Francis Nimmo

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EART164 PLANETARY ATMOSPHERES

• Francis Nimmo

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Last Week Energy Budgets and Temperature
Structures

• Energy Budgets
• Incoming (short l) energy is reflected (albedo),
  absorbed, or re-emitted from the surface at
  longer wavelengths
• Gas giants have extra energy source (contraction)
• Simplified temperature structures
• Convection in troposphere produces an adiabat
• Stratosphere is (approximately) isothermal
  (because it radiates directly to space)
• Adiabatic gradient is affected by condensation
• Deep gas giant structures
• Hydrogen undergoes phase changes at high P,T
• There is a maximum radius for a gas giant

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Key Equations

• Equilibrium temperature

• Greenhouse effect (simple)

• Adiabat (including condensation)

• Adiabatic relationship

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This Week - Chemisty

• A bit of a rag-bag! We will look (briefly) at
• Three important chemical cycles
• And then do a quick tour of
• Mars
• Venus
• Earth
• Jupiter co.
• Titan
• Taylor Ch.6

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Where do planetary atmospheres come from?

• Three primary sources
• Primordial (solar nebula)
• Outgassing (trapped gases)
• Later delivery (mostly comets)
• How can we distinguish these?
• Solar nebula composition well known
• Noble gases are useful because they dont react
• Isotopic ratios are useful because they may
  indicate gas loss or source regions (e.g. D/H)
• 40Ar (40K decay product) is a tracer of
  outgassing

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Atmospheric Compositions
Earth Venus Mars Titan
Pressure 1 bar 92 bar 0.006 bar 1.5 bar
N2 77 3.5 2.7 98.4
O2 21 - - -
H2O 1 0.01 0.006 -
Ar 0.93 0.007 1.6 0.004
CO2 0.035 96 95 1ppb
CH4 1.7 ppm - ? 1.6
CO 0.12 ppm 40 ppm 700 ppm 45 ppm
SO2 0.2 ppb 150 ppm - -
Ne 18 ppm 5 ppm 2.5 ppm 0.3 ppm?
40Ar 6.6x1016 kg 1.4x1016 kg 4.5x1014 kg 3.5x1014 kg
H/D 3000 63 1100 3600
14N/15N 272 273 170 183

• Isotopes are useful for inferring outgassing and
  atmos. loss

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Gas properties
Atomic mass Solidification temperature (K)
He 4 4.2
Ne 20.2 27
O2 32. 54
N2 28. 63
Ar 40. 87
CH4 16. 90.5
Kr 83.3 121
Xe 131.3 166
CO2 44. 195
NH3 17. 195
Pluto (40K)
Titan (94K)
Mars poles (195K)
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A selection of probes
Galileo Probe (Jupiter)
Pioneer Venus
Phoenix (Mars)
Huygens (Titan)
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Three Important Cycles(Terrestrial Planets)
Well talk about the Urey cycle in Week 9
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Sulphur Cycle
UV photon
escape
but aerosols cool the atmosphere!
H2O H2 O
O SO2 SO3
H2O SO3 H2SO4 (condenses)
SO2
?
outgassing

• SO2 is an important greenhouse gas
• Major source of SO2 is volcanic outgassing
• Applications Earth, Venus, early Mars(?)
• Removal of SO2 requires water

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CO cycle
UV photon
CO2 CO O2
H2O OH H
CO 2OH CO2 H2O

• If H2O is present, this limits the amount of CO
• CO is almost non-existent on Earth
• Mars and Venus have a lot less water than Earth,
  and a lot more CO (though still small compared
  with CO2)
• Spatial distribution of CO gives info on dynamics

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Ozone cycle
PRODUCTION
REMOVAL
O2 O O
O3 O2 O
O O2 O3
O O3 2O2

• Ozone (O3) formation mediated by aerosols
• O3 is a good UV absorber
• Spatial distribution of ozone on Earth due to
  dynamics
• Similar photochemical processes important in
  determining oxygen isotope ratios (see next slide)

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Oxygen 3-isotope plot
Slope ½ (expected)
Slope 1
Drake Righter 2002
Solar (appx. -60, -60) (Genesis mission)

• Origin of anomalies uncertain but probably
  involves disk photochemistry - CO
  self-shielding mechanism?
• Useful tracer for discriminating e.g. different
  meteorites

