EART164: PLANETARY ATMOSPHERES

1. EART164: PLANETARY ATMOSPHERES Francis Nimmo

2. 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

3. Key Equations • Greenhouse effect (simple) • Adiabat (including condensation) • Adiabatic relationship Equilibrium temperature

4. 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

5. 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

6. Atmospheric Compositions Isotopes are useful for inferring outgassing and atmos. loss

7. Gas properties Pluto (40K) Titan (94K) Mars poles (195K)

8. A selection of probes Galileo Probe (Jupiter) Pioneer Venus Phoenix (Mars) Huygens (Titan)

9. Three Important Cycles*(Terrestrial Planets) * We’ll talk about the Urey cycle in Week 9

10. 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

11. 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

12. 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)

13. 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

15. Mars 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 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

16. Mars noble gases Encyc. Paleoclimat. 2009 p.72 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

17. 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

19. 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?

20. SO2 on Venus 40mbar height Esposito (1984) 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 Days

21. CO on Venus Less photodissociation at poles 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.

23. Mars Titan CI chondrites D/H and Earth’s hydrosphere After Hartogh et al. 2011 Standard interpretation (asteroids as Earth water source) muddied by comet Hartley 2

24. 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)

25. 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!

27. Gas giant bulk compositions Volume fractions Lissauer & DePater Table 4.6 He depleted in J and S (rain-out) C/H increases with semi-major axis – why?

28. Gas giant volatiles Note enrichment in both C,N,S and noble gases relative to solar After Hersant et al. 2004

29. 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 . . .

30. 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

31. Jupiter composition profiles Incondensible, well-mixed Condensible (freezes out) Produced by photolysis of CH4

33. Titan Composition • 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 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?

34. Titan chemistry Hydrogen escapes Photodissociation • Methane lifetime ~10 Myr – implies recharge • Recharge requires outgassing of CH4 from interior (e.g. by “cryovolcanic” activity or clathrate decomposition) 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?

35. Lakes & channels on Titan 150 km North polar lakes

36. 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)

37. 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)

38. 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

39. End of lecture

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

41. 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: Values are by number of molecules Why is the H/He ratio not constant?

42. Earth atmospheric evolution Solar N/Ne=1 Zahnle et al. (2007)

43. 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

44. 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

45. Atmospheric Structure Particulate haze extends to ~300 km • At lowest temperature (tropopause) all constituents except N2 are condensed (clouds) • For an adiabatic atmosphere we have dT/dz=mg/Cp (derived in next week’s lecture) • For an N2 atmosphere, m=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 Lapse rate ~1 K/km From Owen, New Solar System

46. 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

47. 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) Haze layer