EART164: PLANETARY ATMOSPHERES

Transcription

1 EART164: PLANETARY ATMOSPHERES Francis Nimmo

2 Dynamics Key Concepts 1 Hadley cell, zonal & meridional circulation Coriolis effect, Rossby number, deformation radius Thermal tides Geostrophic and cyclostrophic balance, gradient winds Thermal winds du dt u Ro 2 L sin 1 P x u g T z ft y 2sin v F x

3 Dynamics Key Concepts 2 Reynolds number, turbulent vs. laminar flow Velocity fluctuations, Kolmogorov cascade Brunt-Vaisala frequency, gravity waves Rossby waves, Kelvin waves, baroclinic instability Mixing-length theory, convective heat transport ul Re u l ~(e l) 1/3 2 g dt g NB T dz C p ur / 1/ 2 ~ F 3/ 2 1/ 2 dt dt g ~ C p H dz ad dz T 2

4 This Week - Long term evolution & climate change Recap on energy balance and greenhouse Common processes Faint young Sun Atmospheric loss Orbital forcing Examples and evidence Earth Mars Venus

5 T eq and greenhouse Venus Earth Mars Titan Solar constant S (Wm -2 ) Bond albedo A T eq (K) T s (K) Greenhouse effect (K) Inferred t s T eq S (1 A) 4 e s eq s 1/ 4 T T 1 t 4 Recall that t dz So if =constant, then t = x column density So a (wildly oversimplified) way of calculating T eq as P changes could use: Example: water on early Mars t P g

6 Climate Evolution Drivers Driver Period Examples Seasonal 1-100s yr Pluto, Titan Spin / orbit variations 10s-100s kyr Earth, Mars Solar output Secular (faint young Sun); and 100s yr Earth Volcanic activity Secular(?); intermittent Venus(?), Mars(?), Earth Atmospheric loss Secular Mars, Titan Impacts Intermittent Mars? Greenhouse gases Various Venus, Earth Ocean circulation 10s Myr (plate tectonics) Earth Life Secular Earth Albedo changes can amplify (feedbacks)

7 Common processes Faint young sun Atmospheric loss & impacts Orbital forcing

8 1. Faint young Sun High initial UV/X-ray fluxes (atmos loss) Sun was 30% fainter 4.4 Gyr ago Zahnle et al. 2007

9 Faint Young Sun 4.4 Gyr ago, the Sun emitted 70% of today s flux What would that do to surface temperatures? Venus Earth Mars Titan T eq (K) at 4.4 Gyr B.P Assumed t s T s (K) at 4.4 Gyr B.P T s (K) at present day Albedos assumed not to have changed Effects most important for Earth and Titan Earth would be deep-frozen, and Titan would not have liquid ethane Why might these estimates be wrong?

10 Feedbacks Temperature-albedo feedback can positive or negative Clouds negative feedback (T, A ) Ice caps positive feedback (T, A ) 1/ 4 4 S (1 A) 4eT T eq Aeq 1 4e S A, A eq CLOUDS A A, A eq ICE CAP A eq T A eq A T

11 2. Atmospheric loss An important process almost everywhere Main signature is in isotopes (e.g. C,N,Ar,Kr) Main mechanisms: Thermal (Jeans) escape Hydrodynamic escape Blowoff (EUV, X-ray etc.) Freeze-out Ingassing & surface interactions (no fractionation?) Impacts (no fractionation)

12 Jeans escape Thermal process (in exosphere) Important when thermal velocity of molecule exceeds escape velocity (H,He especially) Leads to isotopic fractionation 2 esc 2 th nvth (1 ) e v 2 GM / R v 2 R T / g is flux (atoms m -2 s -1, n is number density (atoms m -3 )

13 Hydrodynamic escape Other species can be dragged along as H is escaping (momentum transfer) Important at early times (primordial atmos.) Process leads to isotopic fractionation Fractionation is strongest at intermediate H escape rates why?