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Mars

• CO2 pressure shows seasonal variations of 30
• H2O present at 0.03 (also highly variable)
• Variability of both is due to presence of polar
  caps
• Presence of H2O explains lack of CO (see before)
• D/H ratio suggests some loss of H over time
• 15N/14N ratio suggests nitogen loss over time

Summer (H2O ice)
Early spring (CO2 ice)
Ice caps are a few m of CO2 over 100s m of H2O
600 km
North Polar Cap
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Mars noble gases

• Mars has more heavy noble gases (15N, 40Ar) than
  Earth (fractionation), suggesting atmospheric
  loss
• 40Ar story is complicated because source of 40Ar
  is decay from the interior (see later)
• SNC meteorites contain gas inclusions which
  record atmospheric evolution over time

Encyc. Paleoclimat. 2009 p.72
Mars (Viking) Earth
D/H (5-13)x10-4 1.56x10-4
14N/15N 170 272
36Ar/38Ar 4-7 5.3
40Ar/36Ar 3000 296
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Methane on Mars??

• A hot topic a few years ago
• Exciting because methane is produced on Earth
  mainly by biology (though there are also
  non-biological sources e.g. serpentinization)
• But the observations didnt make much sense
• Methanes lifetime against photodissociation is
  300 years, but variability is seen on yearly
  timescales
• Mixing in Mars stratosphere should be rapid, but
  large spatial variations in CH4 were seen
• Zahnle et al. (2012) argue that it is all just
  interference from terrestrial methane (hard to
  remove)
• Mars Science Laboratory may resolve the issue

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Venus

• D/H ratio is 50 times Earths, suggesting Venus
  has lost a lot of H (from photodissociation of
  H2O)
• What happened to the O?
• Either it was also lost, or it ended up bound in
  the crust
• Present-day survival time of H2O is 100 Myr
• So unless we are viewing Venus at a special time,
  there must be a present-day water source. What is
  it?
• 40Ar in Venus atmosphere suggests that the
  interior is only 10 outgassed (cf. 50 for
  Earths mantle). Why the difference?

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SO2 on Venus

• Lifetime of SO2 in Venus atmosphere is short (few
  Myr) (longer than on Earth why?)
• And there are fluctuations on 100 day periods
• So there must be a current source (cf. H2O)
• Most likely present-day volcanism

40mbar height
Esposito (1984)
Days
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CO on Venus

• Photodissociation in stratosphere CO2-gtCOO
• Rapidly removed by reactions with OH at cloud
  tops and also reactions with surface
• But CO levels increase in lower atmosphere why?
• Global circulation draws CO-rich material from
  stratosphere down to poles
• Consistent with latitudinal variations in CO at
  low alt.

Less photodissociation at poles
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D/H and Earths hydrosphere
After Hartogh et al. 2011

• Standard interpretation (asteroids as Earth water
  source) muddied by comet Hartley 2

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Rise of Oxygen

• An abrupt increase at 2.45 Ga
• Had knock-on effects (e.g. removal of methane,
  ice age)
• Biomarkers indicative of photosynthesis have
  (probably) been detected prior to onset of
  oxidation
• Rise of oxygen occurred when sinks of O2 (BIFs?)
  became full
• Still poorly-understood

BIFs
Sessions et al. (2009)
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Outgassing

• 4He and 40Ar are produced by radioactive decay
  (from UTh and 40K, respectively)
• He is rapidly lost from terrestrial planets
  (why?)
• So 4He abundance tells us about recent outgassing
• Ar is too heavy to lose. So 40Ar tells us about
  time-integrated outgassing (half-life 1.25 Gyr)
• Venus has about 5 times less 40Ar in the
  atmosphere than Earth (but much more 36Ar)
• So Venus is less-efficiently outgassed than Earth
• Earth not completely outgassed (3He from mantle)
• Note these calculations assume no Ar loss!

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Gas giant bulk compositions
Solar Jupiter Saturn Uranus Neptune
H2 84 86.4 88 83 82
He 16 13.6 12 15 15
CH4 x102 0.046 0.2 0.45 2.3 3
NH3 x104 1.13 0.7 5 lt1.5 lt1.5

• He depleted in J and S (rain-out)
• C/H increases with semi-major axis why?

