EART 164 PLANETARY ATMOSPHERES Francis Nimmo F Nimmo

EART 164: PLANETARY ATMOSPHERES Francis Nimmo F. Nimmo EART 164 Spring 11

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 F. Nimmo EART 164 Spring 11

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 ~(e l)1/3 F. Nimmo EART 164 Spring 11

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 F. Nimmo EART 164 Spring 11

Teq and greenhouse Venus Earth Mars Titan Solar constant S (Wm-2) 2620 1380 594 15. 6 Bond albedo A 0. 76 0. 4 0. 15 0. 3 Teq (K) 229 245 217 83 Ts (K) 730 288 220 95 Greenhouse effect (K) 501 43 3 12 Inferred ts 136 1. 2 0. 08 0. 96 Recall that So if a=constant, then t = a x column density So a (wildly oversimplified) way of calculating Teq as P changes could use: Example: water on early Mars F. Nimmo EART 164 Spring 11

Climate Evolution Drivers Driver Period Examples Seasonal 1 -100 s yr Pluto, Titan Spin / orbit variations 10 s-100 s kyr Earth, Mars Solar output Secular (faint young Sun); and 100 s yr Earth Volcanic activity Secular(? ); intermittent Venus(? ), Mars(? ), Earth Atmospheric loss Secular Mars, Titan Impacts Intermittent Mars? Greenhouse gases Various Venus, Earth Ocean circulation 10 s Myr (plate tectonics) Earth Life Secular Earth Albedo changes can amplify (feedbacks) F. Nimmo EART 164 Spring 11

Common processes • Faint young sun • Atmospheric loss & impacts • Orbital forcing F. Nimmo EART 164 Spring 11

1. Faint young Sun • High initial UV/X-ray fluxes (atmos loss) • Sun was 30% fainter 4. 4 Gyr ago Zahnle et al. 2007 F. Nimmo EART 164 Spring 11

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 Teq (K) at 4. 4 Gyr B. P. 209 224 198 76 Assumed ts 136 1. 2 0. 08 0. 96 Ts (K) at 4. 4 Gyr B. P. 665 263 201 87 Ts (K) at present day 730 288 220 95 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? F. Nimmo EART 164 Spring 11

Feedbacks • Temperature-albedo feedback can positive or negative – Clouds – negative feedback (T , A ) – Ice caps – positive feedback (T , A ) A, Aeq ICE CAP CLOUDS A Aeq T A T F. Nimmo EART 164 Spring 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) F. Nimmo EART 164 Spring 11

Jeans escape • Thermal process (in exosphere) • Important when thermal velocity of molecule exceeds escape velocity (H, He especially) • Leads to isotopic fractionation f is flux (atoms m-2 s-1, n is number density (atoms m-3) F. Nimmo EART 164 Spring 11

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? F. Nimmo EART 164 Spring 11

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: Here Fext is the particle flux of interest, p. Rext 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 F. Nimmo EART 164 Spring 11

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) F. Nimmo EART 164 Spring 11

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: • 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) F. Nimmo EART 164 Spring 11

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 F. Nimmo EART 164 Spring 11

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 F. Nimmo EART 164 Spring 11

Isotopic signatures Solar N/Ne=1 Zahnle et al. 2007 F. Nimmo EART 164 Spring 11

3. Orbital forcing • Universal process, details vary with planet • For Earth, Milankovitch cycle forcing amplitudes are small compared to (observed) response – feedbacks? F. Nimmo EART 164 Spring 11

1. Earth F. Nimmo EART 164 Spring 11

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 (? ) F. Nimmo EART 164 Spring 11

Faint Young Sun Problem Rampino & Caldeira (1994) ce surfa ure t a r e p m te radiating • How were temperatures suitable for liquid water maintained 4 Gyr B. P. ? • Presumably some greenhouse gas (CO 2? ) • Urey cycle as temperature stabilizer F. Nimmo EART 164 Spring 11

Bombardment • Earth suffered declining impact flux: • Moon-forming impact (~10% ME, ~4. 4 Ga) • “Late veneer” (~1% ME, 4. 4 -3. 9 Ga) • “Late Heavy Bombardment” (0. 001% ME, 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 F. Nimmo EART 164 Spring 11

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? F. Nimmo EART 164 Spring 11

2. Mars F. Nimmo EART 164 Spring 11

Early Mars was Wet Hematite “blueberries” (concretions? ) F. Nimmo EART 164 Spring 11

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 2 O (Urey cycle) • Possible solution is highly acidic waters (? ) Mars lower atmosphere F. Nimmo EART 164 Spring 11

Transient early hydrosphere? • Alternative – cold Mars, with subsurface water occasionally released by big impacts Segura et al. 2002. Most of water is from melting subsurface. F. Nimmo EART 164 Spring 11

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 F. Nimmo EART 164 Spring 11

Evidence for obliquity cycles? Laskar et al. 2002 F. Nimmo EART 164 Spring 11

Snowball Mars? • Moderate obliquity periods may have allowed nearequatorial ice to develop Hellas quadrangle (mid-latitudes) F. Nimmo EART 164 Spring 11

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 F. Nimmo EART 164 Spring 11

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 F. Nimmo EART 164 Spring 11

MAVEN & MSL • MAVEN launches late 2013 • Measures Martian upper atmospheric composition and escape rates • MSL landed Aug 2012 • Measuring Martian rock and atmospheric compositions F. Nimmo EART 164 Spring 11

3. Venus F. Nimmo EART 164 Spring 11

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? F. Nimmo EART 164 Spring 11

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) F. Nimmo EART 164 Spring 11

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) F. Nimmo EART 164 Spring 11

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) F. Nimmo EART 164 Spring 11

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 F. Nimmo EART 164 Spring 11

Key equations F. Nimmo EART 164 Spring 11

End of lecture F. Nimmo EART 164 Spring 11

How about radar image of subsurface ice? F. Nimmo EART 164 Spring 11

Albedo feedback • Main sources of albedo are clouds and ices • Equilibrium gives: • 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) F. Nimmo EART 164 Spring 11

N, Ne and Ar – atmospheric loss 14 N/15 N 36/38 Ar Solar 357 5. 8 Venus - 5. 6 Earth 272 5. 3 Mars 170 5. 5 Jupiter 440 5. 6 Titan 56 - 20 Ne/36 Ar Why do we use isotopic ratios? Planets (except Jupiter) have more heavy N and Ar – loss process 20/22 Ne and 36/40 Ar tell us about radiogenic processes F. Nimmo EART 164 Spring 11

D/H – water loss • Higher D/H suggests more water loss • Not all loss mechanisms involve fractionation! Titan (CH 4) Chondrites Mars Venus (0. 016) Hartogh et al. 2011 F. Nimmo EART 164 Spring 11

Milkovich and Head 2005 F. Nimmo EART 164 Spring 11