14 Blowoff/sputtering (X-ray/UV) Molecules in the exosphere can have energy added by photons (e.g. X-rays, UV etc.), charged particles or neutrals (e.g. solar wind) This additional energy may permit escape Energy-limited mass flux is given by: 2 a ext ext dm R F e dt GM / R Here F ext is the particle flux of interest, R ext 2 is the relevant cross-section, M and R are the mass and radius of the planet and e is an efficiency factor (~0.3) E.g. hot Jupiters can lose up to ~1% of their mass by this process; more effective for lower-mass planets

15 Freeze-out Unlikely unless other factors cause initial reduction in greenhouse gases (solar radiance increasing with time) But potentially important albedo feedbacks Can happen seasonally (Mars, Triton, Pluto?) Mars probably lost a lot of its water via freeze-out as its surface temperature declined Triton freeze-out (Spencer 1990)

16 Ingassing and surface interactions Plate tectonics can take volatiles (e.g. water) and redeposit them in the deep mantle Reactions can remove gases e.g. oxygen was efficiently scavenged on early Earth (red beds) and Mars A very important reaction is the Urey cycle: MgSiO CO MgCO SiO This proceeds faster at higher temperatures and in the presence of water (+ and - feedbacks) Causes removal of atmospheric CO 2 on Earth and maybe Mars (but where are the carbonates?) Reverse of this cycle helped initiate runaway greenhouse effect on Venus (see later)

17 Magma oceans Magma oceans can arise in 4 ways: Close-in, tidally-locked exoplanets (hemispheric) Extreme greenhouse effect (e.g. steam atmosphere) Gravitational energy (giant impacts) (Earth) Early radioactive heating ( 26 Al) (Mars?) Some volatiles (e.g. H 2 O, CO 2 ) are quite soluble in magma Magma oceans can store volatiles for later, long-term release

18 Impact-driven loss Tangent plane appx. Runaway process Much less effective on large bodies (Earth) than small bodies (Mars) Asteroids tend to remove volatiles; comets tend to add Does not fractionate M V V M M 2 i 2 esc 1 4 V V R 2 i 2 esc 2 i R R 0 2 i 2 p H

19 Isotopic signatures Solar N/Ne=1 Zahnle et al. 2007

20 3. Orbital forcing Universal process, details vary with planet For Earth, Milankovitch cycle forcing amplitudes are small compared to (observed) response feedbacks?

21 1. Earth

22 Water on early Earth Hadean Zircons (4.4 Ga) Oxygen isotopes (higher than expected for mantle) Low melting temperatures (Ti thermometer) Isua Pillow Basalts (3.8 Ga) Indicates liquid water present Possible indication of plate tectonics (?)

23 Faint Young Sun Problem Rampino & Caldeira (1994) How were temperatures suitable for liquid water maintained 4 Gyr B.P.? Presumably some greenhouse gas (CO 2?) Urey cycle as temperature stabilizer

24 Bombardment Earth suffered declining impact flux: Moon-forming impact (~10% M E, ~4.4 Ga) Late veneer (~1% M E, Ga) Late Heavy Bombardment (0.001% M E, 3.9 Ga) Atmospheric consequences unclear chondritic material added, but also blowoff? Comets would probably have delivered too much noble gas Bottke et al. 2010

25 Snowball Earth Cap carbonate Tillite Ice-albedo feedback (runaway) Several occurrences (late Paleozoic last one) Abundant geological & isotopic evidence Details are open to debate (ice-free oceans?) How did it end?

26 2. Mars

27 Early Mars was Wet Hematite blueberries (concretions?)

28 Was early Mars warm and wet or cold and (occasionally) wet? Usual explanation is to appeal to a thick, early CO 2 atmosphere, allowing water to persist at the surface How much CO 2 would have been required? Atmosphere was subsequently lost One problem was absence of observed carbonates H (Urey cycle) 2 O Possible solution is highly acidic waters (?) Mars lower atmosphere

29 Transient early hydrosphere? Alternative cold Mars, with subsurface water occasionally released by big impacts Segura et al Most of water is from melting subsurface.

