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Comparative Planetary Atmospheres

Comparative Planetary Atmospheres. 1. Formation and evolution of planets and atmospheres Prof FW Taylor f.taylor@physics.ox.ac.uk. Aim of Course. To set the Earth in its context as a member of the Solar System

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Comparative Planetary Atmospheres

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  1. Comparative Planetary Atmospheres 1. Formation and evolution of planets and atmospheres Prof FW Taylor f.taylor@physics.ox.ac.uk

  2. Aim of Course • To set the Earth in its context as a member of the Solar System • To study the same physics in different atmospheres, and so gain further insight to the processes that control climate RECOMMENDED BOOKS: Lewis, J.S., The Physics and Chemistry of the Solar System, Academic Press, 1995. Chamberlain, J.W. and D.M. Hunten, Theory of Planetary Atmospheres, Academic Press, 2nd Edition 1987.

  3. Outline of Lectures (Weeks 1 & 2,Mondays and Fridays at 11am) Comparative Planetary Atmospheres • Formation and evolution of the solar system, the planets, and their atmospheres • The atmospheres of the terrestrial planets • The atmospheres of the giant planets • Measurements of temperature, composition and energy balance of planetary atmospheres

  4. The Earth and the other planets • Earth is one of a family of 8 planets • The planets form two groups: • 4 small, rocky inner planets • 4 large, gaseous outer planets

  5. Physical Properties of the Planets

  6. Formation of Solar System & Planets • Can we account for similarities and differences in the planets and their atmospheres? • Is it Origin, Evolution, or Both? • First need to understand how they were formed • Contents of the Solar System • Properties to be explained • Formation of Protosolar Nebula • Formation of Planets

  7. Contents of the Solar System • 1 Star (99.95% of SS by mass) • 8 Planets • ~60 satellites & 1 (4?) ring system(s) • ~6000 Asteroids (rocky) • ~1 billion Comets, Oort cloud & Kuiper belt objects (icy) • Gas & dust • Magnetic fields & energetic particles

  8. Properties to be explained • Planetary orbits are nearly all in the same plane (the ecliptic) • Planetary orbits are roughly circular • All of the planets orbit the Sun in the same sense • Planets spin in the same sense as their orbits (not Venus, Uranus) • Sun has almost all the mass, but planets (mostly Jupiter and Saturn) have most of the angular momentum of the solar system • Rocky planets form a family close to the Sun; gas/ice giants form a family far from the Sun. • Relative abundance of ‘heavy elements’ increases in the giant planets from 3 × solar for Jupiter to ~40 × solar for Neptune • Space between planets increases with distance • Solar system is transparent between mass concentrations • Atmospheric compositions and isotopic ratios are different.

  9. Formation of the Solar Nebula • The universe contains a lot of ‘loose’ material, mostly hydrogen and helium, but also some (~2%) heavier elements from old stars • Any local accumulation of mass will tend to grow, forming a region of locally enhanced density (a ‘molecular cloud’) in space • About 4,600 million years ago a molecular cloud collapsed under its own weight to form the Sun and planets, including Earth • First sums for gravitational collapse worked out by Sir James Jeans Born: 11 Sept 1877 in Ormskirk, LancashireDied : 16 Sept 1946 in Dorking, Surrey

  10. Formation of Protosolar Nebula

  11. Molecular Clouds • Molecular clouds are observed in the Universe • Typical values for temperature and density are: • T = 20K, • r ~ 1010 H atoms (10-17 kg) m-3 • gives MJ ≈ 1031kg ≈ 10 solar masses. • R ≈ 40,000 AU or about 1 light year. Molecular Cloud Barnard 68 (VLT)

  12. Formation of Circumstellar Disc

  13. Planet Formation

  14. Circumstellar Discs

  15. Loss of mass and angular momentum in circumstellar discs: The T-tauri phase of the Sun • Problems: • MJ is >> present mass of Solar System. • Distribution of angular momentum between Sun and planets. • Answer: • Soon after the Sun began to fuse hydrogen it entered its ‘T-tauri’ phase with ~ 3 x current luminosity and a very dense, high speed solar wind. • Mass loss of 10-8 MSun/year over 107 years. • Planets formed before the T-tauri phase. • Remaining solar nebula was swept away. • Angular momentum carried away by the solar wind, ‘despinning’ the Sun.

