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EART164: PLANETARY ATMOSPHERES

This text discusses key concepts in planetary atmospheric dynamics and explores the drivers and impacts of climate change on various planets. It covers topics such as the Hadley cell, zonal and meridional circulation, thermal tides, Reynolds number, turbulence, gravity waves, and more. The text also delves into the long-term evolution and climate change on Earth, Mars, and Venus, including processes like albedo changes, atmospheric loss, and orbital forcing. Other topics include the faint young sun problem, isotopic fractionation, freeze-out, ingassing, and impact-driven loss.

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EART164: PLANETARY ATMOSPHERES

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  1. Francis Nimmo EART164: PLANETARY ATMOSPHERES

  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

  3. Dynamics Key Concepts 2 ul ~(e l)1/3 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

  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. Teq and greenhouse Recall that So if a=constant, then t = a x column density So a (wildly oversimplified) way of calculating Teqas P changes could use: Example: water on early Mars

  6. Climate Evolution Drivers Albedo changes can amplify (feedbacks)

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

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

  9. Faint Young Sun 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? 4.4 Gyr ago, the Sun emitted 70% of today’s flux What would that do to surface temperatures?

  10. Feedbacks Temperature-albedo feedback can positive or negative Clouds – negative feedback (T , A ) Ice caps – positive feedback (T , A ) A, Aeq A, Aeq ICE CAP CLOUDS A A Aeq Aeq T 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 f is flux (atoms m-2 s-1, n is number density (atoms m-3)

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

  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: Here Fext is the particle flux of interest, pRext2 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 • This proceeds faster at higher temperatures and in the presence of water (+ and - feedbacks) • Causes removal of atmospheric CO2 on Earth and maybe Mars (but where are the carbonates?) • Reverse of this cycle helped initiate runaway greenhouse effect on Venus (see later) 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:

  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 (26Al) (Mars?) • Some volatiles (e.g. H2O, CO2) 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

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

  23. Faint Young Sun Problem Rampino & Caldeira (1994) surface radiating temperature • How were temperatures suitable for liquid water maintained 4 Gyr B.P.? • Presumably some greenhouse gas (CO2?) • Urey cycle as temperature stabilizer

  24. 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

  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 CO2 atmosphere, allowing water to persist at the surface How much CO2 would have been required? Atmosphere was subsequently lost One problem was absence of observed carbonates (Urey cycle) H2O Mars lower atmosphere • Possible solution is highly acidic waters (?)

  29. 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.

  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. Snowball Mars? • Moderate obliquity periods may have allowed near-equatorial ice to develop 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 Melosh and Vickery 1989 • 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)

  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 (CO2), surface temperature 450oC Earth has similar CO2 abundance, but mostly locked up in carbonates If you take Earth and heat it up, carbonates dissociate to CO2, increasing greenhouse effect – runaway Will this happen as the Sun brightens?

  38. Another runaway greenhouses This one happened first, and involves H2O H2O in atmosphere lost via photodissociation Once the water is lost, then CO2 drawdown ceases and the CO2 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, Energy-limited, Impact-driven, Freeze-out, Surface interactions, Urey cycle) and fractionation Orbital forcing, Milankovitch cycles “Warm, wet Mars”? Earth bombardment history Runaway greenhouses (CO2 and H2O) Snowball Earth

  42. Key equations

  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: • On Earth, 1% change in albedo causes 1oC 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 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. D/H – water loss Venus (0.016) Mars Chondrites Titan (CH4) Hartogh et al. 2011 Higher D/H suggests more water loss Not all loss mechanisms involve fractionation!

  48. Milkovich and Head 2005

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