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Solar System – Planets
Terrestrial planets – Mercury, Venus, Earth, Mars
Venus:
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average distance from Sun (ie. semi-major axis of orbit), a = 0.72 AU  ~ ¾ Earth’s
distance from Sun
 Porb = 0.62 yrs = 225 Earth days  Kepler III: P2 = a3
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Prot = 243 Earth days  Prot > Porb
 Venusian day longer than Venusian year
 very small rotation rate
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Size and Mass similar to Earth  Earth’s “sister planet”
- Radius (R) = 0.95 REarth
 Volume (V)  R3  V = 0.82 VEarth  Mass (M)  0.82 MEarth
 gravitational acceleration at surface (“surface gravity”)
= g/gE = (M/ME)(RE/R) 2  0.91
 Venus can retain an atmosphere
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Atmosphere:
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visible from Earth  appears cloudy, yellowish
atmosphere is opaque in visible band  blocks view of surface
Venus rotates clockwise (CW) as seen from above Earth’s north pole
 Venus’s rotation is in opposite direction to its revolution  has a back-spin
- ALL other planets with upright axes rotate CCW (ie. have a top-spin)
 rotation is retrograde
 rotation axis is upside-down
 clue that Venus was struck in a glancing collision with a large body,
probably during heavy bombardment era
atmosphere thicker than Earth’s atmosphere
 atmospheric pressure at surface, P = 90 Atmospheres
composition:
- Carbon dioxide (CO2) – 96%
- No Oxygen (O2), No Water (H2O) vapor or liquid
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 similar to Earth’s atmosphere in earlier epoch
Surface temperature of Venus, TSurf, determined by balance between solar heating and
Blackbody cooling
-CO2 a Greenhouse gas  Venus has extreme Greenhouse effect
 TSurf = 700K  400oC
- NOTE: albedo, A = 0.59
 with d=0.72 AU  TSurf4 without atmosphere = (1-A)LSun/σ16πd2
 TSurf = 273 K = 0oC
Why do Venus and Earth have such different atmospheres?
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both planets probably began with same primordial atmosphere
 composition due to out-gassing: CO2, H2O, N2
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Runaway Greenhouse effect:
- runaway: when a change made to a system results in the original change being
amplified
 an unstable situation
- also known as a positive feedback loop
- heat baked CO2 out of surface rock (outgassing)
 increased CO2 in atmosphere  increased Greenhouse effect
 increased Tsurf  more CO2 baked out
 increased CO2 in atmosphere  increased Greenhouse effect  …
 a runaway effect
- opposite effect as that of a thermostat: heating leads to changes that cause more
heating
- eventually all CO2 baked out of surface rock
 situation stabilized at 96% atmospheric CO2 and Tsurf = 700K
 gave rise to a thick atmoshere with large P
Venus closer to Sun  warmer
 H2O did not condense out of atmosphere
 no ozone (O3)  UV photons from Sun eventually destroyed H2O
 H2O +   2H + O
 H escaped into space, O bound up in mineral oxides on surface
 modern atmosphere with no H2O, no O2
Yellow clouds:
- high Ts causes sulfur to bake out of rocks and enter atmosphere
 clouds composed of airborne sulfur grains and droplets of sulfuric acid
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Interior and Surface:
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atmosphere transparent in radio band  surface relief studied by radar ranging
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interior is chemically differentiated
T and ρ are stratified with depth
 trapped heat of formation
 geologically active
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Venus volcanically active, BUT no plate tectonics (ie. no faults)
 high Tsurf and atmospheric P
 surface rock plastic (like Earth’s mantle)
 not rigid enough to form tectonic plates
 Venusian volcanoes differ from Earth volcanoes
 lava wells up through plastic surface
 surface geologically young
 most craters destroyed by surface activity (like Earth)
Mars:
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average distance from Sun, a = 1.52 AU  Porb = 1.88 yrs (Kepler III)
Prot = 24.5 hours  length of day similar to Earth
inclination of rotation axis = 25o  seasonal variation similar to Earth’s
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Radius, R = 0.53 REarth  Volume: V/VE ≈ (1/2)3 =1/8
 Density, ρ < ρEarth  Mass, M  1/10 MEarth
 surface gravity: g/gE = (M/ME)(RE/R) 2  0.4
 should retain substantial atmosphere
 BUT, atmospheric pressure at surface, P = 0.006 Atmospheres
 no liquid H2O
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Interior and surface:
