eart164: planetary atmospheres

F.Nimmo EART164 Spring 11

Francis Nimmo

EART164: PLANETARY ATMOSPHERES


• Energy Budgets– Incoming (short ) 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

Last Week – Energy Budgets and Temperature Structures


Key Equations

• Equilibrium temperature

dTdx

p LC

g

dz

dT

4/1

4

)1(

AS

Teq

cP

• Greenhouse effect (simple) eqs TT 4/12

• Adiabatic relationship

• Adiabat (including condensation)


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


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 don’t 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


Atmospheric Compositions

Isotopes are useful for inferring outgassing and atmos. loss

Earth Venus Mars TitanPressure 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 360014N/15N 272 273 170 183


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

Titan (94K)

Pluto (40K)

Mars poles (195K)


A selection of probes

Pioneer Venus

Galileo Probe(Jupiter)

Huygens (Titan)

Phoenix (Mars)


Three Important Cycles*(Terrestrial Planets)

* We’ll talk about the Urey cycle in Week 9


Sulphur Cycle

H2O H2 + O

escapeUV photon

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

**but aerosols cool the atmosphere!


CO cycle

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

UV photon


Ozone cycle

O2 O + O

O + O2 O3

PRODUCTION

O3 O2 + O

O + O3 2O2

REMOVAL

• 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)


Drake & Righter 2002

Solar (appx. -60, -60) (Genesis mission)

Slope ½ (expected)

Slope 1

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

Oxygen 3-isotope plot


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

North Polar Cap

Early spring (CO2 ice) Summer (H2O ice)

~600 km

Ice caps are a few m of CO2 over 100s m of H2O


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

Mars (Viking) Earth

D/H (5-13)x10-4 1.56x10-4

14N/15N 170 27236Ar/38Ar 4-7 5.340Ar/36Ar 3000 296

Encyc. Paleoclimat. 2009 p.72


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 didn’t make much sense– Methane’s 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


Venus• D/H ratio is ~50 times Earth’s, 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 Earth’s mantle). Why the difference?


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

Days

Esposito (1984)


CO on Venus• Photodissociation in stratosphere: CO2->CO+O

• 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


D/H and Earth’s hydrosphere

After Hartogh et al. 2011

Mars

Titan

CI chondrites

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


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

Sessions et al. (2009)

BIFs


Outgassing• 4He and 40Ar are produced by radioactive decay (from U&Th

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!


Gas giant bulk compositions

• He depleted in J and S (rain-out)

• C/H increases with semi-major axis – why?

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 <1.5 <1.5

Volume fractionsLissauer & DePater Table 4.6

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


Gas giant volatiles

After Hersant et al. 2004

Note enrichment in both C,N,S and noble gases relative to solar


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 . . .


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


Jupiter composition profiles

Incondensible, well-mixed

Condensible (freezes out)

Produced by photolysis of CH4


Titan Composition

• Obtained from UV/IR spectra, radio occultation data and Huygens

• Various organic molecules at the few ppm level

• Haze consists of ~1 m 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 C:N ratio is 4-20:1. On Earth, most of the C is locked up in carbonates; where is the C stored on Titan?

Titan Earth

N2 98.4 % 78%

CH4 1.4 % 2 ppm

O2 - 21%

CO2 0.01 ppm 350 ppm

Ar 7 ppm 0.9%


Titan chemistry

• Methane lifetime ~10 Myr – implies recharge• Recharge requires outgassing of CH4 from interior (e.g. by

“cryovolcanic” activity or clathrate decomposition)

26242 HHCCH

Hydrogen escapesPhotodissociation

Ethane condenses at 101 KReactions produce more complex organics

Organic drizzle

Clouds plus organic haze

Ethane etc. lakesUnderground aquifer?

Methane recharged

After Coustenis andTaylor, Titan, 1999

Surface 94K


Lakes & channels on Titan

North polar lakes

150 km


Huygens Isotopic Measurements• 14N/15N~180 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 D:H 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 D:H than solar)


Jeans Escape• Escape velocity ve= (2 g R)1/2 (where’s this from?)

• Mean molecular velocity vm= (2kT/m)1/2

• Boltzmann distribution – negligible numbers of atoms with velocities > 3 x vm

• Nitrogen N2, 94 K, 3 x vm= 0.7 km/s

• Hydrogen H2, 3 x vm= 2.6 km/s

• Titan ve=2.6 km/s – H2 escapes, N2 doesn’t (much)

• A consequence of Jeans escape is isotopic fractionation – heavier isotopes will be preferentially enriched (and now we have the observations)


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• Titan’s problematic methane source


End of lecture


Terrestrial planetsVenus 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 1/300014N/15N 273 170 272

Major constituents tell us about chemistry.Noble gases & isotope ratios tell us about loss processes.


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

Valuesare by number of molecules

Why is the H/He ratio not constant?


Earth atmospheric evolution

Solar N/Ne=1

Zahnle et al. (2007)


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?

It’s locked up in solid H2O at the surface (temperature).

• This is why there’s little CO2 but CH4 instead

26242 HHCCH


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):

3 2 3 2MgSiO CO MgCO SiO • 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


Atmospheric Structure• At lowest temperature

(tropopause) all constituents except N2 are condensed (clouds)

• For an adiabatic atmosphere we have dT/dz=g/Cp (derived in next week’s lecture)

• For an N2 atmosphere, =0.028 kg, Cp~3.5R=30 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 From Owen, New Solar System

Lapse rate~1 K/km

Particulate hazeextends to ~300 km


Clouds

Keck Adaptive Optics images, fromBrown et al. Nature 2002

Predominance of clouds near S pole not yet explained but may be due to local convective columns driven by small changes in surface temperature.

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

Cassini image of clouds near South Pole

450km


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)

– Titan’s 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) H

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eart164: planetary atmospheres

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