  16. Formation of Planets I: Encounter Theories • Propounded by Jeans (and Maxwell) but not generally accepted today • Basic idea • Planets pulled off sun by collision with a comet or by gravitational attraction of passing star. • Explains • Common direction of planets' orbital motion • Planets' nearly circular and coplanar orbits • Disadvantages • Hot gas could not have condensed into planets • Probability of a near encounter in our region of the Galaxy is vanishingly small, less than one in many millions

  17. Formation of Planets II: Condensation and Collapse of Solar Nebulainto Planetesimals* * a planetesimal is a chunk of matter large enough to affect others through gravitation (~a few km across). The theory explains: • The varying composition and size of planets: • Inner planets formed where the nebula was hotter and so only rocky materials could condense. Further from the Sun, volatiles especially water could also condense. • Planetisimals in the outer Solar System grew quickly and could accrete hydrogen and helium before T-Tauri phase removed remaining gas. • Jupiter and Saturn grew more quickly than Uranus and Neptune so the latter are more ice-rich. • The regular motions of the planets and moons: • all revolve in the nearly same plane, • in nearly circular orbits, • in same direction the Sun rotates. • Small bodies (asteroids and comets, KBOs, moons and rings) • Exceptions to the trends (Earth’s Moon, axial tilts, eccentricities) • Problem: recent discovery of ‘Hot Jupiters’ around other stars

  18. Formation of Terrestrial Planet Atmospheres • Did the atmosphere: • form with the planet out of the solar nebula? • outgas later from the interior? • accumulate from the solar wind? • arrive later as icy meteorites and comets? • Obtain clues from the relative abundances and isotopic ratios of the noble gases, allowing that some of these are of radiogenic origin. • For Venus, Earth and Mars it is found that: • the ratio of 20Ne to 36Ar is similar on all 3 planets, but different in the Sun: argues against (1) and (3) • primordial argon decreases by several orders of magnitude from Venus to Earth and from Earth to Mars. Argues against (4). • This leaves (2). Plus, outgassing is still observed (e.g. volcanoes).

  19. Formation of Outer Planet Atmospheres • Unlike the terrestrial planets, the gas giants were too massive, cold, and distant from the Sun to have lost their original atmospheres • If so, the giant planets are made up of primitive material from the solar nebula • Both Jupiter and the Sun are ~85% hydrogen, ~15% helium • The H2/He ratios on all four planets resemble that of the Sun • Heavier element abundances and noble gas ratios are difficult to measure and interpret because of condensation, chemistry, interior processes etc. They remain controversial research topics that are slowly yielding a detailed picture of the evolution of the Solar System.

  20. Processes affecting the evolution of atmospheres to their present state • Thermal escape to space* • Condensation, e.g. on permanent polar caps or as permafrost below the surface • Dissolve in oceans & subsequent removal, e.g. carbonate formation removes CO2 on Earth • Regolith absorption/chemical combination, e.g O2 ➔ rust • Hydrodynamic escape (lighter atoms move heavier ones) • Solar wind erosion (especially if no mag. field, Venus & Mars) • Impact erosion* (incoming mass blasts gases into space) • Sources (e.g. comets, volcanism)

  21. Thermal Escape: Jeans' Formula (1)

  22. Thermal Escape: Jeans' Formula (2)

  23. Characteristic time for reduction of exospheric density by Jeans escape for Earth’s atmosphere (Assumes exobase is at 500km altitude with temperature = 1480K)

  24. Characteristic Jeans escape timesfor different gases on several planets.

  25. Impact Escape • The inner planets suffered a period of heavy bombardment and the energy imparted can drive off some of the atmosphere. • This is most important for Mars and in that case even extends to driving off solid material (the SNC meteorites). If Me is the mass of gas driven off by an impactor of radius R then: where Ma = mass of atmosphere per unit area, ve = (2GM/r)1/2 is the escape velocity, and vsis the impact velocity. N.B. molecular mass is not a factor so no fractionation of species.

  26. Comparative Planetary Atmospheres II. Terrestrial Planets

  27. Mercury • Diameter 1.4 times Moon • Much denser than Moon: 5.43 vs. 3.34 g cm-3 • Temperature range 70 to 700 K • Thin atmosphere: surface pressure ~10-15 bar • Icy polar deposits in shaded craters

  28. Venus • Solid body resembles Earth • Small inclination and eccentricity – no seasons • Complete cloud cover of mainly 75%H2SO4.25%H2O. • No liquid water & very little vapour • Surface temperature ~ 730 K • Net insolation < Earth! • Equilibrium temperature ~ 240 K • 500K greenhouse effect (Earth ~ 30K) • Very thick CO2 atmosphere - 1000 km-atm of CO2 (Earth: 10-3) • Surface pressure 92 bars.