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interior differentiated  heat of formation melted planet in past
 extinct volcanoes on surface (eg. Olympus Mons)  evidence of past
volcanism
BUT: Mars small  most interior heat already escaped
 little or no molten core  no global magnetic field
 no mantle convection  no plate tectonics, no current volcanism
 Mars close to geologically dead
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 has geologically old surface (like Moon)
Liquid water (H2O) on Mars:
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Mars today: atmospheric surface pressure, P = 0.006 atmospheres
 Tliquid = 0 K  liquid H2O cannot exist on surface, like Moon
 most H2O in form of ice, small amount in vapor in atmosphere
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BUT: there is evidence that there was liquid water on Mars in past
- water erosion features, dried up river beds and sea beds
- erosion features due to flash-flooding
- may be caused by large amounts of ice melting quickly
- surface geologically old  water features could have been made billions of years
ago
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explanation:
-Martian atmosphere thicker in past
 larger surface P  Tliquid > 0 K for H2O
 larger Greenhouse effect  surface T > Tfreeze for H2O
 atmosphere must have evolved over time (like Earth’s)
 as atmospheric P reduced  liquid H2O became ice or vapor
Atmosphere:
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evidence of liquid H2O in past
 atmospheric P and surface T higher in past
 atmosphere must have evolved over time
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primordial atmosphere:
- from volcanic outgassing
 similar composition to Earth’s primordial atmosphere
 high concentration of CO2 and H2O vapor
 Primpordial P higher than present P
 CO2 and H2O are greenhouse gases
 greenhouse effect kept Ts higher than present Ts
surface warm enough for liquid H2O
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first steps in evolution same as Earth:
- as Mars cooled  H2O vapor in atmosphere condensed to form liquid oceans on
surface
- atmospheric CO2 dissolved in oceans  precipitated into sedimentary rock
 reduced greenhouse effect  cooled surface further
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important difference between Mars and Earth:
- Mars small  already lost interior heat  already geologically dead
 no more volcanic outgassing to replenish atmospheric CO2
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 on Mars: runaway icehouse effect:
- due to fact that CO2 dissolves in liquid H2O (carbonation of water)
- reduced atmospheric CO2  reduced greenhouse effect  TSurf lower
 more atmospheric H2O condensed out CO2 dissolves in H2O  more CO2
removed from atmosphere by dissolution in water  reduced atmospheric
CO2  reduced greenhouse effect  TSurf lower  …
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- end results:
- most CO2 and H2O removed from atmosphere
 atmosphere thinned out  atmospheric P reduced to 0.006 Atmospheres
- Greenhouse effect reduced  Tsurf very low
- problem: where did the H2O go?
- may be underground, as either liquid or ice
- may be under polar CO2 ice caps
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no liquid water  no plant life evolved
 no photosynthesis  no free oxygen (O2)  no ozone (O3)
 no protection from solar UV-band photons
 remaining atmospheric H2O destroyed: H2O +   2H + O
 H escaped into space
O bound up in FeO (Iron Oxide, “rust”) on surface  gives Mars red
color
Seasonal climate variation:
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rotation axis inclined to perpendicular to orbital plane by 25o  similar to Earth
 on Mars ecliptic inclined by 25o to celestial equator
 Mars has seasons
 has tropical zones between latitudes +25o and -25o
arctic zones north of +65o and south of -65o
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evidence:
- Mars has polar ice caps in arctic zones
- polar caps made mostly of CO2 ice (“dry ice”)
- there may be some H2O ice underneath CO2 ice
- ice caps expand and contract in anti-phase (ie. northern cap large when southern
cap small and visa versa)
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- ice caps vary with a period equal to Martian year (Mars’ Porb)
- during Spring in northern hemisphere
 some CO2 ice in northern cap vaporizes
 northern cap shrinks
- during Autumn in northern hemisphere
 some atmospheric CO2 freezes (condenses to ice) in northern cap
 northern cap expands
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Mars’ moons:
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Phobos & Deimos
both minor moons
- Diameter, D <30 km
 asteroid sized  planetesimals captured by Mars’ gravity