30 Obliquity cycles on Mars Obliquity on Mars varies much more strongly than on Earth (absence of a big moon, proximity to Jupiter) Long-term obliquity is chaotic Mars experienced periods when poles were warmer than equator

31 Evidence for obliquity cycles? Laskar et al. 2002

32 Moderate obliquity periods may have allowed nearequatorial ice to develop Snowball Mars? Hellas quadrangle (mid-latitudes)

33 Atmospheric loss many choices Low surface gravity - easy for atmosphere to escape But further from Sun than Earth lower solar flux Death of dynamo increased atmospheric loss via sputtering (?) Impact erosion probably important runaway process (no fractionation) D/H and N isotope ratios indicate substantial loss with fractionation (see Week 3) Melosh and Vickery 1989

34 Long-term evolution Atmosphere certainly thicker (at least transiently) in deep past, and then declined Large quantities of subsurface ice at present day Details poorly understood Catling, Encyc. Paleoclimat. Ancient Environments

35 MAVEN & MSL MAVEN launches late 2013 Measures Martian upper atmospheric composition and escape rates MSL landed Aug 2012 Measuring Martian rock and atmospheric compositions

36 3. Venus

37 Background Venus atmospheric pressure ~90 bar (CO 2 ), surface temperature 450 o C Earth has similar CO 2 abundance, but mostly locked up in carbonates If you take Earth and heat it up, carbonates dissociate to CO 2, increasing greenhouse effect runaway Will this happen as the Sun brightens?

38 Another runaway greenhouses This one happened first, and involves H 2 O H 2 O in atmosphere lost via photodissociation Once the water is lost, then CO 2 drawdown ceases and the CO 2 greenhouse takes over Did Venus lose an ocean? (D/H evidence)

39 Recent outgassing & climate Venus was resurfaced ~0.5 Gyr ago, probably involving very extensive outgassing How has atmosphere evolved since then? (Taylor Fig 9.8)

40 Afterthought - Exoplanets Mostly gas giants Orbital parameters very different (tidal locking, high eccentricity, short periods) In some cases, we can observe H loss Just starting to get spectroscopic information Inferred temperature structure can tell us about dynamics (winds)

41 Key Concepts Faint young Sun, albedo feedbacks, Urey cycle Loss mechanisms (Jeans, Hydrodynamic, Energylimited, Impact-driven, Freeze-out, Surface interactions, Urey cycle) and fractionation Orbital forcing, Milankovitch cycles Warm, wet Mars? Earth bombardment history Runaway greenhouses (CO 2 and H 2 O) Snowball Earth

42 Key equations 1/ 4 S (1 A) 4 4 T 4 3 eq Ts Teq 1 t s e 4 MgSiO CO MgCO SiO t P g M nvth (1 ) V V 2 i 2 esc R 2 i e H 2 a ext ext dm R F e dt GM / R 0

43 End of lecture

44 How about radar image of subsurface ice?

45 Albedo feedback Main sources of albedo are clouds and ices Equilibrium gives: dt 1 T da' 4 A' On Earth, 1% change in albedo causes 1 o C temperature change more than predicted. Why? Feedbacks can work both ways e.g. Ocean warming more clouds form albedo increases (stable feedback). This feedback is the main uncertainty in climate prediction models. Ice-cap growth albedo increases (unstable feedback)

46 N, Ne and Ar atmospheric loss 14N/15N 36/38Ar 20Ne/36Ar Solar Venus Earth Mars Jupiter Titan 56 - Why do we use isotopic ratios? Planets (except Jupiter) have more heavy N and Ar loss process 20/22 Ne and 36/40Ar tell us about radiogenic processes

47 Titan (CH 4 ) Chondrites Mars D/H water loss Higher D/H suggests more water loss Not all loss mechanisms involve fractionation! Venus (0.016) Hartogh et al. 2011

48 Milkovich and Head 2005