spacecraft.Galileo spacecraft.

– Surprising!Surprising!– Yet appears to be “frozen”Yet appears to be “frozen”

Covered with cratersCovered with craters Absence of interior forces – Absence of interior forces –

geologically dead.geologically dead.

Craters on CallistoCraters on Callisto

Ganymede: Largest, Most VariedGanymede: Largest, Most Varied Largest satelliteLargest satellite Cratered, but less so than Cratered, but less so than CallistoCallisto

¾ of the surface seem to ¾ of the surface seem to have formed “recently”have formed “recently”– 1 billion years old – 1 billion years old – rather than 4.4rather than 4.4

DifferentiatedDifferentiated– Rock and metals sank to Rock and metals sank to form a core about the form a core about the size of the Moonsize of the Moon

– Mantle + crust floating Mantle + crust floating above core.above core.

– Magnetic Field Magnetic Field – – partly molten interiorpartly molten interior

GanymedeGanymede Ganymede is the Ganymede is the

largest moon in the largest moon in the solar system solar system – Diameter = 5262 km Diameter = 5262 km – Slightly larger Slightly larger than Mercury than Mercury

Ganymede should have Ganymede should have little tidal heating little tidal heating due to its distance due to its distance from Jupiter from Jupiter – But, Ganymede But, Ganymede shows evidence for shows evidence for surface alteration surface alteration

Ganymede must have Ganymede must have had more geological had more geological activity in the past activity in the past – Did it once have a Did it once have a more eccentric more eccentric orbit?orbit?

Old Dark Terrain Old Dark Terrain

Old:Old: it is it is covered covered with with craterscraters

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

New Bright New Bright Terrain Terrain

New due to fewer New due to fewer craters craters

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

Craters – varying degradationCraters – varying degradationGeologically Geologically

active.active.– Younger Younger terrain terrain result of result of tectonic and tectonic and volcanic volcanic forces.forces.

– Extensive Extensive Mountain Mountain ranges formed ranges formed by by compression compression of the crust.of the crust.

– Some Some indication of indication of large scale large scale crustal crustal movements.movements.

Why is Ganymede different from Callisto?Why is Ganymede different from Callisto?

Small difference in Small difference in – size.size.– Internal heating.Internal heating.

Gravity of JupiterGravity of Jupiter– Ganymede close enough to Jupiter to Ganymede close enough to Jupiter to have tidal force effects – have tidal force effects – episodically heating the crust.episodically heating the crust.

Europa: Ice-Covered OceanEuropa: Ice-Covered Ocean

real color enhanced color

EuropaEuropa

Europa’s surface is covered with a layer Europa’s surface is covered with a layer of ice of ice

Under the ice is Under the ice is waterwater or warm fluid ice or warm fluid ice Water flows up to the top continually Water flows up to the top continually resurfacing Europa resurfacing Europa

Tidal heating produces the internal Tidal heating produces the internal energy energy – It also has differentiated Europa into It also has differentiated Europa into an iron core, a rocky mantle and an an iron core, a rocky mantle and an icy crust icy crust

Tidal flex may also crack the surfaceTidal flex may also crack the surface

Evidence for Warm Oceans on Evidence for Warm Oceans on EuropaEuropa

Europa has ice rafts Europa has ice rafts where the surface has where the surface has been broken up and been broken up and reassembled reassembled

Galileo has imaged faults Galileo has imaged faults where the ice has pulled where the ice has pulled apart and water as flowed apart and water as flowed up up

Europa also has smooth Europa also has smooth areas where water has areas where water has flowed up and re-frozen flowed up and re-frozen

On Earth simple life On Earth simple life forms evolved under water forms evolved under water at warm deep ocean vents at warm deep ocean vents – Could something Could something similar have happened similar have happened on Europa? on Europa?

