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EART 160: Planetary Science
25 February 2008
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Homework 4 Graded
• Wikipedia is not a peer-reviewed journal
• I gave an incorrect value for H in problem 3, but it didn’t seem to trip anybody up.
• Convection in mantle, Conduction in lithosphere
• Max: 50, Min: 21, Mean 36, St. Dev. 10
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Last Time
• Coriolis Force Demo
• Planetary Atmospheres– Thermal Balance– Origin / Geochemistry– Climate Change
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Jeans Escape• This expression for Jeans Escape IS
correct after all.
• Escape velocity ve= (2 g R)1/2
• Recall: g = GM / R2
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Today
• Giant Planets– Atmospheres– Interiors– Magnetospheres?
• Questions to Ponder:– What determines their internal structure?– How did they form and evolve?– What controls their atmospheric dynamics?
• Moons: Tour of Icy Satellites
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Image not to scale!
Giant Planets
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Basic Parameters
Data from Lodders and Fegley 1998. Surface temperature Ts and radius R are measured at 1 bar level.
a (AU)
Porb
(yrs)
Prot
(hrs)
R
(km)
M
(1026 kg)
Obli-quity
Density g/cc
Ts
K
Jupiter 5.2 11.8 9.9 71492 19.0 3.1o 1.33 165
Saturn 9.6 29.4 10.6 60268 5.7 26.7o 0.69 134
Uranus 19.2 84.1 17.2R 24973 0.86 97.9o 1.32 76
Neptune 30.1 165 16.1 24764 1.02 29.6o 1.64 72
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Compositions• We’ll discuss in more detail later, but briefly:
– (Surface) compositions based mainly on spectroscopy– Interior composition relies on a combination of models
and inferences of density structure from observations– We expect the basic starting materials to be similar to
the composition of the original solar nebula• Surface atmospheres dominated by H2 or He:
(Lodders and Fegley 1998)
Solar Jupiter Saturn Uranus Neptune
H2 83.3% 86.2% 96.3% 82.5% 80%
He 16.7% 13.6% 3.3% 15.2%
(2.3% CH4)
19%
(1% CH4)
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Pressure• Hydrostatic approximation• Mass-density relation• These two can be combined (how?) to get the pressure
at the centre of a uniform body Pc:
)()( rgrdrdP
2)( )(4 rrdrrdM
4
2
8
3
R
GMPc
• Jupiter Pc=7 Mbar, Saturn Pc=1.3 Mbar, U/N Pc=0.9 Mbar
• This expression is only approximate (why?) (estimated true central pressures are 70 Mbar, 42 Mbar, 7 Mbar)
• But it gives us a good idea of the orders of magnitude involved
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Equation of State• If parcel of gas moves up/down fast enough that it doesn’t exchange
energy with surroundings, it is adiabatic• In this case, the energy required to cause expansion comes from
cooling (and possible release of latent heat); and vice versa• For an ideal, adiabatic gas we have two key relationships:
T
RT
P KP Ideal Gas Law Polytropic Law
Here P is pressure, is density, R is gas constant (8.3 J mol-1 K-1), T is temperature, is the mass of one mole of the gas, is a constant (ratio of specific heats, ~ 3/2)
• So:
• Adiabatic Lapse Rate
pC
g
dz
dT
At 1 bar level on Jupiter: T=112 K, g=23 ms-2, Cp = 25 J mol-1 K-1, =0.002 kg mol-1
dT/dz = 1.4 K/km
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Hydrogen phase diagram
• Jupiter – interior mostly metallic hydrogen
Hydrogen undergoes a phase change at ~100 GPa to metallic hydrogen (conductive)
It is also theorized that He may be insoluble in metallic H. This has implications for Saturn.
Interior temperatures are adiabats
• Saturn – some metallic hydrogen
• Uranus/Neptune – molecular hydrogen only
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Compressibility & Density• As mass increases, radius
also increases• But beyond a certain mass,
radius decreases as mass increases.