Solar Jupiter Ratio
20Ne 1.3x10-4 2x10-5 0.15
36Ar 2.8x10-6 1.6x10-5 5.7
84Kr 3.3x10-9 7.6x10-9 2.3
132Xe 3.3x10-10 7.6x10-10 2.3
Volume fractions Lissauer DePater Table 4.6
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Gas giant volatiles
Note enrichment in both C,N,S and noble gases
relative to solar
After Hersant et al. 2004
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Non-solar compositions

• C,N,S were probably accreted as ices (CH4, NH3,
  H2S) and so delivered in solid phase, not as
  gases alongside H,He
• But it is hard to explain the noble gas
  enrichment, because they condense at much lower T
• Maybe the temperatures during Jupiter formation
  were lower than usually thought? Or maybe the
  solid material was brought in from larger
  semi-major axes (lower temperatures)?
• It would help to know whether the O/H ratio was
  solar or not . . .

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Water on Jupiter

• The Galileo probe did measure the O/H ratio on
  Jupiter.
• But unfortunately it appears to have done so in a
  dry spot a region of descending, cold air.
• So the Galileo measurement is not representative
  of the planet as a whole
• The Juno spacecraft (launched 2011) will use
  microwave radiometry to hopefully resolve this
  issue

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Jupiter composition profiles
Condensible (freezes out)
Produced by photolysis of CH4
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Titan Composition
Titan Earth
N2 98.4 78
CH4 1.4 2 ppm
O2 - 21
CO2 0.01 ppm 350 ppm
Ar 7 ppm 0.9

• Obtained from UV/IR spectra, radio occultation
  data and Huygens
• Various organic molecules at the few ppm level
• Haze consists of 1 mm particles, methane
  condensates plus other hydrocarbons (generated by
  photolysis of methane)
• Atmosphere is reducing (e.g. CO2 vs. CH4). Where
  is the oxygen?
• Solar system CN ratio is 4-201. On Earth, most
  of the C is locked up in carbonates where is the
  C stored on Titan?

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Titan chemistry
Hydrogen escapes
Photodissociation
Ethane condenses at 101 K Reactions produce more
complex organics
Clouds plus organic haze
Methane recharged
Organic drizzle
Surface 94K
Ethane etc. lakes
After Coustenis and Taylor, Titan, 1999
Underground aquifer?

• Methane lifetime 10 Myr implies recharge
• Recharge requires outgassing of CH4 from interior
  (e.g. by cryovolcanic activity or clathrate
  decomposition)

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Lakes channels on Titan
150 km
North polar lakes
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Huygens Isotopic Measurements

• 14N/15N180 c.f. 270 for Earth enrichment in
  heavy N2 suggests Titan lost 80 of its
  atmosphere
• 12C/13C 80, similar Earth, Jupiter, Saturn
  outgassing of methane outpaced atmospheric
  methane loss
• 40Ar 43 ppm low outgassing efficiency (why?)
• 36Ar 0.3ppm suggests N2 arrived as NH3 (why?)
• Because N2 condenses at such cold temperatures
  that Ar would have been trapped too. More likely
  NH3 was later photodissociated to N2

• Methane DH ratio about 6 times that of Jupiter
  and Saturn. Half of this is due to Jeans
  fractionation the remainder is probably due to
  cometary additions to the atmosphere (comets have
  higher DH than solar)

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Jeans Escape

• Escape velocity ve (2 g R)1/2 (wheres this
  from?)
• Mean molecular velocity vm (2kT/m)1/2
• Boltzmann distribution negligible numbers of
  atoms with velocities gt 3 x vm
• Nitrogen N2, 94 K, 3 x vm 0.7 km/s
• Hydrogen H2, 3 x vm 2.6 km/s
• Titan ve2.6 km/s H2 escapes, N2 doesnt (much)
• A consequence of Jeans escape is isotopic
  fractionation heavier isotopes will be
  preferentially enriched (and now we have the
  observations)

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Key Concepts

• Cycles ozone, CO, SO2
• Noble gas ratios and atmospheric loss
  (fractionation)
• Outgassing (40Ar, 4He)
• D/H ratios and water loss
• Dynamics can influence chemistry
• Photodissociation and loss (CH4, H2O etc.)
• Non-solar gas giant compositions
• Titans problematic methane source