Io

Io: Volcano WorldIo: Volcano World Io is the most volcanically active world in the solar system

Io has an elliptical orbit, so the tidal forces on it vary with time

These changing forces squeeze and flex Io producing heat

The hot interior produces massive volcanism

The interior heat has also produced a differentiated interior – Io has an iron core surrounded by a molten, rocky mantle

Io Jupiter

Volcanism on Io Io has no impact craters – They have been eradicated by lava

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)

Volcanoes can be very long lived – Some have been observed for 20 years Loki volcano erupts

Pele VolcanoPele Volcano

Io’s Plasma Torus

Io’s volcanoes put lots of ions into its orbit – Ions are atoms that have lost an electron giving them a net electrical charge

The ions are effected by Jupiter’s magnetic field producing a plasma torus

As Jupiter rotates its changing magnetic field produces an electrical current through the torus and interior

Saturn’s Moons and RingsSaturn’s Moons and Rings

30 known satellites 30 known satellites (before Cassini)(before Cassini)–Titan: largest of Titan: largest of Saturn’s satellitesSaturn’s satellitesAlmost as big as Almost as big as GanymedeGanymede

Only satellite with Only satellite with substantial substantial atmosphereatmosphere

Rings of SaturnRings of Saturn

TitanTitan The second largest moon in the solar The second largest moon in the solar system system – Hint: on the test, don’t be fooled by the Hint: on the test, don’t be fooled by the name!name!

Only moon with a significant atmosphere Only moon with a significant atmosphere Thick atmosphere makes the surface Thick atmosphere makes the surface impossible to see impossible to see

Why does Titan have an atmosphere? Why does Titan have an atmosphere? – 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 Titan is cold enough so that the gas in the atmosphere is slow movingin the atmosphere is slow moving

Dense atmosphere: pressure 1.5 times Earth's mostly nitrogen plus 6% argon and a few percent methane. trace amounts of organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water water is formed when methane in Titan's upper atmosphere is exposed to sunlight.chemical activity despite low surface temperature, 94 K (-290 F). like the smog found over large cities, but much thicker.

Conditions like Earth early in its history when life was first getting started. May have the necessary building blocks for life!

Titan: Cloud WorldTitan: Cloud World

Huygens Huygens Probe Probe

Lands on Lands on TitanTitan

Uranus SystemUranus System Ring and Satellite Ring and Satellite tilted at 98tilted at 98oo just just like the planet like the planet itself.itself.

11 rings11 rings– Composed of very Composed of very dark particlesdark particles

– Discovered 1977Discovered 1977– Narrow ribbons of Narrow ribbons of material with broad material with broad gapsgaps

very different very different from the rings of from the rings of SaturnSaturn

20 known satellites – 20 known satellites – none really largenone really large

Neptune SystemNeptune System 8 known satellites8 known satellites

– 6 regular close to the planet6 regular close to the planet– 2 irregular farther out2 irregular farther out

Triton (remember, Neptune carries a Triton (remember, Neptune carries a trident; in mythology, Triton is trident; in mythology, Triton is Neptune’s son)Neptune’s son)– large moon in retrograde orbit,large moon in retrograde orbit,– has an atmosphere, has an atmosphere, – active volcanism,active volcanism,– Bears some resemblance to PlutoBears some resemblance to Pluto

Triton: Triton: Ice WorldIce World

orbit is retrograde: didn’t form with Neptune! •Perhaps a captured Kuiper Belt object •Capture may have shattered another Neptunian moon

Capture scenario accounts for:•Triton's orbit •unusual orbit of Nereid •provides energy to melt and differentiate Triton's interior

Historical connection to Pluto?•similar bulk properties •Pluto has eccentric Neptune-crossing orbit

Is Triton a captured comet? What about Pluto?

Triton: Triton: Ice WorldIce World

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

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

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

Io: sulfur compounds driven by tidal interactions with Jupiter.

plume

Triton's AtmosphereTriton's Atmosphere

Triton’s AtmosphereTriton’s Atmosphere Triton has a very thin nitrogen atmosphere Triton has a very thin nitrogen atmosphere (1.6 X 10(1.6 X 10-5-5 atmospheres of pressure) atmospheres of pressure)

Triton is very cold (37 K) and thus nitrogen Triton is very cold (37 K) and thus nitrogen is mostly frozen on the surface is mostly frozen on the surface

A little bit of nitrogen evaporates to A little bit of nitrogen evaporates to produce the atmosphere produce the atmosphere

Motions of the atmosphere (wind) seem to Motions of the atmosphere (wind) seem to effect the plumes effect the plumes

Tenuous CloudsTenuous Clouds

Summary: Six Large MoonsSummary: Six Large Moons The six large moons of the gas The six large moons of the gas giants resemble the terrestrial giants resemble the terrestrial planets of the inner solar system planets of the inner solar system –They can have volcanoes, They can have volcanoes, atmospheres, and evidence of atmospheres, and evidence of resurfacing resurfacing

In general they are cold and have In general they are cold and have rocky interiors and icy exteriors rocky interiors and icy exteriors

Some produce internal energy Some produce internal energy through tidal heating through tidal heating

Europa and Titan may have the Europa and Titan may have the conditions for life to existconditions for life to exist

PlutoPluto Discovered through systematic Discovered through systematic

search.search.– At P. Lowell observatory in 1930.At P. Lowell observatory in 1930.– Named Pluto after the roman god Named Pluto after the roman god of the underworld (also PL are of the underworld (also PL are the initials of Percival Lowell).the initials of Percival Lowell).