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both moons rotate synchronously (like Earth’s moon)
 Prot = Porb
 due to tidal synchronization
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Jovian Planets – Jupiter, Saturn, Uranus, Neptune
Jupiter
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located in outer solar system
semi-major axis of orbit, a = 5.2 AU
- Kepler III: P(yrs)2 = a(AU)3  P = a3/2
 Porb = 11.9 yrs
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Radius, R = 11.2  REarth
- diameter defined by “surface” where Jupiter becomes opaque
 Volume: V/VE = (R/RE)3 = 1350
Mass, M = 318  MEarth
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largest and most massive planet in solar system
 average density: /ρE = (M/V)/(ME/VE) = (M/ME)/(V/VE) = 318/1350 ≈ 0.25
 Jupiter’s very large volume compensates for its large mass  has low mean
density
 surface gravity (at apparent surface), g/gE = (M/ME)(RE/R) 2 = 318/11.22 = 2.4
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- larger R compensates for larger M  g only a few × larger than gE
 Jupiter is able to retain a thicker atmosphere than Earth
 retains H and He in atmosphere (unlike terrestrial planets)
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composition: similar to solar nebula
- mostly H and He fluid (ie. gas and liquid phase)
- Jupiter held together by force of gravity
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Consequences of Jupiter’s fluid structure:
- rotational deformation:
- rapidly rotating ball of fluid  shape is oblate spheroid
 equatorial D = 11.2 DEarth , polar D = 10.5 DEarth
- differential rotation in latitude:
- Prot at low latitudes (ie. near equator) < Prot at mid latitudes
 ie. Jupiter rotates faster near equator than at mid-latitudes
 Jupiter does not rotate as a solid body, Jupiter constantly sheared along
lines of latitude
Rotation period, Prot = 9H 50m = 0.4 Prot, Earth  short rotation period
- equatorial surface velocity, vrot: velocity of surface at equator due to rotation
= (distance around planet’s surface)/(time to rotate once) = 2R/ Prot
- where: R = equatorial radius of planet
- for Earth: vrot, E = 440 m s-1
- vrot/vrot, E = (2πR/ Prot)/(2πRE/ Prot, E)
= (R/RE)( Prot, E/ Prot)
= 11.2/0.4 = 26
 vrot = 26 × 440 km s-1 = 12000 m s-1
 Jupiter is a rapid rotator!
Atmosphere:
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composition: 92% H, 7.8% He
trace amounts of methane CH4 and ammonia NH3  gas colored
 same composition as solar nebula
 Jovian atmosphere has not evolved, unlike terrestrial atmospheres
weather:
- Jupiter very massive  a lot of heat of formation still trapped in interior (central
T = 25000 K)
 Jupiter emits as Blackbody radiation
- Blackbody flux ~ 2  amount of E that it absorbs from sunlight
- surface T < 1000 K  BB spectrum peaks in IR band (λmax > few
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microns)
 heat flowing out from interior
 drives convection currents in interior and atmosphere
 hot updrafts – high pressure zones
cool down-drafts – low pressure zones
- Jupiter rotating rapidly  fast latitudinal winds
 high and low pressure zones stretched out around planet along lines of
latitude
 “Belt-zone” weather  gives Jupiter banded appearance
- belts are downdrafts, zones are updrafts
- note: on Earth zones are cylonic, due to slower rotation rate
- also has cyclonic downdrafts and updrafts at higher latitudes
- eg. the great red spot
- seen by Galileo in 16th C  storm stable for ~400 years!
Interior:
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Jupiter has large mass  pressure in interior very high
 at a depth of 20000 km P = 1.4106 atmospheres
 P high enough to squeeze H into liquid form
 Jupiter has a liquid H mantle
- liquid H is a very efficient electrical conductor, like metal
 liquid H known as liquid metallic Hydrogen
Jupiter rotating  liquid metallic H circulating
trapped heat of formation flowing outward  liquid metallic H carries convection
currents
 liquid metallic H zone is a dynamo  generates a dipolar magnetic field
 Jupiter has magnetic North and South poles, like Earth
- liquid metallic H an excellent electrical conductor AND mass of liquid H zone
large
 Jupiter’s magnetic field 14  stronger than Earth’s
- field traps electrically charged particles from solar wind
 Jupiter has polar aurorae
at center: solid core of rock  original proto-planet
- mass of rocky core = 8  MEarth BUT R  REarth  mean density,   8  Earth
- large  due to extreme compression due to large central Pressure, P (7107
atmospheres)
The Jupiter system - major moons, minor moons, and rings
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Galilean Moons – major moons
- four largest moons
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- discovered by Galileo in 16th C.