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End of lecture
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Terrestrial planets
Venus Mars Earth
Pressure (bar) 92 0.006 1
CO2 0.96 0.95 0.0003
CO 40 ppm 700 ppm 0.12 ppm
H2O 30 ppm 300 ppm 0.01
SO2 150 ppm 0 0.2 ppb
O2 trace 0.0013 0.21
N2 0.035 0.027 0.77
36Ar (not total Ar) 25 ppm 6 ppm 30 ppm
Total Ar 55 ppm 0.02 0.009
Ne 5 ppm 2.5 ppm 18 ppm
D/H 1/63 1/1100 1/3000
14N/15N 273 170 272
Major constituents tell us about chemistry. Noble
gases isotope ratios tell us about loss
processes.
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Not primordial!

• Terrestrial planet atmospheres are not primordial
  (How do we know?)
• Why not?
• Gas loss (due to impacts, rock reactions or Jeans
  escape)
• Chemical processing (e.g. photolysis, rock
  reactions)
• Later additions (e.g. comets, asteroids)
• Giant planet atmospheres are close to primordial

Solar Jupiter Saturn Uranus Neptune
H2 84 86.4 97 83 79
He 16 13.6 3 15 18
CH4 0.07 0.2 0.2 2 3
Values are by number of molecules
Why is the H/He ratio not constant?
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Earth atmospheric evolution
Solar N/Ne1
Zahnle et al. (2007)
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Atmospheric Chemistry

• Methane gets photodissociated and H2 is lost
  (why?)
• Reactants e.g. ethane will condense and fall to
  the surface
• These two effects mean that the lifetime of
  methane in the present atmosphere is 107 yrs
• So there must be something which is continually
  resupplying methane to the atmosphere
• One suggestion was that this source was a
  methane/ethane ocean at the surface (caused by
  rain-out of the condensing species)
• Methane liquefies at 90.6 K, ethane at 101 K,
  c.f. surface temp. 94 K
• There are other possibilities e.g. comet
  delivery, outgassing
• Atmosphere is reducing because of lack of oxygen.
  Where is the oxygen? Its locked up in solid H2O
  at the surface (temperature).
• This is why theres little CO2 but CH4 instead

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Atmospheric Evolution

• Earth atmosphere originally CO2-rich, oxygen-free
• How do we know?
• CO2 was progressively transferred into rocks by
  the Urey reaction (takes place in presence of
  water)

• Rise of oxygen began 2 Gyr ago (photosynthesis
  photodissociation)
• Venus never underwent similar evolution because
  no free water present (greenhouse effect, too
  hot)
• Venus and Earth have same total CO2 abundance
• Urey reaction may have occurred on Mars (water
  present early on), but very little carbonate
  detected

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Atmospheric Structure

• At lowest temperature (tropopause) all
  constituents except N2 are condensed (clouds)
• For an adiabatic atmosphere we have dT/dzmg/Cp
  (derived in next weeks lecture)
• For an N2 atmosphere, m0.028 kg, Cp3.5R30 J
  K-1 mol-1
• So the lapse rate is 1 K/km
• Temperature increases above tropopause due to
  incoming solar radiation
• Particulate haze makes direct observations of
  clouds hard

Particulate haze extends to 300 km
Lapse rate 1 K/km
From Owen, New Solar System
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Clouds

• Clouds lie beneath the haze layer, at 10-20km,
  and are mainly methane crystals (bright)

Predominance of clouds near S pole not yet
explained but may be due to local convective
columns driven by small changes in surface
temperature.
Keck Adaptive Optics images, from Brown et al.
Nature 2002
450km
Cassini image of clouds near South Pole
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Why Titan?

• Titan is the only satellite to have a significant
  atmosphere. Why?
• Seems to be a combination of three factors
• Local nebular temperatures sufficiently cold that
  primordial atmosphere was able to form (Saturn is
  twice as far from Sun as Jupiter, and is less
  massive)
• Titans mass sufficiently high that it was able
  to retain a large fraction of this original
  atmosphere (and later cometary additions) (Jeans
  escape)
• Surface temperature warm enough to prevent some
  volatiles (e.g. N2) freezing out (c.f. Pluto,
  Triton)

Haze layer