EART 160: Planetary Science

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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 didnt 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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Giant Planets
Image not to scale!
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Basic Parameters

• Data from Lodders and Fegley 1998. Surface
  temperature Ts and radius R are measured at 1 bar
  level.

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Compositions

• Well 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)
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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

• Jupiter Pc7 Mbar, Saturn Pc1.3 Mbar, U/N Pc0.9
  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 doesnt 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

Ideal Gas Law
Polytropic Law
Here P is pressure, r is density, R is gas
constant (8.3 J mol-1 K-1), T is temperature, m
is the mass of one mole of the gas, g is a
constant (ratio of specific heats, 3/2)

• So
• Adiabatic Lapse Rate

At 1 bar level on Jupiter T112 K, g23 ms-2,
Cp 25 J mol-1 K-1, m0.002 kg mol-1 dT/dz
1.4 K/km
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Hydrogen phase diagram
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

• Jupiter interior mostly metallic hydrogen

• 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 HHe (J,S) or H/Heice (U,N)

radius
Constant density
mass
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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 didnt 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?

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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

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

radiation
Temperature (schematic)
Theoretical cloud distribution
mesosphere
80 K
CH4 (U,N only)
stratosphere
140 K
0.1 bar
NH3
tropopause
clouds
230 K
NH3H2S
troposphere
adiabat
270 K
H2O
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Giant planet atmospheric structure

• Note position and order of cloud decks

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Bands and Zones
Jupiters Clouds New Horizons
Different colors indicate different gases Youre
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 Earths
  magnetic field
• Shield Earths surface against high-energy
  particles
• Hazard to satellite navigation, manned space
  missions
• Similar radiation belts observed at other planets
• Jupiters 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 Earths 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 arent 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
1. Ganymede
3. Callisto
4. Io
6. Europa
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
Captured Asteroids
Crater Stickney
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Jupiter The Inner Moons
Metis Adrastea Thebe Amalthea
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Jupiter Io
Tvashtar Plume
The Volcanic Moon
The lavas of violent Io, Though they may look
like pico de gallo, Erupt and then rain On the
sulphurous plain, Looking nothing at all like
Ohio.
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Jupiter Europa
ALL THESE WORLDS ARE YOURS EXCEPT EUROPA ATTEMPT
NO LANDING THERE USE THEM TOGETHER USE THEM IN
PEACE
A promising place is Europa Where astrobiologists
hope a Critter or three May swim in the sea Far
beneath the icy dystopia
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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
MET DR THIP
For a sickeningly huge number of awesome images,
go to http//ciclops.org
Some share orbits!
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Saturn Mimas
Now Mimas has one giant crater Thats sitting
right on the equator Because its a hole It
should be at the pole. It could maybe reorient
later.
Herschel
Thats no moon . . . its a space station!
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Saturn Enceladus
South 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 plume Like 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 streaks Eruptions of Snow?
The ice shell of distant Dione Lies over a core
that is stony. Its wispy terrain In extensional
strain Is as brittle as dry macaroni.
Shares orbit with Helene
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Saturn Titan
Xanadu
Haze
Volcano?
The tholins surrounding old Titan Raise the
following question. Might an Airborne
balloon Get a look at this moon And see if the
dark patches brighten?
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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
Iapetus said to Miranda Youre no place at all
for a lander. Your canyons have rocks Like the
teeth on some crocs Whereas Im black and white
like a panda.
Broken apart and reassembled
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Uranus Ariel and Umbriel
Fluorescent Cheerio (north polar crater)
Interconnected valleys 100s km long, 10 km deep
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Uranus Titania and Oberon
Queen 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

A big KBO in disguise or A moon with a nitrogen
geyser? The retrograde moon Of sea-king
Neptune Triton thinks well all be none the
wiser.
Ice Lava
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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)