Highest inclination to the ecliptic Highest inclination to the ecliptic (17(17oo).).

Largest eccentricity ~ 0.248.Largest eccentricity ~ 0.248. Average distance ~40 AU = 5.9 Average distance ~40 AU = 5.9

billion km.billion km.– Perihelion closer than NeptunePerihelion closer than Neptune

Orbital period ~ 248.6 earth years.Orbital period ~ 248.6 earth years. Rotation : ~ 6.4 days on its side.Rotation : ~ 6.4 days on its side. Pluto's diameter 2240 kmPluto's diameter 2240 km Largest satellite: CharonLargest satellite: Charon

– Charon’s orbit is locked to Charon’s orbit is locked to Pluto: Charon revolves and Pluto: Charon revolves and rotates in the same time as Pluto rotates in the same time as Pluto rotates.rotates.

Also two smaller satellites found.Also two smaller satellites found.

HST Picture

Pluto

Charon

Pluto BasicsPluto Basics

Not visited by spacecraft,Not visited by spacecraft,– very faint, very faint, – observation requires best observation requires best telescopestelescopes

Diameter ~ 2190 km (60% of the Diameter ~ 2190 km (60% of the Moon)Moon)

Density ~ 2.1 g/cm3Density ~ 2.1 g/cm3 Mixture of rocky material and Mixture of rocky material and

water ice water ice Similar to Triton (Neptune)Similar to Triton (Neptune) Highly reflective surface – Highly reflective surface –

– frozen methane, carbon frozen methane, carbon monoxide, nitrogenmonoxide, nitrogen

Surface temperature 50 K/ 60 KSurface temperature 50 K/ 60 K Tenuous atmosphere.Tenuous atmosphere.

Quaoar – new planet?Quaoar – new planet? orbit more orbit more circular than circular than Pluto'sPluto's

closer to the closer to the ecliptic – ecliptic – 7.9 degree 7.9 degree inclination inclination compared to compared to Pluto's 17 Pluto's 17 degrees.degrees.

diameter 1280 diameter 1280 km vs. Pluto's km vs. Pluto's 2240 km2240 km

possibly Pluto possibly Pluto and Quaoar are and Quaoar are both Kuiper both Kuiper belt objectsbelt objects

Pluto's OrbitPluto's Orbit

11.3.4 The Nature of Pluto11.3.4 The Nature of Pluto Pluto is not like the Terrestrial Pluto is not like the Terrestrial or Jovian planets. or Jovian planets.

Pluto, Quaoar, Xena, Charon, and Pluto, Quaoar, Xena, Charon, and possibly Triton, are examples of possibly Triton, are examples of Kuiper belt objects.Kuiper belt objects.

RingsRings All four giant planets have ringsAll four giant planets have rings Each ring is a system of billions Each ring is a system of billions of small particles (moonlets).of small particles (moonlets).

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

The four ring systems are quite The four ring systems are quite different.different.

Ring BasicsRing Basics

SaturnSaturn– Made up of icy particles spread Made up of icy particles spread out into several vast, flat out into several vast, flat rings, with a great deal of fine rings, with a great deal of fine structure.structure.

Neptune/UranusNeptune/Uranus– Made up of dark particles, Made up of dark particles, confined to a few narrow rings, confined to a few narrow rings, with broad empty gaps.with broad empty gaps.

JupiterJupiter– Rings are transient dust bands, Rings are transient dust bands, constantly renewed by erosion of constantly renewed by erosion of dust grains from small dust grains from small satellitessatellites

What causes Rings?What causes Rings?