- in order of increasing distance from Jupiter: Io, Europa, Ganymede, Callisto
- among largest moons in solar system:
- Ganymede: Mass  2  Mass of Earth’s Moon, Radius > R of Mercury
- Callisto: Mass of  1.5  Mass of Earth’s Moon, Radius > R of Earth’s Moon
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- orbital planes lie in plane of Jupiter’s rotational equator
 lie close to plane of Jupiter’s orbit
- moons revolve and rotate CCW
 Jupiter system similar to overall solar system
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- mass of moons determined from gravitational deflection of Voyager spacecraft
 from Earth Jupiter system is viewed almost edge-on
 transit: moon passes in front of Jupiter
(note: this differs from definition of transit of an object on the Celestial
Sphere)
 occultation: Jupiter passes in front of moon
- ingress and egress: condition when moon is partially in transit or
occultation at Jupiter’s limb as occultation or transit is beginning or
ending
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trajectory
 mean density,  = M/V, of moons ranges from 2000 to 3500 kg/m3 = 1/3 to 1/2
ρEarth
  of moons <  of Earth (5500 kg/m3)
 outer solar system moons composed of mixture of volatile (ices) and
refractory (mineral) material throughout interior
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- Synchronizations due to gravitational tides
- all moons rotate synchronously  Prot = Porb
- due to tidal synchronization (like Earth’s Moon)
- orbital resonance:
- Io, Europa, and Ganymede are within 6105 km of each other
 mutual gravitational force causes orbital periods, Porb, to become
synchronized
 PGanymede = 2  PEuropa = 4  PIo
 affects size of orbits, a, via Kepler III  a3  P2
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Minor Moons:
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Ring system:
- several rings
- lie in Jupiter’s equatorial plane
- composed of microscopic dust grains
 dust grains have low albedo (A)  difficult to see
 discovered in late 1970’s by Voyager space craft
- dust grains orbit Jupiter with prograde revolution (CCW) (like Galilean
moons)
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Formation of Jupiter system
- similar to formation of overall solar system
- local region of solar nebula collapsed into rotating disk centered on proto-Jupiter
- major moons are smaller proto-planets that formed in disk
- 44 known minor moons
- asteroid sized
- many have retrograde revolution (ie. orbit clockwise (CW))
- many have orbits inclined by large angles to plane of Jupiter’s equator
 gravitationally captured minor bodies
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 collapse and accretion were heierarchical
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Saturn
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a = 9.5 AU  Porb = 29.5 years  Kepler III
- note: aJupiter = 5 AU  planets of outer solar system widely spaced
R ~ 9  REarth  V ~ 730  VEarth
 second largest and second most massive planet in Solar System (after Jupiter)
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M = 95  MEarth ~ 1/3  MJupiter
  = 687 kg/m3 <  of water at sea level on Earth
 least dense planet in solar system
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Prot = 10h 14m  very rapid rotation (like Jupiter)
 vrot at surface at equator very large
 Saturn is most oblate planet in solar system
- equatorial D = 9.5  DEarth
polar D = 8.5  DEarth
Interior and Atmosphere:
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Saturn less massive than Jupiter  interior pressure, P, smaller
 interior less compressed
 liquid metallic H mantle smaller
 global magnetic field weaker
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tapped heat of formation flowing outward
 convection currents in interior and atmosphere
 belt-zone weather in atmosphere  banded appearance
The Saturn System – Rings and moons
Rings:
- lie in plane of Saturn’s rotational equator
- main rings: three distinct rings visible from Earth
- - in order of decreasing distance from Saturn: A, B and C rings
- - Cassini Division: 4500 km gap between A and B rings
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- composition: rings are belts of small solid fragments  not continuous
- fragment size ≈ 10 cm
- - fragments composed of ices  have large albedo (A)
 easily visible from Earth due to reflected sunlight
- - individual fragments orbit around Saturn (like mini-moons)
 fragments obey Kepler III: P2 α a3
 rings have differential rotation in radial distance  are constantly sheared
- appearance of rings viewed from Earth changes cyclically from prominent to invisible
and back with period = ½ Porb of Saturn
- - rings thin: thickness ≈ 30 km
- - plane of Saturn’s rotational equator inclined to plane of ecliptic (plane of Earth’s
orbit) by 27o
-  when Saturn’s North or South pole inclined toward Sun we see ring plane from
above or below  rings prominent