• This is because the increasing pressure compresses the deeper material enough that the overall density increases faster than the mass
• The observed masses and radii are consistent with a mixture of mainly H+He (J,S) or H/He+ice (U,N)
mass
radius
Con
stan
t den
sity
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From Guillot, 2004
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Giant Planet Formation• Recall the first week of class• Initially solid bodies (rock + ice; beyond snow line)
• When solid mass exceeded ~10 M, gravitational acceleration sufficient to trap an envelope of H and He
• Process accelerated until nebular gas was lost• So initial accretion was rapid (few Myr)• Uranus and Neptune didn’t acquire so much gas because
they were further out and accreted more slowly• Planets will have initially been hot (gravitational energy)
and subsequently cooled and contracted • We can investigate how rapidly they are cooling at the
present day . . .
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Energy budget observations• Incident solar radiation much less than that at Earth• So surface temperatures are lower
• We can compare the amount of solar energy absorbed with that emitted. It turns out that there is usually an excess. Why?
5.4
8.1
13.5
48
2.0
2.6
4.6
14
0.6
0.6
3.5
0.3
0.3
0.6
1.4
incidentreflected
After Hubbard, in New Solar System (1999)All units in W/m2
Jupiter Saturn Uranus Neptune
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Sources of Energy • One major one is contraction – gravitational energy
converts to thermal energy. Helium sinking is another.• Gravitational energy of a uniform sphere is
• So the rate of energy release during contraction is
RGMEg /6.0 2
dt
dR
R
GM
dt
dEg
2
2
6.0
e.g.Jupiter is radiating 3.5x1017 W in excess of incident solar radiation.This implies it is contracting at a rate of 0.4 km / million years
• Other sources?– Tidal dissipation – Radioactive decay small compared to grav. energy
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Uranus – What The Hell?• Why is Uranus’ heat budget so different?
– Perhaps due to compositional density differences inhibiting convection at levels deeper than ~0.6Rp .May explain different abundances in HCN,CO between Uranus and Neptune atmospheres.
– This story is also consistent with generation of magnetic fields in the near-surface region (see earlier slide)
• Why is Uranus tilted on its side?– Nobody really knows, but a possible explanation is an oblique
impact with a large planetesimal (c.f. Earth-Moon)– This impact might even help to explain the compositional
gradients which (possibly) explain Uranus’ heat budget
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Atmospheric Structure• Lower atmosphere (opaque) is dominantly heated from below and will
be conductive or convective (adiabatic)• Upper atmosphere intercepts solar radiation and re-radiates it• There will be a temperature minimum where radiative cooling is most
efficient; in giant planets, it occurs at ~0.1 bar• Condensation of species will occur mainly in lower atmosphere
Temperature (schematic)
tropopause
troposphere
stratosphere
mesosphere
~0.1 bar
radiation
adiabat
CH4 (U,N only)
NH3
NH3+H2S
H2O
80 K
140 K
230 K
270 K
Theoretical cloud distribution
clouds
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Giant planet atmospheric structure
• Note position and order of cloud decks
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Bands and Zones
Jupiter’s Clouds – New Horizons
Different colors indicate different gases
You’re seeing to different depths in the atmosphere
Zonal winds affect opacity
Galileo went into a boring spot – crushed before it got to anything interesting
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Magnetic fields
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How are they generated?• Dynamos require convection in a conductive medium • Jupiter/Saturn – metallic hydrogen (deep)• Uranus/Neptune - near-surface convecting ices (?)• Terrestrial Planets – convection in liquid iron core• Icy Satellites – salty ocean
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Van Allen Belts• Torus of charged particles trapped in Earth’s
magnetic field• Shield Earth’s surface against high-energy
particles• Hazard to satellite navigation, manned space
missions
• Similar radiation belts observed at other planets– Jupiter’s belt poses problem for dedicated Europa
mission
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Aurora
• Collisions between charged particles with the atmosphere.– Gas controls color (Red: N, Green: O)
• Magnetic reconnection between solar magnetic field and Earth’s magnetosphere at poles
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Summary• Jupiter - mainly metallic hydrogen. Rock-ice core ~10
ME.
• Saturn - mix of metallic and molecular hydrogen; helium may have migrated to centre due to insolubility. Similar rock-ice core to Jupiter. Mean density lower than Jupiter because of smaller self-compression effect (pressures lower).
• Uranus/Neptune – thin envelope of hydrogen gas. Pressures too low to generate metallic hydrogen. Densities (and moment of inertia data) require large rock-ice cores in the interior.
• All four planets have large magnetic fields, presumably generated by convection in either metallic hydrogen (J,S) or conductive ices (U,N)
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Moons
• What are they?