Ring = collection of vast numbers of particlesRing = collection of vast numbers of particles– Each particle obeys Kepler’s laws.Each particle obeys Kepler’s laws.– Inner particles revolve fasterInner particles revolve faster– Ring does not rotate as a solid body.Ring does not rotate as a solid body.– Better to consider the revolution of individual moonlets.Better to consider the revolution of individual moonlets.

Particles 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 collisions. speed collisions.

– Gives rise to waves that move across the rings.Gives rise to waves that move across the rings. Two basic theoriesTwo basic theories

– Breakup theory, remains of a shattered satellite.Breakup theory, remains of a shattered satellite.– Make up of particles that did not fuse into a single bodyMake up of particles that did not fuse into a single body

Ring Causes ContinuedRing Causes Continued Gravitation of the planetGravitation of the planet

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

Rings of Saturn, Uranus are close to the planet…Rings of Saturn, Uranus are close to the planet…

Breakup – a satellite, or a passing Breakup – a satellite, or a passing comet may have come too close and torn comet may have come too close and torn apart under tidal forces, or through apart under tidal forces, or through some collision.some collision.

It is believed that some of the rings It is believed that some of the rings are young, and must therefore be the are young, and must therefore be the result of a breakup.result of a breakup.

Rings of SaturnRings of Saturn

Many rings and sub-rings: A, B, CMany rings and sub-rings: A, B, C B-Ring : Brightest, most closely packed particlesB-Ring : Brightest, most closely packed particles A/C-rings : translucent.A/C-rings : translucent. Total mass of B ~ that of icy satellite 250 km in Total mass of B ~ that of icy satellite 250 km in a diameter.a diameter.

A & B separated by a wide gap called Cassini A & B separated by a wide gap called Cassini division.division.

Rings are broad and very thin.Rings are broad and very thin. Main ring ~ 70000 km, thickness ~ 20 m.Main ring ~ 70000 km, thickness ~ 20 m. Composed of water ice.Composed of water ice. Particles range from grains the size of sand up Particles range from grains the size of sand up to house-sized bouldersto house-sized boulders

A handful of narrow rings ~ 100 km, in addition A handful of narrow rings ~ 100 km, in addition to the main rings. to the main rings.

Narrow and blackNarrow and black Almost invisible from EarthAlmost invisible from Earth Nine rings discovered (1977) Nine rings discovered (1977)

during observation of a star – during observation of a star – occultationoccultation

First seen by Voyager (1986)First seen by Voyager (1986) Outermost and most massive Outermost and most massive

called Epsiloncalled Epsilon– 100 km wide, ~ 100 m thick, 100 km wide, ~ 100 m thick, 51000 km from the planet.51000 km from the planet.

Other rings much smaller : 10 km Other rings much smaller : 10 km wide.wide.

Particles are very dark; black Particles are very dark; black carbon and hydrocarbon carbon and hydrocarbon compounds.compounds.

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

Rings of Uranus Rings of Uranus and Neptuneand Neptune

Satellite-Ring InteractionsSatellite-Ring Interactions Rings have intricate structure as Rings have intricate structure as discovered by Voyager.discovered by Voyager.

Structures due to mainly gravitational Structures due to mainly gravitational effects of satellites.effects of satellites.– Without satellites, the rings would be flat Without satellites, the rings would be flat and featureless.and featureless.

– There could even be no rings at all…There could even be no rings at all…– Gaps in Saturn A-ring result from Gaps in Saturn A-ring result from gravitational resonances with smaller inner gravitational resonances with smaller inner satellites. (Mimas)satellites. (Mimas)

Uranus' moon Miranda:Uranus' moon Miranda: innermost and smallest of the five major satellites, just 480 kilometers (about 300 miles) in diameter.

Two major strikingly different types of terrain: old, heavily cratered, rolling terrain with relatively uniform reflectivity.young, complex terrain characterized by sets of bright and dark bands, scarps and ridges (ovoid regions at right and left and the distinctive chevron feature below and right of center). likely due to upwelling of partially melted ices

April 11, 2006April 11, 2006 Astronomy 2010Astronomy 2010 5050

Discussion QuestionDiscussion Question

Astronomers wish to search for life Astronomers wish to search for life in the ocean believed to lie in the ocean believed to lie beneath the ice of Europa. How beneath the ice of Europa. How should we approach this exploration should we approach this exploration to avoid possible cross to avoid possible cross contamination of Earth and Europa contamination of Earth and Europa with organisms (DNA) from each with organisms (DNA) from each other?other?