-  ring plane crossing: when Saturn’s rotation axis lies in plane of sky  we see
rings edge-on  rings invisible
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Why rings? – The Roche limit
- Roche limit: distance from a body within which another body that is larger than a
certain size that is held together mainly by gravity will be destroyed by tidal
forces
- within Roche limit: force due to tides > force of gravity holding body together
- rings of Jovian planets lie within planet’s Roche limit
 consist of fragments that cannot accrete into a larger body due to tidal force
- within a planet’s Roche limit: rings and minor moons
- outside a planet’s Roche limit: major and minor moons
- terrestrial planets have smaller gravitational field
 Roche limit close to planet’s surface
 terrestrial planets do not have rings
- ( also, there is less building material in inner solar system to make rings)
- Ring structure:
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- Voyager spacecraft revealed greater detail
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- additional fainter rings:
- D ring inside orbit of C ring
- F, G, and E rings (in order of increasing distance from Saturn) outside orbit of A
ring
- divisions: rings separated by gaps where density of fragments lower (eg. Cassini
division)
- note: divisions not completely empy
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ring sub-structure:
- each ring made up of 100’s of ringlets
- ringlets braided
- complex structure due to combination of Saturn’s gravity, moons’ gravity and
mutual gravity among ring fragments
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Moon-ring interaction (The cause of the Cassini Division):
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- divisions between rings due to gravity of moons
- eg. Cassini division:
- moons and ring fragments obey Kepler III: Porb2  a3
 at Cassini division: a=1.18×108 m  Porb = 11.3 hrs
at moon Mimas: a=1.85×108 m  Porb = 22.6 hrs
 Cassini division fragments in a 2:1 orbital resonance with Mimas (like
orbital resonance among inner three Galilean moons of Jupiter)
 Cassini division particles aligned with Mimas at same place in orbit
every second orbit  pulled out of division by Mimas’ gravity
- eg. F ring shepherd moons
- F ring bracketed by two minor moons
- Prometheus: a  aF ring; Pandora: a  aF ring
 from Kepler III: PPrometheus  PF ring  PPandora
 Prometheus always overtaking F ring fragments AND F ring fragments
always overtaking Pandora
 F ring fragments pulled forward by Promethues’ gravity  vorb
increases  a increases
 F ring fragments pulled backward by Pandoras’ gravity  vorb
decreases  a decreases
 Prometheus & Pandora keep F ring particles confined 
shepherd moons
Moons:
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28 moons, most minor
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Titan:
- second largest moon in solar system
- surface T very low due to large distance from Sun (recall: gas particle
vave=(3kT/m)1/2  see Kinetic Theory)
 retains thick atmosphere; P = 1.5 Atmospheres
- composition: 90% Nitrogen (N2), methane (CH4, a hydrocarbon)
 solar UV photons + CH4  organic chemicals
 Titan may be promising place to search for primitive life
Pluto
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neither a Jovian nor Terrestrial planet
- solid surface  not Jovian
- interior composed of mixture of rock and ices  not Terrestrial
- similar in composition to a moon of a Jovian planet
- BUT: orbits Sun directly  considered to be a planet
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Radius, R = 1/5 REarth  smaller than Earth’s Moon
Mean density,  = 2000 kg/m3 < 1/2Earth
 even mixture of volatile and refractory material throughout interior
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Orbit:
a = 40 AU  Porb = 250 years  Kepler III
eccentricity of elliptical orbit, e = 0.25; all other planets have e < 0.1
 unusually elongated orbit
 perihelion distance = 30 AU; aphelion distance = 50 AU
 Pluto’s orbit crosses Neptune’s orbit
inclination of plane of orbit to ecliptic, i = 17o
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Pluto’s moon: Charon
- Diameter, D = ½ DPluto  Pluto-Charon system almost a binary planet
- distance from Pluto = 20000 km (DEarth = 12000km)
 Kepler III: Porb = 6.4 days
- Pluto and Charon BOTH have Prot = Porb
 both have rotation rate tidally synchronized to revolution rate
 only one period for whole system: 6.4 days
 both keep same face toward other
 neither rises or sets as viewed from other
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- state that Earth-Moon system is evolving toward
Pluto’s uncertain status:
Kuiper belt:
- belt of icy planetesimals beyond orbit of Neptune
- named after Gerard Kuiper who first speculated that the belt existed before it was
observed
- origin of short period comets
- Pluto and Charon may be largest, most visible members of Kuiper belt
 ie. Pluto not “really” a planet, really a large outer solar system planetesimal