• Natural things that orbit about other natural things that aren’t stars
• Terrestrial planets have few if any
• Jovian planets have whole bunches
• Even some asteroids and comets have them.
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The Magnificent Seven
4. Io 6. Europa
1. Ganymede 3. Callisto
2. Titan 5. The Moon 7. Triton
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Earth: The Moon
Maria: Lava plainsOnly on near side
Terrae: Cratered Highlands
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Mars: Phobos and Deimos
Crater: Stickney
Captured Asteroids
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Jupiter: The Inner Moons
Metis AdrasteaThebe Amalthea
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Jupiter: IoThe Volcanic Moon Tvashtar Plume
The lavas of violent Io,Though they may look like pico de gallo,Erupt and then rainOn the sulphurous plain,Looking nothing at all like Ohio.
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Jupiter: Europa
A promising place is EuropaWhere astrobiologists hope a
Critter or threeMay swim in the sea
Far beneath the icy dystopia
ALL THESE WORLDSARE YOURS EXCEPT
EUROPA ATTEMPT NO LANDING THERE
USE THEM TOGETHERUSE THEM IN PEACE
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Jupiter: Callisto
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Jupiter: The Outer Moons
• Seven in prograde orbits– Themisto, Leda, Himalia, Lysithia, Elara
• 46 in retrograde orbits– Probably Captured– Ananke, Came, Pasiphae, Sinope
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Saturn
Some share orbits!
MET DR THIP
For a sickeningly huge number of awesome images, go to:
http://ciclops.org
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Saturn: Mimas
Herschel
That’s no moon . . . it’s a space station!
Now Mimas has one giant craterThat’s sitting right on the equator
Because it’s a holeIt should be at the pole.
It could maybe reorient later.
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Saturn: EnceladusSouth Polar Plumes
Tiger-Stripes
Possible liquid ocean
Tidally heated?
There once was a moon called Enceladus,Whose Tiger-stripes have cast a spell at us.The south polar plumeLike a vapor mushroom,Has poked its way through
the ice shell at us.
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Saturn: Tethys
•Heavily cratered•Active early on•Some Resurfacing•Dark Belt•Polar caps
Odysseus
Calypso and Telesto on same orbit
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Saturn: Dione and Rhea
Wispy streaksEruptions of Snow?
Shares orbit with Helene
The ice shell of distant DioneLies over a core that is stony.
Its wispy terrainIn extensional strain
Is as brittle as dry macaroni.
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Saturn: Titan
Haze
The tholins surrounding old TitanRaise the following question. “Might anAirborne balloonGet a look at this moonAnd see if the dark patches brighten?”
Xanadu
Volcano?
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Saturn: Iapetus
Black on White or White on Black?
20 km highridge!
Cassini RegioHuge Fracture
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Uranus
• 11 small inner moons– Cordelia, Ophelia, Bianca, Cressida,
Desdemona, Juliet, Portia, Rosalind, Belinda, Puck
• MAUTO– Miranda, Ariel, Umbriel, Titania, Oberon
• 5 small outer moons– Caliban, Sycorax, Stephano, Prospero,
Setebos
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Uranus: Miranda
Broken apart and reassembled
Iapetus said to Miranda“You’re no place at all for a
lander.Your canyons have rocksLike the teeth on some crocsWhereas I’m black and white
like a panda.”
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Uranus: Ariel and Umbriel
Interconnected valleys100s km long, 10 km deep
Fluorescent Cheerio(north polar crater)
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Uranus: Titania and OberonQueen and King of the Faeries
Chasms all over 6 km high mountain,craters
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Neptune: Moons
• 5 small inner moons
• Proteus
• Triton, the big retrograde moon– Captured KBO?– Going to collide – more on this tomorrow
• Nereid (medium size moon)
• 3 small retrograde outer moons
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Neptune: Triton•Orbits retrograde•Unstable orbit•Going to impact Neptune
Ice Lava
A big KBO in disguise orA moon with a nitrogen geyser?
The retrograde moonOf sea-king Neptune
Triton thinks we’ll all be none the wiser.
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Next time
• Icy Satellites
• Ring Systems of Giant Planets
• Tidal Interactions
• Paper Discussions– Namouni and Porco (2002)– Porco et al. (2006)