  29. Earth • Water in all three phases • Widespread water clouds • 70% liquid H2O coverage • N2 – O2 atmosphere • Surface pressure 1 bar • Mean surface temperature 288 K • Life is part of climate

  30. Mars • Thin CO2 atmosphere • Thin CO2 and H2O clouds. • Surface Pressure ~7 mb (variable) • Surface temperature 218 K (very variable)

  31. Radiative equilibrium temperature of a Planet Applying the Stefan-Boltzmann law we obtain for the total radiant power of the Sun: ESun = 4πσRS2(TS)4(W) where σ is equal to 5.670 x 10-8 W m-2 K-4. The solar constantS, is then given by S = σ TS4 (RS/DS)2 ( = 1.37 kW m-2 for DS = 1AU) where DS is the planet’s distance to the sun in AU. For equilibrium EE= 4πσRE2(TE)4 = (1-A)SπRE2 (W), whence the radiative equilibrium temperatureTE, given the albedoA.

  32. Calculating Model Vertical Temperature Profiles Assumptions • No absorption of sunlight in the atmosphere, only at the surface, i.e. atmosphere optically thin at short wavelengths (< 4 µm). • Most of the energy emission to space takes place from a narrow range of altitudes near the tropopause • Troposphere is optically thick, stratosphere is optically thin, in the mid and far infrared (> 4 µm)

  33. Tropospheric Lapse Rate Consider a ‘parcel’ of dry air at pressure p and temperature T rising adiabatically, i.e. so that p is that of its surroundings while T may change. From the first law of thermodynamics for one mole: dQ = CvdT + pdV = 0 And from the perfect gas law pdV + Vdp = RdT = (Cp – Cv)dT Using the hydrostatic equation, for balance between rising and falling CpdT = Vdp = -Vgdz = -Mgdz Hence where Γ is called the adiabatic lapse rate.

  34. Stratospheric Temperature • The observed lapse above the tropopause, where convection stops, tends to zero (i.e. constant temperature with height) • Each layer is heated by radiation from the optically thick atmosphere below, and cooled by radiating to space, to the same degree; to first order height is no longer important. • The stratospheric temperature TS may be estimated by treating the region as if it were a single slab of gas which is optically thin at all wavelengths, rather than just on average, which is the real situation. • TS is related to the effective radiative temperature TE by the expression for the energy balance of the stratosphere, treated as a slab of emissivity and absorptivity ε: εσ (TE)4 = 2εσ (TS)4 whence TS = TE/21/4

  35. A Simple Greenhouse model (Earth) • Need a further fixed point to make profile unique • Represent the atmosphere as a homogeneous slab at temperature Taoverlying the surface at temperature T0 • The atmosphere is completely transparent to solar radiation (at λ<4µm) and completely opaque to thermal radiation (λ>4µm). • At the surface, σ(T0)4 = 2σ (255)4 so T0=20.25 x 255 = 303 K. Transparent λ < 4µm Opaque λ > 4µm T0

  36. Radiative equilibrium of terrestrial planets

  37. Multilayer radiative equilibrium model for Venus

  38. The Temperature profile on Venus • Radiometric temperature = cloud top temperature • Cloud top pressure ~ 0.1 bar • Surface pressure ~ 100 bar • Pressure scale height H = RT/Mg, ≈Earth so ≈ 7 km • Lapse rate = g/cp ≈ 10 K km-1 • Cloud height z ≈ ln (0.1/100)H ≈ 50 km • Surface temperature ≈ 240 + 50*10 = 740 K

  39. Model temperature profiles

  40. Terrestrial Planet temperature profiles • Profiles based on measurements match simple theory • ozone heating on Earth • dissociation & ionisation heats thermospheres • Cool thermospheres on Mars/Venus - more CO2

  41. Radiative-Convective ModelTemperature Profile HEIGHT Optical depth changes TEMPERATURE

  42. Radiative-Convective ModelTemperature Profile HEIGHT Albedo changes TEMPERATURE

  43. Radiative-Convective ModelTemperature Profile HEIGHT Optical depth changes Albedo changes TEMPERATURE

  44. Composition of Terrestrial Planet Atmospheres

  45. Key Questions:(1) CO2 • Terrestrial planet atmospheres are secondary, i.e. outgassed from the interior • Suppose all three atmospheres were about the same at first, as indicated (with complications) by chemically inactive gases e.g. N2 • The primordial gases were primarily CO2 and H2O, plus smaller amounts of nitrogen (possibly as ammonia), argon &c, SO2 • Most of Mars’s CO2 lost by impact, perhaps a few very large impacts • Venus lost its liquid H2O and kept its CO2 in the atmosphere • Earth’s CO2 was dissolved in oceans and became carbonate rocks

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