EART164: PLANETARY ATMOSPHERES

Transcription

1 EART164: PLANETARY ATMOSPHERES Francis Nimmo

2 Last Week Energy Budgets and Energy Budgets Temperature Structures 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 Equilibrium temperature Greenhouse effect (simple) Adiabat (including condensation) T eq dt dz S (1 A) 4 1/ 4 T 2 T s eq g C p L 1/ 4 dx dt Adiabatic relationship P c

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) 40 Ar ( 40 K decay product) is a tracer of outgassing

6 Atmospheric Compositions Earth Venus Mars Titan Pressure 1 bar 92 bar bar 1.5 bar N 2 77% 3.5% 2.7% 98.4% O 2 21% H 2 O 1% 0.01% 0.006% - Ar 0.93% 0.007% 1.6% 0.004% CO % 96% 95% ~1ppb CH ppm -? 1.6% CO 0.12 ppm 40 ppm 700 ppm 45 ppm SO 2 ~0.2 ppb ~150 ppm - - Ne 18 ppm 5 ppm 2.5 ppm 0.3 ppm? 40 Ar 6.6x10 16 kg 1.4x10 16 kg 4.5x10 14 kg 3.5x10 14 kg H/D N/ 15 N Isotopes are useful for inferring outgassing and atmos. loss

7 Gas properties Atomic mass He Ne O N Ar CH Kr Xe CO NH Solidification temperature (K) 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 H 2 O H 2 + O escape **but aerosols cool the atmosphere! O + SO 2 SO 3 H 2 O + SO 3 H 2 SO 4 (condenses) outgassing SO 2? SO 2 is an important greenhouse gas** Major source of SO 2 is volcanic outgassing Applications: Earth, Venus, early Mars(?) Removal of SO 2 requires water

11 UV photon CO cycle CO 2 CO + O 2 H 2 O OH + H CO + 2OH CO 2 + H 2 O If H 2 O 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 CO 2 ) Spatial distribution of CO gives info on dynamics

12 Ozone cycle PRODUCTION REMOVAL O 2 O + O O 3 O 2 + O O + O 2 O 3 O + O 3 2O 2 Ozone (O 3 ) formation mediated by aerosols O 3 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 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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15 Mars CO 2 pressure shows seasonal variations of ~30% H 2 O present at ~0.03% (also highly variable) Variability of both is due to presence of polar caps Presence of H 2 O explains lack of CO (see before) D/H ratio suggests some loss of H over time 15 N/ 14 N ratio suggests nitogen loss over time Early spring (CO2 ice) Summer (H2O ice) Ice caps are a few m of CO 2 over 100s m of H 2 O North Polar Cap ~600 km

16 Mars noble gases Mars has more heavy noble gases ( 15 N, 40 Ar) than Earth (fractionation), suggesting atmospheric loss 40 Ar story is complicated because source of 40 Ar 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)x x N/ 15 N Ar/ 38 Ar Ar/ 36 Ar Encyc. Paleoclimat p.72

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 nonbiological 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

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19 Venus D/H ratio is ~50 times Earth s, suggesting Venus has lost a lot of H (from photodissociation of H 2 O) What happened to the O? Either it was also lost, or it ended up bound in the crust Present-day survival time of H 2 O is ~100 Myr So unless we are viewing Venus at a special time, there must be a present-day water source. What is it? 40 Ar in Venus atmosphere suggests that the interior is only ~10% outgassed (cf. 50% for Earth s mantle). Why the difference?

20 SO 2 on Venus Lifetime of SO 2 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. H 2 O) Most likely present-day volcanism 40mbar height Esposito (1984) Days

21 CO on Venus Photodissociation in stratosphere: CO 2 ->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

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

24 Rise of Oxygen BIFs 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 O 2 (BIFs?) became full Still poorly-understood Sessions et al. (2009)

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

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27 Gas giant bulk compositions Solar Jupiter Saturn Uranus Neptune H He CH 4 x NH 3 x <1.5 <1.5 Solar Jupiter Ratio 20 Ne 1.3x10-4 2x Ar 2.8x x Kr 3.3x x Xe 3.3x x 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 (CH 4, NH 3, H 2 S) 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 semimajor 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 CH 4

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33 Titan Titan Composition Earth N % 78% CH % 2 ppm O 2-21% CO 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. CO 2 vs. CH 4 ). 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 Photodissociation 2CH C H H Hydrogen escapes 2 Ethane condenses at 101 K Reactions produce more complex organics Clouds plus organic haze Methane recharged Underground aquifer? Organic drizzle Surface 94K Ethane etc. lakes After Coustenis and Taylor, Titan, 1999 Methane lifetime ~10 Myr implies recharge Recharge requires outgassing of CH 4 from interior (e.g. by cryovolcanic activity or clathrate decomposition)

35 Lakes & channels on Titan 150 km North polar lakes

36 Huygens Isotopic Measurements 14 N/ 15 N~180 c.f. 270 for Earth enrichment in heavy N 2 suggests Titan lost ~80% of its atmosphere 12 C/ 13 C ~80, similar Earth, Jupiter, Saturn outgassing of methane outpaced atmospheric methane loss 40 Ar 43 ppm low outgassing efficiency (why?) 36 Ar 0.3ppm suggests N 2 arrived as NH 3 (why?) Because N 2 condenses at such cold temperatures that Ar would have been trapped too. More likely NH 3 was later photodissociated to N 2 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 v e = (2 g R) 1/2 (where s this from?) Mean molecular velocity v m = (2kT/m) 1/2 Boltzmann distribution negligible numbers of atoms with velocities > 3 x v m Nitrogen N 2, 94 K, 3 x v m = 0.7 km/s Hydrogen H 2, 3 x v m = 2.6 km/s Titan v e =2.6 km/s H 2 escapes, N 2 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, SO 2 Noble gas ratios and atmospheric loss (fractionation) Outgassing ( 40 Ar, 4 He) D/H ratios and water loss Dynamics can influence chemistry Photodissociation and loss (CH 4, H 2 O etc.) Non-solar gas giant compositions Titan s problematic methane source

39 End of lecture

40 Terrestrial planets Venus Mars Earth Pressure (bar) CO CO 40 ppm 700 ppm 0.12 ppm H 2 O ~30 ppm ~300 ppm 0.01 SO 2 ~150 ppm 0 ~0.2 ppb O 2 trace N Ar (not total Ar) 25 ppm 6 ppm 30 ppm Total Ar 55 ppm Ne 5 ppm 2.5 ppm 18 ppm D/H 1/63 1/1100 1/ N/ 15 N 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: Solar Jupiter Saturn Uranus Neptune H He CH Why is the H/He ratio not constant? Values are by number of molecules

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

43 Atmospheric Chemistry Methane gets photodissociated and H 2 is lost (why?): 2CH C H H 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 ~10 7 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 H 2 O at the surface (temperature). This is why there s little CO 2 but CH 4 instead

44 Atmospheric Evolution Earth atmosphere originally CO 2 -rich, oxygen-free How do we know? CO 2 was progressively transferred into rocks by the Urey reaction (takes place in presence of water): MgSiO 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 CO 2 abundance Urey reaction may have occurred on Mars (water present early on), but very little carbonate detected

45 Atmospheric Structure At lowest temperature (tropopause) all constituents except N 2 are condensed (clouds) For an adiabatic atmosphere we have dt/dz=mg/c p (derived in next week s lecture) For an N 2 atmosphere, m=0.028 kg, C p ~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 Particulate haze extends to ~300 km 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 Cassini image of clouds near South Pole 450km

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. N 2 ) freezing out (c.f. Pluto, Triton)